Launched June 21, 1993
Authors:
Brian Finley, Dr. Carlos Grodsinsky, and Richard DeLombard
The maiden voyage of the commercial Spacehab laboroatory module on-board the STS-57 mission was integrated with several accelerometer packages, one of which was the Space Acceleration Measurement System (SAMS). The June 21st, 1993 launch was the seventh successful mission for the Office of Life and Microgravity Science and Application's (OLMSA) SAMS unit. This flight was also complemented by a second accelerometer system, the Three-Dimensional Microgravity Accelerometer (3-DMA), a Code C funded acceleration measurement system, offering an on-orbit residual calibration as a reference for the unit's four triaxial accelerometers.
The SAMS accelerometer unit utilized three remote triaxial sensor heads mounted on the forward Spacehab module bulkhead and on one centrally located experiment locker door. These triaxial heads had filter cutoffs set to 5, 50, and 100 Hz.
The mission also included other experiment-specific accelerometer packages in various locations. This summary report will not explicitly present data from these packages. However, where available, data from these other packages will be given as a reference.
This report provides an assessment of the low-level acceleration environment on the first Spacehab (SH-1) mission (STS-57) as measured by the Space Acceleration Measurement System (SAMS). There were also other accelerometers on-board STS-57, a brief description of these accelerometers and their associated data acquisition systems will be covered for general information where future publications will present data set comparisons with the SAMS device.
The following will furnish interested investigators with a guide to evaluating the acceleration environment during this mission and identify areas which require further study.
The SH-1 mission was the seventh mission for SAMS with four previous missions in the Spacelab module, one in the middeck and one in the cargo bay. Reports resulting from previous SAMS missions are listed in the bibliography.
Acceleration is the rate of change of velocity and is measured in terms of meters per second per second (m/s2). In this report, all acceleration magnitudes have been normalized to the acceleration of gravity at sea level, approximately 9.8 m/s2.
The acceleration environment experienced on a spacecraft is a complex combination of accelerations caused by a multiplicity of sources. Typical sources of accelerations are gravity, rotating and oscillating machinery, atmospheric drag, thruster jets, vehicle motion, crew motion, etc. The characteristics of the accelerations caused by these sources varies considerably, both in magnitude and frequency, where most time histories are non-stationary. The locations of the vehicle center of mass, the experiment(s), and the acceleration disturbance sources, along with the vehicle dynamics determine the primary acceleration disturbance at an experiment location. The magnitude of accelerations typically varies from parts of a micro-g to milli-g's. The frequency range of accelerations of concern to experiments is typically between quasi-steady accelerations to 300 Hz. Quasi-steady accelerations are characterized by very low frequencies at an orbital period, (i.e., approximately 90 minutes).
Motion sensors can be grouped into two general categories: (1) inertial and (2) non-inertial. An inertial motion sensor provides a measurement of the motion with respect to an inertial reference frame, while a non-inertial motion sensor measures motion with respect to a non-inertial reference frame. The following sections are dedicated to the SAMS accelerometer system which measures the inertial response of the shuttle at the sensor attachment points.
Inertial motion sensors (accelerometers and inertial rate devices) have performance characteristics which vary with the frequency of input excitation. Mathematically, the frequency response of a sensor is described by its transfer function. In general the transfer function is a complex value dependent on frequency. The magnitude and phase of the sensor response to the input excitation are typically plotted separately as a function of frequency. The magnitude of the frequency response is typically sufficient to describe the important features of the sensor dynamics.
The magnitude function is the ratio of the amplitude of the output to the amplitude of the input excitation as functions of frequency. If the sensor is intended to measure velocity, then the ratio of output voltage to the input velocity excitation should ideally be flat over the frequency range of the sensor. Therefore, if the magnitude of the transfer function from input to output for the sensor is constant over the operating range, then the sensor response is said to be flat.
An inertial motion sensor can only exhibit a flat response over some finite frequency range. Above and below the operating frequency range (generally the flat portion of the magnitude function) the response is attenuated, or "rolled off".
