Main Landing Gear Brakes

Each of the orbiter's four main landing gear wheels has electrohydraulic disc brakes and an anti-skid system

Each main landing gear wheel has a disc brake assembly consisting of nine discs, four rotors, three stators, a backplate and a pressure plate. The carbon-lined beryllium rotors are splined to the inside of the wheel and rotate with the wheel. The carbon-lined beryllium stators are splined to the outside of the axle assembly and do not rotate with the wheel.

Each of the four main landing gear wheel brake assemblies is supplied with pressure from two different hydraulic systems. Each brake hydraulic piston housing has two separate brake supply chambers. One chamber receives hydraulic source pressure from hydraulic system 1 and the other from hydraulic system 2. There are eight hydraulic pistons in each brake assembly. Four are manifolded together from hydraulic system 1 in a brake chamber. The remaining four pistons are manifolded together from hydraulic system 2. When the brakes are applied, the eight hydraulic pistons press the discs together, providing brake torque.

In the event of the loss of hydraulic system 1 or 2 source pressure, switching valves provide automatic switching to the standby hydraulic system 3 when the active hydraulic system source pressure drops below approximately 1,000 psi. If hydraulic system 1 is unavailable, it has no effect on the braking system because standby system 3 would automatically replace system 1. Loss of hydraulic system 2 or both 1 and 2 would also have no effect on the braking system because system 3 would automatically switch to replace system 2 or 1 and 2. Loss of hydraulic systems 1 and 3 would cause the loss of half of the braking power on each wheel and additional braking distance would be required. Loss of hydraulic systems 2 and 3 would also cause the loss of half of the braking power on each wheel, requiring additional braking distance.

As in the landing gear deployment, the landing gear isolation valve in hydraulic systems 1, 2 and 3 must be open to allow the applicable hydraulic source pressure to the main landing gear brakes.

The brakes MN A, MN B and MN C switches are located on the flight deck display and control panels O14, O15 and O16 and allow electrical power to brake/anti-skid control boxes A and B. The antiskid switch located on panel L2 provides electrical power for enabling the anti-skid portion of the braking system boxes A and B. The brakes MN A, MN B and MN C switches are positioned to on to supply electrical power to brake boxes A and B and to off to remove electrical power. The antiskid switch is positioned to on to enable the anti-skid system and to off to disable the system.

When weight is sensed on the main landing gear, the brake/anti-skid boxes A and B are enabled, permitting the main landing gear brakes to become operational.

The main landing gear brakes controlled by the commander's or pilot's brake pedals are located on the rudder pedal assemblies at the commander's and pilot's stations. The pedals' positions are adjustable by a handle. The braking commands are accomplished by the commander or pilot initiating toe pressure on the top of the rudder pedal assembly.

Each brake pedal (left and right) has four linear variable differential transducers. The left pedal transducer unit will output four separate braking signals through the brake/skid control boxes for braking control of the two left main wheels. The right pedal transducer unit does likewise for the two right main wheels. When toe pressure is applied to the brake pedal, the transducers transmit electrical signals of zero to 5 volts dc to the brake/anti-skid control boxes. If both right pedals are moved, the pedal with the greatest toe pressure becomes the controlling pedal through electronic OR circuits. The electrical signal is proportional to the toe pressure. The electrical output energizes the main landing gear brake coils proportionally to brake pedal deflection, allowing the desired hydraulic pressure to be directed to the main landing gear brakes for braking action. The brake system bungee at each brake pedal provides the braking artificial feel to the crew member.

Each of the three hydraulic systems' source pressure of 3,000 psi is reduced by a regulator in each of the brake hydraulic systems to 1,500 psi.

The anti-skid portion of the brake system provides optimum braking by preventing tire skid or wheel lock and subsequent tire damage.

Each main landing gear wheel has two speed sensors that supply wheel rotational velocity information to the skid control circuits in the brake/skid control boxes. The velocity of each wheel is continuously compared to the average wheel velocity of all four wheels. Whenever the wheel velocity of one wheel is below 30 percent of the average velocity of the four wheels, skid control removes brake pressure from the slow wheel until the velocity of that wheel increases to an acceptable range. The brake system contains eight brake/skid control valves that receive signals from the brake/skid control boxes. Each valve controls the hydraulic brake pressure to one of the brake chambers. The brake/skid control valves contain a brake coil and a skid coil. The brake coil allows hydraulic pressure to enter the brake chambers. The skid coil, when energized by the skid control circuit, provides reverse polarity to the brake coil, preventing the brake coil from allowing brake pressure to the brake chamber.

Anti-skid control is automatically disabled below 9 to 14 knots (11 to 17 mph) to prevent loss of braking for maneuvering and/or coming to a complete stop.

The anti-skid system control circuits contain fault detection logic. The antiskid yellow caution and warning light located on the flight deck display and control panel F3 will be illuminated if the anti-skid fault detection circuit detects an open or short in a wheel speed sensor, open or short in a anti-skid control valve servocoil or a failure in an anti-skid control circuit. A failure of these items will only deactivate the failed circuit, not total anti-skid control. If the brake power switches are on and the antiskid switch is off , the antiskid caution and warning light will be illuminated.

Insulation and electrical heaters are installed on the portions of the hydraulic systems that are not adequately thermally conditioned by the individual hydraulic circulation pump system because of stagnant hydraulic fluid areas.

