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New Ion Engine Enters Space Race
Bibek Paudel brings us a BBC report on the development and testing of an new ion engine by a security firm named Qinetiq. The engine will be used in an ESA spacecraft tasked with mapping the Earth's gravitational field from orbit. Only a handful of ion drives have been used for space missions before, some of which we have discussed. Quoting:
"Cryogenic pumps can be heard in the background, whistling away like tiny steam engines. Using helium gas as a coolant, they can bring down the temperature in the vacuum chamber to an incredibly chilly 20 Kelvin (-253C). The pressure, meanwhile, can drop to a millionth of an atmosphere. Ion engines ... make use of the fact that a current flowing across a magnetic field creates an electric field directed sideways to the current. This is used to accelerate a beam of ions (charged atoms) of xenon away from the spacecraft, thereby providing thrust."
I'll have to keep an ion this.
Xenon isn't exactly in great supply. I think they might want to rethink that and design it with a more common material. But sweet that they're finally testing an actual ion drive.
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As long as the ion drive has to carry all the mass it'll ever use it will never be useful for seriously long trips. It would need to vacuum up stray particles as a mass source for that. But it's mildly interesting anyway.
TIE Fighter's, anyone? (Twin Ion Engine, for those of you who are not true geeks)
TIE Fighter's, anyone? (Twin Ion Engine, for those of you who are not true geeks)
You mean it's NOT because they're shaped like bow ties?
Darn!
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
Any kind of rocket propulsion will inherently have these limits. Whether its a chemical rocket at 3000m/s with lots of thrust, an ion thruster (which aren't that uncommon, not sure why the article says it is) with 30000m/s and fractions of newtons, or something advanced like VASMIR with high exit velocities and relatively high thrust (and large power requirements), you run into the basic problem that you have to carry all your fuel with you, and the thrust you get out of it is limited by the velocity of the fuel, for which the power requirements increase my v^2 (E=1/2 m v^2).
The only ways I can see to get away from this rather immutable law of nature is to use something like solar sails, which are cool but have a lot of engineering work still needing to be done, or designing a whole new kind of physics that lets us warp space to our needs. I'm pretty sure thats how even the impulse engines work on Star Trek, since if it wasn't, the Enterprise would probably have to be mostly fuel tanks. Of course if you're working on those physics, good luck, I hope you figure out.
Ion engines ... make use of the fact that a current flowing across a magnetic field creates an electric field directed sideways to the current.
No it doesn't. It creates a MECHANICAL FORCE directed sideways to the current. It's the Faraday effect, which is what drives electric motors.
It's also how you can use the Hall effect to determine whether the majority current carrier is positive or negative: The carriers are accelerated toward the same side of the conductor, so the sign of the hall voltage tells you whether you have more + or - charge carriers.
(IIRC It's how they showed that Franklin guessed wrong when he assigned + and - to charges, leading to the sign of "classical current" and the points of arrows on semiconductor diagrams being opposite to the direction of electron flow.)
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
http://en.wikipedia.org/wiki/Bussard_ramjet
http://en.wikipedia.org/wiki/Tales_of_Known_Space
There are a lot of unanswered questions about this technology, but I'd quote AC Clarke at you if someone says it's impossible.
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From wikipedia:
Xenon is the preferred fuel for ion propulsion of spacecraft because of its low ionization potential per atomic weight, and its ability to be stored as a liquid at near room temperature (under high pressure) yet be easily converted back into a gas to fuel the engine. The inert nature of xenon makes it environmentally friendly and less corrosive to an ion engine than other fuels such as mercury or caesium. Xenon was first used for satellite ion engines during the 1970s. It was later employed as a propellant for Europe's SMART-1 spacecraft[16] and for the three ion propulsion engines on NASA's Dawn Spacecraft.
No, mass is important. The fact that xenon is so massive is exactly why it's used- that and the fact that it has a very low ionization potential. The only better material is mercury, but they stopped using that several years ago for obvious reasons.
No, the only thing that matters is momentum. If you shine a 3 MW laser out the back of the spacecraft for 1 s it is only going change the momentum of the spacecraft by 3 MJ/c = 0.01 kg*m/s. If you toss a 145 g baseball out the back of your spacecraft at 30 m/s (KE of baseball = 130.5 J) you will gain 4.35 kg*m/s of momentum, 435 times what the laser would do.
In the case above p = sqrt(2m * E). While E is a function of charge alone, the momentum is a function of both mass and kinetic energy. But it is a sqrt so you need to take into account your ion charge and its mass. A +16 charge is only twice as good as a +4 charge and 16 u is only twice as good as 4 u. Once you take this into account you will find that the difference between Xenon's 131.3 u mass and lead's 207.2 u mass is not as significant as other factors (like ease of use or ease of ionization).
Since these thrusters only operate in a vacuum, I'm afraid they don't make any sound at all...
of the NRX program. NRX (NERVA [NERVA - Nuclear Engine for Rocket Vehicle Application] Reactor-Experimental Research) was the engine that would power the spacecraft that was supposed to take us to Mars and beyond. Unfortunately it was cancelled because of 'environmental concerns' http://www.fas.org/nuke/space/kiwi.gif.
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The T5 is hardly a new thruster- it's probably been around for 10 years or more. And it's not that impressive in terms of performance for an ion thruster. More impressive ion thrusters exist, like the NSTAR thruster they used on Deep Space 1. That provided main propulsion and lasted way longer than expected, so DS1 got a lot done. Or look at the nuclear-reactor powered ion thrusters that were under development until Bush decided we were going to Mars (NEXUS and HiPEP).