As mentioned earlier, an inertial motion sensor is limited in the range of excitation frequencies that it can be used to measure reliably. In addition, a given sensor can only measure excitations which fall within a limited range of amplitudes. The smallest change in the measured quantity which a sensor is capable of measuring accurately is called the sensor resolution. A motion sensor can reliably measure any excitation which falls within this range of amplitudes bounded at one end by the resolution of the sensor, and bounded at the other end by the amplitude which would result in the maximum output range of the sensor. If the excitation acting on the sensor location is too large (i.e. has too large an amplitude), then the sensor will saturate, creating an output which is usually equal to the maximum output voltage of the sensor. If the excitation is too weak (i.e. has an amplitude smaller than the resolution of the sensor), then factors such as static electric fields and magnetic interference which are not related to the excitation of interest (called noise) have more influence on the output of the sensor than the excitation. This situation leads to a poor correlation between the excitation and the sensor output. In order to measure large excitations, sensor components are generally made stiffer, and the electronics less sensitive therefore, the amplitude range of the sensor is very important to consider in that the sensor noise floor must be at least an order of magnitude less than the smallest excitation amplitude of interest and that the largest amplitude expected is within the amplitude range of the sensor used.
The measurement of motions having very small amplitudes is difficult. Most accelerometer manufacturers publish values of the sensitivity and maximum allowable impact for the various models they produce. The sensitivity is merely the value of the magnitude function in the flat portion of the curve. When there is no flat portion on the curve, sensors are often calibrated by experimentally fitting a straight line through data showing the sensor voltage output as a function of the magnitude of the input. The slope of this line is also the sensitivity of the sensor, and is equivalent to the definition given previously. The units of the sensitivity are output per unit input.
It is logical that one could measure as low an amplitude excitation as desired, as long as the sensitivity of the sensor were high enough. Unfortunately, even an accelerometer with a very high sensitivity will fail to provide an accurate measurement when the excitation becomes sufficiently small due to its noise performance. In addition, the amplitude at which the measurement breaks down varies with frequency. What is needed is a plot of the amplitude at which the measurement provided by a sensor breaks down versus frequency. This function is called the incoherent power spectrum, and is obtained using signal processing techniques.
One of the problems with highly sensitive inertial sensors is that they tend to sense motions that they are not intended to sense. For example, a rotational velocity sensor will have some sensitivity to linear motion. This undesired response is called cross-axis, or parasitic sensitivity. [1].
The Sunstrand QA-2000 transducer is an inertial, linear accelerometer which functions on an electromagnetic principle. Figure 2.1 provides an illustration of the operating principle behind this proof-mass accelerometer. The inertial mass is composed of a magnetized material, and thus produces a magnetic field. An opposing magnetic field, controlled by varying the current in a coil, supports the inertial mass due to the repulsive force between the permanent and controlled magnetic fields. The current in the coil is continuously adjusted in a closed loop, such that the repulsive magnetic force restores the relative displacement to a null reading. By altering the current in the coil, the inertial mass is held fixed with respect to the sensor case (to within some tolerance). Therefore, the current required to maintain this condition, is at all times proportional to the acceleration of the case. The voltage drop across a resistor in the coil circuit is used to determine the voltage signal, which is proportional to the current by Ohm's law.
The frequency response of this sensor type was generated as defined in section 2.1.1 and is shown in Figure 2.2. The peaks at high frequencies were caused by the test apparatus, and not by the sensor itself. The phase loss however, is caused by the sensor, but is not limiting since the sensor bandwidth will be rolled-off. The noise floor of the QA-2000 was also measured as described in section 2.1.2 over a limited frequency band. The measured noise floor is presented in Figure 2.3.
