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Handbook of Turbomachinery Second Edition Revised - Ventech!

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using products <strong>of</strong> decomposition <strong>of</strong> hydrogen peroxide (H2O2) as working<br />

fluid. Expanding its gas through a pressure ratio <strong>of</strong> approximately 22, this<br />

turbine delivered just under 800 hp at a shaft speed <strong>of</strong> 4,800 rpm. The most<br />

serious limitation suffered by systems <strong>of</strong> this sort is relatively low-power<br />

output due to low specific energy <strong>of</strong> the turbine working fluid. Despite this<br />

comparative limitation, monopropellant drive turbomachinery remains in<br />

quite high-volume use in countries other than the United States.<br />

Rocket engines employing the gas generator cycle comprise the bulk <strong>of</strong><br />

the engines being produced in the United States today, or that have been<br />

produced in the past. In this cycle, a small combustor is supplied with fuel<br />

and oxidizer in parallel with the engine’s main combustion chamber. The<br />

resulting products <strong>of</strong> combustion are fed directly to the turbine inlet<br />

manifold. Thus, the turbine inlet pressure is, for practical purposes, set at<br />

the turbopump discharge pressure less the pressure losses in the gas<br />

generator combustion system. Typical values <strong>of</strong> inlet pressure for gas<br />

generator cycle turbines have ranged from 400 to 1,000 psi. The turbine<br />

designer could, <strong>of</strong> course, reduce the turbine inlet pressure by means <strong>of</strong><br />

pressure reducing or regulating devices in the gas generator propellant<br />

systems, although this is not common practice.<br />

The turbine exhaust pressure in gas generator cycle turbines may or<br />

may not be dependent on other engine cycle considerations. In the simplest<br />

instances, turbine exhaust gases are simply transported to the base plane <strong>of</strong><br />

the engine by a suitable exhaust duct, where they vent to atmosphere. In<br />

these cases, turbine exhaust pressures are only slightly above atmospheric<br />

pressure. Such systems also typically include a restricting orifice in the<br />

turbine exhaust duct. This orifice, along with the turbine exhaust pressure<br />

level, is sized to induce sonic flow through the orifice. This technique ensures<br />

that the turbine pressure ratio (and thus turbine power) will not vary with<br />

altitude. The other popular method <strong>of</strong> disposal <strong>of</strong> gas generator cycle<br />

turbine exhaust gas involves transporting the gas to an annular manifold<br />

around the divergent section <strong>of</strong> the engine exhaust nozzle. Here, the gas is<br />

injected into the nozzle’s supersonic flow stream. The turbine exhaust is cool<br />

relative to the total temperature <strong>of</strong> the main chamber gas and is used in this<br />

instance to provide a boundary layer adjacent to the nozzle wall that is cool<br />

enough to negate the requirement for regenerative cooling in the lower<br />

portion <strong>of</strong> the nozzle section. This practice is referred to as dump cooling<br />

and has certain advantages. The smaller the fraction <strong>of</strong> the nozzle length<br />

that must be regeneratively cooled, the lower is the discharge pressure<br />

requirement <strong>of</strong> the propellant pump that is delivering the coolant fluid.<br />

Since the regenerative cooling process depends on high coolant velocities,<br />

the pressure losses across a large nozzle’s coolant circuit can be substantial.<br />

Even a relatively small reduction in the regeneratively cooled nozzle length<br />

Copyright © 2003 Marcel Dekker, Inc.

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