10-05-2005 23:35									#1
	A question to pchang about plasma propulsion Full PM-box? Change here!

You have stated that plasma does not form well unless in vacuum. Because of this fact, plasma propulsion does not work -- at least as a first stage -- to get into orbit.

However, how much of a vacuum is really required?

Consider an airship of dimensions about 1 mile wide and long filled with helium or hydrogen, sitting at 150,000 feet altitude in near space.

There is a fellow who claims he can get to orbit in such a vehicle. There is some research going on about airships and electrostatic atmospheric ion propulsion at DARPA (the WALRUS program, f.e., see below) of which this fellow has a small piece, but it is surmised in the experimental spacelift community that the atmospheric drag during the days-long accelleration via ion propulsion obviously would be insurmountable. Because of this, most are keeping a sort of open mind about it, but can't help but think of this fellow as a crackpot.

http://www.darpa.mil/TTO/Programs/walrus.html
http://www.globalsecurity.org/milit...raft/walrus.htm

However, if a high-thrust plasma engine were operable at such an altitude, perhaps it would be possible to gain the necessary delta-v before falling back to Earth.

Any thoughts?

Last edited by DanS on 11-05-2005 at 00:28

	11-05-2005 00:05									#2
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I thought plasma engines provided a low thrust and were useful only over long distances. Wouldn't an engine like that be useless in getting to orbit?

	11-05-2005 00:11									#3
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Yes, most plasma engine concepts are low thrust and, as such, are useless in getting into orbit.

Some concepts are high thrust, but I don't know how high of thrust is attainable as a theoretical matter.

Last edited by DanS on 11-05-2005 at 00:23

	11-05-2005 03:20									#4
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Bump.

	11-05-2005 08:14									#5
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Btw, thought you might be interested to know that Aerojet just got a $30 million contract to build a 600 kW hall effect thruster cluster of 4 x 150 kW engines.

	11-05-2005 08:18									#6
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You could PM him as well to get his attention to this thread .

	11-05-2005 08:19									#7
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Or continue to bump it, lest it plummet towards oblivion due to its lack of nudity, Kucipwnage, or anti-Bush tirade.

	11-05-2005 08:24									#8
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150,000 feet seems like the lower limit of altitude. But, I would feel more comfortable if he could get up to 200,000 feet plus with the balloon part. The real problem is the weight of the power plant. I don't know of any that would be light enough to provide the require thrust to weight ratio necessary to reach orbit.

	11-05-2005 08:57									#9
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He says he will be able to get to 200,000 feet on buoyancy, but has only demonstrated 100,000 feet so far for one of his relatively primitive balloons constructed of Mylar.

It seems from what you say that the airship would have to rely on lift to counteract the drag for his system to work -- as you say, the plasma engine would not provide the adequate thrust/weight to leave the atmosphere entirely. But going those speeds in the atmosphere as you accellerate (or skipping along just above it) would seem to require a damn impressive structurally strong airship.

Bah. He's probably forgetting something.

Last edited by DanS on 11-05-2005 at 09:02

	11-05-2005 17:45									#10
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Once you get up that high, there's hardly any atmosphere at all. Thus, wind related forces are pretty weak despite the high velocities involved. You don't need much structural strength (any internal pressure so passengers could breath would greatly sitffen any material). However, you do need a lot of thermal tolerance. Your flight plan doesn't really matter. Sure, you can get atmospheric lift for a little boost, but the closer you get, the less atmospheric lift there is. Sooner or later you must have sufficient thrust to weight to escape. Jets use the atmosphere as a free source for a decent % of their fuel. Current plasma engine designs cannot.

	11-05-2005 17:53									#11
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There is no theoretical limit to the thrust you can get from an MPD. It is dependent upon how much fuel you want to go through and how much power you have. Practically, the faster you go through fuel, the shorter your flight. As for power, the only big power supplies I could envision getting into space are the nuclear power generators on submarines.

	11-05-2005 18:06									#12
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Wouldn't a mile long craft be exceedingly likely to break, regardless?

	11-05-2005 18:15									#13
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Your biggest stresses would be from vibration and buffeting. Thus, you just need a structure that is flexible to relieve the stresses.

	11-05-2005 18:17									#14
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quote: Originally posted by DanS .....edit Bah. He's probably forgetting something.

