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NASA Ground Tests Ion Engine
herda05 writes "New Scientist reports from a press release by NASA on a successful ground test of the HiPEP (High Power Electric Propulsion) ion engine, which is the first 'major milestone' for Project Prometheus. Also some pictures and more info on the HiPEP engine."
This I think makes it only usefull for long flights. No tie fighters. Fortuanly real space flight has a lot of long flights.
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The third inplication is that the chamber is a vacuum and the xenon is stored in a tank and expelled to generate thrust, as in actually the case.
what is "hydrogent"? :) The reason Hydrogen is not used and Xenon IS, is because xenon is ~130 times more massive per atom than Hydrogen is. Therefore you'll get much more momentum from accelerating the Xenon out the back of an ion engine at a given speed than you will a Hydrogen atom. And for that matter why stop at Xenon? This guy is working on Bismuth powered Hall thrusters.
- "Hear that?! The percolations are imminent! Cease your ingress!"
Before anyone has visions of nimble fighters, we must remember that ion engines have extremely low thrust. A quick calc based on the numbers in the article, which I hope I did correctly, suggests that the thrust is only about 0.3 Newtons (1 ounce for you Imperialists). What makes these engines exciting is that they can sustain that thrust of years. Estimated fuel consumption is only about 14 grams per hour.
Slow and steady wins the race.
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One of the earlier "nuclear" test engines was this puppy, the nuclear thermal rocket. If you have seen a video of it when it was being tested, you would know that that thing is one mighty beast. The soviets also attempted to design one (both the Soviet and the American versions had the purpose of getting people to mars) which looks a lot cooler. I would love to have a model of that sitting on my desk. :)
Anyway, this nuclear propulsion is somewhat related to the newer Xenon method albeit with lower specific impulse but much higher acceleration.
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Ion propulsion works very differently than any other type of normal rocket. The idea with ion engines is that they're able to take individual atoms of their "propellant" and accelerate them to high velocities, using electricity as the energy, rather than some sort of chemical reaction.
Rockets in space work by taking some mass and throwing it in the opposite direction you want to go. Imagine yourself floating in space holding a bowling ball. You wouldn't have to push the ball away from you very hard to get yourself moving, since it's very heavy. But what if you only had a ping-pong ball: to get yourself moving quickly, you'd have to "throw" the ping-pong ball away from you very very very fast, to make up for its very small mass.
Chemical rockets take some combination of chemicals that react strongly together, creating heat. The result is a hot gas at high pressure, which blows out in the direction of the rocket nozzle, providing thrust the other direction.
The xenon ion engine takes xenon gas at very very low pressure, ionizes the atoms so that they're electrically charged, and then uses electric force to fling them at VERY high speed out into space. The velocity is much higher than in any chemical rocket. But ion engines aren't very strong -- the process works with just a little tiny bit of xenon at a time, so the engine as a whole winds up giving just a very gentle push. But since not much xenon is used up, the xenon that you have will last a LONG time.
That's the "specific impluse": a measure of how much a rocket can push you "per pound" of fuel. This page says that the space shuttle's chemical engines have a specific impulse of 460. This latest ion engine has a specific impulse of 6000!
So with the same weight of fuel, the ion engine would get you going about 13 times faster by the time you used up the fuel.
- Peter
INsigNIFICANT
Close, but...
The speed isn't a constant for different gasses. What's important, as you point out is momentum.
Change of momentum = impulse = Force * time.
The advantage of heavier ions is that they accelerate slower, thus staying within the engine for longer. As a result, the force is applied to the ions for longer, therefore the change in momentum of the ions is higher, therefore, the change in momentum of the vehicle is also higher.
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