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)

[Smidge204]

As the AC mentioned, Argon is the propellant and not the fuel.

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.

All rockets work by ejecting mass at high velocities. Take Argon, strip the electrons from it (ionize) and then accelerate it through a electric field potential. The advantage here is you can (potentially) get much higher velocities - and therefore more momentum - using this method than using a chemical fuel. That more "umph" per kilogram of propellant (in traditional rockets, the fuel is the propellant as well).

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. But ion engines can potentially eject mass at much higher speeds.

The energy for both the ionization and field generation are what required the power source. Which could, for example, come from solar power. As long as the extra power/mass ratio of the whole system is better than traditional fuel systems, you're coming out a winner.
=Smidge=

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:The interesting part (to me anyway)

[Anonymous Coward]

This isn't a flame per se. The plasma isn't burning, it's being heated by an external energy source (the radio waves referenced above) and the resultant expansion drives it out the nozzle at very high velocities. Where this system is efficient is in its use of propellant. If you are using a rocket for transportation, you have to carry and then output some type of reaction mass. At higher exit velocities you need less mass to achieve the same increase in speed. However, the device to create the radio heating and its power source are also part of the picture. In space, you usually care about weight, so this system has to compete with others based on the sum of the small mass of plasma propellant and everything else that is required to heat it up and shoot it out the back of the rocket.

Re:The interesting part (to me anyway)

[Anonymous Coward]

I have no idea what that even means, or is even supposed to mean.

It means that in order to use the fuel, you have to carry the fuel with you. Seems obvious. The more fuel you need to carry, the more fuel you need to waste to move the fuel you're carrying. It's an exponential problem, and it's the reason rockets are mostly fuel with a tiny payload on top.

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.

Not quite so obvious. Newton's third law doesn't care how fast you're moving, the exhaust velocity is relative to the engine nozzle, and moving at the same speed as the exhaust velocity doesn't make the engine stop working. The real reason lower exhaust velocity limits the speed of the rocket is that in order to keep accelerating you need to add more fuel, which means you're adding more mass, and weighing down the rocket. You get rapidly diminishing returns if you try to get more speed by adding more fuel. That's why higher impulse engines are better. They get more thrust per pound of fuel, which means you use less fuel, and you waste less fuel pushing around other fuel.

Re:The interesting part (to me anyway)

[Migraineman]

Your top speed isn't limited to the exhaust velocity. Regardless of your current speed, energy is conserved if you tip mass overboard. For the force used to displace the exhaust, the reaction force is applied to your vehicle.

Ion, plasma, arc-jet, and the like are all about taking a small reaction mass (aka propellant) and ejecting it out the back at the highest speed possible. F=ma dictates that you can achieve a large force by tossing a large mass at a relatively low acceleration, or by tossing a small mass at a relatively high acceleration. With a big solar array or a nuclear reactor, you've got a whole lot of electricity with which to expel your relatively small amount of propellant out the back.

Re:The interesting part (to me anyway)

[deander2]

it's not really the heat, but the velocity. you can have cold-as-ice propellant if you can throw it away from you fast enough.

of course, with chemical rockets, there is usually a relationship between heat and velocity, but that's not necessarily true for plasma engines.

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.

But where does the energy come from?

[Kristian T.]

This is all fine with regards to the rocket equation, but that's just about conservation of momentum. You still have to provide it with energy, and 2*H2+O2 -> 2*H2O happens to be as good as you can get in terms of energy/mass ratio. As I see it, this plasma rocket is not really useful without a nuclear power source of some kind.

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.

Re:The Numbers

[ceoyoyo]

Your VASIMR ISP figures are out by 1000. It's 3000 to 30000 seconds, or 3 to 30 k seconds, NOT 3000 to 30000 k seconds.

Generally, if you're going a decent distance, you SAVE time with a high ISP engine, even if it is low thrust. "Decent distance" depends on the specifics, but for most of these engines Mars is far enough away, the moon, probably not.

Re:The risk

[Roger W Moore]

They are using Argon which is a noble gas so, other than the pressure they keep it at, there is no chance of an explosion. In fact if there was a fire and the gas was released it would probably put out the fire.

Re:Constant Boost?

[doug]

If it launched from a station in orbit, it will need to accelerate from 5mps to 7mps to break orbit. Does this slow acceleration imply that VASIMR power ships will have to circle the earth a few times to build up speed?

Re:Specific Impulse?

[Anonymous Coward]

Ad Astra hasn't put up a whole lot of detailed information on their website (I assume they're busy doing engineering and test work...PR can come later), but wikipedia gives an unsourced number of 3000 to 30,000 seconds, presumably depending on the settings of the engine since one of the touted advantages of VASIMR is the ability to switch between "high" thrust or high efficiency settings for the same size power supply. 3000 seconds is on par with existing ion engines, and slightly below other under development models like HiPEP, while 30,000 is quite significantly better.

It should be pointed out that VASIMR takes a lot of power to realize its full potential. The model in development is rated at 200 kW, which is about the same as the entire ~150,000 pound ISS solar array system. To justify using the VASIMR, you either need a lighter weight power supply (which should be possible even with solar because the ISS truss structure is more than just the solar arrays), or to be going for a lot of delta-V (over 150,000 pounds of fuel worth).

Efficiency is an area where ion rockets excel, but power is where chemical rockets excel. The first stage of the Saturn V actually burned propellent at a rate of about 190,000 MW, which is equivalent to nearly 200 commercial nuclear power plants. However, converting all that power to electricity so it can be effectively used in an ion thruster would be horribly impractical.

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:Something Interesting

[Elder Lazarus]

Coincidence? Likley not, IIRC "ad astra" is latin for "to the stars"

Re:Something Interesting

[32771]

Funny how you guys have forgotten the work and effort which goes into this or how the saying goes,

"Per aspera ad astra"

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...

Re:Lowest possible Orbit?

[32771]

While I don't have an answer to my question, let me mention that I somehow had the dream of ion engines being the best thing since sliced bread since they have such a high exhaust velocity (v_e).

Since v_e affects the delta v linearly as opposed to logarithmically like the mass fraction this is just such a nice knob to play with.

Unfortunately the power plant weight of the ion engine is something we are stuck with forever, so there is no nice mass ratio involved with ion engines.

The other thing is that according to a Wiki article

http://en.wikipedia.org/wiki/Spacecraft_propulsion

the propulsion efficiency behaves as follows:

"if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space ^ and has no kinetic energy; and the propulsive efficiency is 100%- all the energy ends up in the vehicle"

So there is little point in using it early in the mission if you want to be efficient.

This quote also motivates the existence of VASIMIR though - you can adjust the exhaust velocity to the current spacecraft speed.

I can't offer any quantitative analysis but I don't expect my previous question to be particularly useful or applicable to any realistic mission.

Re:Plasma Rockets Suck.

[YttriumOxide]

Mine tastes like blue and sounds like the smell of rose petals...

Although, in honesty, unless I take very large doses (greater than 400 micrograms), I find the only synaesthesia I get is seeing sounds. I love "watching" Halcyon and On and On towards the end of a good trip... There's some really nice wavy bits there and the colours in the vocal sounds are quite incredible.
With increased dosage, I've experienced almost every other kind of synaesthesia, but I'm not sure I've seen "happy" (although I may have tasted it...)