MadSci Network: Physics
Query:

Re: How/why is Smart 1's 'ion' drive better than compressed gas?

Date: Thu Dec 16 12:57:50 2004
Posted By: Steve Sell, Staff, Aerospace Engineering, Payload Systems Inc.
Area of science: Physics
ID: 1100614330.Ph
Message:

Gordon,

Imagine you were sitting on a skateboard holding a ball, you throw the ball one direction and this causes you (and the skateboard) to roll the opposite direction. You can imagine that the faster you throw the ball, the faster you will be "pushed" backwards.

This is the same principle behind rocket propulsion, the rocket engine "throws" mass (usually a gas of some sort) through the nozzle and the rocket is then pushed in the opposite direction. The amount of thrust you get is determined by Newton's second law - the familiar F=ma equation. Force (thrust) is equal to the mass you are expelling multiplied by the acceleration you apply to it. Another way to write F=ma is F=massFlow*Velocity... so basically thrust is the how much gas per second the engine uses multiplied by how fast the gas leaves the engine.

In the case of chemical rocket engines, they can expel the gases up to about 10,000 miles per hour or so and you are expelling lots of gas at once (you see this in a rocket launch with the huge plume of flame behind the Space Shuttle or any other rocket).

An Ion engine, on the other hand, can expel the gas up to 70,000 miles per hour, so you get 7 times the acceleration. However, it's only operating on molecules of gas at a time. So, the mass flow and therefore the thrust is low. An ion engine produces thrust on the order of hundredths of a pound - but it can keep this up for hours, days, months, even years!

So, If you started with the same amount of fuel as a chemical rocket and used it all, you would have taken the same amount of mass, but accelerated it 7 times more! So you'd get 7 times more velocity - or conversely you could get the same velocity for 1/7th the amount of fuel.

Go back to the ball and skateboard example for a moment... imagine you had 1 ball that was 20kg. If you tried to throw that ball, it would be hard to do, you'd have to be strong enough to lift it and then you might not be able to throw it very fast. But what if you had 20 balls each with a mass of 1kg? You could easily throw them and a lot faster than than you could the 20kg ball. However, it would also take you longer to throw 20 balls than just 1.

In the end, both can propel you across the floor; one way is easier for you to do, but takes you longer, the other is harder to do, but you get it done in one shot. It's basically the same principle for ion engines. Also, with an ion engine, you don't need to withstand high temperatures and pressures, so the structure of the rocket itself can be lighter. So in space, where you take days to travel between the Earth and the Moon - or Months from Earth to Mars, or other planets, etc. an Ion engine is a very attractive option. the reason they are not used more often is that they are a relatively new technology, and are therefore considered a high risk. But as more missions like SMART-1 use them, more confidence will be gained and mission planners will feel more comfortable using them.

I hope this helps clear it up. Oh, and I'm not sure why they refer to it as a beam - it could be that it's more like the electron guns that draw pictures in your TV tubes - and they refer to those as electron "beams" or "rays" - basically once you're dealing with things on a molecular level, it seems more fitting than a "jet"

Here are some links with more information:
http://www-istp.gsfc.nasa.gov/stargaze/Sionrock.htm
http://ipp.nasa.gov/innovation/Innovation45/engine.htm

-Steve


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