Apollo 15 Flight Summary	Journal Home Page	Earth Orbit and Translunar Injection

Apollo 15

Launch and Reaching Earth Orbit

Corrected Transcript and Commentary Copyright © 1998 by W. David Woods and Frank O'Brien. All rights reserved.

[26 July 1971, first day of mission.]

[The morning launch of Apollo 15 from Pad A at Launch Complex 39 avoids the high heat of a Florida summer but the weather is clear and bright, with few scattered clouds, which will allow the crowds watching the spectacle to track the flight for much of its ascent to orbit. Launch Complex 39 at the Kennedy Space Center was built with 2 fully operational launch pads; in the expectation of a much higher launch rate than was ever achieved. Only Apollo 10 departed from Pad B at a particularly busy time in the Apollo program, leading up to the launch of Apollo 11. Every other Apollo/Saturn V launch, including Apollo 15, began from Pad A.]

[The spacecraft/vehicle stack is designated AS-510, signifying that it is the tenth launch of the Apollo/Saturn V combination. As the Apollo spacecraft components came off the production line, they were also assigned serial numbers to help distinguish them. Each had modifications which built on the experience gained from previous missions or which came from requirements of their particular mission. Therefore, each had checklists pertaining to their individual configurations.]

[This mission was originally intended to be similar in scope to Apollo 14, both classed as 'H' missions, the extent of lunar exploration being limited by the walking range of the landing crew. However, the H mission for Apollo 15 was cancelled on 2 September 1970 as part of NASA's budgetary cutbacks and the advanced 'J' missions were brought forward to take advantage of the enhanced exploration promised by the Lunar Roving Vehicle, a 2-man buggy which would allow astronauts to explore up to 10 kilometres from the Lunar Module. Because of this, Apollo 15 is jumping the numerical sequence of modules, missing out CSM (Command/Service Module) 111 and LM (Lunar Module)-9. LM-9's destiny is to be located in the Apollo/Saturn V Center at KSC, where it now forms part of an impressive visitor attraction dedicated to the Apollo Program.]

[CSM 112, among its upgrades, includes an array of scientific instruments and cameras called the SIM (Scientific Instrument Module), installed in a previously largely vacant sector of the Service Module. The Command Module includes controls for operating the SIM bay. The Lunar Module for this mission, LM-10, also has extensive modifications to permit an extended stay on the Moon, including improved propulsion and guidance, as well as the Lunar Roving Vehicle, cleverly folded on its side. The launch vehicle, AS-510, also has modifications to allow it to lift this heavier Apollo spacecraft to Earth orbit and then on to the Moon.]

[The timing of an Apollo launch to the Moon falls within certain 'windows' or periods of time which are influenced by daily and monthly factors. The daily restriction to the window is due to the rotation of the Earth bringing the launch site to the correct relationship with the Moon's position in its orbit, to allow enough of a parking orbit around the Earth before the boost to the Moon. The monthly factors are the lighting requirements at the landing site. The landing should take place in the early lunar morning so that the Sun will be behind the astronauts as they approach from their east-to-west orbit. A low Sun-angle will also produce shadows on the lunar terrain that allow the Commander to recognise landmarks as well as aiding speed and distance perception. With a lunar day lasting 29.5 Earth days, the correct conditions for the landing only occur monthly.]

[The first launch window for Apollo 15 begins at 09:34, Eastern Standard Time, on 26th July, 1971, and lasts 2 hours, 37 minutes. If technical problems or poor weather delay the launch, they can try again on the 27th, otherwise, they must wait for two opportunities in late August, or three more in late September.]

[Before the voyage commences, there are two sections to the CSM Launch Checklist to be followed. The lift-off configuration is set and verified by the back-up CMP (Command Module Pilot), in this case Vance Brand, who checks that each switch, knob, adjustment and talkback indicator is correctly set for the arrival of the prime crew. The CSM launch Checklist systematically covers each of the 57 panels in the Apollo 15 CSM. There are 454 lines in the checklist. Some panels are covered by a single line, while panel 2 on the main display console requires 78 lines.]

Command Module Main Display Console from Apollo Operations Handbook Block II Spacecraft (October 15, 1969). This console comprises panels 1, 2 and 3, and is very similar, though not identical, to the console in the Apollo 15 Command Module. This and other diagrams of the Apollo spacecraft are available from the Diagram page of the NASA History Website

[At first glance, the Main Display Console of the Command Module is utterly overwhelming. But for those to whom an aircraft cockpit is unfamiliar territory, the "front office" of a Boeing 747 is also a visual racket. Not unlike the tales of people who regain their hearing after a lifetime of deafness, and perceive sounds as a jumble and without structure, finding order in the Apollo spacecraft is a concentrated learning process. When viewed with the skilled eye of the Apollo 15 crew, the cockpit quickly turns into, perhaps not a thing of beauty, but an entity born of structure and function.]

[In February 2004, Dave Scott joined us for a review of the Journal.]

[Scott - "People don't have any idea how much stuff was in there."]

[Three hours before launch, the prime crew enters the spacecraft and once settled, they continue the prelaunch checks. CDR (Commander), Dave Scott, takes the left couch facing the major flight controls and the abort handle. CMP, Alfred Worden occupies the centre couch, facing the caution and warning panel and ready to monitor the computer's display during the critical minutes of ascent. The electrical and environment systems are monitored during launch by the LMP (Lunar Module Pilot), Jim Irwin, who takes the right couch. The checklist for boost preparation will be performed starting at T - 20 minutes.]

[Each stage of the Saturn V launch vehicle has shaped explosive charges attached to its outer surface which, in the event of an abort, rupture the fuel and oxidiser tanks, dispersing their contents into the atmosphere rather than allow them to impact the Earth with dangerous loads still on board. The charge for the S-IC (the designation of the first stage) cuts a longitudinal breach in the fuel tank on the opposite side of the vehicle from that for the oxidiser tank so as to minimise their mixing during dispersion. Charges for the S-II (second stage) cut a 9 metre longitudinal opening in the hydrogen fuel tank and a series of lateral 4 metre ruptures in the squat LOX (liquid oxygen) tank. Those for the S-IVB (third stage) make two parallel 6 metre openings in the fuel tank and a 1.2 metre diameter hole in the LOX tank. These charges are fired only after the Command Module has separated from the launch vehicle. During a normal ascent, the destruct system is safed soon after the Launch Escape Tower is jettisoned; after about 3½ minutes of flight.]

[Early in the Apollo program, the Air Force had insisted that the LM, a vehicle from which every ounce of unneeded weight had been trimmed and which would be a home to men on the lunar surface, also had to have destruct ordnance attached. This was based on the premise that, in the event of a launch abort, it was better for propellants to be consumed before reaching the ground. After all, the LM would be unmanned at this stage. NASA pointed out the weight and safety penalties of this arrangement and eventually won what was a substantial argument by pointing out that the LM was hardly likely to survive the destruction of the S-IVB stage anyway. A similar conflict occurred over demands by the Air Force that the Service Module also carry destruct charges.]

[Journal contributor Henry Spencer - "The actual countdown first appeared in Fritz Lang's movie 'Frau im Mond' (usual English translation 'The Girl in the Moon', although 'Woman' would be more accurate), and it appears to have been Lang's invention as a way to add suspense to a silent movie. The rocket experimenters at the VfR (Verein für Raumschiffahrt or Society for Space Travel) picked it up, and it spread from there, gradually elaborated into a lengthy pre-launch schedule."]

[Early countdowns did not have built-in holds. However, the countdown, even in the early years of rocketry, was not just a clock ticking down toward launch, it specified when various things happened pre-launch. During development, operational missiles suffered inevitable delays due to minor snags cropping up, despite their supposedly predictable schedules. While difficulties were ironed out, the count had to be paused, holding up other events. By building pauses into the count at carefully chosen times, engineers could use this time to appraise problems and deal with them appropriately when this would not cause trouble. A simple way to implement such a pause is to stop the clock.]

[The built-in hold became firmly established in the launch event schedule during the Gemini program, when it was important that launches of the spacecraft and their rendezvous target vehicles, the Agenas, were carefully choreographed and had to be on time.]

Public Affairs Officer - "This is Kennedy Launch Control at T minus 31 minutes and counting. T minus 31 on mission with Apollo 15; still Go. This is the fourth flight intended for a lunar landing and all is going well. [Strictly speaking, this is the fifth flight, as Apollo 13 was intended for a lunar landing until its explosion aborted the mission.] We're still aiming toward our planned T-zero at 9:34 a.m. EDT, when, if all goes well, those 5 big engines in the first stage of the Saturn 5 will ignite, generating more than 7.7 million pounds [34,250 kN] of thrust to start us on the way on a long trip to the Moon. 30 minutes, 25 seconds and counting. This is Kennedy Launch Control."

