VASIMIR, need help with some math

[Skipjack]

There is a new article on some progress the VASIMIR guys are making.
http://www.aviationweek.com/aw/generic/ ... %20Strides

Other testing in the company’s large vacuum chamber verified efficiencies of greater than 50% in the conversion of electricity to thrust through 112 kw.

Now, I really dont quite get how you have have 112 kW of thrust. I would expect newtons, or pounds, or something like that.
Anyway, I am just getting rid of the speed component here and so I would end up with ~10 tons ~ 20,000 pounds of thrust?
So, assuming you had a vehicle mass of 10 tons, you could keep that afloat against earths gravity (as long as you have power and fuel). Did I get that right, or am I missing something here?

[kcdodd]

What they probably mean is conversion to kinetic energy of the exit gas. But without knowing the velocity of the exiting gas you can't convert to thrust.

[Antice]

yes you are missing something. the article linked does not mention anything about thrust. all it talks about is progress towards getting the VX-200 operational.

I'd fetch the real numbers on thrust for you, but due to this news article the adastrarocket company's website is swamped into oblivion with visitors.

[GIThruster]

"Other testing in the company’s large vacuum chamber verified efficiencies of greater than 50% in the conversion of electricity to thrust through 112 kw. . .

. . .The efficiency predictions climb to 60% at 200 kw., or full thrust."

The point is efficiency in converting electrical to kinetic energy. They're saying they have 50% efficiency and expect 60%. It's very unlikely this sort of thruster will ever be able to lift spacecraft from the surface of the Earth. What Tom was saying the other day holds completely for this sort of thruster as well as for fission thrusters like TRITON. Since the energy of the system is set by the power supply, electrical in the case of VASIMR, the fission process in the case of TRITON and the fusion process in the case of a Poly thruster; rather than being set by the reaction of the propellants in the case of a chemical rocket; these other sorts of thrusters are more efficient with less thrust because higher Isp is a more efficient use of propellant.

TRITON is the ultimate expression of this in that it not only works with variable propellant but you can then enhance by dumping LOX into it.

IMHO, this is what we ought to be developing right now, since we're not paying for Constellation anymore:

http://www.engineeringatboeing.com/data ... 4-3863.pdf

which is just what I recommended to the Augustine commission. (No, I don't have any special position of influence here, I just wrote them a note like thousands of others.)

[D Tibbets]

For what it's worth I heard that ~ 2000 watts was needed to generate one pound of thrust. I don't know what efficiency this would represent. I think it was in reference to an ion engine. Hopefully it is not too far off.

If this is the case, and 50% efficiency is similar to ion engines, then ~ 120 KW = ~ 60 lbs of thrust. The question then is how many ion engines could be clustered to utilize 120 KW of power, how would the thrust compare, and the system weight compare?

Dan Tibbets

[Skipjack]

Ok, thanks for the clarification everyone. I guess I missunderstood what they were saying. Sometimes the language barrier still strikes.

[GIThruster]

It's very unlikely this sort of thruster will ever be able to lift spacecraft from the surface of the Earth.

I stand corrected. Snipped from a note from a NASA buddy:

". . .if fully integrated with a high power P-B11 Polywell reactor in the 1.0+ GW output power range, while utilizing direct alpha particle to RF power conversion technology that would pump the two VASIMR RF inputs directly, vehicle thrust to weight ratios greater than one might be obtained if all other vehicle subsystem masses were vigorously restrained. I’m not saying it would be easy, but given a 1.0+ GW Polywell reactor with a power gain of 10+, it’s conceivable in the long term."

So VASIMR seems to hold more promise than I'd imagined.