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Path: utzoo!linus!decvax!decwrl!augeri@gwen.DEC (Mike Augeri)
From: augeri@gwen.DEC (Mike Augeri)
Newsgroups: net.space
Subject: Fuels for rocket propulsion
Message-ID: <384@decwrl.UUCP>
Date: Tue, 10-Sep-85 01:30:22 EDT
Article-I.D.: decwrl.384
Posted: Tue Sep 10 01:30:22 1985
Date-Received: Thu, 12-Sep-85 08:21:18 EDT
Sender: daemon@decwrl.UUCP
Organization: Digital Equipment Corporation
Lines: 127


There is an interesting article in the September 1985 issue of Space
World, the magazine published in cooperation with the National Space
Institute.  The article is "Propulsion Future" (pp. 17-19).  It talks
about past, present and future fuels for rocket propulsion.  The article
does not provide any equations to support the claims that it makes, and in
some cases, it makes statements without any supporting data.  In spite of
these shortcomings, I thought the article was a good summary.  I have some
doubts about some of the statements in the article, but since I am not a
qualified critic, I leave this to our readers.

The article says that "[r]ocket performance is measured in many ways", but
the best measure of the fuel efficiency is specific impulse, usually
written as I(subscript sp) and has units of seconds.  It says that thrust
depends more on the design of the engine whereas specific impulse depends
more on the energy content of the fuel and the pressure and temperature
conditions under which it is used.

Many fuel formulas have been tried over the years, but the best and safest
performance to date is with a fuel first proposed by Konstantin
Tsiolkovsky way back in 1903 -- namely, liquid oxygen and liquid hydrogen.
This fuel combination produces a specific impulse of 400-450.  The next
best is hydrogen and fluorine at 480, but fluorine is a very difficult
substance to handle.

The problem with these fuels is that they are impractical if what you want
to do is plan a manned mission to the outer solar system or to the stars.

Many people are familiar with the NERVA (Nuclear Engine for Rocket Vehicle
Application) program back in the 1960s.  The NERVA program developed the
"fission solid core" nuclear rocket.  This rocket used hydrogen as a
reaction mass and a uranium fueled reactor.  The reactor was operated at a
temperature as hot as possible without producing a meltdown.  Liquid
hydrogen was pumped into a jacket surrounding the reactor core where it
was heated to a gas.  The gas then flowed through holes in the reactor
core to collect more heat, and emerged as a very hot gas that was expelled
through an exhaust nozzle.  The NERVA engine was first operated under full
power in 1966 and produced a specific impulse of 850.  Because of the
massive weight of the engine and the amount of shielding it would have
required, it was believed that the maximum specific impulse that could
have been achieved in a practical engine would have been about 650.  The
NERVA program died around 1970 with the massive cutbacks in all
space-related programs.

Since then some theoretical work has been done on a "fission gas core"
design.  No information was given about this design other than saying that
it used gaseous fuels.  The article claims that such an engine "could make
Mars in about 30 days with a five man crew."  But a rocket of this type
has never been developed and due to reasons that are said to be complex,
some experts say it never will be developed.

Others are pinning their hopes on fusion engines.  It is claimed that a
fusion reaction can liberate from 3 to 5 times the amount of energy
liberated by a fission reaction per unit mass.  Two major problems exist:
first, we have not yet achieved a sustained, controlled fusion reaction,
and second, we have not perfected a means to utilize fusion reactions in a
rocket.  However, in 1972 a couple of people at Lawrence Livermore Labs
described a scheme that would burn small deuterium-tritium pellets by
heating them with a laser beam.  The pellets are injected into a thrust
chamber at the rate of 500 per second where they are hit by laser pulse of
one billionth of a second duration.  The theoretical performance of this
engine is an incredible specific impulse of 2,640,000.  No one thinks that
we could build such a perfect engine, but they do think that we could
build an engine with a specific impulse of 1,000,000.  Such an engine
would revolutionize rocket travel.  "A single stage rocket with a fuel to
mass ratio of one to twenty ... could reach one tenth the speed of light.
At full howl, Pluto would only be five days away, but allowing time for
getting up to speed and slowing down at the destination, the real mission
time might be more like three weeks."

However, even this kind of performance is inadequate when you consider the
requirements for an interstellar voyage.  Assuming we could carry enough
fuel (say 50,000 tons of helium-3 and deuterium), it would take about 50
years to reach the nearest star.

Other propulsion systems we hear about are solar sails and ion engines.
Both systems have been tested in space, at least in principle, and they do
work.  However, their acceleration is slow and they are not too practical
for manned missions.

Beyond the fusion engine, solar sails, and ion engines, we have a big
question mark.  To dream about interstellar travel is one thing -- to
develop an engine to actually do it is another.  "To launch a one-pound
payload to one quarter the speed of light with an engine with [specific
impulse] of 2000 would require a fuel load far greater than the mass of
the universe.  More efficient powerful engines is one answer; free fuel is
another."

One idea for a rocket engine is analogous to the atmospheric ramjet.  The
idea is that a rocket would travel through space and scoop up the
interstellar hydrogen using a magnetic-field scoop generated aboard the
rocket.  Its a great idea, but many people doubt the feasibility of such
an engine.  "Energy losses involved in producing the fields seem likely to
cancel out any net gain in velocity" and scoop sizes of "one million
kilometers to half a light year in diameter" have aroused grave doubts.

So what's left?  "If one pound of fuel could be converted entirely into an
exhaust beam, the result would be five billion times the energy released
per unit mass in the best chemical rocket."  But we all know that 100%
efficiency is impossible to achieve.  A close approximation is the
ultimate in propulsion systems:  the matter-antimatter engine.  Such an
engine would be about ten times more efficient than the deuterium-tritium
fusion engine.  For a quick trip to Mars "a 1000 ton vehicle using 4000
tons of water" for a reaction mass, heated by the matter-antimatter
reaction, would require "about a gram of antimatter."

However, there are some significant problems associated with using such an
engine.  First, about "half the [matter-antimatter] reaction is gamma ray
radiation plus electrons and positrons, and we don't have any idea how to
focus a gamma-ray exhaust beam."

"Furthermore, the other half of the reaction is in neutrino form, and
neutrinos can penetrate anything (including any shielding we can think of,
and astronauts' bodies).  Neutrinos also refuse for the most part to be
directed by electric, magnetic, or any other sort of fields, so they are
hard to get rid of."

Aside from these problems we have the problem of producing "enough
antimatter to power a spaceship.  At present, the world's supply of
antimatter is a few thousand antiprotons stored for a few days."  The
problem here is to make it cheap enough, make enough of it, and figure out
how to store it.

If we can figure out how to solve the problems of the matter-antimatter
engine, maybe someday we can make it to the stars.

	Mike Augeri (DEC, Maynard Massachusetts)