Subject:
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Spacecraft propulsion (was: Ship Power Core)
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Newsgroups:
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lugnet.space
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Date:
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Thu, 16 Dec 1999 06:25:30 GMT
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Viewed:
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806 times
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Hi there,
You've got to love these fantasy-tech discussions that spring out of the
desire to model spaceships in Lego!
To all of you who have posted pictures: love your designs. They have
the no-nonsense look of real machines.
Mr L F Braun wrote:
>
> "Tobias Möller" wrote:
>
> > "cabrionic engines"? "matter hypercompressor generator"?
> > Can someone please tell me what´s wrong with the good old-fashioned rocket
> > engines :-)?
O.K., Tobias, and anyone else who is curious... Did I mention that
model rocketry -- you know, the kind with cardboard tubes, balsa wood
fins, and gunpowder engines -- is another one of my oh-so-numerous
hobbies?
What does a rocket engine do? It accelerates mass. The faster you can
eject the mass, the less fuel you need to generate the same amount of
thrust. Furthermore, the top speed that a rocket can achieve is roughly
equal to the exit velocity of the fuel.
> To a certain extent, ramjets and ion drives are rocket-ish.
Ion drives are real today. They have been in use for satellite
station-keeping for decades. It was only last year, however, that an
ion propulsion system was used as a spaceraft's PRIMARY propulsion. The
lucky spaceship was NASA's "Deep Space 1."
http://nmp.jpl.nasa.gov/ds1/
How does ion propulsion work? Actually, exactly like a conventional
rocket. The difference is in the propellant. Xenon atoms are stripped
of an electron, rendering them positive in charge. The positive ions
are accelerated toward a negatively-charged grille at the opening of the
engine. The ions pick up a lot of speed in a short distance. The
spacecraft moves in the opposiite direction.
Ion drives have two limitations. First, they must operate in a vaccuum,
which makes them useless in Earth's atmosphere. Second, they are very
efficient but have a low PEAK thrust, which means that you can't
accelerate hard. Thus ion drives are not much use anywhere near planets
-- you would have to thrust a long time to escape Earth's gravity, for
example, even if you were already in orbit. The best use for ion
engines is to move around in "empty" space, just as Deep Space 1 did.
Ramjets, at least the kind that operate in space, are still imaginary.
These are cool because they carry no fuel. It is known that "empty"
space is not truly empty. There is estimated to be about one atom of
atomic hydrogen in every cubic centimeter of deep space. If one could
just figure out how to do nuclear fusion (a feat we've, alas, been
working on for decades now), you could possibly scoop up the free
hydrogen in front of your spaceship as you go along and crush it into
helium, generating heat, energy, and thrust. Such vehicles have to be
moving pretty fast in the first place just to get enough matter for
fusion... which means that they have to have SOME fuel in order to get
started... but anyway...
So, to answer Tobias' question... the main problem with a "good
old-fashioned rocket engine" is that it generates a lot less thrust per
pound of engine plus fuel than you would like. There's a magic number
that describes the efficiency of thrust-type rocket engines called the
specific impulse. Basically, this number is the answer to the question,
"If you burn one kilogram of the fuel on this rocket at a rate that
generates one newton of thrust, how long will the fuel last?" Or, for
those of you still using Imperial measurements, substitute "pound" for
both "kilogram" and "newton."
For a conventional rocket engine -- say, the Space Shuttle's
liquid-hydrogen, liquid-oxygen fuel system -- the specific impulse is
about 475 seconds, at sea level. The efficiency drops off with
increasing altitude. There's little room left for improvement in
chemical thrusters. I've found some web pages that refer to a change in
the exhaust nozzle geometry that they call the "linear aerospike"
engine. This will eliminate the loss of efficiency with altitude.
There are solid rocket chemical systems, such as beryllium hydride, that
presently achieve a specific impulse of 700 seconds (again, at sea
level) -- but the Challenger disaster taught us that solid-rocket fuel
systems are probably not something we want to continue using on manned
spacecraft. The best solid-fuel systems are a bit more efficient than
liquid-fuel, but they're also very difficult to turn off when something
goes wrong.
Ion engines are much more efficient -- the ion drive on Deep Space 1 had
a specific impulse of 3,000 seconds. But now imagine going somewhere
REALLY distant, like another star, and imagine trying to get there in a
reasonable amount of time -- say, under one human lifetime. You have to
carry fuel. But until you throw the fuel out of the rocket, you have to
carry it along with you as you accelerate, which means you need fuel to
help you carry the fuel. And then you need to carry fuel to help you
carry the fuel to help you carry the fuel... get the picture? Fuel
needs grow exponentially.
