The system that distributes the propellants to the RCS thrusters
consists of fuel and oxidizer tanks, tank isolation valves, manifold
isolation valves, crossfeed valves, distribution lines and filling
and draining service connections.
Each RCS contains two spherical propellant tanks, one for fuel
and one for oxidizer, constructed of titanium and 39 inches in
The nominal full load of the forward and aft RCS tanks in each
pod is 1,464 pounds in the oxidizer tanks and 923 pounds in the
fuel tanks. The dry weight of the forward tanks is 70.4 pounds.
The dry weight of the aft tanks is 77 pounds.
Each tank is pressurized with helium, which expels the propellant
into an internally mounted, surface-tension, propellant acquisition
device that acquires and delivers the propellant to the RCS thrusters
on demand. The propellant acquisition device is required because
of the orbiter's orientation during boost, on orbit, and during
entry and because of the omnidirectional acceleration spectrum,
which ranges from very high during boost, entry or abort to very
low during orbital operation. The forward RCS propellant tanks
have propellant acquisition devices designed to operate primarily
in a low-gravity environment, whereas the aft RCS propellant tanks
are designed to operate in both high and low gravity, ensuring
propellant and pressurant separation during tank operation.
A compartmental tank with individual screen devices in both the
upper and lower compartments supplies propellant independent of
tank load or orientation. The devices are constructed of stainless
steel and are mounted in the titanium tank shells. A titanium
barrier separates the upper and lower compartments in each tank.
At orbiter and external tank separation and for orbital operations,
propellant flows from the upper compartment bulk region, into
the channel network, to the upper compartment transfer tube and
into the lower compartment bulk region. Flow continues from the
upper compartment until gas is ingested into the upper compartment
device and transferred to the lower compartment.
The lower compartment of the forward RCS propellant tanks will
expel propellant to depletion, as in the case of the upper compartment;
however, orbital operations are terminated with the forward RCS
at an expulsion efficiency of 91 percent to preclude gas ingestion
to the forward RCS engines.
The aft RCS propellant tanks' lower compartment is not used on
orbit, but is required for entry. The aft RCS tank propellants
are positioned approximately 100 degrees away from the tank outlet
because of the influence of up to 2.5-g acceleration. As the acceleration
builds up, the channel screen in the ullage area of both devices
breaks down and ingests gas. As entry expulsion continues, propellant
is withdrawn from the lower compartment until a 96.5-percent expulsion
efficiency is achieved.
The aft RCS propellant tanks incorporate an entry collector,
sumps and gas traps to ensure proper operation during abort and
entry mission phases. Because of these components, the aft RCS
propellant tanks are approximately 7 pounds heavier than the forward
RCS propellant tanks.
The left, forward and right RCS fuel and oxidizer tank ullage
pressures can be monitored on panel O3. When the rotary switch
on panel O3 is positioned to RCS prplnt , the pressures are displayed
on the RCS/OMS press fuel, oxid meters on panel O3. The pressures
will illuminate the left RCS, fwd RCS or right RCS red caution
and warning light on panel F7, respectively, if that module's
tank ullage pressure is below 200 psia or above 312 psia.