The performance goal for these acceleration measurements in orbit is presented in Figure 2.3. It is desirable to employ a sensor having a noise floor that is at least an order of magnitude below the smallest disturbance to be measured. Thus, if at some frequency the performance goal is 1 micro-g, then the level of the noise floor should be at most, 0.1 micro-g. This requirement specifies a performance curve for the sensor noise. In Figure 2.3 an acceleration measurement performance goal, the sensor noise goal, and the noise floors for a variety of inertial sensor is presented on the same plot for comparison. As shown in Figure 2.3 the Sunstrand QA-2000 exhibits an acceptable noise floor. [1].
The shuttle has several axis coordinate systems associated with it. One such system which is typically used for citing the location of equipment on the shuttle is the shuttle structural coordinate system, shown in Figure 2.5 [2]. The location and orientation of the SAMS triaxial sensor head (TSH) are typically cited in this axis coordinate system.
Each TSH has a fixed coordinate system associated with it in line with the sensitive axes of the three orthogonal proof-mass-accelerometers. However, for this mission the TSHs were aligned with the structural coordinate system as seen in Figure 2.5. Table 3 of section 4.1.1 gives the exact fixed axis alignment of all the TSHs. The data is measured and recorded in these fixed coordinates. The data is then translated from the TSH fixed coordinate system to the shuttle structural coordinate system. This report is published with all data converted to the shuttle structural coordinate system for ease of referencing the acceleration data to the various shuttle experiments.
The measured acceleration signal is converted to a digital data stream which is recorded on the optical disks of the SAMS unit. In some cases this data is downlinked to the ground. After the mission, this raw flight data is converted to engineering units and Compact Disk-Read Only Memory (CD-ROM) disks are produced for dissemination to users.
For orbiter missions, two time systems are used. Many experiments with internal clocks have data recorded in terms of Greenwich Mean Time (GMT) based on standard time at the zero meridian. Experiments also use time synchronization signals from the shuttle to reference Mission Elapsed Time (MET), where MET begins at zero, at shuttle lift off and has the format of 000/00:00:00 (day/hour:minute:second).
The internal SAMS clock is initialized when power is applied to the SAMS unit. Depending on the shuttle resources available for a particular mission, SAMS time may or may not be synchronized with MET and another external time reference would have to be utilized to corollate the sams data in time.
If the SAMS time is not synchronized with MET, then a post-mission synchronization is applied based on the MET time at which power was applied to the SAMS unit.
On STS-57, SAMS was synchronized with MET and stored on the mission CD-ROMS accordingly.
On June 21, 1993 at 9:07 a.m. EST, STS-57 was launched from the NASA Kennedy Space Center (KSC). The touchdown at KSC was on July 1, 1993 at 8:51 a.m. EST. The planned duration was for a nominal seven days with one contingency day. Bad weather conditions at the landing site forced two delays for the re-entry and landing. The Orbiter Endeavour was utilized for this mission.
A primary objective of STS-57 was the retrieval of the European Space Agency's free-flying platform named the European Retrievable Carrier (EURECA).
Also on STS-57, Endeavour carried into orbit the commercial laboratory facility called SPACEHAB, a small pressurized module situated in the forward section of Endeavour's payload bay. It was designed and constructed by the privately financed corporation, SPACEHAB, Inc.
In addition there were several other experiments including the Superfluid Helium On Orbit Transfer (SHOOT) demonstration experiment and ten Get-Away Special (GAS) experiments in the payload bay. Another of Endeavour's mission objectives was to allow two of the astronauts to perform a spacewalk for training and practice of deploy and retrieval techniques which will support Space Station assembly and the Hubble telescope servicing mission [3].
The primary objectives for STS-57 were the retrieval of the European Space Agency's EURECA free flyer spacecraft, operation of the Spacehab module, operation of the equipment, and experiments contained within the module.
The EURECA satellite was retrieved on the third day of the mission. The EURECA mission was primarily for microgravity payloads in material processing and life sciences payloads. The shuttle's remote manipulator arm was used to grapple the satellite and lower it into Endeavour's cargo bay. It was then stowed for the return to Earth. The EURECA satellite had been on-orbit collecting data since its deployment during shuttle mission STS-46 in July 1992. The microgravity environment for this period is discussed in section 6.3.
