From: Bruce Dunn <bruce_dunn@mindlink.bc.ca>
Newsgroups: sci.space.tech
Subject: Re: RLV engines (was Re: X-33 Concepts: Lockheed, Mac Dac, Rockwell)
Date: Fri, 21 Jun 1996 18:55:11 -0700

Andy Haber wrote:

> 
> This seems like somewhat of a "disconnect" to me.  If a RL-10A-3-3A will
> do 10 starts and 4000 seconds with no maintenance, then why, after a *lot*
> of work, does a SSME need 3/4 of million dollars worth of maintenace after
> 1 start and 500 seconds of use?
...
  Granted, the SSME generates 27 times the thrust of the RL-10A-3-3A,
> and SSTO could use a engine a little bigger than the RL-10.

It isn't the thrust that makes the SSME a piece of machinery running on
the ragged edge of failure, but the chamber pressure.  Give or take a bit,
the RL-10 and the SSME have about the same thrust to weight ratio.  However,
the RL-10 was designed to operate only in space, where a high chamber
pressure is not needed.  It thus gets by with a low stress, expander cycle
turbopump and a wimpy chamber pressure of about 30 atmospheres.  The SSME
was designed to operate in the atmosphere, and in order to avoid massive
nozzle losses while doing so operates at a chamber pressure of about 200
atmospheres.  This requires developing something like an order of magnitude
more pump power for the same amount of weight.  Not only are the turbopumps
running at a massively higher stresses, the chamber is at about 7 times the
pressure, increasing the amount of heat per unit wall area which needs to
be dealt with.  SSME development was a series of failure after failure,
and NASA was so distrustful of the original turbopumps that it has spent
massive sums on designing and building a completely new design of turbopump
from another manufacturer.

For Ariane V, the Europeans looked at high pressure, staged combustion engines
and said "thanks but no thanks".  The Ariane V hydrogen oxygen engine is a
medium pressure engine running at 105 atmospheres using a very simple (but
not particularly efficient) gas generator cycle.  The Japanese had the same
choice for their H2 engine, and decided to go for broke and develop a new,
high pressure staged combustion engine with a planned chamber pressure of
145 atmospheres.  Broke unfortunately was the operative word, indicating their
general test stand experience (including killing a test engineer in one
of the test stand explosions).  The engine that eventually flew was behind
schedule, overweight, and had been sharply derated from the original design
to get something that would hold together long enough to get to orbit
(for example the combustion chamber pressure was scaled back to 130 atmospheres).

It is chamber pressure, not thrust or size which makes an engine difficult
to build.  In turn, it is the SSTO requirement for the last bit of Isp and the
requirement for operation in both the atmosphere and in space which drive
the use of high chamber pressure engines.

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Mercury Atlas launch question
Date: Wed, 29 Dec 1999 17:57:47 GMT

In article <hrak6s8lc8ud4dd6uo6llk54ltb6che2ia@4ax.com>,
Michael J. Gallagher  <mikejoe@odyssey.net> wrote:
>> .... the "staged combustion" scheme used in the shuttle main engines ....
>>have their own problems, like very high pump pressures...
>
>Would that be why the SSMEs kept exploding during static tests in the
>late '70s?

The development problems with the SSME were more a matter of tight funding
and poor management... but they certainly were aggravated by the use of
the staged-combustion cycle.  The high pressures make the engineering more
demanding and the failures more destructive, and the complex interactions
between different subsystems make it difficult to debug them separately.

For example, the SSME start sequence is astonishingly complex, an
elaborate dance of precisely-timed valve motions (with valves opening
partway, pausing, closing slightly, pausing again, then opening wide), and
as a friend of mine put it, "for every bend in those curves, there's a set
of burned-out engine hardware to prove that it's necessary".
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)