[Of the 110.6 metre height of the entire Apollo 15/Saturn V stack, 42.1 metres comprise the S-IC first stage. Five F-1 engines are clustered at the bottom of the stage to provide 34,025 kN (7,700,000 pounds) of thrust in total. The propellants used are RP-1 (Rocket Propellant-1 or highly refined kerosene) as the fuel and LOX as the oxidiser. The lower of two tanks is filled with 810,000 litres of RP-1 at T-13 hours with a final topping up occurring at T minus 1 hour. At nine hours before launch, the larger upper tank has nitrogen gas pumped through it to purge it of air and water vapour contaminants. Six and a half hours before launch, it is precooled to prepare it for the loading of 1.3 million litres of LOX at a temperature of -183° C. Initially, the LOX is fed at a slow rate of 95 litres per second until the tank is sufficiently chilled to retain the liquid up to 6.5% full, then the tank is filled to 98% at a rate of 630 litres per second, a process lasting over 45 minutes. The slow fill rate is reestablished until the tank is full at about 4 hours 55 minutes before launch. From then on, until three minutes before launch, the level is replenished as the volatile LOX boils off. Both of the first stage tanks are then pressurised prior to launch using helium; the fuel tank at T-96 seconds, the LOX tank at T-72 seconds.]

Public Affairs Officer - "This is Kennedy Launch Control; T minus 20 minutes, 56 seconds and counting. All aspects of the count are still Go, still aiming toward our plan T-zero at the appointed time of 9:34 a.m. Astronaut Al Worden ... Worden, the Command Module Pilot in the middle seat, has completed his checks of the pressurization system for the reaction controls of the Apollo spacecraft and all is still going well. Here in the Launch Control Center, the crew has started a sequence to chill down the upper two stages of the Saturn V because of the extremely low temperatures of the liquid oxygen and liquid hydrogen involved in the propellant system and it is necessary to condition the engine chambers in both the second and third stages so that they will be at a lower temperature when the propellants are introduced at ignition time during the powered stage of the flight. We'll have more than 11 minutes of powered flight in the first stage of the mission [not the first stage of the launch vehicle], before the spacecraft - the Apollo spacecraft is placed into a parking orbit some 90 nautical miles [166 km] high, still attached to the third stage [of the launch vehicle]. A second burn of that third stage of the Saturn V will place the spacecraft on its proper translunar trajectory. Our status; 19 minutes, 40 seconds and counting, all aspects still Go. This is Kennedy Launch Control."

[The second, or S-II, stage of Apollo 15's Saturn V vehicle is 24.9 metres tall and is powered by the combustion of LH₂ (liquid hydrogen) and LOX in a cluster of five J-2 rocket motors which generate a total thrust of 5,115kN (1.15 million pounds). A million litres of LH₂, cooled to -253°C to get it into a liquid state, is loaded into the large, upper tank of the stage while 331,000 litres of LOX is loaded into the smaller, squat tank below. These tanks share a single insulated structure with only an insulated, common bulkhead between them. With both propellants being so cold - LH₂ is only 20 degrees above absolute zero - the tanks must be prepared and chilled down before they can be filled.]

[Air and water vapour is purged from the tanks by repeated pressurisation and venting with helium. Helium is used because nitrogen would freeze in the presence of liquid hydrogen. Once clear of contaminants, the tanks are cooled to accept the propellants by first passing cold gas through the system then feeding propellant at a slow rate and allowing it to boil off, taking heat with it. Seven hours before launch, LOX is fed at 31.5 litres per second until it is 5% full, then the fill rate goes to 315 litres per second to take the tank to 96% full. This takes about 25 minutes and then the tank is topped up at 63 litres per second. Five hours before launch and after purging and cooling, LH₂ enters at 63 litres per second, further cooling its tank so that propellant begins to remain liquid and rise in level in a process similar to that for the LOX tanks. Once the level of liquid propellant reaches 5 percent, the fill rate is increased to 630 litres per second until the tank is 98 percent full, when the fill rate reduces again to 63 litres per second to top off the tank's load. To compensate for loss due to boil-off, both tanks are replenished until about three minutes before launch when the tanks are pressurised. Up to the launch, pressurising helium gas is supplied from the ground. After launch, the boil-off of the propellants is enough to maintain pressure until the engines are ignited 2 minutes and 40 seconds into the flight.]

[Atop the S-IVB stage, a ring 1 metre high and the same diameter as the stage, called the IU (Instrument Unit), carries the guidance and control electronics for the Saturn V. Particularly important among these are the LVDC (Launch Vehicle Digital Computer) and the inertial platform, the ST-124, which has to be aligned to the intended trajectory before launch. The angle between north and the groundtrack of the vehicle's flight path is called the flight azimuth. For this launch, it is to be 80.088° or slightly north of east. The gyroscopically stabilised platform at the heart of the ST-124 is aligned with the intended azimuth just prior to launch by rotating the X stable member with reference to a theodolite mounted some distance away from the launch pad. There is a small window in the skin of the Instrument Unit for this purpose.]

[At T - 20 minutes, the Boost Preparation checklist deals with alignment of the X Stable Member Azimuth and checks that the various RCS (Reaction Control System) thrusters on the side of the Service Module (SM) are powered from the two main electrical busses in such a fashion to allow maximum RCS control, should one bus lose power.]

[The crew is checking the final status of the electrical system and the setup of the FDAI (Flight Director Attitude Indicator). This instrument, often called the "8-ball", is similar to the ball-style artificial horizon found on many aircraft and, likewise, allows determination of the spacecraft's attitude with respect to a desired frame of reference, usually this will be the IMU (Inertial Measurement Unit) though it may be the GDC (Gyro Display Coupler), a device which displays the spacecraft's attitude compared to a crew preselected attitude. The FDAI will also show attitude errors and the rates of change of attitude.]

[Astronaut Vance Brand, the back-up CMP, communicates with the spacecraft from the LCC (Launch Control Center), five kilometres from the launch pad. In Mission Control, Houston, astronaut Gordon Fullerton is designated the CapCom for the first part of the mission and is awaiting the moment when the ascending vehicle will clear the tower it has stood next to and control of the mission transfers from the LCC to Houston.]

[The third stage of the Saturn V, called the S-IVB for historical reasons, could be described as a smaller version of the S-II stage in that it also consists of a single tank structure with a common bulkhead between the LH₂ and LOX compartments. These propellants, which are stored at the same supercold temperatures as for the S-II, are burned in a single J-2 engine which yields a thrust of 890 kN (a shade over 200,000 pounds). The engine's capability for restarting is utilised for the boost out of Earth orbit to the Moon. The construction of the S-IVB's propellant tank differs from the S-II stage by having the insulation on the inside of the tank's metal skin, a detail which made manufacture easier by not having to develop a bonding system which had to work at only 20 degrees above absolute zero. With the insulation between it and the propellant, it would be substantially warmer. About 8 hours before launch, the cryogenic systems of the S-IVB stage are purged, including the engine feeds and pump cavities. At T minus seven and a half hours, the LOX tank is precooled by pumping LOX onboard at 31.5 litres per second and allowing its conversion to a gas to take away heat from the tank. When enough liquid remains to fill the tank to 5%, the fill rate goes to 63 litres per second, the fast fill rate, until the tank is 98% full. Finally the tank's total capacity of 92,350 litres is reached at a slow fill rate of up to 19 litres per second, and after that, replenished at a rate of up to 2 litres per second. The LOX tank filling takes about 25 minutes. Filling of the 253,200 litres of LH₂ follows a similar process beginning 4 hours and 11 minutes before launch. Tank precooling and filling to 5% is achieved with a fill rate of 31.5 litres per second, before the fast filling of the tank at 190 litres per second takes the tank's quantity to 98% three and a half hours before launch. The slow rate of fill is reestablished to top off the tank and keep it replenished. LH₂ tank pressurization is maintained, during initial flight, by the boil-off of the fuel, then later with helium from a collection of spheres mounted on the exterior of the thrust structure at the base of the stage. The LOX tank is pressurised from heated helium fed from cold storage tanks within the LH₂ tank.]

[The Saturn V launch vehicle is assembled, transported on, and launched from the Mobile Launcher. This structure consists of a base platform 48.8 x 41.1 metres and 7.6 metres high with a 13.7 metre square hole over which the vehicle is mounted. (The platforms were later converted for use by the Space Shuttle.) Sprouting from one end of this platform is the LUT (Launch Umbilical Tower). This 116 metre tower bears nine swing arms which provide the ground crew with access points to the vehicle, and a wide range of services including fuel, LOX, hydraulics, electrical power and various gases for purging and pressurization. These arms are articulated so they can swing away from the vehicle to give it clearance as it rises, and to protect them from the rocket's white hot exhaust gases. The crew enter the spacecraft via the top, or ninth, arm, which carries an environmentally controlled room at its end. Known as the "white room", it covers the CM hatch until the crew is aboard. 43 minutes before launch, it is swung away from the spacecraft by 12°. Five minutes before launch, it completes its retraction to 180°, on the opposite side of the tower from the Saturn V.]

