Plasma Rocket Successful Full Power Test

Matt_dk writes "VASIMR is a new high-power plasma-based space propulsion technology, initially studied by NASA and now being developed privately by Ad Astra. A VASIMR engine could maneuver payloads in space far more efficiently and with much less propellant than today's chemical rockets. Ultimately, VASIMR engines could also greatly shorten robotic and human transit times for missions to Mars and beyond."

Comments

The interesting part (to me anyway)

[mcgrew]

The VASIMR engine works with plasma, a very hot gas at temperatures close to the interior of the Sun. Plasmas are electrically charged fluids that can be heated to extreme temperatures by radio waves and controlled and guided by strong magnetic fields. The magnetic field also insulates any nearby structure; so temperatures well beyond the melting point of materials can be achieved and the resulting plasma can be harnessed to produce propulsion. In rocket propulsion, the higher the temperature of the exhaust gases, the higher their velocity and hence the higher their fuel efficiency. Plasma rockets feature exhaust velocities far above those achievable by their chemical cousins, so their fuel consumption is extremely low and their fuel-related costs substantially reduced.

Re:The interesting part (to me anyway)

[Anonymous Coward]

The Argon is not an energy source, it is merely propellant. Argon is chosen due to ionization potential. There is no splitting of argon (that consumes energy rather than producing it)

Power would have to be carted up separately, and in the case of a plasma drive it would presumably be nuclear.

Re:The interesting part (to me anyway)

[Anonymous Coward]

There are only two things that matter in determining how much delta V you can get from a given rocket, Exhaust velocity and Propellant mass fraction.

The exhaust velocity is the mean velocity of the exhaust.

The propellant mass fraction is the fraction of vehicle launch mass that is propellant that will be slung out the back.

Now, in a conventional rocket, the propellants are typically accelerated by a simple thermodynamic gas generator (turbopumps and a chamber to burn the propellants to create high pressure hot gas) followed by a nozzle to convert the pressure and temperature into velocity. There are inherent limits in this process, not least that the heat source and reaction mass flow are inexorably coupled.

VASIMIR is essentially an ion drive variant which separates the reaction mass from the power source, and which allows the specific impulse to be varied (the number of NewtonSeconds of thrust per kg of fuel), this is useful as it allows for high thrust burns at relatively low specific impulse and low thrust burns at much better fuel economy to be mixed at will with the same motor.

The electrical power generator is an interesting problem, as most thermal generators on that scale would seem to need a vast amount of radiator to dump the waste heat from the condensers or equivalent. I suppose you could dump some of it into the fuel before it hits the injector, but that is going to be limited. Most likely the plan is to charge batteries with solar power, then discharge them rapidly to give a series of short burns.

I could see some sort of high temperature nuclear plant being flown, but as radiated power rises as the 4th power of absolute temperature, the radiators would have to run really hot to get good overall specific impulse from the complete propulsion assembly (Which means a relatively poor thermodynamic efficiency for the overall electrical plant), this might be a reasonable tradeoff.

Of course the political problems with launching a small reactor would be 'interesting'.

HTH.

Regards, Dan.

Re:Constant Boost?

[jollyreaper]

Typically these rockets are more efficient than their chemical cousins. For a given reactant mass, rockets will give you more thrust (can't get into orbit with anything but rockets at this point) but the plasma and ion engines are more efficient, low-thrust but higher change in velocity (delta-v.)

As it was described, a mars mission using an ion engine would not leave the space station with a dramatic blast of flame. The captain would say "turn the engine on," go, get coffee, watch a movie, look out the window and still be able to wave back to people at the station. Tune back in three weeks later and he'll be moving at a clip that would make chemical rockets weep in jealousy.

Re:But where does the energy come from?

[bornyesterday]

no fusion is involved here. they ionize hydrogen gas and turn it into a plasma and then heat it even further by applying radio waves (i.e. they basically put it in the microwave) and then they let the plasma out through a ring of magnets which focus and accelerate the exhaust. there isn't much in the way of specific data regarding this, but i don't think that the amount of energy needed to create radio waves or to ionize the hydrogen gas is really that great. the majority of the acceleration force is inherent in the energy of the particles since they are at over 100 million degrees F and that force is then amplified by magnets which themselves likely require little to no electrical power

Re:But where does the energy come from?

[mshannon78660]

As I see it, this plasma rocket is not really useful without a nuclear power source of some kind.

I think they've already solved that one.

SNAP

RTG

Nuclear Reactors for Space

Re:Are there useful numbers on this?

[bornyesterday]

check out http://www.adastrarocket.com/Jared_IEPC07.pdf

it's a paper that the researchers published last year describing what they had done with the previous version of the engine and what they planned on doing with this version

The Numbers

[StefanJ]

The Wikipedia entry says it can be tuned for an Isp of 3,000k seconds to 30,000k seconds.

A liquid fueled chemical rocket has an Isp of about 500 seconds. A really good fission thermal rocket, maybe 1000 seconds. The Deep Space 1 ion rocket could do 3.1k seconds.

How to turn this into usable numbers:

Find the exhaust velocity. Vex. Multiply the Isp by "g". So, your chemical rocket has an exhaust velocity of about 5 kps, and your VASIMIR 30 kps.

The figure out the velocity change you want. Vd.

Then:

M(o)/(M(o)+M(f)) = e^(Vd/Vex)

M(o) = Mass of spaceship without reaction mass
M(f) = Mass of reaction mass
e = natural log number, about 2.178

A Hohmann orbit trip to Mars orbit from Earth orbit without need for aerobreaking of the like might require 20 kps. Hohmann orbit to Mercury, 40 kps.

Drawback to ion drives and VASIMIR is a really, really low thrust. You might be better off with lower efficiency but higher thrust or you'll lose the fuel (uh, reaction mass) savings in consumables, and/or risks to your crew from flares.

Physics doesn't work like you seem to think

[MarkusQ]

accelerating fuel forward so you can spit it back later.

I have no idea what that even means, or is even supposed to mean. I quote it only to highlight that the source of your skepticism seems based entirely on a gross misunderstanding of the technology involved.

I'm not the person to whom you were responding but I suspect the misunderstanding is on your end, not his. The meaning of the phrase is quite clear; in a system with sustained thrust the fuel (and reaction mass) used in a later portion of the trip has to be accelerated (along with the rest of the ship) for the whole proceeding portion of the trip. This means that, early in a long trip, the majority of the fuel/reaction mass you use accelerating the remainder, and only a small fraction is accelerating the payload. That's why large rocket use stages.

The other advantage is maximum top speed. If your hydrazine rocket can expel mass at, say, 1000 mph (making numbers up here) then the top speed of your rocket is 1000mph for reasons I hope are obvious.

The "reasons" may be obvious to you, but they aren't valid. The actual relationship between final speed (from a standing start in some reference frame) and the exhaust velocity has as a factor the natural log of the starting mass over the payload mass. So (to use your made up numbers) if you started with a ship that was 90% hydrazine (by mass) your final velocity would be 1000*ln(100/10) mph or about 2300 mph, over twice your exhaust velocity. If the ship was 99% fuel, the final velocity would be 4600 mph, and so on.

--MarkusQ

Re:top speed is HUH?!

[Rick Bentley]

In fact, the equation for top speed is:

top speed = v * ln(M/m) + v0

where:
v = exhaust gas speed
M = starting mass of rocket + fuel
m = ending/empty mass of rocket
v0 = initial velocity

so the exhaust gas might be only 1000mph but you can go pretty much up to the speed of light if you can get ending mass to 0...