NASA leased a privately-developed mid-deck augmentation module known as SPACEHAB. The primary objective was to support the agency's commercial development of space program by providing additional access to crew-tended, mid-deck locker and experiment rack space. This access is necessary to test, demonstrate and evaluate techniques or processes in a "microgravity" environment.
NASA's secondary objective is to foster the development of a space infrastructure which can be marketed by private firms to support commercial microgravity research payloads. In this instance, SPACEHAB, Inc., has the capability of leasing SPACEHAB facility space to other commercial customers on upcoming flights of the module.
The experiments flying in this first SPACEHAB module flight included investigations in drug improvement, feeding plants, cell splitting, the first soldering experiment in space by American astronauts, and the high-temperature melting of metals.
Microgravity Sciences and Applications Division (MSAD) chose to include SAMS in the Spacehab module on the first two flights to characterize the microgravity environment of this new carrier.
There were five secondary payloads on STS-57, the Get-Away-Special (GAS) bridge assembly, containing twelve payloads, a superfluid helium experiment, spacewalks for construction and repair practice, a fluid management, and an optical ranging test.
There were six middeck experiment payloads in the orbiter middeck which are listed below:
a) Commercial Refrigerator/Incubator Module-Vapor Diffusion Apparatus (CR/IM-VDA), two refrigerator/incubator modules which produced protein crystals.
b) Bioserve Pilot Lab (BPL), R/IM module and multiple syringe "kits" used to conduct biomedical and fluid studies.
c) Physiological Systems Experiment (PSE-03), two animal enclosure modules which contained rats to study microgravity's effect on organ systems.
d) Solution Crystal Growth (SCG), room temperature growth of same crystals as SCG. The SCG experiment included a furnace in the Spacehab.
e) Application Specific Preprogrammed Experimental Culture System (ASPECS), bioreactor which grew human cells and tissue cultures.
f) Thermal Enclosure System-Crystal Observation System (TES- COS), observed equilibrium rates of the crystal growth process.
The STS-57 mission included a GAS bridge assembly with ten GAS payloads from the U.S., Canada, Japan and Europe. Also on the bridge was one secondary commercialization payload and one GAS Can with a ballast payload. Table 1 presents a summary of the GAS payloads for STS-57.
The ballast payload contained a small accelerometer package furnished by NASA Goddard Space Flight Center (GSFC) recording accelerations during the mission for these specific GAS can carriers [3].
Superfluid Helium On-Orbit Transfer (SHOOT) is an experiment designed to develop and demonstrate the technology required to re-supply liquid helium containers in space. In addition, components developed for SHOOT may find use in future space cryogenic (low temperature) systems.
The SHOOT experiment consists of two vacuum insulated containers, each holding 207 liters (55 gallons) of liquid helium. The two Dewars are connected by a vacuum insulated transfer line. Liquid helium is pumped from one Dewar to another at rates from 300 to 1000 liters per hour (1.3 to 4.4 gallons per minute). Each of the Dewar's plumbing, including pumps, valves and instrumentation, is nearly identical so that each Dewar in turn may act as the supply or receiver Dewar. The SHOOT Dewars are attached to a Hitchhiker bridge which spans the width of the orbiter bay.
There were several shuttle maneuvers utilized as part of the SHOOT experiment operations, one pitch rotation and three translations along the shuttle x-axis. These maneuvers were utilized to force the liquid helium in a certain direction and as disturbances during the SHOOT operations. The impact of these acceleration disturbances on the microgravity environment are presented in section 6.2.
STS-57 crew members David Low and Jeff Wisoff performed a 4-hour extravehicular activity (EVA) on the fourth day of the flight as a continuation of a series of spacewalks NASA plans to conduct preparing for the construction of the space station. The impact of these acceleration disturbances on the microgravity environment is presented in section 6.5.
