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NASA Plans Test of New Plasma Drive
Sallust writes "Flightglobal has an interesting article about the testing of a new electrically powered plasma engine called the Vasimir. It's being developed by former astronaut Franklin Chang-Diaz and promises to greatly reduce the time and fuel required for interplanetary journeys. According to the article: 'The Vasimir involves the injection of a gas such as hydrogen into an engine that turns it into a plasma. That plasma is then energised further using radio signals as it flows through the engine, a process controlled by electromagnetic waves from superconducting magnets. Accelerated and heated through this process the plasma is focused and directed as exhaust by a magnetic nozzle. Vasimir is many times more efficient than conventional chemical rockets and far less fuel is needed.' The developers are finalising an agreement with NASA to fit a scaled-down version of the engine to the ISS to conduct operational tests. There is also a concept video on YouTube suggesting a journey time for a manned craft to Mars on the order of 60-70 days."
We can guess that it is similar to the DS-1 Ion drive which propels Xenon ions at 30 km/sec.
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The VASIMIR has been in developement since 1979.
http://en.wikipedia.org/wiki/Variable_specific_impulse_magnetoplasma_rocket
I wonder where they will get the 200 kW to drive it from?
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Wiki - the source of all knowledge - has some good details.
http://en.wikipedia.org/wiki/Variable_specific_impulse_magnetoplasma_rocket
VASIMR (Variable Specific Impulse Magnetoplasma Rocket) is a variation on the Magnetoplasmadynamic thruster (MPDT) that has been in development for a decade or two. These thrusters are coveted for their rare combination of high specific impulse (i.e. efficiency) and thrust to weight ratio (power). Such a combination makes them ideal for manned missions, as they allow space craft to fly under constant, high powered thrust. This is the "holy grail" of space travel because it cuts down the TIME in flight significantly. Thus the summary's mention of a 90 day flight time to Mars. (Normal flight time using a minimal Hohmann Transfer is ~1.5 years.)
Obviously, the exact thrust numbers depend upon the rocket. However, the VASIMR rockets have a range of specific impulses from 3,000 to 30,000 seconds. You can see how that compares to Ion Thrusters here. Chemical rockets tend to top out at 500 seconds and thus don't even place when compared to VASIMR or Ion thrusters. The only reason why we want to keep using chemical rockets once we have better thruster technology is that all these new technologies lack the thrust to weight ratio to get a rocket off the ground. i.e. They are only good for space travel. Atmospheric flight need not apply.
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Ion drives use electromagnetic fields to accelerate particles with an accelerator. Most times they use heavy atoms like xenon.
VASIMIR makes plasma from light elements like hydrogen and then use electromagnetic fields to heat the plasma... by controlling the temperature you can change the thrust/momentum the engine creates.
(at least that's what I heared).
From wiki:
Current VASIMR designs should be capable of producing specific impulses ranging from 3,000 to 30,000 seconds (jet velocities 30 to 300 km/s). The low end of this range is comparable to some ion thruster designs. By adjusting the manner of plasma production and plasma heating, a VASIMR can control the specific impulse and thrust. VASIMR is also capable of processing much higher power levels (megawatts) than existing ion thruster electric propulsion designs. Therefore it can provide orders of magnitude higher thrust, provided a suitable power source can be provided.
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I think the parent poster was trying to differentiate between an RTG (like Voyager has) which relies on the natural decay of radioactive isotopes and a full-bore nuclear fission reactor which induces decay with a neutron chain reaction.
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a better link.
Those people who think they know everything are a great annoyance to those of us who do. (Isaac Asimov)
So you can perhaps power VASIMIR even using the sun.
No doubt, small nuclear reactor would be an excellent idea too, especially for Mars and further missions....
(Normal flight time using a minimal Hohmann Transfer is ~1.5 years.)
I think you forgot to divide by two. The duration of the whole Hohmann Earth/Mars transfer orbit is about this long, but you only use half of it to get there.
Let's see, a kiloton of TNT = 4 x 10^12 joules all released in a fraction of a second.
A megawatt of power for 30,000 seconds (8 hours) = 3 x 10^10 joules, so no nuking here.
