From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Propane-Oxygen Propellant Data?
Date: Tue, 12 Mar 1996 16:19:22 GMT

In article <4hdlb2$5e5@cloner4.netcom.com> greason@ix.netcom.com (Greason) writes:
>A. Giovanetti, L. Spadaccini, and E. Szetela, "Deposit Formation and
>   Heat-Tranfer Characteristics of Hydrocarbon Rocket Fuels", 
>   _Journal of Spacecraft & Rockets_, v. 22, no. 5 (Sept-Oct 1985),
>   p. 574-580.

I finally dug this up and read it, and it contains a couple of interesting
things that Jeff didn't mention.  Much the most interesting result, for
propane enthusiasts anyway :-), is that the coking tendencies of propane
drop spectacularly -- to vanishingly small levels -- with even slight
pre-chilling of the propane.  Their tentative conclusion is that the
coking is not a problem of the propane itself, but is probably caused by
sulfur-containing impurities which are being frozen out by the chilling.

This would fit with their other observations, including the tendency of
room-temperature propane to corrode their copper plumbing slightly --
this is a strong indication of sulfur-based impurities at work, a problem
which is well known in the commercial propane industry.

When dealing with something labelled "propane", one has to be careful
about its actual composition.  The composition of commercial propane can
vary quite widely, and sulfur-containing impurities can cause significant
problems even at quite small concentrations.

Incidentally, sulfur-compound corrosion can be an issue with methane, too.
Aerojet did some work with methane and propane for the STME program (no, I
don't have references, indeed there may not be any -- I heard by private
channels), and had serious corrosion problems with their methane but none
with propane.  The problem was traced to sulfur-based impurities in the
methane, and quick tests showed that if you added the same amount of
sulfur compounds to propane or RP-1, you got the same results.  (The RP-1
spec has a requirement for quite low sulfur levels, although I'm told that
nobody is now sure whether that was due to known problems or just caution.)
-- 
Space will not be opened by always                 |       Henry Spencer
leaving it to another generation.   --Bill Gaubatz |   henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Propane-Oxygen Propellant Data?
Date: Thu, 14 Mar 1996 15:22:43 GMT

In article <4i5p49$7pl@knot.queensu.ca> 3cdk2@qlink.queensu.ca (Kossowski Craig D) writes:
>: ...Their tentative conclusion is that the
>: coking is not a problem of the propane itself, but is probably caused by
>: sulfur-containing impurities which are being frozen out by the chilling.
>
>So, would chilled LNG also display the same prcipitation and resultant 
>low coaking levels?  Any informed guesses?

It depends somewhat on the nature of the sulfur impurities.  Those folks
may have been lucky in having mostly impurities that would freeze out of
propane readily.  The fact that Aerojet did run into sulfur problems with
methane -- which they were surely using as a liquid -- argues that you
can't absolutely depend on low temperatures to do the job.  Depending on
what impurities you get, you might also have to use chemical purification
(or convince your fuel supplier to use it). 
-- 
Space will not be opened by always                 |       Henry Spencer
leaving it to another generation.   --Bill Gaubatz |   henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Propane-Oxygen Propellant Data?
Date: Sun, 17 Mar 1996 18:18:57 GMT

I wrote:
>Incidentally, sulfur-compound corrosion can be an issue with methane, too.
>Aerojet did some work with methane and propane for the STME program (no, I
>don't have references, indeed there may not be any -- I heard by private
>channels), and had serious corrosion problems with their methane...

Aha.  Jeff, you might want to read "Compatibility of Hydrocarbon Fuels
with Booster Engine Combustion Chamber Liners", S.D. Rosenberg & M.L. Gage,
in the Nov-Dec 1992 Journal of Propulsion and Power.  This is the Aerojet
people reporting on their investigation of the problem.

The one-sentence summary of the paper is that neither methane nor propane
shows any coking in the traditional sense of the term, but even tiny sulfur
impurities in either will give you copper-sulfide deposits which have much
the same effect.

They tested RP-1, n-dodecane, propane, and methane.  The n-dodecane was
tested because the spec for RP-1 is tight by petroleum standards but not
by chemical standards, and so they wanted to know whether a tighter RP-2
spec would change its behavior; n-dodecane is essentially what RP-1 would
be if it were a single compound.  As it happens, they found no difference
between RP-1 and n-dodecane, so the complex composition of RP-1 is not
an issue.

They ran some static tests, which generally showed minimal problems with
pure fuels, slight corrosion with traces of air or water added, and heavy
corrosion with small amounts of sulfur impurities.  The really interesting
part were the dynamic tests, in a simulated cooling channel.

