Gas Pressure Feed Systems

One of the simplest and most common means of pressurizing the propellants is to force them out of their respective tanks by displacing them with high-pressure gas. This gas is fed into the propellant tanks at a controlled pressure, thereby giving a controlled propellant discharge. Because of their relative simplicity, the rocket engines with pressurized feed systems can be very reliable. Reference 6-3 includes a design guide for pressurized gas systems.

A simple pressurized feed system is shown schematically in Fig. 1-3. It consists of a high-pressure gas tank, a gas starting valve, a pressure regulator, propellant tanks, propellant valves, and feed lines. Additional components, such as filling and draining provisions, check valves, filters, flexible elastic bladders for separating the liquid from the pressurizing gas, and pressure sensors or gauges, are also often incorporated. After all tanks are filled, the high-pressure gas valve in Fig. 1-3 is remotely actuated and admits gas through

206 LIQUID PROPELLANT ROCKET ENGINE FUNDAMENTALS TABLE 6-2. Typical Features of Liquid Propellant Feed Systems

Enhance Safety

Sniff devices to detect leak of hazardous vapor; used on Space Shuttle orbiter

Check valves to prevent backflow of propellant into the gas tank and inadvertent mixing of propllants inside flow passages Features that prevent an unsafe condition to occur or persist and shut down engine safely, such as relief valves or relief burst diaphragms to prevent tank overpressurization), or a vibration monitor to shut off operation in the case of combustion instability Isolation valves to shut off a section of a system that has a leak or malfunction Burst diaphragms or isolation valves to isolate the propellants in their tanks and positively prevent leakage into the thrust chamber or into the other propellant tank during storage Inert pressurizing gas

Provide Control

Valves to control pressurization and flow to the thrust chambers (start/stop/throttle) Sensors to measure temperatures, pressures, valve positions, thrust, etc., and computers to monitor/analyze system status, issue command signals, and correct if sensed condition is outside predetermined limits Manned vehicle can require system status display and command signal override Fault detection, identification, and automatic remedy, such as shut-off isolation valves in compartment in case of fire, leak, or disabled thruster Control thrust (throttle valve) to fit a desired thrust-time profile

Enhance Reliability Fewest practical number of components/subassemblies

Ability to provide emergency mode engine operation, such as return of Space Shuttle vehicle to landing

Filters to catch dirt in propellant lines, which could prevent valve from closing or small injector holes from being plugged up or bearings from galling. Duplication of unreliable key components, such as redundant small thrusters, regulators, check valves, or isolation valves Heaters to prevent freezing of moisture or low-melting-point propellant

Long storage life—use propellants with little or no chemical deterioration and no reaction with wall materials

Provide for Reusability Provisions to drain remaining propellants or pressurants

Provision for cleaning, purging, flushing, and drying the feed system and refilling propellants and pressurizing gas in field Devices to check functioning of key components prior to next operation Features to allow checking of engine calibration and leak testing after operation Features for access of inspection devices for visual inspection at internal surfaces or components

Enable Effective Propellant Utilization

High tank expulsion efficiency with minimum residual, unavailable propellant

Lowest possible ambient temperature variation or matched propellant property variation with temperature so as to minimize mixture ratio change and residual propellant Alternatively, measure remaining propellant in tanks (using a special gauge) and automatically adjust mixture ratio (throttling) to minimize residual propellant Minimize pockets in the piping and valves that cannot be readily drained the pressure regulator at a constant pressure to the propellant tanks. The check valves prevent mixing of the oxidizer with the fuel when the unit is not in an upright position. The propellants are fed to the thrust chamber by opening valves. When the propellants are completely consumed, the pressurizing gas can also scavenge and clean lines and valves of much of the liquid propellant residue. The variations in this system, such as the combination of several valves into one or the elimination and addition of certain components, depend to a large extent on the application. If a unit is to be used over and over, such as space-maneuver rocket, it will include several additional features such as, possibly, a thrust-regulating device and a tank level gauge; they will not be found in an expendable, single-shot unit, which may not even have a tank-drainage provision. Different bipropellant pressurization concepts are evaluated in Refs. 6-3, 6-A, and 6-5. Table 6-2 lists various optional features. Many of these features also apply to pump-fed systems, which are discussed in Section 6..6. With monopropellants the gas pressure feed system becomes simpler, since there is only one propellant and not two, reducing the number of pipes, valves, and tanks.

A complex man-rated pressurized feed system, the combined Space Shuttle Orbital Maneuver System (OMS) and the Reaction Control System (RCS), is described in Figs 6-3 and 6-A, Ref. 6-6, and Table 6-3. There are three locations for the RCS, as shown in Fig. 1-13: a forward pod and a right and left aft pod. Figures 6-3 and 6-4 refer to one of the aft pods only and show a combined OMS and RCS arrangement. The OMS provides thrust for orbit insertion, orbit circularization, orbit transfer, rendezvous, deorbit, and abort. The RCS provides thrust for attitude control (in pitch, yaw, and roll) and for small-vehicle velocity corrections or changes in almost any direction (translation maneuvers), such as are needed for rendezvous and docking; it can operate simultaneously with or separate from the OMS.