Ion thrusters (and electric propulsion) have been around since the 60s. Back then, they used mercury for propellant and they had grid voltages of 13kV. Tons of ion thrusters have flown already and are already doing stationkeeping on satellites right now.
Smaller molecular weight typically preferred for space thrusters, due to the higher exhaust velocities for similar amounts of energy or momentum imparted. p=mv and E=mv^2 and all.
Which in turn means higher specific impulse.
Which in turn means greater delta-v budget for the same mass.
The price for pushing fewer molecules at higher speeds? Lower thrust at the same power level. But if you've got "unlimited" energy (solar) or "nearly unlimited" (RTG), you can take afford to take the time.
In fact, there are transfers calculated that take less time, despite taking longer to get up to speed, due to the greater delta-v.
Since double-ionzation is much more difficult than single ionization, different atoms have different work functions, and there is a limit to the electric field you can practically achieve, charge:mass ratio is a design constraint.
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It's more complicated than that. To good approximation, ion engines add energy, not momentum or velocity, to the particles they accelerate. So heavier ions leave slower, resulting in lower Isp. Thus, Xenon has relatively low Isp. However, it has the huge advantage of being easy to ionize, a gas, and nontoxic (mercury manages the first two but not the third (at ion engine pressures it's a gas), and adds the downside of tending to dissolve the engine too much).
However, for most ion engine applications, Isp isn't the primary concern -- thrust is. Ion engines easily manage more Isp than they need, but the solar cells to power them are heavy. It would be simpler and produce a shorter flight time to lower the Isp, not to mention reducing the delta-v required (orbital transfers using very long burns, as with ion engines, pay a penalty in delta-v for doing some of their burn higher in the gravity well than they have to; this can be as much as 50% iirc).
In short, Xenon is chosen because it's easy to work with and not too expensive; the heavy mass is a plus in many applications, but the reasons are more complicated than most people realize.
So...you're proposing that we propel spacecraft with baseballs?
I just read Slashdot for the articles.
First of all, it does create an electric field, just as the summary said. I have no idea what you meant by a "MECHANICAL FORCE", since the only thing that can create a force on ions is either an electric or magnetic field-- so one of those two has to be there! Also, it is not called the "Faraday effect", that is an effect of magnetic fields on light polarization-- nothing to do with this. It's the Hall effect, which you later mention anyway.
The reason it creates an electric field is the same as the reason there is an electric field in the Hall effect. The negative ions (electrons) are much more mobile than the positive ions (charged xenon atoms), since they are much less massive-- same as in a solid, except here the positive ions are slightly mobile (not in a crystal lattice.) So, the electrons whiz away from the positive ions, causing a net electric field to form.
But, you were spot on about the hall effect, and you did recall correctly about Franklin.
The only thing that matters is VELOCITY, not momentum. Mass flow RATE. At least if the test is "efficiency". That is what we (rocket scientists) call "Specific Impulse" (Isp). When you do the Delta-V equation, it's only
DeltaV = Isp*ln (m1 - m2) if memory serves. If not, someone will fix it for me. Nothing about momentum. The difference in mass is the only factor for a given propellant/engine combo
Whatever you can get out of the poopchute the fastest is the most efficient. Without speaking of the ionization process, hydrogen is prolly the best, being the lightest, BUT it's density is so low that the mass to contain it lowers the return. Recall that Clarke's Discovery had ammonia instead of hydrogen as Sakharov propellant, because it was denser (smaller, lighter tanks). And thus, it didn't leak out after 9 years (2010 - 2001)
Xenon is probably an optimum of mass and density. Plus whatever they said about ionization.
When it comes to ion propellant, the important figure of merit is the ratio of ionization energy to atomic weight. For each unit of energy spent creating an ion, you want a relatively heavy ion to propel for some "oomph." Historically, mercury and argon have also been used, though they are less efficient (and in mercury simply undesirable).
"Open the pod by doors, Hal" > "I'm afraid I can't do that, Dave" sudo "Open the pod bay doors, Hal" > alright
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Hmmm... I think I really need to re-watch my episodes of Star Trek TOS... I know they covered the whole Ion Engine thing in one of them... THEN I can come back and comment intelligently on this topic.
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And you're just feeding trolls. :)
Are these ion engines more efficient in turning the power stored in their fuel into kinetic energy of the vehicle than the efficiency of, say, liquid fuel rockets we use to launch satellites and the Space Shuttle?
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They would be called Spaceballs.
They may be harder to ionize than other substances, but it's not impossible to do so. Heck, if they could not be ionized, it would mean that they hang on to their electrons with infinite force
The focus of this story is completely wrong. Ion propulsion is kinda old hat, there has been more than just 'a handful' of satellites flying with some form of it, unless your hand is really big. Granted, most of them have been as a secondary propulsion mode and for stationkeeping, but now it is also increasingly being taken up as primary probpulsion for deep space missions.
What is really interesting is the satellite GOCE.
Tasked with mapping out the gravitational pull of earth with very high fidelity, it needs to fly as close to the earth as possible without being dragged out of orbit by the athmosphere, and to remain stable in this very low orbit.
For this reason this is the only satellite I know of where a major design driver was that it be aerodynamic! The ion propulsion is primarily to counteract the constant drag so the satellite maintains it's orbit, and to this end it is projected to be thrusting almost continuously.
A minor point, to be sure, but mach numbers relate speed with the speed of sound in the same medium. Since sound does not travel in a vacuum, using mach as a unit of speed in space is meaningless.
Cheers!
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