Space Forum / Shuttle / August 2004

Turbo pumps on SSME (Space Shuttle Main Engine) and most other engines, burn
some of the fuel to make a high-pressure gas that's used to drive the pumps.
The rest of the propellant goes into the combustion chamber to be burned
there.

There are several schemes used in liquid fuel engine design:

1. staged combustion (closed cycle)
2. simple "open" cycle
3. expander cycle (closed cycle--usually)
4. tapped cycle (open cycle, I think)
5. pressure fed (no turbopump)

Think of the Shuttle's engines, which use a 'staged combustion' cycle, as
a jet engine with a >really big< afterburner.  Both the oxygen and
hydrogen turbopumps have a hot gas generator that uses some of the
propellant to drive the pumps; the hydrogen-rich exhaust is ducted into
the main combustion chamber with the rest of the propellants.

This is a two stage combustion cycle in which all of the propellants are
efficiently used as high velocity reaction mass.  It makes for a more
complex engine that has to work at very high pressure, but achieves the
highest performance for its weight.

The SSME is one example of 'staged combustion' using hydrogen and oxygen.
The Russians have several engines using oxygen and kerosene, which must
run oxygen-rich, and storable propellants: hydrazines and nitrogen
tetrooxide.  The RD-180 used on the Atlas V is one example, as is the
RD-256 on the Proton; the Russians are very big on staged combustion
engines and have more types than I can list.

The most common design simply burns some propellant to drive the
turbopumps and the hot gas is dumped through a duct at low velocity.  The
large majority of rocket engines use this 'open cycle' design.  The
absolute  performance isn't the highest, but it makes for a simpler engine
design that's good enough in most cases.

The Saturn V's F-1 engines are an example of this 'open cycle' design; the
turbopump exhaust is ducted into the exhaust nozzle, but some designs dump
the exhaust through steerable nozzles to provide roll control, as
with the Delta IV's RS-68.  The older Delta and Atlas rockets have a
simple duct exhaust which produces a bright yellow carbon-rich plume.

Rarely used is a tapped cycle that uses some of the hot gas from the
combustion chamber to drive the turbopumps.  In fact, I can't think of

simply overlooked it.  Possibly it has practical problems that haven't
been worth solving.

A final type is the 'expander cycle', almost exclusively used with
hydrogen and oxygen.  The hydrogen is first used to cool the combustion
chamber and exhaust nozzle; the warm hydrogen is expanded through a
turbine to drive the pumps and then dumped into the main combustion
chamber to be burned with the oxygen.  There is no hot gas generator,
which makes for a simpler design that also is fully closed and achieves a
high exhaust velocity.

The best known expander cycle engine is the Pratt and Whitney RL-10 used
on the Centaur upper stage, also on the Delta IV's upper stage.  Similar
engines have been developed by Japan, Europe and Russia, with thrust in
the 10,000 - 25,000 lb range. 60,000 lb/thrust engines are in development;
P&W thinks it can build a 300,000 lb/thrust engine using a variant of this
method.

(As a footnote, the SSME's also use expander cycle turbopumps as booster
pumps, necessary to prevent cavitation at the main turbopump inlets.)

It's possible to make a rocket engine of any size with no turbopump at
all, but in practice a turbopump-driven engine is needed to achieve very
high thrust and helps to achieve the most efficient overall design of a
rocket launcher.  That said, pressure-fed liquid fuel engines do make
for the simplest possible design.  (Let the counter arguments now begin!  ;)

--Damon

They don't call it rocket science for nothing.

Hydrogen and oxygen.  As you may know, each turbopump has the power of
something like 21 train locomotives, so yes, they are powered a
combustion chamber and another smaller turbopump.  I think this smaller
pump is powered by hydrazine or something.  There are numerous variations
in design, just like car engines.  I sort of think that the Russian RD
180 is a neat engine because it uses one kerosine and one oxygen pump to
supply two nozzles.

To me the turbopump is the greater part of the whole engine.  The problem
facing rocket folks is how to generate pressure in a fluid to 6000 psi
over the area of a garbage can lid.  That is an amazing proposition.  The
pay-off is trips into space.

Yeah they use the exhaust for this and that....it was over my head. But
no, the pumps are driven by an turbine engine.  Google can find how
Saturn, shuttle engines work.  Apollo Archives for one.