From: yqg023@mrbig.rockwell.com (Jim F. Glass x60375)
Newsgroups: sci.space.policy
Subject: Re: Advantages of The Expander Cycle Engine
Date: 29 Jan 1997 16:22:16 GMT

In article <E4BwHy.9w3%spenford@zoo.toronto.edu>, Henry Spencer
<henry@zoo.toronto.edu> writes:

|> In article <5bpb3o$cau@infoserv.rug.ac.be>,
|> Filip De Vos <fidevos@eduserv1.rug.ac.be> wrote:
|> >You seem to assume that an expander cycle engine must neccesarily use the
|> >fuel as the working fluid to be expanded. This may not be the case. Is
|> >there any reason why an expander cycle engine cannot use the oxydant
|> >(LOX, say) as the fluid? ...

The problem with LOX (GOX) as the working fluid in an expander engine
is that it is an awful working fluid.  Hydrogen's Cp is over an order-
of-magnitude higher; meaning that it is much easier to put energy in--
and take energy out--of H2 than O2.  So you get driven to high turbine
pressure ratios in pure O2 expanders--which means high pump pressures,
and on and on and badness.  There are numerous concepts for "hybrid"
expanders which use H2 expander cycle on the fuel side and O2 expander
on the oxid side (less horsepower, therefore less penalty for  using
O2 as the working fluid).


|> There is considerable superstition in the US about oxidizer-cooled engines,
|> the theory being that a leak into the fuel-rich chamber will automatically
|> become a catastrophic failure.  The theory is, in fact, wrong -- it's been
|> tried, with deliberately-inflicted leaks, and nothing awful happens -- but
|> it's still an unpopular choice.
|>
|> There is also some unhappiness about driving turbines with hot oxidizing
|> mixtures, although the Russians seem to have solved that one fairly well.

This is not an expander cycle.  The russians use ox-rich preburners in
staged-combustion cycles, and seem to have licked the problem of getting
their turbines to live happily in the hot oxidizing environment.  One benefit
is that there is LOTS of flowrate to play with, making the overall system
easier to balance.


|> Oxygen's critical pressure is a bit high by expander standards, which would
|> make it a bit harder to use in such a system, unless you accepted two-phase
|> flow in the cooling passages.
|>
|> >Heating O2 to drive the turbine will abolish  all
|> >problems with coking denser fuels (like kerosine) at a stroke.

See above. "When God made working fluids, he decided that O2 would be
a bad choice"-- me.

Here are some numbers.  At 500 psia, 500 R, the Cp of H2 is 3.61 BTU/lb-R.
At the same point, the Cp of O2 is 0.235 BTU/lb-R!  H2 is thus 15.4 times better
than O2 as a working fluid in a turbine.  At a MR of 6.0, the advantage is
"only" (15.4/6) or 2.56 times better for hydrogen than oxygen.  That's almost
three times better.  Think that doesn't matter?  Let me assure you that it
most certainly DOES.


|> Methane and propane (the major non-hydrogen candidates for expanders) don't
|> coke, although sulfur impurities in them will corrode copper cooling passages
|> unless protective coatings are used.
|>
|> >Thinking further about dense propellants, why not use H2O2 to drive a
|> >turbine? Decomposing it exothermally, by pushing it over a catalyst, can
|> >drive the turbine...

Yes it has been done; of course the problem is the low energy content of
peroxide.  A secondary problem is that, today in the US, the highest
commercially-available peroxide concentration is 70%; the Germans make 85%.

I called every vendor I could find to inquire about 90%, 95%, and 98%
H2O2.  Some laughed; one hung up on me.  Yes; you can concentrate it
yourself from the low-octane variety; is it worth it?  I don't think so.


|> It's been done; the British peroxide/kerosene launchers used peroxide to
|> drive their turbines, although I believe the turbine exhaust was dumped
|> rather than burned.  They also decomposed *all* the peroxide before
|> injecting the fuel, which gave hypergolic ignition and near-total freedom
|> from high-frequency combustion instability.
|>
|> >Cooling the nozzle must be done by some other method, then.
|>
|> Actually, no, the Brits cooled their nozzles and chambers with peroxide.
|> It's an excellent coolant, provided you limit its maximum temperature.
|> --
|> "We don't care.  We don't have to.  You'll buy     |       Henry Spencer
|> whatever we ship, so why bother?  We're Microsoft."|   henry@zoo.toronto.edu


Jim Glass
Opinions my own, as if you could doubt it.