[The Flight Plan indicates that in the CM, the five launch vehicle indicator lights are illuminated at T - 4 minutes, 10 seconds. Throughout powered flight, these lights, arranged to resemble the pattern of the engine clusters on the S-IC and S-II stages, will provide the commander with cues about the progress of the boost and the status of each engine in each stage. Readers can familiarize themselves with this subpanel through the movie Apollo 13. Two events in the film that focus on this panel are when Tom Hanks (as Jim Lovell, Apollo 13's commander) manually jettisons the Launch Escape Tower, and when the inboard engine of the S-II stage prematurely shuts down. Then we see one of the few flaws in this movie, otherwise known for its incredibly high standards of accuracy. The central light is shown to blink on and off. In reality, it simply changes from on to off or vice versa.]

[Dave Scott, from 1998 correspondence - "In Apollo 13, the movie, the light was purposely made to blink to get the viewers attention - the movie-makers knew the actual operation, but chose to take this license for dramatic effect (actually a pretty good license, as otherwise, the viewer would have missed the point!)."]

[At T - 2 minutes, 15 seconds; glycol coolant is routed to bypass a radiator on the surface of the Service Module. The radiator will be heating up from aerodynamic friction during the passage through the atmosphere and therefore will not work as a cooling device. Coolant flow to the radiator will be reinstated once the spacecraft is in Earth orbit.]

[The Q-ball cover is not unlike a pitot tube cover on an aircraft, it covers and protects eight openings at the top of the Launch Escape Tower. These openings lead to instruments which gauge air pressure. As well as providing dynamic pressure (known as "Q") information during powered flight, they help determine the angle of incidence of the tower during an abort event.]

[The CM batteries are connected across the two main power busses in the spacecraft to supply the extra power required during this particularly busy period. They also ensure that systems will continue to be powered in the event of a fuel cell failure during powered flight. The batteries will be recharged later, once the mission settles down to a lower power regime.]

[In preparation for the higher noise level during launch, the volume of the radio in the astronauts' headsets is increased.]

[The FDAI is aligned so the vehicle's attitude can be monitored. The attitude of the launch site, at the time of launch, with respect to the stars, is the frame of reference used for this alignment. At launch, the FDAI will display roll, 178° (launch azimuth of 88° plus 90°); pitch, 90°; yaw, 0°.]

Public Affairs Officer - "Now 25 seconds. We have complete clearance to launch. We are Go. 20."

[At T - 17 seconds, the Saturn's inertial guidance platform is released from the systems which have up to now been holding it in the correct orientation for the flight.]

[The ignition sequence of an F-1 engine is a complicated affair with many interrelated events happening almost simultaneously.]

[First, a description of the engine. A large combustion chamber and bell have an injector plate at the top, through which, RP-1 fuel and LOX are injected at high pressure. Above the injector is the LOX dome which also transmits the force of the thrust from the engine to the rocket's structure. A single-shaft turbopump is mounted beside the combustion chamber. The turbine is at the bottom and is driven by the exhaust gas from burning RP-1 and LOX in a fuel-rich mixture in a gas generator. After powering the turbine, the exhaust gas passes through a heat exchanger, then to a wrap-around exhaust manifold which feeds it into the periphery of the engine bell. The final task for these hot gases is to cool and protect the nozzle extension from the far hotter exhaust of the main engine itself. Above the turbine, on the same shaft, is the fuel pump with two inlets from the fuel tank and two outlets going, via shutoff valves, to the injector plate. A line from one of these 'feeds' supplies the gas generator with fuel. Fuel is also used within the engine as a lubricant and as a hydraulic working fluid, though before launch, RJ-1 ramjet fuel is supplied from the ground for this purpose. At the top of the turbopump shaft is the LOX pump with a single, large inlet in-line with the turboshaft axis. This pump also has two outlet lines, with valves, to feed the injector plate. One line also supplies LOX to the gas generator. The interior lining of the combustion chamber and engine bell consists of a myriad of pipework through which a large portion of the fuel supply is fed. This cools the chamber and bell structure while also pre-warming the fuel. Lastly, an igniter, containing a cartridge of hypergolic fluid with burst diaphragms at either end, is in the high pressure fuel circuit and has its own inject point in the combustion chamber. This fluid is triethylboron with 10-15% triethylaluminium.]

[At T minus 8.9 seconds, a signal from the automatic sequencer fires four pyrotechnic devices. Two cause the fuel rich turbine exhaust gas to ignite when it enters the engine bell. Another begins combustion within the gas generator while the fourth ignites the exhaust from the turbine. Links are burned away by these igniters to generate an electrical signal to move the start solenoid. The start solenoid directs hydraulic pressure from the ground supply to open the main LOX valves. LOX begins to flow through the LOX pump, starting it to rotate, then into the combustion chamber. The opening of both LOX valves also causes a valve to allow fuel and LOX into the gas generator, where they ignite and accelerate the turbine. Fuel and LOX pressures rise as the turbine gains speed. The fuel-rich exhaust from the gas generator ignites in the engine bell to prevent backfiring and burping of the engine. The increasing pressure in the fuel lines opens a valve, the igniter fuel valve, letting fuel pressure reach the hypergol cartridge which promptly ruptures. Hypergolic fluid, followed by fuel, enters the chamber through its port where it spontaneously ignites on contact with the LOX already in the chamber.]

[Rising combustion-induced pressure on the injector plate actuates the ignition monitor valve, directing hydraulic fluid to open the main fuel valves. These are the valves in the fuel lines between the turbopump and the injector plate. The fuel flushes out ethylene glycol which had been preloaded into the cooling pipework around the combustion chamber and nozzle. The heavy load of ethylene glycol mixed with the first injection of fuel slows the buildup of thrust, giving a gentler start. Fluid pressure through calibrated orifices completes the opening of the fuel valves and fuel enters the combustion chamber where it burns in the already flaming gases. The exact time that the main fuel valves open is sequenced across the five engines to spread the rise in applied force that the structure of the rocket must withstand.]

[As fuel and LOX flow increase to maximum, the rise in chamber pressure, and therefore thrust, is monitored to confirm that the required force has been achieved. With the turbopump at full speed, fuel pressure exceeds hydraulic pressure supplied from ground equipment. Check valves switch the engine's hydraulic supply to be fed from the rocket's fuel instead of from the ground.]

[NASA image KSC-71PC-543 shows the vehicle on the pad as the engines build up to their full thrust.]

[One second to lift-off, the five launch vehicle indicator lights in the spacecraft have gone out, announcing to the crew that the thrust is OK and the stack is about to be let go of the hold-down clamps. Another lamp is illuminated at the point of lift-off as a verification for the crew.]

[The stack is held onto the pad two ways. Four hold-down arms clamp the base of the S-IC, each with a force of 350 tonnes, anchoring the vehicle until full thrust is confirmed. A pneumatic device, backed up by an explosive, collapses the lever linkage to allow the arm to rise. Additionally, a number of controlled-release mechanisms (up to 16, depending on the mission) prevent the vehicle from accelerating too rapidly in the first moments of motion. These consist of tapered pins mounted to the pad which are pulled through dies mounted on the vehicle. The deformation of the pins controls the initial acceleration for the first 150 mm of flight; a simple and ingenious arrangement.]

[Flight Plan page 3-001]

[Apollo 15 launch (336KB) in Real Video]

[Apollo 15 launch (14.7MB) in MPEG format.]

[Once the launch vehicle begins to rise, even fractionally, it cannot safely settle back onto the pad. Intentional engine shutdown will not occur, so of the nine access arms, the five which have remained attached up to this point, must now detach their umbilicals from the vehicle and swing clear. The first two centimetres of travel trigger the release of the umbilical connector plates which in turn triggers retraction of the arms.]

[At lift-off, one of the most important areas of the main display console that the commander must monitor is the subpanel which annunciates the progress of the launch. From top to bottom, this subpanel contains the Abort light, Mission Event Timer, Launch Vehicle Engine lights, lift-off light and critical switches (with safety covers) that are used to override the automatic abort sequences.]

[With a line that has come from crews since Alan Shepard's first flight for America, Dave Scott is confirming that the MET (Mission Event Timer) has begun counting. The timer receives a signal that the vehicle has lifted off, begins incrementing and the Flight Plan calls for this to be reported.]

000:00:04 Worden (onboard): Okay ... lift-off ...

