Propellant Storage and Distribution

The propellant storage and distribution system consists of one fuel tank and one oxidizer tank in each pod. It also contains propellant feed lines, interconnect lines, isolation valves and crossfeed valves.

The OMS propellant tanks of both pods enable the orbiter to reach a 1,000-foot- per-second velocity change with a 65,000-pound payload in the payload bay. An OMS pod crossfeed line allows the propellants in the pods to be used to operate either OMS engine.

The propellant is contained in domed cylindrical titanium tanks within each pod. Each propellant tank is 96.38 inches long with a diameter of 49.1 inches and a volume of 89.89 cubic feet unpressurized. The dry weight of each tank is 250 pounds. The propellant tanks are pressurized by the helium system.

Each tank contains a propellant acquisition and retention assembly in the aft end and is divided into forward and aft compartments. The propellant acquisition and retention assembly is located in the aft compartment and consists of an intermediate bulkhead with communication screen and an acquisition system. The propellant in the tank is directed from the forward compartment through the intermediate bulkhead through the communication screen into the aft compartment during OMS velocity maneuvers. The communication screen retains propellant in the aft compartment during zero-gravity conditions.

The acquisition assembly consists of four stub galleries and a collector manifold. The stub galleries acquire wall-bound propellant at OMS start and during RCS velocity maneuvers to prevent gas ingestion. The stub galleries have screens that allow propellant flow and prevent gas ingestion. The collector manifold is connected to the stub galleries and also contains a gas arrestor screen to further prevent gas ingestion, which permits OMS engine ignition without the need of a propellant-settling maneuver employing RCS thrusters. The propellant tank's nominal operating pressure is 250 psi, with a maximum operating pressure limit of 313 psia.

A capacitance gauging system in each OMS propellant tank measures the propellant in the tank. The system consists of a forward and aft probe and a totalizer. The forward and aft fuel probes use fuel (which is a conductor) as one plate of the capacitor and a glass tube that is metallized on the inside as the other. The forward and aft oxidizer probes use two concentric nickel tubes as the capacitor plates and oxidizer as the dielectric. (Helium is also a dielectric, but has a different dielectric constant than the oxidizer.) The aft probes in each tank contain a resistive temperature-sensing element to correct variations in fluid density. The fluid in the area of the communication screens cannot be measured.

The totalizer receives OMS valve operation information and inputs from the forward and aft probes in each tank and outputs total and aft quantities and a low level quantity. The inputs from the OMS valves allow control logic in the totalizer to determine when an OMS engine is thrusting and which tanks are being used. The totalizer begins an engine flow rate/time integration process at the start of the OMS thrusting period, which reduces the indicated amount of propellants by a preset estimated rate for the first 14.8 seconds. After 14.8 seconds of OMS thrusting, which settles the propellant surface, the probe capacitance gauging system outputs are enabled, which permits the quantity of propellant remaining to be displayed. The totalizer outputs are displayed on the OMS/RCS prplnt qty meters on panel O3 when the rotary switch is positioned to the OMS fuel or oxid positions.

When the wet or dry analog comparator indicates the forward probe is dry, the ungaugeable propellant in the region of the intermediate bulkhead is added to the aft probe output quantity, decreasing the total quantity at a preset rate for 98.15 seconds, and updates from the aft probes are inhibited. After 98.15 seconds of thrusting, the aft probe output inhibit is removed, and the aft probe updates the total quantity. When the quantity decreases to 5 percent, the low-level signal is output.

Parallel tank isolation valves in each pod located between the propellant tanks and the OMS engine and the OMS crossfeed valves permit propellant to be supplied to the OMS engine and OMS crossfeed valves or isolate the propellant. The left or right OMS tank isolation A switch on panel O8 controls the A fuel and A oxidizer valve in that pod, and the B switch controls the B fuel and B oxidizer valve in that pod. When the left or right tank isolation switches in a pod are positioned to GPC , pairs of valves are automatically opened or closed upon command from the orbiter computer. When a pair of valves is opened, fuel and oxidizer from the corresponding propellant tanks are allowed to flow to that OMS engine and OMS crossfeed valves; and when that pair of valves is closed, fuel and oxidizer are isolated from the OMS engine and OMS crossfeed valves. The switch positions open, GPC and close are permanent-position switches. Electrical power is provided to an electrical motor controller assembly, which supplies power to the ac-motor-operated valve actuators. Once the valve is in the commanded position, logic in the motor controller assembly removes power from the ac-motor-operated valve actuator. A talkback indicator above each tank isolation switch on panel O8 indicates the status of the fuel valve and oxidizer valve. The talkback indicator is controlled by microswitches in each pair of valves. The talkback indicator indicates op when that pair of valves is open, barberpole when the valves are in transit or one valve is open or closed, and cl when that pair of valves is closed. The open and close positions of each left or right tank isolation A, B switch permits manual control of the corresponding pair of valves (one for fuel and one for oxidizer).

In each pod, parallel left or right OMS crossfeed valves are controlled by the left, right crossfeed A, B switches on panel O8. The A switch controls the A fuel and A oxidizer ac-motor-operated valve actuators in the pod selected, and the B switch controls the B fuel and B oxidizer valve in the pod selected. When the A or B switch in a pod is positioned to GPC , the A or B pair of fuel and oxidizer valves is automatically opened or closed upon command from the orbiter computer. For example, when the A or B pair of crossfeed valves in the left pod is opened, fuel and oxidizer from the left pod are routed to the OMS crossfeed valves of the right pod; thus, a pair of A or B crossfeed valves in the right pod must be opened to permit the left pod fuel and oxidizer to be directed to the right OMS pod engine. A talkback indicator above the pod crossfeed switches on panel O8 indicates the status of the selected pair's fuel and oxidizer valves. The talkback indicator indicates op when both valves are open, barberpole when the valves are in transit or one valve is open and one closed, and cl when both valves are closed. The left, right crossfeed A, B open/close switches on panel O8 permit manual control of the corresponding pair of fuel and oxidizer valves.

The left and right OMS crossfeed A, B switches also provide the capability to supply OMS propellants to the left and right aft RCS engines. The left and right aft RCS will not be used to supply propellants to the OMS due to differences in pressures between the OMS and RCS.

The OMS crossfeed fuel and oxidizer line pressures are monitored on telemetry and are transmitted to the flight deck CRT.

There are 64 ac -motor-operated valve actuators in the OMS/RCS nitrogen tetroxide and monomethyl hydrazine propellant systems. Each valve actuator was modified to incorporate a 0.25-inch-diameter stainless steel sniff line from the actuator to the mold line of the orbiter. The sniff line permits the monitoring of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of each valve actuator during ground operations.

There are sniff lines in the 12 ac -motor-operated valve actuators in the forward RCS and in the 44 actua tors in the aft left and aft right RCS. The remaining 0.25-inch-diameter sniff lines are in the eight OMS tank isolation and crossfeed ac-motor-operated valve actuators in the left and right orbital maneuvering systems. The 44 aft left and right RCS sniff lines and the eight OMS left and right sniff lines are routed to the respective left and right OMS/RCS pod Y web access servicing panels.

During ground operations, an interscan can be connected to the sniff ports to check for the presence of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of the ac-motor-operated valve actuators.

An electrical microswitch in each of the ac-motor-operated valve actuators signals the respective valves' position (open or closed) to the onboard flight crew displays and controls as well as telemetry. An extensive improvement program was implemented to reduce the probability of floating particulates in the electrical microswitch portion of each ac-motor-operated valve actuator. Particulates could affect the operation of the microswitch in each valve and, thus, the position indication of the valves to the onboard displays and controls and telemetry.