Newsgroups: sci.space.policy
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Advantages of The Expander Cycle Engine
Date: Thu, 30 Jan 1997 15:55:14 GMT

In article <5cntfo$5sn@bugle.nb.rockwell.com>,
Jim F. Glass x60375 <yqg023@mrbig.rockwell.com> wrote:
>...There are numerous concepts for "hybrid"
>expanders which use H2 expander cycle on the fuel side and O2 expander
>on the oxid side (less horsepower, therefore less penalty for  using
>O2 as the working fluid).

One reason why people keep proposing concepts like this is that nobody is
very keen on having rotating shaft seals separating oxidizer from hot
fuel-rich turbine gas, so it would be nice to drive the oxidizer pump with
oxidizer-rich gas.  That would eliminate some complicated and paranoid
engineering on the shaft seals (which typically includes a need for an
ongoing supply of helium as purge gas -- if memory serves, the SSME people
had hoped to get away without that, but decided that they couldn't).

>|> There is also some unhappiness about driving turbines with hot oxidizing
>|> mixtures, although the Russians seem to have solved that one fairly well.
>
>This is not an expander cycle...

True, although the gas in an expander cycle could get pretty warm if you
pushed things.  (Or if you cheat a bit -- P&W has proposed an "augmented
expander" cycle in which there is a little bit of preburning done.)
--
"We don't care.  We don't have to.  You'll buy     |       Henry Spencer
whatever we ship, so why bother?  We're Microsoft."|   henry@zoo.toronto.edu

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: engines and fuels (was Re: Proton/Apollo)
Date: Wed, 7 Jun 2000 14:37:53 GMT

In article <8hcak9$ako$1@nnrp1.deja.com>,  <jakemcguire@my-deja.com> wrote:
>...And the expander cycle is really quite
>simple compared to staged combustion, even with LH2.  And there is no
>such animal (to my knowledege) as an expander-cycle RP engine.

It would theoretically be possible to build an expander-cycle RP engine,
by using the LOX in the expander cycle.  As far as I know, nobody's built
such a thing.

You do need at least one propellant which can go from liquid to gas
(preferably by the supercritical route rather than boiling) without
thermal deterioration, which RP-1 can't.

If you're willing to use lower hydrocarbons, it's not a problem.  RL10
variants have been run experimentally on both methane and propane.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Turbopump questions
Date: Sat, 23 Sep 2000 20:15:13 GMT

In article <39C40506.BE33F094@pacbell.net>,
Michael P. Walsh <mp_walsh@pacbell.net> wrote:
>The expander cycle, in my opinion, provides the most benign environment
>for a system used to drive a turbopump, but has inherent energy
>limitations as long as the energy is obtained from heat transfer from
>the engine cooling system.

Ah, but you *don't* need to rely solely on the cooling system.  People
have, experimentally, put heat exchangers in the chamber to extract more
energy; it seems to work.  As long as your pump-drive power is coming from
combustion of some of the fuel -- which it almost invariably does -- there
is much to be said for putting all the combustion in one place (the
chamber) and avoiding secondary burner systems, extra hot-gas plumbing,
multiple hot-gas streams, etc.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Turbopump questions
Date: Mon, 25 Sep 2000 13:08:29 GMT

In article <39CEA829.5A77C37E@home.com>,
Michael Walsh  <mp1walsh@home.com> wrote:
>> Ah, but you *don't* need to rely solely on the cooling system.  People
>> have, experimentally, put heat exchangers in the chamber...
>
>You are adding some complication and squeezing a bit more out of
>the expander cycle.

Yes, it certainly does add complications.  On the other hand, describing
things like preburners as "complications" is inadequate somehow. :-)

>I remain skeptical of any high chamber pressure expander cycle
>engines, meaning in the 2500 psi and above range.  I perhaps
>should back off from the "inherent energy limitations"
>comment and stick with a, I don't believe it will be practical.

The basics have checked out experimentally to about 1500psi, which ought
to be enough for anyone except the truly demented. :-)  The practicality
in an operational engine remains to be proven, agreed, although a cynic
might say the same of the alternatives.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)