Rocketderived Propulsion

Rocket-derived propulsion systems begin with the liquid propellant rocket. Propel-lants are injected into a combustion chamber to burn at high pressure and temperature then exit via a sonic throat into an expansion nozzle that is designed to match the nozzle exit static pressure to the ambient atmospheric pressure, as shown in Figure 4.2. For maximum performance the nozzle exit pressure should be equal to the surrounding ambient pressure. However atmospheric pressure ranges from 14.696 psi (101.3 kPa) at the surface to zero in space. Normally the nozzle size is specified by the area ratio, i.e., the exit area divided by the sonic throat area. The area ratio determines the ratio of the nozzle exit pressure to the chamber pressure. Once the chamber pressure is determined, then the exit pressure is determined. If the nozzle exit pressure is higher than the ambient pressure the nozzle is termed "under-expanded" and the result is the nozzle flow suddenly expands upon exiting the nozzle. When you see a picture of a rocket at high altitude or in space and see the exhaust blossoming into a large plume, this is an under-expanded nozzle. If the nozzle exit pressure is lower than the ambient pressure, the nozzle is termed "over-expanded" and the nozzle flow separates from the nozzle wall at a location that yields the approximate correct area ratio for the ambient pressure. If you see a picture of a rocket lifting of from a launching pad, you can see the flow exiting the nozzle is smaller in diameter than the actual nozzle diameter, a sign that this is an over-expanded nozzle. Engines such as the Pratt & Whitney RL-10-3 have a two-position nozzle. At lower altitudes the nozzle area ratio is small (10 to 20). As the altitude is increased and the area ratio becomes too small, a nozzle extension slides over the nozzle increasing the area ratio (50 to 60). Thus there are two altitude regions where the engine is matched to the ambient pressure. For most high-thrust rockets the propellants are a fuel and an oxidizer. For some space maneuver and station-keeping rockets the fuel is a monopropellant, that is decomposed by a catalyst into gaseous products.

Rocket-derived propulsion involves installing the rocket as a primary nozzle in an air ejector system. The rocket induces airflow in the secondary air system increasing the total mass flow through the system. These systems are generally operated up to Mach 6 or less because of pressure and temperature limits of the air induction system. At Mach 6 the inlet diffuser static pressures can typically equal 10 to 20 atmospheres and 3,000°R (1,666 K). These propulsion systems can offer major advantages when applied to existing rocket launchers [Czysz and Richards, 1998].

1. Chemical rocket. Figure 4.10 represents a typical turbopump-fed liquid propel-lant rocket. A turbopump is generally a centrifugal compressor to pressurize the fuel,

Liquid Propel lain Rocket Air Augmented Rocket

Ram Rocket

Figure 4.10. Rocket-derived propulsion.

Liquid Propel lain Rocket Air Augmented Rocket

Ram Rocket

Figure 4.10. Rocket-derived propulsion.

coupled to an expansion turbine driving the pump. The turbopump pressurizes the propellant feed system to the pressure required for engine operation. For the turbopump to function some fuel and oxidizer are burned in a separate combustion chamber to generate the hot gases necessary to power the turbine, powering in turn the pump. Because this burned propellant does not contribute to the primary thrust of the rocket engine, the turbopump cycle rocket (such as Rocketdyne J-2 for Saturn V) has the lowest specific impulse (7sp) for a given propellant combination. A hydrogen/oxygen high-pressure engine has an Isp of about 430 s. In the so called "Topping cycle'' (such as in the Rocketdyne SSME) the turbopump exhaust, which is still rich in fuel, is introduced into the rocket motor, contributing to the engine total thrust. A hydrogen/oxygen high-pressure engine using this cycle has an Isp of about 455 s. In an "expander cycle'' (such as Pratt & Whitney RL-10) a liquid fuel, such hydrogen, is vaporized and raised in temperature by passing through the engine cooling passages. The hot gases then drive an expansion turbine to drive the turbopump before being introduced into the combustion chamber. This engine has the highest Isp for a specific propellant. A hydrogen/oxygen high-pressure engine has an Isp of about 470 s. Some representative propellants are given in Table 4.5 with

Table 4.5. Representative propellants and their characteristics.



Isp (sec)

Sp. gr.-/spa


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