Overview

The landing gear system on the orbiter is a conventional aircraft tricycle configuration consisting of a nose landing gear and a left and right main landing gear. Each landing gear includes a shock strut with two wheel and tire assemblies. Each main landing gear wheel is equipped with a brake assembly with anti-skid protection. The nose landing gear is steerable. The nose landing gear is located in the lower forward fuselage, and the main landing gear are located in the lower left and right wing area adjacent to the midfuselage.

The nose landing gear is retracted forward and up into the lower forward fuselage and is enclosed by two doors. The main landing gear are also retracted forward and up into the left and right lower wing area, and each is enclosed with a single door. The nose and main landing gear can be retracted only during ground operations.

For retraction, each gear is hydraulically rotated forward and up during ground operations until it engages an uplock hook for each gear in its respective wheel well. The uplock hook locks onto a roller on each strut. Mechanical linkage driven by each landing gear mechanically closes the respective landing gear doors. All three landing gear doors have high-temperature reusable surface insulation thermal protection system tiles bonded to their outer surface with thermal barriers to protect and prevent the landing gear and wheel well from the high-temperature thermal loads encountered during the shuttle's entry into the atmosphere.

For deployment of the landing gear, the uplock hook for each gear is activated by the flight crew initiating a gear-down command. The uplock hook is hydraulically unlocked by hydraulic system 1 pressure applied to release it from the roller on the strut to allow the gear, assisted by springs and hydraulic actuators, to rotate down and aft. Mechanical linkage released by each gear actuates the respective doors to the open position. The landing gear reach the full-down and extended position within 10 seconds and are locked in the down position by spring-loaded downlock bungees. If hydraulic system 1 pressure is not available to release the uplock hook, a pyrotechnic initiator at each landing gear uplock hook automatically releases the uplock hook on each gear one second after the flight crew has commanded gear down.

The landing gears are deployed only after the spacecraft has an indicated airspeed of less than 300 knots (345 mph) and an altitude of approximately 250 feet.

The shock strut of each landing gear is the primary source of shock attenuation at landing. The struts have air/oil shock absorbers to control the rate of compression extension and prevent damage to the vehicle by controlling load application rates and peak values.

Each main landing gear wheel contains an electrohydraulic disc brake assembly with anti-skid control. The main landing gear brakes are controlled by the commander or pilot applying toe pressure to the rudder pedals; electrical signals produced by rudder pedal toe pressure control hydraulic servovalves at each wheel and allow hydraulic system pressure to perform braking. Main landing gear brakes cannot be applied until weight on the main gear has been sensed. The anti-skid system monitors wheel velocity and controls brake torque to prevent wheel lock and tire skidding. The braking/anti-skid system is redundant in that it utilizes system 1 and 2 hydraulic pressure as the active system with system 3 as standby and also utilizes all three main dc electrical systems.

The nose landing gear contains a hydraulic steering actuator that is electrohydraulically steerable through the use of the onboard general-purpose computers, the commander's or pilot's rudder pedals in conjunction with the orbiter flight control system in the control stick steering mode, or through the use of the commander's or pilot's rudder pedals in the direct mode. If hydraulic system 1 is inoperative, nose wheel steering changes to caster mode, and the commander or pilot would then apply toe pressure to the brake pedals to apply hydraulic pressure to the left and right main gear brakes as required for directional control using differential braking.

Each landing gear shock strut assembly is constructed of high-strength, stress- and corrosion-resistant steel alloys, aluminum alloys, stainless steel and aluminum bronze. Cadmium and chromium plating and urethane paint are applied to the strut surfaces for space flight protection. The shock strut is a pneudraulic shock absorber containing gaseous nitrogen and hydraulic fluid. Because the shock strut is subjected to zero-g conditions during space flight, a floating piston separates the gaseous nitrogen from the hydraulic fluid to maintain absorption integrity.

Landing gear wheels are made in two halves from forged aluminum and are primed and painted with two coats of urethane paint.

The LG hyd isol vlv 1, 2 and 3 switches on panel R4 control the corresponding landing gear isolation valve in hydraulic systems 1, 2 and 3. When the LG hyd isol vlv 1 switch on panel R4 is positioned to close , hydraulic system 1 is isolated from the nose and main landing gear deployment uplock hook actuators and strut actuators, nose wheel steering actuator and main landing gear brake control valves. A talkback indicator next to the switch would indicate cl. The landing gear isolation valves will not close or open unless the pressure in that system is at least 100 psi. When the LG hyd isol vlv 1 switch is positioned to open , it allows hydraulic system 1 source pressure to the main landing brake control valves and to the normally closed extend valve. The normally closed extend valve is not energized until a gear-down command is initiated by the commander or pilot on panel F6 or panel F8. The talkback indicator would indicate op . The LG hyd isol vlv 1 switch is left in the close position during the mission to prevent inadvertent gear deployment.