[On this, and all Saturn V launches, the stack can easily be seen leaning away from the LUT as it ascends from the launch pad. This yaw, begun 1.35 seconds after lift-off, maneuvers the vehicle 1.25° from vertical in a direction away from the tower to ensure clearance in case a gust of wind pushes it back or a swing arm doesn't fully retract. Then nine seconds into the flight, almost as the rising rocket clears the tower, the stack is brought vertical again. To those who were not prepared for it, this fully intended yawing of over 110 metres of metal, filled to the brim with exotic fuels and oxidisers, could sometimes cause some consternation. The launch checklist for the CSM calls for the initiation of this yaw to be reported to the ground but the transcript does not carry it.]

[Scott, from 1998 correspondence - "The approved, required, authorized, etc. procedures during launch were on the cue cards. For A-15, there were 45 "CSM Cue Cards," all of which were prepared (in a single book) by the Crew Procedures Division. The Cue Cards represent the proper crew procedures when applicable. During launch, the A-15 CDR Cue Card indicated :00 - lift-off, and then next :11 - Roll. Therefore, in summary, there was no "yaw" transmission to the ground. How this got into the "checklist" I do not know, but as the missions wore on, "interested parties" were able to get into the loop. During the early days of Apollo, the crew only prepared the "checklist," which was later approved by the Crew Procedures Change Board. An interesting part of the Apollo 'culture.'"]

[All Saturn V launches were extensively photographed from every conceivable angle and s71-41356 is a fine shot of the rising Apollo 15 stack from Kipp Teague's web site. Eric Jones, editor of the Apollo Lunar Surface Journal was one of those lucky enough to witness the launch.]

[Jones, from 1999 correspondence - "I was on the beach about 10 miles from the pad for Apollo 15 and I will never forget listening to PAO as the excitement built and then seeing the engines fire and then feeling - more than hearing - that incredible noise as the Saturn V began to rise. I was screaming 'Go, go, go', completely unable to hear myself. Awesome!"]

[Woods, from 1999 correspondence - "I never knew you were actually there at launch. Care to tell us more about your experience that morning - for the Journal, of course?"]

[Jones, from 1999 correspondence - "Actually, I don't remember a great deal about it other than the magnificence of the spectacle. I made the mistake of trying to take pictures which, as you experienced during the eclipse, actually detracts. I do remember that it was an interesting mix of middle-aged Middle Americans in their campers and hippies. There was a lovely girl with a kitten on a string wading in the shallow water while we also listened to the countdown via the many radios that folks had with them. And the traffic leaving afterwards was incredible."]

Public Affairs Officer - "The tower is clear."

[Once the ascending vehicle has cleared the Launch Umbilical Tower, control of the mission is transferred to Mission Control in Houston, Texas. Gordon Fullerton assumes the role of CapCom from Vance Brand.]

[Scott, from 1998 correspondence - "The 'Tower Clear' call is made by the Launch Director - this is a very critical transmission in terms of both safety and responsibility: (1) safety, of course, at the instant of the call, and by visual observation of the Launch Director, the Saturn is clear of a major obstacle (such failures as an engine hardover would probably be catastrophic prior to Tower Clear); and (2) this is the official transfer of mission responsibility from the LCC (Launch Control Center) at the Cape (the Launch Director) to the MCC in Houston (the Flight Director). The Flight Director would have acknowledged this over the comm link with the Launch Director. This is very important topic, as few people seem to realize the significance of the call."]

000:00:15 Fullerton: Thank you.

[Dave has reported that a guidance program in the Saturn V's Instrument Unit (IU) has begun to maneuver the stack in roll. The launch pads at Launch Complex 39, Kennedy Space Center, are aligned to the points of the compass with the LUT north of the vehicle. Therefore, at launch, the vehicle's frame of reference, its azimuth, is 90° east of north and the purpose of this roll maneuver is to align the launch vehicle with the desired trajectory, with an azimuth 88.088° east of north, before it begins to pitch over. These maneuvers place the spacecraft into a "heads down" attitude.]

[Scott, from the 1971 Technical Debrief - "My evaluation, compared to Apollo 9 [on which he flew as CMP], was that the lift-off itself was softer and quieter. When the tiedowns went, we could feel definite motion, but it didn't seem like as much as it was on Apollo 9."]

[Woods, from 1998 correspondence with Scott - "Could the difference between your perception of Apollo 9's and Apollo 15's lift-off be due to the greater weight of the stack in the latter mission?"]

[Scott, from 1998 correspondence - "Unlikely - there are probably many other variables with greater influence on the 'feel' of a launch - each launch seemed to have its own unique characteristics, even each mission. The crew reports after each mission were, among other uses, factored into the simulators - which to subtle, but perceptible, degrees were changed in some form after each mission - like sounds of thrusters, sometimes louder and sometimes softer. Also, the added mass in percent was probably less than temperature and wind changes in percent - on any given launch. "]

000:00:21 Scott: Thanks, Gordo. Roll's complete.

000:00:24 Fullerton: Roger.

000:00:27 Scott: And we have a pitch program.

000:00:28 Fullerton: Roger. Pitch. [Pause.]

[KSC-71PC-547 shows the vehicle well into its vertical flight.]

[Scott, from 1998 correspondence - "During launch, all transmissions between the spacecraft and MCC are made to and from the Commander and the CapCom, only. This is essential to maintain continuity, clarity, command, and control of the existing situation as well as potential and actual abort situations. In the spacecraft, just as in MCC, only one person communicates during time-critical situations."]

[Ascent has been nearly vertical up to now, except for the yaw maneuver for tower clearance. The vehicle has rolled to the required azimuth and, under control of the IU, is pitching over to begin gaining horizontal velocity.]

[The guidance of the launch vehicle is monitored and controlled by the Launch Vehicle Digital Computer in the IU and its programs, and not by the CM guidance and control system, which only monitors for now. The value of this arrangement was dramatically demonstrated on Apollo 12 when, during the early seconds of the flight, lightning temporarily knocked out the CM's electrical system, causing the CM's Inertial Measurement Unit (IMU) to tumble and lose its pointing reference. The IU, clearly unaffected by the enormous pulse of electricity which passed through the vehicle, continued to faithfully guide the Saturn towards orbit while the crew worked to reset the CM's guidance system.]

000:00:42 Fullerton: Mark. One Bravo.

000:00:43 Scott: Roger. One Bravo. [Long pause.]

[The role of Public Affairs Officer has also transferred to Houston.]

['Booster' is one of the posts on the front row of Mission Control. After the Saturn V finishes its task, and Apollo 15 is on its way to the Moon, the three booster consoles are vacated.]

[Throughout the powered ascent of the launch vehicle, there are various modes of aborting the mission, each of which are appropriate to the current height and speed. For the first two of these modes, IA and IB, the Flight Plan defines the safe range of vehicle motion rates as not exceeding ±4° per second in pitch and yaw, ±20° per second in roll. Motion rates exceeding these limits will entail an abort.]

[The initial 42 seconds, to an altitude of about 3,000 metres (10,000 feet) are flown in abort Mode IA (one alpha). If a dangerous situation occurs within this period, the CM would separate from the SM, and the LET (Launch Escape Tower, or just 'tower'), which is the solid-fuelled rocket mounted on top of the CM, would carry it up from the wayward launch vehicle while a small 'pitch control' motor at the top of the LET steers the assembly east out over the ocean and away from a possibly exploding booster below. The tower would be jettisoned only 14 seconds after the initiation of the abort. While this is going on, the highly dangerous hypergolic propellants of the Command Module's RCS would quickly and automatically be dumped overboard as they would be harmful to the recovery forces. The CM would then descend on parachutes to a normal splashdown.]

[Abort Mode IB extends from 42 seconds into the flight to an altitude of 30.5 km (16.5 nautical miles) as defined by the abort checklist. With the vehicle being further downrange and tilted over, the pitch control motor would not be required in the event of a IB abort. However, it had been discovered during hypersonic testing, that the CM/LET stack could be aerodynamically stable in a tower-first as well as a base-first attitude so a pair of canards were added which would be deployed automatically to force the combination into an attitude where the base of the CM is facing the direction of travel, ready for the safe deployment of the drogue and main parachutes. While the canards have little effect in a low altitude abort, they become increasingly important as the Saturn V gains speed through the IB mode.]

[Scott, from 1998 correspondence - "The 'abort' function was so very critical in terms of success/failure that many people thought there should be no crew function, and it should all be automatic (which in turn would introduce other more consequential failure modes). The most difficult simulations during the entire training process were 'launch aborts' - even more so than lunar landings (the landing itself was more difficult than launch, but not for 'aborts'). More crews 'bought it' during launch sims than any other area, by far!"]

000:01:03 Irwin (onboard): Here...

000:01:03 Fullerton: 15, Houston. Everything looks perfect down here.

000:01:06 Scott: Okay. Looks cleared up here, Gordo. [Long pause.]

Scott (onboard): Okay, the altimeter is six zero.

000:01:12 Worden (onboard): Well, how about that?

000:01:28 Irwin (onboard): Exciting!

000:01:30 Scott (onboard): Okay. We're through Max Q.

