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
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
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
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.