Main Propulsion System Helium Subsystem
The MPS helium subsystem
consists of seven 4.7-cubic-foot helium supply tanks; three 17.3-cubic-foot
helium supply tanks; and associated regulators, check valves, distribution
lines and control valves. Four of the 4.7-cubic-foot helium supply
tanks are located in the aft fuselage, and the other three are located
below the payload bay liner in the midfuselage in the area originally
reserved for the cryogenic storage tanks of the power reactant storage
and distribution system. The three 17.3-cubic-foot helium supply
tanks are also located below the payload bay liner in the midfuselage.
The tanks are composite structures consisting of a titanium liner
with a fiberglass structural overwrap. The large tanks are 40.3
inches in diameter and have a dry weight of 272 pounds. The smaller
tanks are 26 inches in diameter and have a dry weight of 73 pounds.
The tanks are serviced before lift-off to a pressure of 4,500
Each of the larger supply tanks is plumbed to two of the smaller
supply tanks (one in the midbody, the other in the aft body),
forming three sets of three tanks for the engine helium pneumatic
supply system. Each set of tanks normally provides helium to only
one engine and is commonly referred to as left, center, or right
engine helium, depending on the engine serviced. Each set normally
provides helium to its designated engine for in-flight purges
and provides pressure for actuating engine valves during emergency
The remaining 4.7-cubic-foot helium tank is referred to as the
pneumatic helium supply tank. It normally provides pressure to
actuate all of the pneumatically operated valves in the propellant
There are eight helium supply tank isolation valves grouped in
pairs. One pair of valves is connected to each engine helium supply
tank cluster, and one pair is connected to the pneumatic supply
tank. In the engine helium supply tank system, each pair of isolation
valves is connected in parallel, with each valve in the pair controlling
helium flow through one leg of a dual-redundant helium supply
circuit. Each helium supply circuit contains two check valves,
a filter, an isolation valve, a regulator and a relief valve.
The two isolation valves connected to the pneumatic supply tanks
are also connected in parallel; however, the rest of the pneumatic
supply system consists of a filter, the two isolation valves,
a regulator, a relief valve and a single check valve. Each engine
helium supply isolation valve can be individually controlled by
its He isolation A left , ctr , right open , GPC , close and He
isolation B left , ctr , right , open , GPC, close switches on
panel R2. The two pneumatic helium supply isolation valves are
controlled by a single pneumatic He isol , open, GPC, close switch
on panel R2.
All of the valves in the helium subsystem (with the exception
of the supply tank isolation valves) are spring loaded to one
position and electrically actuated to the other position. The
supply tank isolation valves are spring loaded to the closed position
and pneumatically actuated to the open position. Valve position
is controlled via electrical signals from either the onboard GPCs
or manually by the flight crew. All of the valves can be controlled
automatically by the GPCs, and the flight crew can control some
of the valves.
The helium source pressure of the pneumatic, left, center and
right supply systems can be monitored on the helium , pneu , l
(left), c (center), r (right) meters on panel F7 by positioning
the tank, reg (regulator) switch below the meters to tank . In
addition, the regulated pressure of the pneumatic, left, center
and right systems can be monitored on the same meters by placing
the switch to reg.
Each of the four helium supply systems operates independently
until after main engine cutoff. Each engine helium supply has
two interconnect (crossover) valves associated with it, and each
valve in the pair of interconnect valves is connected in series
with a check valve. The check valves allow helium to flow through
the interconnect valves in one direction only. One check valve
associated with one interconnect valve controls helium flow in
one direction, and the other interconnect valve and its associated
check valve permit helium flow in the opposite direction. The
in interconnect valve controls helium flow into the associated
engine helium distribution system from the pneumatic helium supply
tank. The out interconnect valve controls helium flow out of the
associated engine helium supply system to the pneumatic distribution
Each pair of interconnect valves is controlled by a single switch
on panel R2. Each He interconnect , left , ctr , right switch
has three positions- in open/out close , GPC , and in close/out
open. With the switch in the in open/out close position, the in
interconnect valve is open and the out interconnect valve is closed.