The LG hyd isol vlv 2 and 3 switches on panel R4 positioned to close isolate the corresponding hydraulic system from only the main landing gear brake control valves. The adjacent talkback indicator would indicate cl . When switches 2 and 3 are positioned to open, the corresponding hydraulic system source pressure is available to the main landing gear brake control valves. The corresponding talkback indicator would indicate op.

Thus, only hydraulic system 1 is used to deploy the nose and main landing gear and for nose wheel steering. When the nose- and main-landing-gear-down command is initiated by the commander or pilot on panel F6 or F8, hydraulic system 1 pressure is directed to the nose and main landing gear uplock hook actuators and strut actuators (provided that the LG hyd isol vlv 1 switch is in the open position) to actuate the mechanical uplock hook for each landing gear and allow the gear to be deployed and also provide hydraulic system 1 pressure to the nose wheel steering actuator. The main landing gear brake control valves receive hydraulic system 1 source pressure when the LG hyd isol vlv 1 switch is positioned to open. If hydraulic system 1 is unavailable, a pyrotechnic actuator attached to the nose and main landing gear uplock actuator would deploy the landing gear automatically one second after the gear-down command, actuate the mechanical uplock hook for each landing gear and allow the gear to be deployed. Because powered nose wheel steering would not be functional, directional control for steering would be accomplished by differential braking to caster the nose wheel.

The GPC position of the LG hyd isol vlv 1, 2 and 3 switches on panel R2 permits the onboard computer to automatically control the valves in conjunction with computer control of the corre sponding hydraulic system circulation pump. The LG hyd isol vlv 2 and 3 switches provide fluid circulation to only the main landing gear brake system, which dead-ends at the brake control valves. The LG hyd isol vlv 1 switch is left closed to prevent inadvertent gear deployment.

The normally open hydraulic system 1 redundant shutoff valve is a backup to the retract/circulation valve to prevent hydraulic pressure from being directed to the retract side of the nose and main landing gear uplock hook actuators and strut actuators if the retract/circulation valve fails to open during nose and main landing gear deployment.

The normally closed hydraulic system 1 dump valve is energized open to allow hydraulic system 1 fluid to return from the nose and main landing gear areas when deployment of the landing gear is commanded by the flight crew.

The activation/deactivation limits of the hydraulic fluid circulation systems can be changed during the mission by the flight crew or the Mission Control Center-Houston. The program also includes a timer to limit the maximum time a circulation pump will run and a priority system that automatically monitors hydraulic bootstrap pressure to allow all three circulation pumps to be on at the same time. The software timers allow this software to be used in contingency situations for ''time-controlled'' circulation pump operations in order to periodically boost an accumulator that is losing hydraulic fluid through a leaking priority valve or unloader valve.

During entry, if required, LG hyd isol vlv 1, 2 and 3 are positioned to GPC . At 19,000 feet per second, the landing gear isolation valve automatic opening sequence begins under GN&C; software control. If the landing gear isolation valve is not opened automatically, the flight crew will be requested by the Mission Control Center to open the valve by positioning the applicable LG hyd isol vlv to open. Landing gear isolation valve 2 is automatically opened six minutes and 37 seconds later, and this is followed by the automatic opening of landing gear isolation valve 1 when orbiter velocity is at 800 feet per second or less. Landing gear isolation valve 3 is automatically opened at ground speed enable. Landing gear isolation valve 1 is next to last to ensure that an inadvertent gear deployment would occur as late (low airspeed) as possible.

Note that the hydraulic system 1 retract/circulation valve would be automatically closed when the landing gear system is armed for deployment.

The commander and pilot have a landing gear deployment arm and dn (down) guarded push button switch/light indicators and landing left, nose and right indicators. The commander's controls and indicators are on panel F6, and the pilot's controls and indicators are on panel F8. The dn push button, when depressed, energizes the hydraulic system 1 normally closed extend valve, permitting hydraulic system 1 source pressure for gear deployment and nose wheel steering.

The proximity switches on the nose and main landing gear doors and struts provide electrical signals to control the landing gear nose, left and right indicators on panels F6 and F8. The output signals of the landing gear and door uplock switches drive the landing gear up position indicators and the backup pyrotechnic release system. The output signals of the landing gear downlock switches drive the landing gear dn position indicators. The landing gear indicators are barberpole when the gear is deploying (or retracting).

The left and right main landing gear weight-on-wheels switches produce output signals to the guidance, navigation and control software to reconfigure the flight control system for landing.

