VASIMR for LEO Launches? - What prevents it?

Alcon · 2005-01-31 00:10:59

Why isn't any one talking about using the VASIMR for Earth to LEO launches? Reports on it I've seen claim that it can reach either very high Isp or very high trusts, so why wouldn't it work well for sending ships to LEO?

Euler · 2005-01-31 00:55:31

When people say that VASIMR can have a high thrust, they mean that it has a high thrust compared to other high-Isp electric engines. The thrust is still very low compared to chemical engines, and it is not enough to overcome the Earth's gravity.

Alcon · 2005-01-31 01:30:54

Yeah... that makes sense, should have caught that, but my brain isn't operating on full function this late at night. Ok next question: why do thrust and specific impulse seem to be inverses? Why do engines like the VASIMR and ion drives that have such huge specific impulses have small thrusts. The equations I have in front of me in Zubrin'ss Entering Space seem to indicated that a high Isp would lead to a high trust. C = g(Isp) and T = mC where T is thrust, m is the propellant mass flow, c is the exhaust velocity and g is the acceleration due to gravity. Is there an equation relating propellant mass flow to Isp? I've read that they aren't always inverses just in the case of the VASIMR. Why is that the case?

SpaceNut · 2005-01-31 05:21:04

You have forgotten to take into account the Earth's gravity well and air drag when going to orbit. The equation once in orbit is a lot closer.

GCNRevenger · 2005-01-31 07:51:50

Yeah... that makes sense, should have caught that, but my brain isn't operating on full function this late at night. Ok next question: why do thrust and specific impulse seem to be inverses? Why do engines like the VASIMR and ion drives that have such huge specific impulses have small thrusts. The equations I have in front of me in Zubrin'ss Entering Space seem to indicated that a high Isp would lead to a high trust. C = g(Isp) and T = mC where T is thrust, m is the propellant mass flow, c is the exhaust velocity and g is the acceleration due to gravity. Is there an equation relating propellant mass flow to Isp? I've read that they aren't always inverses just in the case of the VASIMR. Why is that the case?

The reason they are inverse is simply a consequence of thermodynamics concerning the convervation of energy. It is really fairly simple to understand, that since your power source has a limited output, you can either push a small amount of propellant up to a very high speed (high Isp but low thrust) or push a large amount of propellant to a low speed (high thrust but low Isp). This goes for any engine or any power source, not just for VASIMR. No propulsion scheme that uses a propellant can cheat thermodynamics.

Alcon · 2005-01-31 14:03:06

Well, theoretically, with enough power, one could do both: heat a very large amount of propellant to a very high tempurature. But its just not within range of the VASIMR I guess. There's a good reason for my not seeming to think terribly clearly as of last night: a 16 page paper on space launch, that I'd spent about 19 hours working on under the influence of about 6 cans of Dr. Pepper and a couple of cups of tea.

Thanks to everyone who responded.

GCNRevenger · 2005-01-31 16:47:18

Perhaps not even then, if the VASIMR engine weighs more then a fraction of its own maximum thrust, which it does, then if you have more and more power available to run it the engine must be larger to handle the extra power, which in turn is heavier and keeps your tail on the ground.

No practical concieved power plant, even a fusion reactor, would give you enough power to push the weight of the reactor off the ground either with VASIMR. It will be beyond any doubt limited to in-space maneuvering only without some far-future super fusion reactor. A secondary engine with much more thrust would be needed.

Also, unless someone comes up with a portable fusion reactor or a next-generation (or perhaps a revolutionary) fission reactor, then there are no practical power plants light enough to operate a VASIMR even for efficent space-only flight.

Euler · 2005-01-31 17:56:07

The power it takes to accelerate the fuel is equal to the mass flow rate times the square of the exhaust velocity(P=mC^2). In other words, power equals thrust times exhaust velocity(P=TC). This is a basic result of kinematics and applies to all types of engines, not just VASIMR. All electric engines are basically power-limited, so you have to make a trade off between thrust and Isp.

There is a lot of hype around VASIMR, but it helps to remember that it is just another electric engine, and that it is bound by the same restrictions that apply to other electric engines. According to its proponents, VASIMR will be thottleable through a wider range of Isp than current electric engines and have a higher power density. It will also not have any problems will grids wearing out and having to be replaced. However, as nice as these advantages are, they are all evolutionary rather than revolutionary. Modern ion engines are already at least 60% efficient, and they typically weigh less than 20% as much as the power plants that they need to run them. Since electric engines have already gotten this advanced, much more improvement can be gained through work on power plants than on the electric engines themselves.

No practical concieved power plant, even a fusion reactor, would give you enough power to push the weight of the reactor off the ground either with VASIMR. It will be beyond any doubt limited to in-space maneuvering only without some far-future super fusion reactor. A secondary engine with much more thrust would be needed.

Definitely true. Even a nuclear thermal engine has difficulty getting off the ground on Earth, and nuclear thermal engines can have a much higher specific power than any nuclear electric engine due to the lack of things like radiators and turbines.

Also, unless someone comes up with a portable fusion reactor or a next-generation (or perhaps a revolutionary) fission reactor, then there are no practical power plants light enough to operate a VASIMR even for efficent space-only flight.

Current/near term power plants would be able to operate a VASIMR in space without any problems. Just don’t expect it to get you to Mars in a month or anything like that.

GCNRevenger · 2005-01-31 18:04:48

Also, unless someone comes up with a portable fusion reactor or a next-generation (or perhaps a revolutionary) fission reactor, then there are no practical power plants light enough to operate a VASIMR even for efficent space-only flight.
Current/near term power plants would be able to operate a VASIMR in space without any problems. Just don’t expect it to get you to Mars in a month or anything like that.

I suppose a cluster of low megawatt range solid-core reactors could generate enough power to make it practical, but unless VASIMR were to offer signifigant advantages over solid-core nuclear thermal engines, I do not see VASIMR as being worthwhile.

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#1 2005-01-31 00:10:59

Re: VASIMR for LEO Launches? - What prevents it?

#2 2005-01-31 00:55:31

Re: VASIMR for LEO Launches? - What prevents it?

#3 2005-01-31 01:30:54

Re: VASIMR for LEO Launches? - What prevents it?

#4 2005-01-31 05:21:04

Re: VASIMR for LEO Launches? - What prevents it?

#5 2005-01-31 07:51:50

Re: VASIMR for LEO Launches? - What prevents it?

#6 2005-01-31 14:03:06

Re: VASIMR for LEO Launches? - What prevents it?

#7 2005-01-31 16:47:18

Re: VASIMR for LEO Launches? - What prevents it?

#8 2005-01-31 17:56:07

Re: VASIMR for LEO Launches? - What prevents it?

#9 2005-01-31 18:04:48

Re: VASIMR for LEO Launches? - What prevents it?

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