Pertinent mass property data for this mission are given in Table 2. The data is given in the orbiter structural coordinate system. The center of gravity varies no more than two inches in the X and Z axes and no more than one inch in the Y axis for the various payload configurations listed in the following Table 2.
The SAMS data acquisition system is used to measure and record microgravity accelerations at as many as three experiment locations simultaneously. Each SAMS unit is comprised of a main unit, up to three remote triaxial sensor heads (TSHs), and sensor head cables to connect the heads to the main unit. Each TSH contains three accelerometer sensors in an orthogonal orientation.
The TSH's and main unit may be independently configured with one of six low-pass filter cutoff frequencies of 2.5, 5, 10, 25, 50 and 100 Hertz.
Each axis in the TSHs also contain a programmable amplifier. The possible gains are 1, 10, 100 and 1000 with full scale ranges of 0.5 g, 0.05 g, 0.005 g, and 0.0005 g, respectively [4].
For SH-l, the SAMS flight unit A was installed on the forward bulkhead in location FP08 (see Figure 4.1).
Three SAMS TSHs were flown on SH-l, one mounted to a structural member of the forward bulkhead in location FP01 (starboard side), one mounted to a stowage locker in location FC05 on the forward bulkhead (center section), and one mounted to the Environmental Control and Life Support System (ECLSS) Flight Experiment (EFE) locker in location FP01 on the forward bulkhead (port side). The mounting, alignment, and pass band of the three TSHs is given in Table 3 showing the orientation of the SAMS TSHs with respect to the Orbiter structural coordinate system.
One of the primary reasons for SAMS to acquire data on STS-57 was to characterize the microgravity environment of the Spacehab module. This information is used by MSAD to assess the assignment of MSAD sponsored experiments to this carrier in deference to other carriers. For this purpose, TSHs A and C were mounted to the Spacehab module structure. TSH B was mounted to a locker with the ECLSS so that the ECLSS PI would have microgravity data for correlation to ECLSS experiment data.
The frequency response and data sampling rate for the three TSHs are given in Table 3. The four gain stages (1, 10, 100, 1000) of the TSHÕs were enabled for all three TSHs. Sundstrand QA-2000-030 accelerometers were used for the SH-1 TSH accelerometers.
The error budget for a TSH calibration [5] is given by Table 4. There are two sources of error which affect the accuracy of the SAMS TSH data. They are the errors in the measurement system used in the calibration and errors inherent to the Sunstrand QA-2000 accelerometer, mainly bias and scale factor errors. A third error source is the axis misalignment which is negligible in comparison to the other accumulated errors.
At a gain of 1 the 0.03 percent scale factor error (150 micro-g's) is the major contributor to the total full-scale error. At gains of 10,100, and 1000 the major error contribution is a result of the +/- 98.6 micro-g bias error. The maximum full scale error for a gain of 10, 100, and 1000, is 15, 2.5, and 2.1 micro-g for the associated 0.03, 0.05, and 0.42 percent maximum error of full scale.
All TSH data was recorded on optical disks which were stored in lockers and removed after the mission.
The SAMS unit operated a total of 162 hours 13 minutes during the SH-1 mission. Recording began at MET 000/06:57:20 and ended at MET 007/15:07:02. One anomaly occurred on day 4 of the mission, when a disk drive began to record data intermittently. A malfunction procedure was followed for the remainder of the mission minimizing the loss of data. A total of 13 hours 31 minutes of data was lost, the majority between MET 004/07:17:00 and 004/20:48:00.
An autoranging algorithm is used in the SAMS unit along with a hardware peak detect to ensure the optimal gain range is selected. The system worked well with the exception of the X axis of head A. The gain on this axis continually changed between a gain of 10 and 100 during the mission. Post flight analysis revealed that this was caused by a hardware problem in the peak detect circuit of the X axis. A capacitor had been added to the X axis circuit to suppress a noisy clock signal which caused the circuit to operate improperly. The problem was successfully diagnosed by an analysis of the data, and corrective action has been taken.
