Index Home About Blog
From: Henry Spencer <>
Subject: Re: Russian rocket engines
Date: Fri, 26 Dec 1997 04:17:39 GMT

In article <>,
Michael P. Walsh <> wrote:
>I am unfamiliar with the terms "Open Cycle" and "Closed Cycle"
>with regard to rocket engines.
>From the context I assume "Open Cycle" means a gas generator cycle
>which does not feed directly into the main combustion chamber but
>dumps exhaust gases either through a separage out let or someplace
>downstream in the main nozzle.

Open Cycle means anything that dumps the turbine gas separately, without
running it through the combustion chamber.  That covers several different
cycles, although the gas-generator cycle is the commonest.

>I assume "Closed Cycle" refers to what I know as staged combustion
>or preburner cycles where a high pressure gas generator is exhausted
>to the main combustion chamber.

Closed Cycle means anything where the turbine gas goes into the chamber.
Again, there are more possibilities than staged combustion; for example,
the expander cycle is also a closed cycle.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

Date: Tue, 12 Oct 1999 23:19:31 -0700
From: Doug Jones <>
Subject: Re: "Kistler's vehicle now 80% complete" (IAF congress)

Samuel Paik wrote:
> Jeff Greason wrote:
> > Turbojets are, after all, higher energy density devices than
> > reciprocating piston engines, yet far outpace them in reliability.  A
> > turbopump rocket engine is, essentially, a turbine pump attached to a
> > rocket combustion chamber.  Given a suitable choice of turbine drive gas,
> > the turbine drive environment can be as benign as that of a turbojet engine
> > (or even more so, in the case of some propellants in expander cycles).
> Am I correct in believing that most rocket turbopumps turbines
> are driven by the coolant flow?  This seems to be a very benign
> environment compared to turbojets where the turbines use the
> propellent exhaust directly, at temperatures often exceeding the
> melting point of the materials the turbines are made out of.

Pumpfed engines have three main sources for the gases that drive the
turbine.  In rising order of difficulty:

Expander cycle- all of one cryogenic propellant (typically hydrogen) is
pumped to about 2x the combustion chamber pressure.  It then flows through
the coolant passages in the combustor walls, is vaporized, then spins the
turbine and enters the combustor.  The turbine gases are just a little
above room temperature, and are clean- very easy on the turbine. All
propellants pass through the combustor.

Gas generator cycle- a small amount of both propellants is burned (usually
at a very rich mixture ratio) to power the turbine.  The (very dirty black
sooty) spent turbine gases are vented overboard, wasting about 2% of the

Staged combustion cycle- all of one propellant is, like the expander cycle,
pumped to very high pressure, but in addition to passing through the
coolant passages to be warmed up, a small amount of the other propellant is
burned with it to increase the volume before passing through the turbine.
This allows much higher combustion pressures by making more work available
for pumping the propellants.  The turbine gases are at *very* high pressure
and are quite hot.  All the propellants pass through the combustor.

Some staged combustion engines use the oxygen to drive the turbine, while
the fuel cools the combustor- thus it's not just an expander cycle with
additional heat.  Details of engine start and shutdown sequencing get kinda
hairy, because enough energy must be supplied to start the turbopump under
load, ignition must be reliably achieved in at least two places, and the
pump must be shut down without overspeeding, overheating, or shock chilling
the turbine (spraying LOX on hot spinning metal is Not Good), or allowing a
mixture ratio excursion that could burn out the chamber.

As Jeff said though, we have some clever ideas that we won't talk about-
it's really just some very intense plumbing ;)

Doug Jones
Rocket Plumber, Xcor Aerospace

Index Home About Blog