The systems feature various redundancies, an automatic RCS thruster selection system, various safety devices, automatic controls, sensors to allow a display to the Shuttle's crew of the system's status and health, and manual command overrides. The reliability requirements are severe. Several key components, such as all the helium pressure regulators, propellant tanks, some valves, and about half the thrusters are duplicated and redundant; if one fails, another can still complete the mission. It is possible to feed up to 1000 lbm of the liquid from the large OMS propellant tanks to the small RCS ones, in case it is necessary to run one or more of the small reaction control thrusters for a longer period and use more propellant than the smaller tanks allow; it is also possible to feed propellant from the left aft system to the one on the vehicle's right side, and vice versa. These features allow for more than nominal total impulse in a portion of the thrusters, in case it is needed for a particular mission mode or an emergency mode.

The compartmented steel propellant tanks with antislosh and antivortex baffles, sumps, and a surface tension propellant retention device allow propellant to be delivered independent of the propellant load, the orientation, or the

RCS propellant manifold valves

Gimballed OMS engine (1 per aft pod)

RCS propellant manifold valves

Gimballed OMS engine (1 per aft pod)

Space Shuttle Main Propulsion Engine

Vernier thrusters (2 per aft pod)

RCS pressurlzatlon components

RCS oxidizer OMS

tank oxidizer tank

Primary thrusters (12 per aft pod)

OMS helium tank

FIGURE 6-3. Simplified sketch at the left aft pod of the Space Shuttle's Orbiting Maneuvering System (OMS) and the Reaction Control System (RCS). (Source: NASA.)

Vernier thrusters (2 per aft pod)

RCS pressurlzatlon components

RCS oxidizer OMS

tank oxidizer tank

Primary thrusters (12 per aft pod)

OMS helium tank

FIGURE 6-3. Simplified sketch at the left aft pod of the Space Shuttle's Orbiting Maneuvering System (OMS) and the Reaction Control System (RCS). (Source: NASA.)

acceleration environment (some of the time in zero-g). Gauges in each tank allow a determination of the amount of propellant remaining, and they also indicate a leak. Safety features include sniff lines at each propellant valve actuator to sense leakage. Electrical heaters are provided at propellant valves, certain lines, and injectors to prevent fuel freezing or moisture forming into ice.

A typical RCS feature that enhances safety and reliability is a self-shutoff device is small thrusters that will cause a shutdown in case they should experience instability and burn through the walls. Electrical lead wires to the propellant valves are wrapped around the chamber and nozzle; a burnout will quickly melt the wire and cut the power to the valve, which will return to the spring-loaded closed position and shut off the propellant flow.

The majority of pressurized feed systems use a pressure regulator to maintain the propellant tank pressure and thus also the thrust at constant values. The required mass of pressurizing gas can be significantly reduced by a blow-down system with a "tail-off' pressure decay. The propellants are expelled by the expansion of the gas already in the enlarged propellant tanks. The tank pressure and the chamber pressure decrease or progressively decay during this adiabatic expansion period. The alternatives of either regulating the inert gas pressure or using a blowdown system are compared in Table 6^t; both types

RCS helium tanks (2)

OMS helium tank

4 4 /Dual helium pressure regulators (6 sets) <1 0<

Set of 4 series-parallel check valve (4 places)

RCS helium tanks (2)

OMS helium tank

Propellant Feed System

Relief valve (4 places)

Cross feeds to right-side pod, each with dual isolation valves

OMS thrust chamber (gimballed)

FIGURE 6-4. Simplified flow diagram of the propellant feed system flow for the left aft pod of the Orbital Maneuvering System (OMS) and Reaction Control System (RCS) of the Space Shuttle Orbiter Vehicle. Solid lines: nitrogen tetroxide (N204); dash-dot lines: monomethylhydrazine (MMH); short dashed lines: high-pressure helium. (Source: NASA.)

Relief valve (4 places)

Cross feeds to right-side pod, each with dual isolation valves

OMS thrust chamber (gimballed)

FIGURE 6-4. Simplified flow diagram of the propellant feed system flow for the left aft pod of the Orbital Maneuvering System (OMS) and Reaction Control System (RCS) of the Space Shuttle Orbiter Vehicle. Solid lines: nitrogen tetroxide (N204); dash-dot lines: monomethylhydrazine (MMH); short dashed lines: high-pressure helium. (Source: NASA.)

TABLE 6-3. Characteristics of the Orbital Maneuver System (OMS) and the Reaction Control System (RCS) of the Space Shuttle in One of the Aft Side Podes

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Responses

  • sophia
    How to inspect the coordinates of thrust chamber?
    3 years ago
  • sven
    Where used gas pressure feed syster?
    3 years ago
  • ayden
    What used gas pressure feed system?
    3 years ago
  • Weronika
    What is gas pressure feed system in rockets?
    3 years ago
  • fausta
    What is regulated pressure in pressure feed systems?
    2 years ago
  • emppu
    What is blowdown propellant feeding system?
    2 years ago

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