These constant gain changes, exhibited as step functions in the data, caused an order of magnitude increase in the broadband energy. As a result, the x axis and vector magnitude spectrographs show this increase in broadband energy. (See Appendix B).
3-DMA is a data acquisition system designed to measure data from three remote triaxial accelerometer heads. The main unit also houses three orthogonal accelerometer sensors which are invertible. The system can provide up to 50 sample per second accelerometry data from all of its sensors. The sampling rate is set pre-flight for each sensor.
At the time that this report was published, data from the 3-DMA was not avail- able for inclusion.
GAS ballast payloads are flown for stability when a GAS payload drops out and no GAS payload is available to replace it. This ballast payload contains a small accelerometer package furnished by GSFC to record accelerations during the mission.
At the time that this report was published, data from the GAS can accelerometer was not available for inclusion.
The raw data recorded by SAMS is processed to compensate for temperature and gain related errors of bias, scale factor and axis misalignment. The processing utilizes a fourth order temperature model, see reference 1, to compensate the data and convert the raw digitized data into engineering units. This results in data of fractions of one "g" versus time. The data is then transformed to the shuttle structural coordinate system. CD-ROMS are then produced to distribute the data to Principal Investigators.
The data from the CD-ROMS was processed further to produce the plots for this report. The processing for each type of plot is described in the following sections.
The color spectrographs in appendix B were produced using the Sensor A data from the SAMS SPACEHAB-01 mission. The data was taken in 2 hour periods and an amplitude spectrum was performed in 10 second intervals. The spectrum data was then scaled by taking the log of each data point and assigning a color to the integer result. Eight colors were used for eight intervals between 0.1 micro-g and 1 milli-g. In using this method a range of values are assigned the same color. For example 0.1-0.3 micro-g values are assigned the color black, 0.3-1 micro-g values are assigned the color purple. Apparent resolution can be adjusted by using more or less colors.
The average plots, in appendix C, were also produced using the Sensor A data from the SAMS SPACHEAB-01 mission. The plots were produced by taking the average of each 10 second interval of data. The average produces 1 data point for every 10 seconds (N=5000 points) of data for this 100 Hz head. The following equation was used to calculate the 10 second moving window average.
Where X is the vector magnitude of the x, y, and z axis data, respectively.
The RMS plots, in appendix C, were also produced using the Sensor A data from the SAMS SPACHEAB-01 mission. The plots were produced by taking the Root Mean Square(RMS) of 10 second intervals for the two hour period. The root mean square of a discrete time series for over 10 seconds was calculated as follows:
Where X is the vector magnitude of the x, y, and z axis data, respectively.
Various activities from this mission were characterized so that the effects may be considered for planning of future missions and experiments.
The crew work schedule was based on all crew members working during the same twelve hour work shift. The other twelve hours of each day were occupied with pre-sleep, sleep, and post-sleep activities.
During these sleep periods, non-essential pieces of equipment were turned off and attitude maneuver changes were minimized or eliminated. This results in a quiet microgravity environment on-board the shuttle. Figure 6.1 illustrates 2 hours of a typical sleep period during SH-1. The plots show a very quiet microgravity environment in the lower frequency ranges of 0.1 to 15 Hz with the levels gradually increasing with increasing frequency.
The two vertical lines spaced an hour apart in the plot are caused by the SAMS unit resetting its gain at 1 hour intervals.
The SHOOT experiment requested four inertial maneuvers of the shuttle in order to force the experiments liquid contents to each end of the Dewars during the course of the experiment. These four maneuvers consisted of a pitch rotation at 3 deg/sec. and three x translational accelerations of the orbiter. These maneuvers are described in Table 5.
The pitch rotation of the orbiter was initiated at MET 000/21:40:53 with two intermediate rotational rates where the nominal 3 deg/sec. rotation was achieved at MET 000/21:42:39. The 3 deg/sec. pitch rotational rate lasted for 13 minutes and 57 seconds until the rotation of the orbiter was slowed down and eventually stopped to the rotational deadband of the orbiter. Figure 6.2 gives the moving average of the x, y, and z acceleration of the 5 Hz accelerometer head during the rotational maneuver. Appendix D contains some calculations confirming the acceleration levels during this maneuver.