000:O1:34 Worden (onboard): Okay. 1:32. Program looks good, Dave. Out at 11 miles.

000:01:40 Scott (onboard): Okay. Good shot. Two and a half g.

000:01:44 Irwin (onboard): Roger.

000:01:47 Scott (onboard): Pitch profile looks good.

000:01:50 Irwin (onboard): Roger.

000:01:52 Worden (onboard): 1:50.

Public Affairs Officer - "Going through maximum dynamic pressure at this time."

[The aerodynamic forces acting on the launch vehicle have been rising as the vehicle gains speed. However, the air around it is thinning rapidly with its increasing altitude. The interaction of these two changing values results in a maximum degree of pressure on the vehicle's skin at 1 minute, 22 seconds; at a speed of about Mach 1.7 and an altitude of 13.7 km. This moment of maximum dynamic pressure is also often described as Max Q.]

[Scott, from the 1971 Technical Debrief - "The noise was relatively low-level, and none of us had any trouble with the comm at all. We had vibrations within the S-IC which were just about the same frequency as the noise you hear standing on the ground. You hear the reverberations from the engines - or the S-IC vibrations were about the same frequency, low amplitude - just something you could feel. Going through max Q was noisy, but we still had good comm. And it didn't seem to me that that was as loud as it was on Apollo 9 either. I could hear Jim call 'cabin pressure relieving' very clearly. You could hear pretty well all the way through there, too."]

[Irwin, from the 1971 Technical Debrief - "Yes, I thought the comm was excellent."]

[Worden, from the 1971 Technical Debrief - "I guess the shaking of the S-IC was a little bit more than I expected. More lateral shaking, a little more vibration than I expected right at lift-off. When we got away from the tower and got away, maybe from some ground effects, whatever it was, it smoothed down."]

[During the ascent through the atmosphere, it is important that the rocket points straight into the direction of its motion, and avoids flying sideways, even slightly, lest aerodynamic forces overload its structure. This is achieved by flying the first stage, and the start of the second stage, according to a carefully calculated, preprogrammed tilt schedule.]

[Scott, from the 1971 Technical Debrief - "During the launch phase we went right into the Sun. At one point during launch, I put my hand up to shield my eyes so I could see the ball [the FDAI]. I was surprised."]

000:01:54 Fullerton: Stand by for Mode One Charlie.

000:01:57 Fullerton: Mark. One Charlie now.

000:01:58 Scott: Roger. One Charlie.

[Mode IC is used for aborts occurring between 30.5 km (16.5 nautical miles) and the jettison of the tower. As the air is now very thin, the airflow around the pair of canards at the top of the tower would have little aerodynamic effect during an abort, so the Command Module's RCS would be used to control the orientation of the spacecraft until they become effective. The safe range of vehicle motion rates are now defined as not exceeding ±9° per second in pitch and yaw, ±20° per second in roll.]

000:02:01 Scott: EDS Auto to Off.

000:02:03 Fullerton: Roger. [Long pause.]

[As the S-IC nears the end of its burn, Al Worden is inhibiting the EDS (Emergency Detection System) with a switch directly below the computer keypad. The EDS is only needed for flight through the thickest part of the atmosphere where high aerodynamic forces and the structural load they impart to the vehicle could cause loss of control to turn catastrophic too quickly for the crew to react in time. With EDS switched off, any required aborts must be initiated by the crew.]

000:02:14 Worden (onboard): Okay. The program's looking good.

Public Affairs Officer - "Each of the five S-IC engines [are] gulping 3 tons of fuel [means propellant] per second."

[This value is split into about a ton of fuel and two tons of oxidizer each second for each F-1 engine. Across the five engines, the stage was consuming 13.5 tonnes of propellant each second.]

000:02:17 Fullerton: Roger. Inboard. [Long pause.]

[Towards the end of the S-IC's boost phase, two factors serve to gradually increase the acceleration experienced onboard. First, the atmosphere is becoming essentially a vacuum, reducing the backpressure the exhaust gases have to fight against compared to the air pressure at sea level. This improves the efficiency of the engines so that from launch to cut-off, S-IC thrust rises 19% from 34,250 kN to about 40,700 kN. Second, the S-IC tanks are emptying, lightening the launch vehicle and presenting less inertia to the thrust. To alleviate these rising acceleration forces and the shock of all five engines shutting down simultaneously, the centre engine of the cluster is shut down 23.5 seconds earlier than the outboard engines. The outboard engines cutoff at 2:39.6, staging occurs at 2:40.7 and the second stage ignition command is at 2:41.8.]

[Another effect of the reduced air pressure on the S-IC is visible on movie coverage of the launch as the base of the vehicle appears to be progressively consumed by the conflagration. Near the ground, the plume is constrained by air pressure into a narrow flame extending rearwards. With decreasing air pressure, the hot gases are able to expand into an ever widening plume. Towards the end of the S-IC's flight, the air is so thin and the slipstream so negligible that a small amount of exhaust is able to expand forwards up the side of the rocket's structure giving the appearance, on TV coverage, of the rocket's base being consumed by the plume.]

[The spent S-IC has lifted the vehicle to about 70 km; above most of the atmosphere. The S-II will continue the ascent to the orbital height of about 170 km, as well as accelerating the vehicle to about 90% of the velocity required to achieve a stable orbit.]

000:02:36 Scott (onboard): Five seconds.

Public Affairs Officer - "Flight dynamics reports Go for staging."

000:02:43 Worden (onboard): Man, you aren't kidding.

000:02:44 Irwin (onboard): Boy.

000:02:46 Scott: Good stage.

000:02:47 Fullerton: Roger. [Pause.]

[The term "Good stage" is used to mean that one stage of the stack is spent and has been separated from the rest of the rocket to allow the stage above it to take over the task of acceleration. The tilt sequence, which has defined the trajectory throughout the flight so far, has brought the rocket's slow, pre-programmed tilting motion to a halt so that during staging, it's attitude is stable which avoids complicating the event with unwanted rotation of the vehicle.]

[The S-IC/S-II staging sequence for AS-510 differed markedly from earlier missions. On previous flights, the interstage, or skirt, a 4.9-metre tall ring matching the 10-metre diameter of the S-IC and S-II that it sits between, carried solid-fuel rockets which fired shortly after the first stage separated to settle the S-II propellants in their tanks. AS-501 and AS-502 had eight of these ullage rockets, while AS-503 to AS-509 had four. They were deleted from the Apollo 15 launch vehicle, along with four of the eight retro rockets built into the conical engine fairings around the base of the S-IC, in order to save weight and increase payload. Ullage is a brewers' term for the portion of a barrel occupied by air, not liquor. The separation did not quite go according to plan. After the F-1 engines were shut down, the thrust they generated during the tail-off period was greater than expected. The engines don't instantly stop thrusting when they receive their cut-off command. After a quick drop to about 2% thrust, they take over four seconds to decay to zero. As the engines expired, the acceleration imparted to the, now separate, empty and therefore light S-IC stage was above the predicted value. Despite deliberately coasting for longer than usual between separation and S-II ignition, the distance between the two stages was less than engineers had planned and the blast of hot gases from five J-2s against the top of the empty stage disabled a telemetry package with which the S-IC was to be monitored until its impact with the Atlantic Ocean.]

[Staging of the S-IC and S-II is technically described as a "dual plane separation," as the vehicle is cut across two geometrical planes. The first plane is between the skirt and the S-IC, with the S-II engines starting 1.1 seconds later. The second plane separation, when the second stage loses the skirt, occurs at 3 minutes, 10.7 seconds; 30.0 seconds after the S-IC separation. This time allows the S-II's attitude to stabilise because if either part of the launch vehicle were to be yawing or pitching excessively, there would be a danger of the engine bells striking the S-IC and skirt as the two great metal cylinders coast along before ignition of the S-II. The skirt provides clearance above the first stage's LOX tank for the five J-2 engines of the S-II stage.]

000:02:58 Scott: Okay. [Long pause.]

Public Affairs Officer - "73 [nautical] miles [135 km] downrange, 47.9 [88.7 km] altitude."

000:03:00 Worden (onboard): Okay, Dave. You got TFF 3:26.

000:03:02 Scott (onboard): Good show. We'll go on time.

[Scott, from the 1971 Technical Debrief - "The staging was as we expected, I guess. It was what I'd call violent when the S-IC shuts down and everything recoils there, and that was almost identical to Apollo 9. It was really just a big bang. We saw the fireball come up to the BPC [Boost Protective Cover]; I saw it in my left side window. I saw the fireball out the front window, too."]

[The Boost Protective Cover over the Apollo 15 CM had windows in it to allow visibility through both the CM hatch window (window 3) that the CMP could look through, and the left-hand rendezvous window (window 2).]

[Scott, from the 1971 Technical Debrief - "Right after, or just prior to, the S-II ignition, there was a lateral motion, attitude-wise, in the vehicle. Sometime in this staging sequence, we got a slight yaw."]

