Handbook of Turbomachinery Second Edition Revised - Ventech!

lifetime. With very few exceptions, rocket engine turbines have lives that are
exceedingly short when compared with more conventional turbine applications.
A German engineer who participated in the development of the F-1
engine turbine once told me, ‘‘The design lifetime of our machines was
intended to be 25 years [he was referring to hydrodynamic power plant
turbines in Europe]. Then I start to work on this machine, and I asked about
the intended design life. They told me it was 160 seconds!’’ While the current
trend is toward lives considerably in excess of this, the basic idea is still
valid.
Rapidity of startup is another somewhat unique problem area. Rocket
turbines are often required to transition from a ‘‘cold start’’ to rated power
output in 2–3 sec. The transient loading resulting from this very rapid power
buildup has been the cause of turbine blade failures. It should also be
pointed out that the startup loads issue has a strong thermal component as
well as a mechanical one. Since rocket turbines are normally powered by
gases that are products of combustion, the gas temperature increases from
nearly ambient to its full value within the same 2- to 3-sec time period. This
may be accompanied by transient overshoots in both gas temperature and
chemical composition due to the action of the engine’s control system. The
turbine must tolerate these, and they must not compromise design life.
Rocket turbines operate with working fluids of a very high energy
content, and this energy must be extracted in as efficient a manner as
possible. This is somewhat problematic (as mentioned earlier) due to
relatively strict size and weight limitations placed on the turbine package.
With the advent of hydrogen-based working fluids, old problems are
compounded and new ones created. For a number of reasons, blade speed in
rocket turbines is normally lower than desired. Also, due to hydrogen’s high
specific heat value, gas spouting velocities tend to be higher in hydrogenoperated
turbines than in turbines operated with other propellant
combinations over the same pressure ratio. This combination of factors
tends to move the operating point for a turbine stage toward a lower range
of velocity ratio, where maintaining a given efficiency is difficult. In
addition, a heated hydrogen environment under elevated pressure creates
the potential for a turbine blade environment that is chemically adverse. A
great many metallic materials are subject to premature embrittlement and
failure in a hydrogen environment. Although this failure mechanism may
not represent a serious problem for an expendable rocket turbine, it may
have to be considered in the design of longer-lived and reusable machinery.
What represents ‘‘consideration’’ varies from case to case. In one instance, a
problem may be avoided by wise choice of blade or disk material and/or
processing methods. In a more acute case, blade speed may be limited or derated
by the necessity of keeping blade loading below a threshold value.
Copyright © 2003 Marcel Dekker, Inc.