As shown by the three curves the y axis acceleration of the orbiter which was perpendicular to the rotational plane was not stable in that the rotation of the orbiter about the y axis must have caused an oscillatory disturbance out of the rotational plane and therefore the control thrusters must have been activated to keep the rotational plane perpendicular to the orbital flight path. As seen in this figure thrusters were fired in the y axis direction about every 200 seconds to stabilize the pitch rotational maneuver of the shuttle. The impact of these intermittent thruster firings can be seen in the acceleration histories of the other two axes with a significantly reduced magnitude.
The three x translational accelerations for the SHOOT experiment were accomplished by firing primary thrusters translating the shuttle in the direction of the velocity vector. The first x translational acceleration consisted of firing an aft primary thruster causing a negative x acceleration while the second and third translational maneuvers were accomplished by firing forward thrusters causing positive x accelerations in the global shuttle structural coordinate system. The first x acceleration SHOOT maneuver began at MET 000/22:28:39 and lasted 14 seconds having a magnitude of 7.1669x10-3 g's while the second translational maneuver began at MET 001/21:07:29 and lasted for 20 seconds. This second translational acceleration was characterized by two distinct acceleration levels during the 20 second maneuver having magnitudes of 3.3306x10-3 and 7.2506x10-3 g's, respectively. The third x maneuver began at MET 001/23:22:36 and lasted for 20 seconds, again having two distinct acceleration levels of 3.3517x10-3 and 6.8404x10-3 g's, respectively.
Figure 6.3 shows all three translational maneuvers comparing the acceleration magnitudes where the same time slice was taken and normalized to time zero for a 120 second duration window and low pass filtered to 0.5 Hz with a stopband of 1 Hz and a time sample resolution of 25 Hz. The first translational acceleration is much cleaner than the second and third exercises. The second and third primary thruster burns from the forward end of the shuttle seem to have a 3 second disturbance. Figure 6.4 shows the non-filtered maneuvers giving the stable aft thruster firing and the 3 second pulsed forward primary thruster responses for the three x inertial motions. As can be seen by this plot the second and third x acceleration maneuver are almost identical as they should be, since the same thrusters were used in both maneuvers. To demonstrate this the mirror image of the second translational SHOOT exercise was taken and plotted with the third acceleration maneuver giving a comparison of the orbiters response at different times to the same input forcing function as seen in Figure 6.5.
The Spacehab-1 mission exercise equipment aboard the orbiter was a ergometer device which was attached to the middeck floor. The ergometer faces the y dimension thus, the astronauts are facing the side wall of the middeck when exercising. The pedaling rate is set by the astronaut and was not recorded. However, from the spectral content of the acceleration disturbances during an exercise period, the pedaling rate was approximately 1.25 Hz. In addition, a 1.25 Hz rocking motion of the ergometer was detected caused by the side-to-side motion of the astronaut at the pedal rate and the disturbance caused by the pedaling was at 2x the pedaling rate or 2.5 Hz. This specific crew exercise period began at MET 001/22:14:17 and lasted for approximately 20 minutes. Figure 6.6 through 6.8 gives an average of four spectrums for the x, y, and z axis showing the 2x pedal rate disturbances at 2.5 Hz in the y and z directions and the pedal rate of 1.25 Hz in the x axis, respectively.
In order to establish an average disturbance environment amplification spectrum for exercise periods, specific to this mission and with reference to the Spacehab-1 module, a non-exercise period was analyzed and a spectral ratio of these two curves were calculated for the x, y, and z directions giving Figures 6.9 through 6.11, respectively. These figures give a measure of the amplified environment as a function of frequency where the acceleration environment of the Spacehab-1 module is not significantly affected except at the discrete ergometer operating frequencies of 1.25 and 2.5 Hz. As seen in these curves the x, and z dimensions are the least affected, as expected, while the y dimension has the most amplification from nominal crew activity.
