Al Fin had a big of an interview about VASIMR up, and it got me thinking. One issue with VASIMR is that it's plasma based, and plasmas don't do well in atmosphere. Very useful in space, but you still have to get up there first.
Tesla invented a vacuum tube that utilized various forms of aerodynamic flow to keep maintain the vacuum.
Would it work to stick a VASIMR engine inside such a tube, to be used for lift-off? This could even allow such systems to be used elsewhere, anywhere you need a really hot flame. Someone posted about that flame drill, why not use a plasma?
VASIMR and opne vacuum tubes?
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kunkmiester
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VASIMR and opne vacuum tubes?
Evil is evil, no matter how small
1) VASIMR is designed for in-space propulsion. The thrust isn't high enough for atmospheric work.
2) The annular airflow of which you speak constitutes a propulsion element. In all probability it produces far more thrust than the plasma engine, and has a higher specific impulse combined with lower power requirements, because the air is freely available and doesn't have to be carried on board. So if you can do that, why bother with VASIMR until you're above the atmosphere anyway?
If you're generating the annulus from onboard material, you're wasting your time using VASIMR because the annulus kills your specific impulse.
3) If something like VASIMR were designed for sea-level high-thrust use, it would probably look very different, and like modern chemical rockets it would have no trouble at all dealing with the relatively low pressure of Earth's atmosphere.
A flame is a plasma. That dim blue glow is recombination. (The bright yellow glow is just soot...) You could try using exotic heating techniques and magnetic confinement to get a really hot flame, but if the flame is dense enough to be useful as a cutter it shouldn't be necessary to shield the generating mechanism from the atmosphere. I suppose I could be wrong...
On the other hand, an RBCC based on a Polywell-powered plasma rocket is distinctly possible - the difference is that instead of 'protecting' the rocket from the atmosphere with a jet of air, you use the rocket exhaust to accelerate the jet of air up to the range where the ramjet principle is useful. And then of course you pour a bunch more power into the airstream to heat it up and actually use the ramjet principle.
2) The annular airflow of which you speak constitutes a propulsion element. In all probability it produces far more thrust than the plasma engine, and has a higher specific impulse combined with lower power requirements, because the air is freely available and doesn't have to be carried on board. So if you can do that, why bother with VASIMR until you're above the atmosphere anyway?
If you're generating the annulus from onboard material, you're wasting your time using VASIMR because the annulus kills your specific impulse.
3) If something like VASIMR were designed for sea-level high-thrust use, it would probably look very different, and like modern chemical rockets it would have no trouble at all dealing with the relatively low pressure of Earth's atmosphere.
A flame is a plasma. That dim blue glow is recombination. (The bright yellow glow is just soot...) You could try using exotic heating techniques and magnetic confinement to get a really hot flame, but if the flame is dense enough to be useful as a cutter it shouldn't be necessary to shield the generating mechanism from the atmosphere. I suppose I could be wrong...
On the other hand, an RBCC based on a Polywell-powered plasma rocket is distinctly possible - the difference is that instead of 'protecting' the rocket from the atmosphere with a jet of air, you use the rocket exhaust to accelerate the jet of air up to the range where the ramjet principle is useful. And then of course you pour a bunch more power into the airstream to heat it up and actually use the ramjet principle.
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kunkmiester
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I'd imagine you'd use air while you can. Especially since it looses the requirement to hold a vacuum as you go up, and you reach a natural vacuum.
Big problem is that chemical motors can only get things so hot. The atmosphere also limits the heat of a plasma, though I'm sure people know ways around that.
Using the vacuum tube would let you get the high temperature plasmas you can only get in a vacuum--there's a reason tokomaks are vacuum pumped. This would be one source of heat for the engine itself, though probably one of the more inefficient ways.
Big problem is that chemical motors can only get things so hot. The atmosphere also limits the heat of a plasma, though I'm sure people know ways around that.
Using the vacuum tube would let you get the high temperature plasmas you can only get in a vacuum--there's a reason tokomaks are vacuum pumped. This would be one source of heat for the engine itself, though probably one of the more inefficient ways.
Evil is evil, no matter how small
If air insists on leaking into your plasma engine, great - it's free propellant.kunkmiester wrote:The atmosphere also limits the heat of a plasma, though I'm sure people know ways around that.
If you have to carry the propellant on board, then for a given power input, Isp increases with temperature. If you don't have to carry the propellant on board, then for a given power output, thrust (and thus Isp) decreases with temperature (because you're pumping the same energy into less mass, fighting 0.5*mdot*v^2 to get mdot*v).