Today we’re going to put our scientific eye-on-propulsion. (See what I did there?)
I don’t know about you, but when I see a nice blue-colored flame coming from ANYTHING, my immediate response is as follows:
So today, in honor (or honour, as Ben would say) of the successful Hayabusa mission, I’m going to give you a VERY brief, VERY simple explanation on Ion Propulsion technology.
So the first thing you’ve gotta know about Ion Propulsion, is that it’s OLD.
Ion thrusters have been in constant use on Russian satellites since the 1970s, so they’re not anything new, per-se, but their more recent implementations are much more exciting than the old ones.
Does anyone have any questions? Remember, there are no stupid questions…
So Jacob, if ion thrusters are so old, then why don’t I see any jets powered by ion thrusters?
Thank you for that stupid question, anonymous-person. First, a “jet” by definition is an aircraft powered by a “jet engine”, which is not an “ion propulsion” engine… duh…
Secondly, ion propulsion is SLOWWWWW.
A typical full-scale ion thruster may only produce 100 to 1000 milli-Newtons of thrust. So for the most part, ion thrusters have only been used in satellites for station-keeping (orientation changes and small altitude changes). Compare this to the Pratt and Whitney F135 Axial-Flow, Dual-Spool, Afterburning Turbofan engine which produces 191 kilo-Newtons of thrust, almost 200,000 times more thrust than an ion propulsion engine. There is no way on Earth that you could get something as big as a jet or a spacecraft moving with only 1 Newton of thrust.
Luckily, Ion propulsion engines aren’t typically used on Earth ;-)
The true advantage to an Ion propulsion engine lies in something called its Specific Impulse. Specific Impulse is a measure of how much force is created given a certain amount of propellant. You could think of it as a fuel-efficiency for rockets and other propellant-based forms of propulsion.
Rocket engines typically see specific impulses on the order of 200-500 seconds, (btw, don’t worry about the units, just understand that a higher specific impulse is better) while Ion thrusters have demonstrated specific impulses as high as 30,000 seconds.
So for spacecraft, what this means is that you can carry minuscule amounts of fuel, and go a looong way with it. Unfortunately, because of the low thrust, it might take a while to accelerate to an decent speed in the first place.
OK Jacob, we get it… Ion thrusters are old and slow, whatever… What makes them cool?
Well, I was going to draw you another turtle picture, BUT NOT ANYMORE!
Ion propulsion is COOL because it derives it’s propulsive force from an electrical power source. This is great in space because solar panels provide a pretty reliable source of electrical energy. So if you have an engine that uses very little propellant, and can be powered by the sun, you can theoretically design a spacecraft that could be propelled for hundreds of years on very little propellant.
Wasn’t this article supposed to be about how Ion Propulsion worked? Who is this guy?
I’VE HAD A LONG WEEK ALRIGHT!?!?
Different ion thruster designs vary in how exactly they create their propulsive force and the specific propellants they use, but the basic idea behind them all is this…
- Take a lightweight, gaseous particle like xenon
- Give it an electrical charge, either by rubbing it on your head, or bombarding it with electrons from an adjacent hot filament, whichever fits your engine design best
- Now place that charged particle into an electromagnetic field
- The electromagnetic field will impart a force onto the particle, and if aligned properly, the particle will be accelerated out of the engine.
- Due to the Newtons Third Law, since the spacecraft has imparted some force to the particle, the particle in-turn imparts some force to the space craft (equal-and-opposite reaction), and therefore accelerates the craft.
The blue lines represent the lines of magnetic flux, and the force imparted on the Xe ion would be out-of-the-screen, into your face.
Did you follow that? If you happen to know anything about these engines, you’ll know that I’ve skipped over a lot of details, so feel free to discuss the details with me in the comments.
So basically, you can think of the way that an Ion thruster works by imagining yourself on a frictionless(!) skateboard, tearing off bits of clothing and throwing them behind you to make yourself move… You’re not going to go very fast. And on Earth, due to air resistance, you probably won’t move at all… but in space, you’ll slowly accelerate until you’re naked, and continue at that velocity until the end of the universe.
Luckily, we engineers know how to take something that barely works, and turn it into something actually useful. So how do you make a weak, not-very-powerful engine into something useful? By using 4 to 500 of them at a time!
Newer deep-space probes like Dawn carry three or more ion thrusters, but future missions plan to use upwards of 500 smaller ion thrusters in an array to attempt to provide thrust on the same order of magnitude as rocket engines. Combine this with a nuclear power source, and we may just have found our path to the stars…