Redundant electrical heaters are installed on the main landing hydraulic flexible lines located on the back side of each main landing gear strut between the brake module and brakes. These heaters are required because the hydraulic fluid systems are dead-ended and fluid cannot be circulated with the circulation pumps. In addition, on OV-103 and OV-104, the hydraulic system 1 lines to the nose landing gear are located in a tunnel between the crew compartment and forward fuselage. The passive thermal control systems on OV-103 and OV-104 are attached to the crew compartment, which leaves the hydraulic system 1 lines to the nose landing gear exposed to environmental temperatures, thus requiring electrical heaters on the lines in the tunnel. Since the passive thermal control system on OV-102 is attached to the inner portion of the forward fuselage rather than the crew compartment, no heaters are required on the hydraulic system 1 lines to the nose landing gear on OV-102.

The hydraulics brake heater A, B, and C switches on panel R4 enable the heater circuits. On OV-103 and OV-104, hydraulics brake heater switches A, B and C provide electrical power from the corresponding main buses A, B and C to the redundant heaters on the main landing gear flexible lines and the hydraulic system 1 lines in the tunnel between the crew compartment and forward fuselage leading to the nose landing gear. Thermostats on each electrical A, B and C system cycle the heaters automatically off or on.

The hydraulics brake heater A, B and C switches on panel R4 enable the heater circuits on only the main landing gear hydraulic flexible lines on OV-102.

Because problems were encountered with the main landing gear braking system in the majority of the first 24 landings, an improvement program has been implemented for the main landing gear and braking system in addition to a long-term improvement program for the main landing gear brakes.

Main landing gear axle stiffness has been increased to reduce brake-to-axle deflections to preclude brake damage, which occurred in previous landings. This should also minimize tire wear. With the increased axle thickness, existing axle/bearing and axle/sensor interfaces are maintained. All main landing gear axles will be changed before the three orbiters return to flight.

Six orifices were added to the hydraulic passages in the brake hydraulic piston housing to restrict circular fluid flow within the chambers in order to stop the whirl phenomenon, which has been identified as the cause of brake damage.

The electronic brake control boxes were modified to provide hydraulic pressure balancing between adjacent brakes in order to equalize energy applications. This results in higher efficiency and allows full capability of adjacent brakes. The anti-skid circuitry that reduced brake pressure to the opposite wheel if a flat tire was detected was removed.

The previous, thinner, carbon-lined beryllium stator discs are being replaced in two positions with thicker discs to provide a significant increase in braking energy capability. The additional material added to the stators improves heat capacity, with resulting lower temperatures, and provides the stators with greater strength. Note that the main landing gear brakes, which were exposed to two 14-million- foot-pound wear-in cycles added before installation on the orbiter, reduced damage to the brakes during landing. The thicker stator discs will provide approximately 65 million foot pounds of energy absorption, which is a significant increase over the thinner stator discs.

A long-term structural carbon brake program is in progress to provide higher braking capability by increasing maximum energy absorption capability to 82 million foot pounds and to reduce refurbishment costs. These new brakes will consist of a five-rotor, disc-type carbon configuration for each main landing gear wheel brake. The goal is to demonstrate that the carbon heat sink brake design will have the capability of providing a one-time stop of 100 million foot pounds. The go-ahead for the carbon brake design was given in January 1986, with delivery scheduled in late April 1988.

Upon the return to flight of the space shuttle, end-of-mission landings are planned for Edwards Air Force Base in California until the performance of the landing gear system is fully understood and a higher confidence in the weather prediction capability is established at the Kennedy Space Center Shuttle Landing Facility runway area in Florida.

Strain gauges have been added to each nose and main landing gear wheel in order to monitor tire pressure and provide the status of tire pressures during launch pad stay, launch, orbit, deorbit and landing to the flight crew and Mission Control Center in Houston.

A landing gear tire improvement and runway-surface study is in progress at NASA's Langley Research Center to determine how best to decrease tire wear experienced during previous Kennedy Space Center landings and improve crosswind landing capability. Six new tire designs will be evaluated at the Langley Research Center with various yaw and tilt angles at speeds up to 253 mph. Additional tests at Langley Research Center are to provide the ability to mathematically model tire side-force characteristics.

Modifications were made to the Kennedy Space Center Shuttle Landing Facility runway. The full 300-foot width of 3,500-foot sections at both ends of the runway were ground to smooth the runway surface and remove cross grooves. The corduroy ridges are smaller than those they replace and run the length of the runway rather than across its width. The existing landing zone light fixtures were also modified, and the markings of the entire runway and overruns were repainted. The primary purpose of the modifications is to enhance safety by reducing tire wear during landing.

The current strength of the orbiter landing system is a maximum of 240,000 pounds. Evaluations for landing loads of 256,000 pounds, associated with abort landings, are to be completed by the spring of 1988.

Other studies in progress are arrest barriers at landing sites (except lake bed runways) to provide safe stops in the event of main landing gear brake failure or unforeseen wet runway conditions. The barrier net study will determine whether the barrier can safely stop a 256,000-pound orbiter traveling at 100 knots (115 mph) at the end of runway. Also under study are (1) the installation of a skid on the landing gear that could preclude the potential for a second blown tire on a gear on which one tire has blown, (2) a rim that would provide a predictable roll in the event of the loss of both tires on a single or multiple gear and (3) the addition of a drag chute.