RP-1 gave no deposits at 291C, a bit at 302C, a lot at 372C.  The deposits
were "classical" coking, mostly carbon with some oxygen.  When they added
50ppm of a mercaptan -- the sulfur limit in the RP-1 spec -- the picture
changed, with heavy corrosion and copper-sulfide deposits even at the lower
temperatures.

Technical-grade methane (which has 4.8ppm O2, 9.0ppm H2O) gave no deposits
or corrosion even at 501C.  This didn't change when they threw in 5% of
ethylene to encourage the stuff to polymerize.  It did change when they
added sulfur.  With 200ppm of CH3SH, deposits clogged their test cooling
channel completely.  10ppm gave heavy deposits.  Even 1ppm at 384C gave
thin deposits and microscopic surface damage.  These deposits were copper
sulfides, with no detectable carbon or oxygen content. 

Instrument-grade propane (6ppm O2, 200ppm H2O) gave copper-sulfide
deposits under all conditions tried!  The precise nature of the sulfur
impurity is not known, and it must be pretty small -- the spec says <2ppm
sulfur, and in fact they found none with an assay sensitive to 0.2ppm --
but the deposits were definitely sulfides, not classical coking.  Unlike
the methane deposits, these were light, powdery, and weakly adhering.  At
the higher coolant velocities (150-200 ft/s), the amounts were modest
(although the one thing I find unsatisfying about this paper is that they
aren't very quantitative about the deposition problems) and the stuff was
mostly found in the downstream filter.  At lower rates, it was on the
channel walls.  Chilling of the propane to about -100C reduced the
deposits by an order of magnitude.  (Alas, they didn't try going colder.)

My take on this is that anybody using any of these fuels in a reusable
rocket is going to have to keep a really sharp eye out for sulfur, and in
particular will need to get fuel which is free of the "stenchants" added
to commercial propane and natural gas.  (Stenchants are there to make the
stuff smell, for safety, but they're invariably sulfur compounds, often at
tens of ppm.  Worse, they are often a legal requirement, which might mean
you'd have to get code exemptions and/or purify the stuff yourself.)  Bulk
fuel prices should be taken with a grain of salt, because the bulk fuels
quite possibly aren't pure enough. 

(Also of note is the very next paper in that issue of JP&P, which concludes
that SSTOs using methane have considerably higher dry masses than ones using
propane.)
-- 
Space will not be opened by always                 |       Henry Spencer
leaving it to another generation.   --Bill Gaubatz |   henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Propane-Oxygen Propellant Data?
Date: Mon, 25 Mar 1996 19:10:06 GMT

I wrote:
>...you might want to read "Compatibility of Hydrocarbon Fuels
>with Booster Engine Combustion Chamber Liners", S.D. Rosenberg & M.L. Gage,
>in the Nov-Dec 1992 Journal of Propulsion and Power...

As I noted earlier, for "1992", read "1991".  However, it turns out that the
Nov-Dec 1992 issue has another paper from them:  "Hydrocarbon-Fuel/Copper
Combustion Chamber Liner Compatibility, Corrosion Prevention, and
Refurbishment", by Rosenberg, Gage, Homer, and Franklin.  It recaps the
earlier paper briefly, and adds some more.  Briefly:

Copper appears to catalyze "classical" coking of RP-1, because the threshold
temperature for it in their copper tests (about 600F) is considerably lower
than that seen in traditional hydrocarbon engines (which aren't copper).

The copper-sulfide deposits in sulfur corrosion are fairly pure Cu2S.

Mil-spec RP-1 can have up to 500ppm sulfur, 50ppm of it as mercaptans.
This is much too high, at least for copper engines.

After some experimenting with possible metals for surface coatings, they
settled on gold:  it's a relatively good match for copper in thermal
properties, and the technology for plating it onto copper is mature.  They
in fact had trouble getting good coatings -- the gold itself behaved well,
but the nickel layer underneath it, meant as a diffusion barrier, gave
difficulties -- but decided that what they had was adequate for early
testing.  It was quite successful; under conditions that clogged uncoated
channels badly, no deposits at all were seen in coated channels. 

They also experimented with cleaning sulfide deposits out of channels.
Most cleaning agents either attacked the copper or didn't remove the
sulfide.  Dilute aqueous sodium cyanide (!) left the copper alone and
removed the sulfide quickly and completely.  However, the final surface is
rough and porous, and as you would expect, the result is slightly better
cooling performance at the price of slightly higher flow resistance.

