Propellant System

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 diameter.

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.