Subject:
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Re: Spacecraft propulsion (was: Ship Power Core)
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lugnet.space
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Date:
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Thu, 16 Dec 1999 17:31:25 GMT
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879 times
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> 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.
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. There were ideas in the works
for fission-pulse engines prior to the ban on atmospheric nuclear
testing. The thing is, if the projects are constructed in orbit or on the
moon, then it could be done safely without risking atmospheric
contamination.
One of the ideas behind a nuclear rocket is to take fissionable
material and eject it out the rear of the spacecraft and detonate
it.. the resulting shock against a blast-plate propels the vehicle
forward. By sending out a rapid sequence of small fissionable
pellets, a relatively constant thrust can be achieved. The largest
problem is how to detonate the material effectively since in
a conventional nuclear blast, a certain "critical mass" is required,
and for propulsion you want to use small but frequent bursts.
An idea that was presented is to use anti-protons as the catalyst,
which will annihilate in contact with protons contained within the
nuclei of the fuel's atoms to start a rapid fission process...
Very intriguing.. :)
--
-Bones-
= http://www.necrobones.com/ = NecroBones Enterprises
John J. Ladasky Jr. wrote in message <385885CF.C9B03369@my-deja.com>...
> 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 2 Replies:
 | | Re: Spacecraft propulsion (was: Ship Power Core)
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| I'm gonna go out on a limb here... and speculate that Tobias was only trying to call attention to the overly complex technologies and names people give there "LEGO" creations... I think Tobias (and please correct me if I'm wrong Tobias) was (...) (25 years ago, 16-Dec-99, to lugnet.space)
| | | Re: Spacecraft propulsion (was: Ship Power Core)
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| (...) Why fission? Just because it's easier for us right now to garner energy from fissionable heavy minerals rather than the fusion of light ones? The concepts for nuclear pulse propulsion for fission almost always have to be external to the (...) (25 years ago, 17-Dec-99, to lugnet.space)
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Message is in Reply To:
| | Spacecraft propulsion (was: Ship Power Core)
|
| 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. (...) O.K., Tobias, (...) (25 years ago, 16-Dec-99, to lugnet.space)
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