[Worden, from the 1971 Technical Debrief - "After being briefed several times of what to expect at separation, it didn't seem as violent as I was really expecting it to be."]

[Scott, from the 1971 Technical Debrief - [speaking to Worden] "Which one?"]

[Worden, from the 1971 Technical Debrief - "The first one."]

[Scott, from the 1971 Technical Debrief - "I thought you agreed that it was pretty violent."]

[Worden, from the 1971 Technical Debrief - "It was pretty violent, but I guess I was expecting something even more than that."]

[Irwin, from the 1971 Technical Debrief - [speaking to Scott] "You had us so well briefed, Dave, that we were expecting it."]

[The S-II stage carries five J-2 uprated engines which burn LH₂ and LOX to produce up to 1,041 kN (234,000 pounds) thrust each. They are capable of being restarted in flight but this feature is only implemented in the engine used in the S-IVB.]

[The thrust chamber and bell of each engine is fabricated from stainless steel tubes brazed together in a single unit. Supercold LH₂ is pumped through these tubes to cool the thrust chamber and simultaneously prewarm the fuel. The engine carries two separate turbopumps, both powered in turn by the exhaust from a gas generator which burns the stage's main propellants. The hot gas exhaust is fed from the gas generator, first to the fuel turbopump, then to the LOX turbopump before being routed to a heat exchanger and finally into the engine bell. The fuel and LOX outputs of both turbopumps are fed, via main control valves, to the thrust chamber injector via the LOX dome. Unlike the solid steel injector of the F-1, the J-2 injector is fabricated from layers of stainless steel mesh sintered into a single porous unit. A solid LOX injector behind this carries 614 posts which pass LOX through the injector and into the combustion chamber. Each post has a concentric fuel orifice around it and these orifices are attached to the porous injector. The fuel delivery is arranged to ensure that about 5 percent seeps through the injector face to cool it, the rest passing through the annular orifices.]

[The ASI (Augmented Spark Igniter), fed with propellant and mounted to the injector face, provides a flame to initiate full combustion. Valves are provided to bleed propellant through the supply system well before ignition to chill all components to their operating temperatures otherwise gas would be formed which would interfere with the engine's use of propellant as a lubricant in the turbopump bearings. A tank of gaseous helium is fabricated within a larger tank of gaseous hydrogen. This is the Start Tank. The helium provides control pressure for the engine's valves while the hydrogen spins up the turbopumps before the gas generator is ignited. A PU (Propellant Utilization) valve on the output of the LOX turbopump can open to reduce the LOX flowrate. This adjusts engine thrust down to 890 kN (200,000 pounds) during flight to optimise engine performance.]

[To start the J-2 engine, spark plugs in the ASI and gas generator are energised. The Helium Control and Ignition Phase valves are actuated. Helium pressure closes the Propellant Bleed valves, it purges the LOX dome and other parts of the engine. The Main Fuel valve and the ASI Oxidiser valves are opened. Flame from the ASI enters the thrust chamber while fuel begins to circulate through its walls under pressure from the fuel tank. After a delay to allow the thrust chamber walls to become conditioned to the chill of the fuel, the Start Tank is discharged through the turbines to spin them up. This delay depends on the role of the engine. A one second delay is used for the S-II engines. Half a second later, the Mainstage Control Solenoid begins the major sequence of the engine start. It opens the control valve of the gas generator where combustion begins and the exhaust supplies power for the turbopumps. The Main Oxidiser valve is opened 14° allowing LOX to begin burning with the fuel which has been circulating through the chamber walls. A valve which has been allowing the gas generator exhaust to bypass the LOX turbopump is closed allowing its turbine to build up to full speed. Finally, the pressure holding the Main Fuel valve at 14° is allowed to bleed away and the valve gradually opens, building the engine up to its rated thrust.]

[The interstage ring or skirt has been jettisoned.]

000:03:17 Scott: Tower Jett.

000:03:18 Fullerton: Roger. We confirm the Skirt Sep. You're Mode II.

000:03:21 Scott: Roger. Mode II.

[A single, small, solid-propellant motor near the top of the tower fires for one second, jettisoning the entire LES (Launch Escape System) and the checklist moves to abort mode II.]

[The LES consists of the tower, with all its rocket motors, instrumentation and canards; and the BPC (Boost Protective Cover), which is a shroud over the entire Command Module which protects the spacecraft from the friction generated heat of ascent, and from the exhaust of the Launch Escape Motor should the tower be used for an abort. The BPC includes two windows to allow visibility through the left hand and central windows of the CM. The central window is mounted in the main hatch of the CM. Only when the LES is jettisoned are the other three windows uncovered.]

[The main rocket motor of the LES, if used, would burn for eight seconds, generating 654 kN (147,000 pounds) through four nozzles which are angled to direct the exhaust away from the CM.]

[Abort Mode II lasts from the jettisoning of the tower to the decision to stage from the S-II to the S-IVB. In a Mode II abort, the Command and the Service Modules will separate from the launch vehicle and the SM main engine or its RCS engines will be used to get the spacecraft away from the launch vehicle. Then the CM and SM will separate before the CM completes a normal splashdown on the ocean.]

000:03:28 Fullerton: Roger. [Long pause.]

[Up to now, the IU has been steering the vehicle according to a predetermined trajectory which minimises sideways movements though the air while tilting it along its flight azimuth. It has not been using any information about its position and velocity to alter its flight path. The control is "open loop." Dave Scott's call of "Guidance initiate," marks the point where the IU begins the IGM (Iterative Guidance Mode). The control loop is closed as the IU begins using position and velocity data to help it guide the vehicle to where it wants to get to - an accurate Earth orbit.]

[Scott, from the 1971 Technical Debrief - "The S-II was very smooth, and all the way through the S-II burn, we had a very light - I'd guess in talking about it - we figured 10- to 12-cycle-per-second vibration, something in that range, low-amplitude, something you could just feel, but it was continuous all the way through. There was no pogo, no change in the oscillation. Tower jett was smooth and came away very cleanly."]

[Being a huge, long, metal structure in a very dynamic environment, with large quantities of fluid moving inside at speed, the stack is prone to vibrations along its length. This lengthwise stretching and squeezing is known as "Pogo" from its similarity to the spring action of the "Pogo stick," a contemporary child's toy. The structure itself vibrates at particular frequencies which, as the fuel tanks empty, sweep through a substantial range. Other variables, such as vehicle and payload mass, and propellant characteristics, make it exceedingly difficult to predict the frequency and magnitude of these pogo oscillations.]

[Many launch vehicles suffer pogo problems during development and the Saturn V was never completely free of it. The S-II, in particular, exhibited severe pogo which plagued the early flights. One partial cure for this is to shut down the centre engine earlier than the others.]

000:03:37 Scott (onboard): How we doing, Al?

000:03:38 Worden (onboard): We're doing fine; 63 miles.

000:03:41 Scott (onboard): Good.

000:03:48 Irwin (onboard): Everything looks good over here.

000:03:50 Scott (onboard): Okay.

000:03:51 Worden (onboard): Just - looks just about a hundred feet per second down on the H-dot, but everything else looks fine.

000:03:56 Scott (onboard): Okay, I got the big fireball going by at staging. I don't know whether you saw it or not. That beauty really goes.

000:03:59 Worden (onboard): Yes.

000:04:01 Fullerton: 15, Houston. At 4 [minutes], the guidance has converged. The CMC is Go. Everything looks good.

000:04:06 Scott: Okay, Gordo. Looks good up here. [Long pause.]

[The CMC (Command Module Computer) will be closely involved in computation of the spacecraft's trajectory throughout the mission. However, during launch and ascent, the IU handles trajectory computation and vehicle control. The CMC is accessed by the crew via two DSKYs (Display and Keyboard), one of which is on the main instrument panel, the other being on the LEB (Lower Equipment Bay). The LM carries a computer of very similar design.]

[Since they left the pad, the CMC has been running Program 11 for two main purposes. It lets the crew monitor the progress of the ascent, and it gives Dave the option of having the CMC control the ascent instead of the IU, even with crew input.]

[Scott, from 1998 correspondence - "The CMC computed the entire launch profile such that if the IU failed (at any time) the entire Saturn V, or subsequent stages, could be flown into orbit manually by using primarily the FDAI and DSKY displays and one of the two RHCs [Rotational Hand Controllers]. This is a little-known, but very important capability; and we spent a fair amount of time training for this. The DSKY display also provided a monitor of the IU performance, and if deviations were too large, [you could] disengage the IU completely and proceed with a manual launch insertion."]

[There is a note in the launch checklist, page L2-8, that gives the simple procedure for changing to manual control of the booster. The LV Guidance switch at the bottom left of Panel 2 is switched away from IU to CMC, then Verb 46 in the computer is executed, allowing control signals from the Rotational Hand Controllers to be processed and routed to the Saturn V.]