Also, unless I misunderstand this, you don't get more energy as thrust than you can put in as electricity. So to get that megawatt of power out, you need a megawatt of electricity in. Solar panels will only get you 1300 W / m^2 with 100% efficiency at the Earth's orbit. I guess you need a really long extension cord.
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A more fuel efficient way to nuke things from orbit is to haul the nuke to the target in a wagon pulled by a horse.
I think you're mixing up specific impulse with thrust.
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Good luck with that. Not only a nuclear reactor, but a heat exchanger, a turbine, and a generator. That's a lot of complexity for a space mission.
Look up the SP-100, which was an interesting NASA project designing a small nuclear fission reactor specifically for safe space use. One of the nice things about running a reactor in space is the fact that you only need radiation shielding in the direction where you've got sensitive equipment (i.e. crew). One of the bad things about running a reactor in space is trying to keep it cool.
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The VASIMIR isn't new... it was developed by NASA over 10 years ago, but the power-thrust/mass ratio didn't justify funding it. It just sounds like someone who wouldn't let it die. You're right, this is an ion drive with the difference that the net fuel charge is zero, as opposed to a true ion drive which ionizes a neutral fuel (usually a noble gas like Xe or Ar) and uses the excess charge repulsion to generate thrust. This disassociates neutral hydrogen using RF coils, so no charge is introduced, but the reactive plasma is used to generate an Isp. The plasma propulsion effect is much more charge than heat. Your goal is to minimize heat release. Thermal expansion on sparse plasmas in deep space doesn't generate much thrust (we're talking PV cooling), it just causes a cooling problem, trying to keep the propulsion system properly controlled (thermally).
flamebait? fuck off, you ingnorant moderators.
as for amouth - you really seem to have no idea how tenuous the intersteller medium (or solar wind, for that matter) is. at one astronomical unit, there are only about 7 protons per cubic centimeter. The further one goes from the sun, the more tenuous the solar wind gets. by the time you've passed the heliopause, the particle density is roughly one per cubic centimeter. Since even at earth distances the solar wind is so thin, your 'jet intake' (the actual collection radius of your "scoop) will have to be several hundred kilometers wide to bring in enough fuel to be useful. There is drag involved with such a scoop - both from the particles entering the scoop, as well as drag from the galactic magnetic field. You still have to provide power for the magnetic field, and if you're not fusing the material from the solar wind, you have to provide power to run your microwave heater to turn the fuel into plasma. there isn't enough material in between planets (or stars, for that matter) to be used as fuel for anything less than a technology-as-magic level.
I can think of a number of reasons:
an ion drive takes atoms of an element and strips off some of the electrons in order to make the particle charged (ions). These charged particles are then directed through an aperture with an opposing charge, accelerating them. When an ion with a positive charge comes in contact with a free electron, it captures it, and the atom reverts back to a charge-neutral state.
a plasma is a gas that is energetic enough that the positively-charged nuclei and negatively-charged electrons circulate in a gas. As long as the energy levels are high enough, the nuclei will not capture the free electrons. the amount of charge carried in a plasma is a function of the ratio of ions and electrons versus neutral particles in the gas.
I assume you meant "power in the exhaust", not "energy as thrust"?
Actually, the main reason to have a variable Isp (specific impulse) engine is because there's a direct trade-off between Isp, and energy.
Note that momentum transfer increases linearly with velocity and mass, but kinetic energy increases linearly with mass and with the square of velocity. Thus, to make a given amount of mass go faster requires a lot more energy.
A more concrete example: You want to impart a total impulse of 2MV. You can either get it throwing out a mass M at 2V (mass efficient), or by throwing out a mass 2M at V (energy efficient).
In the first case, you use 1/2 M (2V)^2 = 1/2 M 4V^2 = 2MV^2 energy, while in the second case, you use 1/2 2M V^2 = MV^2 energy, or half as much, for the same total impulse. This only gets worse the bigger the velocity difference gets.
Often, the limiting factor in electric propulsion technologies like these is the power supply, not the thruster. Generally, to get the max Isp all the time you'd need a power supply so massive (like a nuclear reactor) as to completely throw off the mass fraction.
So generally, the way these systems are proposed to be operated is with a fixed power budget, and switched between mass efficient, low thrust mode (for long term delta-V), and mass inefficient, high thrust mode (for certain maneuvers).