Orbiter 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 psi.

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 pneumatic shutdown.

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 management subsystem.

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 system.

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.