The two weight-on-nose-gear signals run to the main landing gear brake/skid control boxes to prevent the main landing gear brakes from being applied until the nose gear is in contact with the runway and also to the GN&C; software, which computes a nose wheel steering enable signal. This enable signal is then sent to the NWS control box to prevent NWS until the nose gear is in contact with the runway.

The six group 1 switches are signal conditioned by the landing gear proximity sensor electronics box 1, located in avionics bay 1. The six group 2 switches are signal conditioned by the landing gear proximity sensor electronics box 2, located in avionics bay 2.

Landing gear deployment is initiated when the commander or pilot depresses the guarded arm push button switch/light indicator and then the guarded dn push button switch/light indicator at least 15 seconds before predicted touchdown and at a speed no greater than 300 knots (345 mph).

Depressing the arm push button switch/light indicator energizes latching relays that close the hydraulic system 1 landing gear retract/circulation valve and the normally open redundant shutoff valve to the retract/circulation valve. It also arms the nose and main landing gear pyrotechnic initiator controllers and illuminates the yellow light in the arm push button switch/light indicator.

The dn push button switch/light indicator is then depressed. This energizes latching relays that open the hydraulic system 1 landing gear normally closed extend control valve, permitting the fluid in hydraulic system 1 to flow to the landing gear uplock and strut actuators and nose wheel steering. The relays also open the normally closed dump valve, allowing the landing gear retract line fluid to flow in to the hydraulic system 1 return line. The green light in the dn push button switch/light indicator is illuminated.

Hydraulic system 1 source pressure is routed to the nose and main landing gear uplock actuators, which releases the nose and main landing gear and door uplock hooks. As the uplock hooks are released, the gear begins its deployment and mechanical linkage attached to the doors and fuselage is powered by landing gear strut camming action, during gear extension, which opens the landing gear doors. There are two landing gear doors for the nose gear and one for each main gear. The landing gear free falls into the extended position, assisted by the strut actuators and airstream in the deployment. The hydraulic strut actuator incorporates a hydraulic fluid flow through orifice (snubber) to control the rate of landing gear extension and thereby prevent damage to the gear's downlock linkages.

If hydraulic system 1 fails to release the landing gear within one second after the dn push button is depressed, the nose and left and right main landing gear uplock sensors (proximity switches) will provide inputs to the pyro initiator controllers for initiation of the redundant NASA standard detonators (nose, left and right main landing gear pyrotechnic backup release system). They release the same uplock hooks as the hydraulic system. The nose landing gear, in addition, has a PIC and redundant NSDs that initiate a pyrotechnic power thruster two seconds after the dn push button is depressed to assist gear deployment.

The landing gear drag brace overcenter lock and spring-loaded bungee lock the nose and main landing gear in the down position.

The ldg gr/arm/dn reset switch positioned to reset on panel A12 unlatches the relays that were latched during landing gear deployment by the landing gear arm and dn push button light/switch indicators. This is primarily a ground function, which will be performed only during landing gear deactivation.

The reset position also will extinguish the yellow light in the arm push button switch/light indicator and the green light in the dn push button switch/light indicator. In addition, the hydraulic system 1 landing gear dump valve is closed, the extend control valve is closed, the retract/circulation valve is opened only if the switch is in the open position, and its redundant shutoff valve is opened (de-energized) and de-energizes the landing gear PIC circuits.

The nose landing gear tires are 32 by 8.8 inches and will withstand a burst pressure of not less than 3.2 times the normal inflation pressure of 300 psi. The inflation agent is gaseous nitrogen. The maximum allowable load per nose landing gear tire is approximately 45,000 pounds and rated at 224 knots (258 mph) landing speed.

The nose landing gear shock strut has a 22-inch stroke. The maximum allowable derotation rate is approximately 9.4 degrees per second or 11 feet per second, vertical sink rate.

The main landing gear tires are 44.5 by 16 and 21 inches. The normal inflation pressure is 315 psi, and the inflation agent is gaseous nitrogen. The maximum allowable load per main landing gear tire is 123,000 pounds. If the orbiter touches down with a 60/40 percent load distribution on a strut's two tires, with one tire supporting the maximum load, then the other tire can support a load of only 82,410 pounds. Therefore, the maximum tire load on a strut is 205,410 pounds with a 60/40 percent tire load distribution. The tires are rated at 225 knots (258 mph).

The main landing gear shock strut stroke is 16 inches. The allowable main gear sink rate for a 212,000-pound orbiter is 9.6 feet per second; for a 240,000-pound orbiter, it is 6 feet per second. With a 20-knot (23-mph) crosswind, the maximum allowable gear sink rate for a 212,000-pound orbiter is 6 feet per second; for a 240,000-pound orbiter, it is approximately 5 feet per second.

The landing gear tires have a life of one landing.