000:04:53 Scott (onboard): I hope so by now.

Public Affairs Officer - "Now at 35 percent of the velocity needed to orbit. Downrange 190 [nautical] miles [351.6 km]. Altitude 79 [nautical miles, 146.2 km]."

000:04:59 Fullerton: 15, Houston. Five minutes. Everything looks nominal. You're Go.

000:05:02 Scott: Okay, Gordo; thank you. Looks good up here. Got a smooth ride so far.

[The crew is reporting their current status each minute of the ascent, as called for in page 2-8 of the CSM Launch Checklist.]

Public Affairs Officer - "40 percent of velocity needed [to achieve orbit]."

000:05:09 Fullerton: Roger. [Long pause.]

Public Affairs Officer - "Downrange 271 [nautical miles, 501 km] altitude 88 [nautical miles, 163 km]."

[Apollo 15 has just about reached the correct height for the Earth parking orbit. This orbit will be very low, but it is sufficient for the two hours and fifty minutes that the crew and Mission Control require to check all the vehicle's systems before they leave for the Moon. The stack is travelling almost horizontally, skimming through the upper atmosphere and accelerating towards an orbital velocity of 7,800 metres per second.]

000:05:44 Irwin (onboard): Okay.

Public Affairs Officer - "Predicting nominal shutdown on the S-II stage."

000:05:43 Fullerton: 15, Houston. Times are nominal. The level sense arm will be 8 plus 34, and S-II cut-off at 9 plus 09. Over.

[This method of relating GET (Ground Elapsed Time) is a common shorthand used throughout the mission. To some extent, the context determines whether the first figure is hours or minutes. These times are 0 hours, 8 minutes and 34 seconds; and 0 hours, 9 minutes and 9 seconds.]

[Each propellant tank in the S-II has five sensors near the bottom which signal when they are uncovered by the draining liquid. When it receives two such signals from the same tank, the IU computer begins a sequence which will lead to engine cut-off. However, this engine cut-off system is not armed until the measured level has fallen below a certain threshold so as to inhibit the possibility of a false shutdown. Fullerton is telling the crew when Mission Control expects the cut-off system to be armed, based on current consumption, and when the engines will subsequently be shut down.]

000:05:57 Fullerton: And stand by for S-IVB to COI.

[COI stands for Contingency Orbit Insertion. This is another way of saying "abort Mode III". The S-IVB now has the capability to take the stack to a point where the Service Module's large SPS engine can ignite and place the CSM into Earth orbit. However, in the event of such an abort, and without the S-IVB, the spacecraft would not be able to depart for the Moon, instead embarking on a planned for, but hopefully unrequired Earth orbit mission.]

000:05:59 Fullerton: Mark. You have S-IVB to COI now.

000:06:01 Scott: Roger. S-IVB to COI. [Long pause.] [To Irwin:] Jim?

000:06:04 Irwin (onboard): Ready. l, 2, 3, 4.

000:06:11 Worden (onboard): On and trimmed.

000:06:22 Scott (onboard): Okay, we're right - right down the line.

000:06:26 Irwin (onboard): Okay.

000:06:27 Worden (onboard): Coming up on 15,000 V_i, 260 H-dot. That's right where we ought to be, and that gives us an SPS COI.

000:06:32 Scott (onboard): Okay. Keep your eye up for S-IVB to orbit.

000:06:38 Worden (onboard): Yes.

Public Affairs Officer - "380 [nautical] miles [703 km] downrange, altitude 94.5 [nautical miles, 174.9 km], velocity 15,000 feet per second."

[Apollo 15 is now above their nominal orbital height and for a time, the attitude of the vehicle will be nose down.]

[The crew is at the top of page 2-9 of the CSM Launch Checklist. Communication with Mission Control is through a ground station in Bermuda and is via one of four antennas spaced around the Command Module at 90° intervals, designated A, B, C and D. The Flight Plan calls for Omni(directional) antenna D to be used if the launch azimuth is less than 96°, antenna C if greater than 96°. The actual launch azimuth was 88.088° so antenna D should be used.]

[The four flush mounted S-band antennas are used for near Earth communication and as a back-up to deep space communication. There is a capability for the ground to switch between antenna D and one which has been manually selected by the crew. Normally, the crew would select antenna B, which is on the opposite side of the spacecraft to D. Then, as one antenna goes away from line of sight of the Earth, the other will come into view and the ground can take care of antenna selection. However, if the crew manually selects antenna D, then the ground effectively lose control of which antenna is being used at any particular time.]

000:06:46 Fullerton: Mark. You have it now.

000:06:48 Scott: Roger; S-IVB to orbit. [Long pause.]

[Should the S-II cut out early, the S-IVB now has the ability to place itself, the CSM and LM into a safe orbit. Of course, there would not then be sufficient propellant onboard to boost to the Moon. However, the crew will switch to an alternate mission plan.]

[Scott, from 1998 correspondence - "There were several Alternate Mission Timelines in Section 6 of the Flight Plan to be used in the event there was no TLI. They were very straightforward and were essentially an extension of the pre-TLI timeline. We spent very little time on them, only to be aware that they were there and the general nature of the requirements. Although Al Worden did probably work on the non-lunar-return reentry to ensure that he was proficient in that area too."]

000:07:34 Worden (onboard): V_i's 18,000 .... We're at 96 miles.

Public Affairs Officer - "Downrange 479 [nautical] miles, [886.5 km], altitude 96 [nautical] miles, [177.7 km], now approaching 65 percent of velocity needed for orbit. The velocity now is 16,700 feet per second [5,090 metres per second]. Official time of lift-off [9 hours] 34 minutes, 00 seconds .79. [Eastern Daylight Time]"

000:07:40 Scott: Inboard.

000:07:41 Fullerton: Roger. Inboard. [Long pause.]

Public Affairs Officer - "Downrange 660 [nautical] miles [1,221.5 km], altitude 95 [nautical] miles [175.8 km], velocity 78% velocity required for orbit."

[As with the first stage, the centre or inboard engine of the S-II is cut-off early; in this case, to minimise the vehicle's pogo oscillation tendencies late into the burn. The inboard engine cut-off was at 7:39.5, 1 minute, 29.5 seconds before the outboard engines.]

[At 8:03.8, the PU (Propellant Utilization) valves open, reducing the LOX flowrate and therefore the mixture ratio to the engines from 5.5:1 to 4.8:1. This results in a perceptible change in the g-forces felt by the crew. It is part of a strategy to make sure that as little propellant as possible is left in the tanks when the second stage has done its work. The AS-510 Saturn V Flight Manual explains that this changeover occurs when the S-II stage has achieved a predetermined velocity change, or Delta-V. Earlier flights of the S-II used the readings from onboard sensors to calculate the most appropriate time for PU shift.]

000:08:ll Worden (onboard): Yes.

000:08:12 Fullerton: 15, Houston. 15, Houston. Go ahead. Say again, 15?

000:08:25 Scott: Houston, 15. We didn't call. You got something?

000:08:29 Fullerton: You've had PU shift, and the thrust looks good.

000:08:32 Scott: Okay.

[CapCom Gordon Fullerton had thought he heard the crew speak. With the situation clarified, he informs Dave that PU valves have operated to ensure optimal engine performance.]

[Scott, from the 1971 Technical Debrief - "We didn't notice the PU shift; when we went through it, I couldn't feel anything ... I remember on Apollo 9, we also didn't feel the PU shift, but I guess other crews have felt it."]

[Fullerton is informing the crew that the "level sense arm" signal has been sent to the IU. The engines will be shut down once two probes in one of the tanks have been uncovered by the dwindling propellant. They can expect shutdown shortly.]

Public Affairs Officer - "About 6 seconds to staging."

000:09:11 Fullerton: Stand by for Mode IV capability.

000:09:15 Mark. You have Mode IV now.

[Mode IV is the abort mode where the crew have been given a Go decision to continue to orbit using the S-IVB, and should that stage deviate from its allowed limits, the CSM will separate from the Saturn and use the SPS (Service Propulsion System) to continue into Earth orbit.]

[The sequence of events for the first ignition of the single J-2 engine in the third stage is essentially the same as for the engines in the S-II (see 000:02:58). The main change is that the supercold fuel is allowed to flow through the walls of the thrust chamber to condition it for three seconds, instead on one, before the Start Tank discharges through the turbines, spinning them up in preparation for operation.]

[The outboard engines cutoff at 9:09. One second after outboard cutoff, the S-IVB separates from the S-II. Ignition of the S-IVB's single J-2 engine occurs a tenth of a second later.]

[Although part of the S-IVB in terms of construction, the conical aft interstage is left with the S-II at separation. Unlike the earlier staging, this is a single plane separation as the vehicle is essentially outside the effects of the atmosphere. Also, as there is only one engine, there is no possibility of an unbalanced thrust across a cluster of engines skewing the S-IVB's attitude.]

