Handbook of Turbomachinery Second Edition Revised - Ventech!

emembered that the second (and any subsequent) rotor rows depend on
upstream rotor exit swirl for work-generating capability. Under these
conditions, downstream rotor rows will be relatively low work rotors. For
example, in a typical two-row velocity-compounded design, a rotor work
split of 70–30 or 75–25 might be considered typical.
The staged combustion and expander cycles tend to place greater
demands on turbine performance than the monopropellant or gas generator
cycles. The performance of the gas generator cycle (a so-called open cycle by
virtue of its atmospheric turbine exhaust) is relatively insensitive to turbine
efficiency. However, the staged combustion and expander cycles (the socalled
closed cycles) involve the turbine package quite intimately in the
overall engine system. In this regard, the problems presented by the
‘‘closed’’ rocket engine cycles are not unlike the turbine component
matching problem in jet engine design. Not only must the turbine package
be capable of sufficient component level efficiency, but it must be
‘‘matched’’ to deliver this efficiency within the range of operation of the
rocket engine system.
The first production engine incorporating a closed cycle was the RL-10
engine (also known as the Centaur engine for the stage that it powers)
designed and built by the Pratt and Whitney Division of United
Technologies. This engine operates on a classic version of the expander
cycleandwasshownschematicallyinFig.4.Incidentally,thisenginehasthe
honor of being the first production liquid-hydrogen-powered rocket engine
in the world. As one can see from the figure, LH 2 is discharged from the fuel
pump into the cooling circuit for the engine’s combustion chamber and
nozzle. The flow of heat energy from the chamber’s combustion reaction is
absorbed by the hydrogen, resulting in a substantial enthalpy increase (as
well as a static pressure drop) at the exit from the cooling jacket. Now comes
the tricky part. The enthalpy increase across the cooling jacket must be
sufficient that the turbine enthalpy drop results in proper turbine
performance with a turbine outlet pressure equal to the pressure required
in the fuel manifold of the engine’s injector. This is probably easier to
absorb by looking at the figure than by giving a verbose discussion.
Since we try to minimize required pump discharge pressure in the
closed engine cycles, the operating point of a typical expander cycle turbine
will tend to be at a considerably higher velocity ratio than that of an opencycle
engine. In the case of the RL-10 (circa 1970), the use of two turbine
stages permitted astage velocity ratio of around 0.35. From Fig. 9, it can be
seen that this value is near the optimum for an impulse stage. As a result, a
turbine efficiency of almost 75% (total to static efficiency) was obtained. The
relevant design conditions were a power output of approximately 660 hp at a
shaft speed of 28,670 rpm with a blade pitchline velocity of approximately
Copyright © 2003 Marcel Dekker, Inc.