The current DoD rocket-methane spec simply says "max 1ppm sulfur", and
this is not good enough.  Based on their experience, they suggest "max
0.1ppm H2S, max 0.2ppm mercaptan sulfur, max 0.5ppm total sulfur". 

Bulk methane, even top grades, does not meet even the DoD spec, never
mind their suggested spec.  There was one exception, at the time they
did this work:  Quadren Cryogenic Processing sells bulk methane that
actually exceeds their spec, with total sulfur less than 0.1ppm.

I get the impression they were mostly interested in methane, and didn't
pursue propane very hard after their initial difficulties.
-- 
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |   henry@zoo.toronto.edu

From: Bruce Dunn <bdunn@genastro.bc.ca>
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Mon, 03 Jan 2000 14:34:14 GMT

Charles R Martin wrote:

> The problem is that propane is a waxy solid at LOX temperatures....

Actually, no.  The propane melting point is 85.5 K, while LOX boiling
point is 90 K or so (depending on exact atmospheric pressure).  This
allows a vehicle in which the propane is kept at or near LOX temperature
by thermal contact between the LOX tank and the propane tank.

At 300 K (equatorial launch site temperature), propane has a density of
490 kg/m^3 and an inconveniently high vapor pressure (near ten
atmospheres).  When cooled to 90 K, the density increases to 790 kg/m^3,
and the vapor pressure is negligable.


--
Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
http://www.genastro.com/
Reliable, low-cost transportation to low Earth orbit and beyond

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Wed, 5 Jan 2000 20:31:54 GMT

In article <3872A7A2.B8D7A411@indra.com>,
Charles R Martin  <crmartin@indra.com> wrote:
>My mistake -- I allowed having seen solid something-or-other when dumping
>propane at the old family propane plant to mislead me.  Paul Dietz's theory is
>that it was longer-chain hydrocarbons I was seeing.

Quite likely.  Commercial "propane" typically is nowhere near pure C3H8,
it's a mixture of all manner of light hydrocarbons.  (For example, in
warmer climates like Mexico, it tends to contain a lot of butane to keep
the vapor pressure down.)
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Tue, 4 Jan 2000 21:36:22 GMT

In article <3870FC84.5039AF37@Gramlich.Net>,
Wayne Gramlich  <NoSpam@Gramlich.Net> wrote:
>Could I ask a slightly more general question?  What was it about
>LOX/propane that prevented it from making the transition from
>experimental engines to operational engines.

Mostly, the fact that the US hasn't built any new major hydrocarbon-fueled
rocket systems since the early 1960s.  New fuels were not wanted for the
Saturn lower stages, which were quite adequate development challenges with
familiar fuels.  Upper-stage work then and since has concentrated heavily
on hydrogen.  And the few substantial in-space maneuvering systems -- the
area where the FLOX/propane work was aimed -- which have been built have
stressed simplicity and low development risk rather than top performance.
Propane doesn't have enough performance advantage over kerosene to make it
compelling for applications where kerosene will do.

In general, nobody has valued the combination of high fuel density,
long-term in-space storability, and high performance enough to finish
developing a new fuel combination whose Isp is inferior to LOX/LH2 and
which does involve cryogenics.

>Is liquid propane at LOX boil off temperatures nasty to work with?

Not particularly.  It's a bit more viscous than you might like, and the
very low vapor pressure means that you need some other gas (not air!)
above it to avoid tank collapses, but it's not a big problem.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: "Paul F. Dietz" <dietz@interaccess.com>
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Sat, 15 Jan 2000 09:06:42 -0600

Ken Doyle wrote:

>   The performance advantage of sub-cooled propane over good old kerosene is
> minimal and not worth the handling hassle involved in getting it that cold
> except to crazy people.

The advantage I've heard of propane over kerosene is not
its performance, but rather for cleaning reusable engines.
Propane is easily cleaned from the system after use --
just let it boil off.  Residual kerosene is harder to remove.
And you don't want any of that residual kerosene getting
into the oxidizer plumbing.

	Paul

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Wed, 19 Jan 2000 15:16:55 GMT

In article <38808D02.530A3800@interaccess.com>,
Paul F. Dietz <dietz@interaccess.com> wrote:
>The advantage I've heard of propane over kerosene is not
>its performance, but rather for cleaning reusable engines.
>Propane is easily cleaned from the system after use --
>just let it boil off...

Propane's low boiling point also lets you do some other useful things --
e.g. converting it to a gas via chamber cooling, so you can inject it as a
gas -- which are normally thought of as hydrogen specialties.  (Injecting
one propellant as a gas is a big win, because that lets you spray the
other one into a high-velocity gas stream, which gives better and faster
mixing than conventional liquid/liquid injection.)
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)