The in close/out open position does the reverse. With the switch
in GPC, the out interconnect valve opens automatically at the
beginning of the liquid oxygen dump and closes automatically at
the end of the liquid hydrogen dump.
In a return-to-launch-site abort, the GPC position will cause
the in interconnect valve to open automatically at MECO and close
automatically 20 seconds later. If an engine was shut down before
MECO, its in interconnect valve will remain closed at MECO. At
any other time, placing the switch in GPC results in both interconnect
valves being closed.
An additional interconnect valve between the left engine helium
supply and pneumatic helium supply would be used if the pneumatic
helium supply regulator failed. This crossover valve would be
opened and the pneumatic helium supply tank isolation valves would
be closed, allowing the left engine helium supply system to supply
helium to the pneumatic helium supply. The crossover helium valve
is controlled by its own three-position switch on panel R2. The
pneumatics l (left) eng He xovr (crossover) switch positions are
open, GPC and close. The GPC position allows the valve to be controlled
by the flight software loaded in the GPCs.
Manifold pressurization valves located downstream of the pneumatic
helium pressure regulator are used to control the flow of helium
to propellant manifolds during a nominal propellant dump and manifold
repressurization. There are four of these valves grouped in pairs.
One pair controls helium pressure to the liquid oxygen propellant
manifolds, and the other pair controls helium pressure to the
liquid hydrogen propellant manifold.
The liquid hydrogen RTLS dump pressurization valves located downstream
of the pneumatic helium pressure regulator are used to control
the pressurization of the liquid hydrogen propellant manifolds
during an RTLS liquid hydrogen dump. There are two of these valves
connected in series. Unlike the liquid hydrogen manifold pressurization
valves, the liquid hydrogen RTLS dump pressurization valves cannot
be controlled by flight deck switches. During an RTLS abort, these
valves are opened and closed automatically by GPC commands. An
additional difference between the nominal and the RTLS liquid
hydrogen dumps is in the routing of the helium and the place where
it enters the liquid hydrogen feed line manifold. For the nominal
liquid hydrogen dump, helium passes through the liquid hydrogen
manifold pressurization valves and enters the feed line manifold
in the vicinity of the liquid hydrogen feed line disconnect valve.
For the liquid hydrogen RTLS dump, helium passes through the RTLS
liquid hydrogen dump pressurization valves and enters the feed
line manifold in the vicinity of the liquid hydrogen inboard fill
and drain valve on the inboard side. There is no RTLS liquid oxygen
dump pressurization valve since the liquid oxygen manifold is
not pressurized during the RTLS liquid oxygen dump.
Each engine helium supply tank has two pressure regulators operating
in parallel. Each regulator controls pressure in one leg of a
dual-redundant helium supply circuit and is capable of providing
all of the helium needed by the main engines.
The pressure regulator for the pneumatic helium supply system
is not redundant and is set to provide outlet pressure between
715 to 770 psig. Downstream of the regulator are two more regulators:
the liquid hydrogen manifold pressure regulator and the liquid
oxygen manifold pressure regulator. These regulators are used
only during MPS propellant dumps and manifold pressurization.
Both regulators are set to provide outlet pressure between 20
to 25 psig. Flow through the regulators is controlled by the appropriate
set of two normally closed manifold pressurization valves.
Downstream of each pressure regulator, with the exception of
the two manifold repressurization regulators, is a relief valve.
The valve protects the downstream helium distribution lines from
overpressurization if the associated regulator fails fully open.
The two relief valves in each engine helium supply are set to
relieve at 785 to 850 psig and reseat at 785 psig. The relief
valve in the pneumatic helium supply circuit also relieves at
785 to 850 psig and reseats at 785 psig.
There is one pneumatic control assembly on each of the three
space shuttle main engines. The PCA is essentially a manifold
pressurized by one of the engine helium supply systems and contains
solenoid valves to control and direct pressure to perform various
essential functions. The valves are energized by discrete on/off
commands from the output electronics of the associated SSME controller.
Functions controlled by the PCA include the high-pressure oxidizer
turbopump intermediate seal cavity and preburner oxidizer dome
purge, pogo system postcharge and pneumatic shutdown.