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
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
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
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.