And the best guess is that the peak exhaust velocity of the best ion
engine we will ever design will be about 200 km/sec. This is not even
1/1,000th the speed of light, and the nearest star is 4.3 light-years
away. Even an ion engine isn't enough. The acceleration to maximum
velocity, by itself, would take on the order of 33 years. The rest of
the journey would take 6,500 years.
> The reasoning for
> alternate powerplants is because rockets are extremely high consumers of space,
> power, and so forth;
And, actually, mass is the issue here. If you have to throw fuel out
the back of your rocket, you have to carry the fuel.
> if one can derive one's power from the fields all around
> them, and basically "ride" the curvature of space, the power supply is virtually
> inexhaustible.
Lindsay is right on this one: ideally, we would like to find a way to
propel a spacecraft WITHOUT carrying ANY fuel on the ship. Ramjets and
solar sails (see below) -- are in this category. There's also a great
fantasy of deriving energy from the theoretical "zero-point field," the
energy available in free space when particles are spontaneously and
transiently created from nothing -- yes, this is really believed to
occur. Usually, these particles are also rapidly destroyed, but if you
could just intervene between the creation and the destruction...
> Gene Mallove and Greg Matloff wrote an excellent book on subluminal flight
> methods--they discount superluminality for the reason that it changes the game,
> and that there's nothing superluminal even on the horizon so it's pie in the sky
> even to worry about it right now (John and Tim, want to chime in here on your
> subluminal ships' propulsion systems?).
Well, I guess Lindsay remembers my brief remarks about building a Lego
model of a Bussard ramjet. Actually, it was a combination solar-wind
electromagnetic (more on this some other time, but the idea is already
in test by NASA) and Bussard system. I've looked hard at the pieces
I've accumulated, and concluded that I don't have enough for now. For
the time being, I'm having a serious mental block when it comes to
imaginary high-tech propulsion that I can actually model. I'm working
instead on a model of a single-stage-to-orbit, liquid-fuel vehicle of
this type...
http://www.venturestar.com/pages/gallery/imgspecs/
... the likes of which I believe will be operational within a century.
> It's called _The Starflight Handbook_
> (New York: Wiley and Sons, 1989). While it's a bit old, it's still a very good
> primer to the issues and physics surrounding nuclear rockets (or nuclear pulse
> propulsion), ion drive, solar sails,
Have you ever seen a radiometer in action?
http://www.edmundscientific.com/Products/DisplayProduct.cfm?productid=1483
Then you basically know how a solar sail works. Photons can transfer
momentum. NASA has done a small-scale test of solar sails in orbit.
The thrust is measurable. Of course, to get a spaceship clear of the
Sun's gravity using this system, the sail would have to be tens of
kilometers wide and EXTREMELY lightwight and thin...
> and cryogenics. I suppose a "rocket" would
> still be viable, if it had a sufficiently high specific impulse (ratio of power
> to mass) and thus a good acceleration.
It depends where you want to go, and how fast. Nothing that we have now
is remotely capable of interstellar travel on a time scale that we can
bear to wait for results.
--
John J. Ladasky Jr., Ph.D.
Department of Structural Biology
Stanford University Medical Center
Stanford, CA 94305
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Message has 3 Replies:
 | | Re: Spacecraft propulsion (was: Ship Power Core)
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| (...) Cool, thanks for the interesting propulsion discussion. It's been a while since I've thought about these things, but it's always been an interest of mine. Personally I'd like to see some work being done on nuclear propulsion again as well. (...) (25 years ago, 16-Dec-99, to lugnet.space)
| | | Re: Spacecraft propulsion (was: Ship Power Core)
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| (...) This is due to the shape of the engine bell -- how gases tend to collect inside instead of just spewing out, increasingly so the farther you get from optimal pressure. This waste gas not only robs from the total thrust mass, but also (...) (25 years ago, 16-Dec-99, to lugnet.space)
| | | Re: Spacecraft propulsion (was: Ship Power Core)
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| John J. Ladasky Jr.: (...) I think you have some miscalculations there. Assuming that the reaction mass is 99% of the total initial mass of the vessel, and that the peak exhaust velocity is 2*10^5 m/s, the peak velocity of the vessel will be (...) (25 years ago, 23-Jan-00, to lugnet.space)
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Message is in Reply To:
| | Re: Ship Power Core
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| (...) To a certain extent, ramjets and ion drives are rocket-ish. The reasoning for alternate powerplants is because rockets are extremely high consumers of space, power, and so forth; if one can derive one's power from the fields all around them, (...) (25 years ago, 14-Dec-99, to lugnet.space)
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