A major mission activity during the Spacehab-1 mission was the retrieval of the European free-flyer EURECA. This activity entailed significant maneuvering of the shuttle causing inertial motion of the orbiter. The EURECA capture will be organized into four distinct orbiter acceleration disturbance profiles, before, during and after retrieval of the EURECA satellite. These four phases of the retrieval are the target track, grapple, pre-berthing, and berthing of the satellite. The target tracking profile of the disturbance environment was predominantly driven by a target track mode of navigation by the shuttle to rendezvous with EURECA where at MET 002/23:34:00 the shuttle was taken over by the commander and flown under manual controls. Figure 6.12 shows a number of these manual maneuvers consisting of numerous pitch rotations and altitude changes causing most of the accelerations in the x, and z dimensions where the shuttle x axis was being flown into the velocity vector. The figure begins at MET 002/23:33:19, showing the x, y, and z axes with a 10 sec moving average giving a filtered response of the data at 0.1 Hz.
The grapple of EURECA was accomplished at MET 003/00:45:00. The grapple phase of the EURECA retrieval began after the shuttle was positioned above the satellite. There was no noticeable shock input to the shuttle, detected by the SAMS sensors, during the EURECA capture.
A four hour period including the capture and latching of the EURECA satellite is presented in Figure 6.13. The high acceleration levels during the chase maneuvers by the shuttle may be seen in the first 45 minutes of the data.
EVA operations were analyzed from MET 004/02:00:00 to MET 004/03:00:00, for two astronauts tethered to the remote manipulator arm. The arm was maneuvered during this period. Figure 6.14 shows a moving 10 second average of the 5 Hz accelerometer head for the structural x, y, and z coordinates during the first 24 minutes of day four, hour two. There was a verbal reference from the astronauts that the arm was cleared to reposition itself back to antenna one position at MET 004/02:14:00 and that the arm was in-position at MET 004/02:18:00. This reference to an inertial event was from a SAMS mission log book and therefore, the actual time of the event is suspect by a few minutes. However, from the acceleration data plot one can plainly see a shift in the x axis quasi-steady level from MET 004/02:11:00 to MET 004/02:18:30. The significant deviations from the constant rigid body motion of the shuttle, due to the mass motion of the arm, are most likely from arm corrections during the end effectors path from point A to B.
This report serves as a road map to the SAMS data acquired during the STS-57 mission. Further analysis of specific events and comparisons with other missions will be performed and published under future documents.
There were two primary payloads for the STS-57 mission, the retrieval of the EURECA free-flyer and the operation of the first SPACEHAB module. SAMS was onboard STS-57 to support the SPACEHAB-1 payloads where three triaxial sensor heads were mounted to the forward bulkhead of the pressurized enclosure. These triaxial sensor heads were configured with a 5, 50, and 100 Hz low pass filter, respectively.
A mission summary of the vector magnitude RMS, and average accelerations for the entire mission was produced for the 100 Hz head. Spectrographs were also produced on head A to give a spectral time history for the entire mission. Significant events were chosen to give a more detailed look at the acceleration disturbances at the sensor head locations in the SPACEHAB module. These events were typical sleep periods, numerous scheduled astronaut activities such as exercise, EVA's and the EURECA retrieval. In addition, a secondary payload named SHOOT required significant quasi-steady acceleration disturbances which were analyzed for the resulting SPACEHAB accelerations. These disturbances were on the order of a milli-g with significant duration. The middeck exercise period was fairly benign in its disturbance of the SPACEHAB environment. The EVA and EURECA phases of the mission were characterized by certain "DC" shifts of the acceleration levels pertaining to mass maneuvers or orbiter attitude changes, respectively.
To produce the spectral and acceleration time histories, approximately 3.5 gigabytes of data were processed. The RMS and average time histories were processed for only the 100 Hz head, while the spectral time histories were processed for the vector magnitude and the associated x, y, and z axes.