[Scott, from the 1971 Technical Debrief - "The S-II to S-IVB staging was about a quarter to a fifth the force of the S-IC staging. It was again a positive kind of feeling, but it wasn't a violent crash like we felt on the S-IC; I didn't think. We had the same light 10- to 12-cps vibration on the S-IVB all the way into orbit. The shutdown was smooth. All the sequences throughout the launch were nominal and as expected. All the lights worked good; controls and displays were good; comfortable."]

[Irwin, from the 1971 Technical Debrief - "The noise and the vibration were less than I was expecting; it was much less. I was impressed about the lateral vibration on launch. It was much greater on the S-IC than it was on the S-II. Just a shaking, back and forth, lateral vibration all the way through the launch. It was a pretty smooth ride."]

Public Affairs Officer - "Booster reports thrust okay on the S-IVB stage."

000:09:24 Fullerton: 15, Houston. You've had - you have good thrust on the S-IVB.

000:09:28 Scott: Roger.

[Comm break.]

000:10:46 Fullerton: 15, Houston. Everything's looking perfect. Predicted [S-IVB engine] cut-off time [is] 11 plus 37. Over.

000:10:54 Scott: Roger; 11 plus 37. [Long pause.]

Public Affairs Officer - "1,281 [nautical] miles [2,372 km] downrange, 93.4 [nautical miles, 173 km] altitude, 97 percent of velocity... 98 percent of velocity required. [Current velocity is] 25,143 feet per second."

000:11:36 Scott: Okay. Cutoff. 11 plus 34.

000:11:39 Fullerton: Roger. [Long pause.]

Public Affairs Officer - "Booster confirms the S-IVB has shutdown."

[This first burn of the S-IVB was 3.8 seconds shorter than was predicted. This was due to the over-performance of the previous stages, and higher than expected thrust from the S-IVB's J-2 engine. For sixty seconds during its burn, the engine has been refilling the spherical Start Tank with gaseous hydrogen. This will be used to respin the turbines when the engine is restarted for the boost out of Earth orbit. The helium tank within the Start Tank does not need to be refilled as there is sufficient in it for three or four restarts.]

[The crew are now experiencing prolonged weightlessness for the first time in the mission after a ride that has subjected them to loads of up to 4g.]

[The above graph was derived from the Flight Evaluation Report for AS-510 and displays how the acceleration of the vehicle changed throughout the boost, from about 1g on leaving the launch pad to weightlessness 11½ minutes later. The key events in the graph are:

1. Launch with ignition of the S-IC. Note how the acceleration rapidly rises with increasing engine efficiency and reduced fuel load.
2. Cut-off of the centre engine of the S-IC.
3. Outboard engine cut-off of the S-IC at a peak of 4g.
4. S-II stage ignition. Note the reduced angle of the graph for although the mass of the first stage has been discarded, the thrust of the S-II stage is nearly one tenth of the final S-IC thrust.
5. Cut-off of the centre engine of the S-II.
6. Change in mixture ratio caused by the operation of the PU valve. The richer mixture reduces the thrust slightly.
7. Outboard engine cut-off of the S-II at a peak of approximately 2.7g.
8. S-IVB stage ignition. Note again the reduced angle of the graph caused by the thrust being cut by a fifth.
9. With the cut-off of the S-IVB's first burn, the vehicle is in orbit with zero acceleration.

The 4g reached during boost is not the highest that will experienced during the mission. Entry through the Earth's atmosphere decelerates the Command Module by about 6½g.]

000:11:59 Fullerton: Roger; 40 and 31.

[During firing, pressurisation of the LOX tank comes from eight helium spheres in the LH₂ tank which, via a heat exchanger in the engine, feed helium into the ullage space above the LOX. 40 psi is within the range of normal operation but between now and the restarting of the engine, the LOX pressure will be allowed to decay. During ascent on the previous stages, LH₂ tank pressure is maintained by boil-off of the liquid hydrogen itself. After S-IVB ignition, GH₂ (gaseous hydrogen) is generated by the engine for tank pressurisation. 31 psi is within allowable limits. Up to the time of S-IVB restart, the CVS (Continuous Vent System) will regulate tank pressure at 19.3 psi, propulsively venting GH₂ generated by boil-off of LH₂. This vent also provides a force to settle propellants in the tanks.]

000:12:24 Fullerton: Roger, Al. Copy. [Long pause.]

000:12:46 Fullerton: 15, Houston. IU shows you in a 92.5 by 91.5 [nautical mile orbit]. Radar confirms that. And the booster is safed.

[The crew is consulting their computer's DSKY (display and keyboard), as required by the checklist, which tells them that they have achieved a slightly eccentric orbit of 93.7 nautical miles (173.4 km) by 88.9 nautical mile (164.5 km) and their velocity is currently 25,595 feet per second. H dot is the derivative of height with respect to time - essentially the rate of change of height - and, at 8 feet per second, is small as would be expected for a nearly circular orbit. The current altitude is 93.2 nautical miles (172.5 km). The IU is giving a slightly different answer which Mission Control have observed through telemetry and which has been confirmed by radar. This is a very low orbit and one which cannot be sustained for long due to the friction with the tenuous air at this altitude. Indeed, there is significant heating on the skin of the vehicle due to atmospheric friction. However, this is considered acceptable for the three hours duration of the parking orbit and the low altitude allows the Saturn to carry a greater payload.]

000:13:01 Fullerton: Roger. [Long pause.]

000:13:28 Fullerton: 15, Houston. The booster is configured for orbit. Over.

000:13:35 Scott: Roger.

[Comm break.]

[On attaining orbit, the APS (Auxiliary Propulsion System, the S-IVB's own attitude control system), begins a maneuver to rotate the vehicle from the 18° pitched down attitude at shutdown of the S-IVB, to the local horizontal (i.e. with respect to the Earth's surface below). This maneuver is greater on Apollo 15 than the 6° to 10° noted on previous flights, due to the lower altitude of the final orbit, and is carried out at a rate of 0.3° per second. However, the large size of the maneuver helps develop a slosh wave within the nearly three quarters full LOX tank and about 220 kilograms of LOX is lost through an intentionally open vent. Future missions would rotate the stack at only 0.14° per second to minimise propellant sloshing.]

[The APS then sets up a slow pitch rotation at a rate which matches the vehicle's rate of rotation about the Earth (therefore called 'orb rate') so that throughout the orbit, the same side keeps facing Earth. In this case the crew is in a 'heads down' attitude and the vehicle is pointing in the direction of travel.]

[There seem to be a number of interrelated reasons for the orb rate maneuver during the Earth parking orbit. As we have seen, making large attitude changes to the very full S-IVB is to be avoided to inhibit sloshing in the tanks. Already, nearly a quarter of a tonne of LOX has been lost. They entered orbit nearly "pointy-end-forward" and will boost to the Moon (the TLI or Translunar Injection burn) in a similar attitude. Staying in that same attitude, relative to the local horizontal, avoids excessive attitude changes. Also, this attitude presents a small cross section to the extremely thin atmosphere that the spacecraft and S-IVB is still moving through, and this minimises friction induced heating. It may be preferable for the small thrust resulting from the venting of LH₂ to push the vehicle in the same direction as orbital motion. Finally, keeping the CSM in a 'heads down' attitude ensures that the spacecraft's optics, mounted on the opposite side of the Command Module from the hatch, are facing towards the stars so Al can sight on them during the upcoming navigation tasks.]

[Woods - "there was a propulsive vent going on the whole time while you were in Earth orbit. They were venting the boil-off as a propulsive vent, partly to settle the propellants. And I wondered if you had any sense of there being a microgravity, a drift to the aft end of the spacecraft?"]

[Scott - "No."]

[Woods - "Was it too small?"]

[Scott - "Don't remember it. I'm not sure I didn't stay strapped in the whole time. Al was the only guy that got out, wasn't he?"]

[The checklist moves to page 2-11, 'Insertion and Systems Checks.']

000:14:53 Worden: Okay; go ahead.

000:14:55 Fullerton: Minus decimal 1 degrees. One tenth of a degree, minus.

000:15:02 Scott: Roger. Minus .1.

000:15:06 Fullerton: That's correct.

[Woods - "As soon as you get to orbit, you're given a Z-torquing angle. What is that?"]

[Scott - "That torques the CSM guidance platform to your nominal orientation - the orientation you want after insertion because it may have drifted during launch. Before launch, you gyrotorque down to the last minute so that everything's aligned. And then you go through the launch and you get all this vibration and all that stuff. I think what they do when you get into orbit is to torque it again to null again all the drift and bias built up during launch."]

[Communication is about to be lost through the Bermuda ground station but will be reacquired in about a minute through a station on Gran Canaria, one of the Canary Islands just off the coast of North Africa.]

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