A question to pchang about plasma propulsion

[DanS]

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/military/systems/aircraft/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?

[Kuciwalker]

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?

[DanS]

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.

[DanS]

Bump.

[DanS]

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.

[Imran Siddiqui]

You could PM him as well to get his attention to this thread .

[Verto]

Or continue to bump it, lest it plummet towards oblivion due to its lack of nudity, Kucipwnage, or anti-Bush tirade.

[pchang]

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.

[DanS]

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.

[pchang]

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.

[pchang]

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.

[chequita guevara]

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

[pchang]

Your biggest stresses would be from vibration and buffeting. Thus, you just need a structure that is flexible to relieve the stresses.

[child of Thor]

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.

[child of Thor]

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?