As Che asks.

By Airship i keep thinking of a Zeplin

But escape velocity and or disturbance for such a large craft.......i find it hard to imagine it would make it through the upper atmosphere intact? That exit/entry part always looks like hell for spacecraft.

	11-05-2005 18:19									#15
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quote: Originally posted by pchang Your biggest stresses would be from vibration and buffeting. Thus, you just need a structure that is flexible to relieve the stresses.

Hmmm so what i should be thinking is the bigger and bendier the craft, the slower it would need to travel to escape the earths gravity, and the less trauma on exit/entry it would feel? something like that?

	11-05-2005 19:10									#16
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quote: Originally posted by child of Thor As Che asks. By Airship i keep thinking of a Zeplin

Yes. A huge Zeppelin. An airship. A dirigible. The things the NAZIs built. A mile wide and long. This is the easily doable and inexpensive part. Nobody's laughing at this part of the plan, even though it's sure a lot of sewing.

quote: Wouldn't a mile long craft be exceedingly likely to break, regardless?

No. It's doable. The WALRUS, above, is conceived as 1/5 of a mile long and is intended to operate at ground level. This would be to move military equipment worldwide in less than a week.

At great heights, as pchang points out, the air is very thin. The orbital airship is conceived as taking off from about 150,000 feet in altitude rather than from the ground, so could be built merely to survive the atmosphere at those heights.

quote: But escape velocity and or disturbance for such a large craft.......i find it hard to imagine it would make it through the upper atmosphere intact? That exit/entry part always looks like hell for spacecraft.

ITYM, orbital velocity rather than escape velocity. But other than that, you're repeating what I said above. The structure would have to be strong enough to withstand acceleration to orbital velocity in an atmosphere over a week-long period.

	11-05-2005 20:04									#17
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Is Bush planning on taking over the world with this?

someone should tell him its been tried

I thought with you talking about plasma drives this would be a space based project. But i get it now, its a world conquest with giant balloons thing

wont they be quite vunerable, being so large?

Last edited by child of Thor on 11-05-2005 at 20:19

	11-05-2005 20:26									#18
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A mile wide blimp, now that would be quite a sight.

	11-05-2005 20:37									#19
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quote: Originally posted by child of Thor Is Bush planning on taking over the world with this? someone should tell him its been tried I thought with you talking about plasma drives this would be a space based project. But i get it now, its a world conquest with giant balloons thing wont they be quite vunerable, being so large?

Two slightly related topics. (1) The Walrus, which is a DARPA program to experiment with worldwide heavy lift and is an example of a very large airship, with possible use of electric propulsion. (2) The orbital airship, which is one man's conception of a way to go to orbit using a quite sizeable high-altitude airship and electric propulsion.

Plasma propulsion is a subset of electric propulsion that requires vacuum or near-vacuum to operate. It tends to allow for higher thrust than other electric propulsion concepts, so I was exploring whether #2 was less crack-pottish under a plasma propulsion scenario.

Besides, Bush will be out of office before completion, so he won't be the one to have the fun with #1.

	11-05-2005 20:49									#20
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All geniuses are crackpots but not all crackpots are geniuses

	11-05-2005 21:05									#21
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Well, even few balloons have operated at those altitudes, so there's a possibility that he found something through testing that nobody else knows. I'm always hesitant to rubbish novel ideas that were arrived at through testing, because we don't know a lot more than we know for certain.

Sometimes that may mean giving crackpots more coverage than they deserve.

NB: If I were looking at it from an investor's eye, I would be immensely more skeptical.

	12-05-2005 00:07									#22
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If the balloon doesn't get the ship to a high enough altitude, the remaining 50,000 - 100,000 feet is no big deal. A conventional jet goes to 30,000 feet, flies thousands of miles, and yet it's fuel compartmant is only a few percent of its crafts weight. Even Spaceship one made it to 63 miles, looking more like a jet than a spceship. You only need huge fuel tanks when going into orbit or further. Plama propulsion will bear most of this burden even if no ballons were used.

	12-05-2005 00:13									#23
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Not quite sure what your point is. Could you clarify?

	12-05-2005 00:14									#24
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If you see me up there someday, waving back down at you, that's the signal to unleash all the raccoons.