New Ion Engine Enters Space Race 168
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."
Interesting... (Score:5, Funny)
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bad idea (Score:3, Insightful)
Re:bad idea (Score:5, Informative)
Re:bad idea (Score:5, Informative)
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If xenon is an inert gas, will any inert gas do?
Re:bad idea (Score:4, Informative)
You'd best bone up on your Newtonian physics.
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Re:bad idea (Score:5, Informative)
The 3.8 day half-life might cause some difficulty. Not to mention that the short half-life implies a high radiation output. Generally, it's a good thing not to have your propellant tanks glow on their own.
Besides, $6000 per milliliter is expensive, even by aerospace standards.
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Xenon is a good propellant since it's easy to ionize.
Brett
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I suppose the only way around this would be a solar sail, or perhaps such a thing powered by
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It uses Xenon atoms
http://en.wikipedia.org/wiki/Ion_engine#Deep_Space_1 [wikipedia.org]
Still just a curiosity... (Score:3, Insightful)
Re:Still just a curiosity... (Score:5, Funny)
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Ions and magnetic fields (Score:2)
You've got to say these obvious things because the space boffins seem to forget now and then.
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You've got to say these obvious things because the space boffins seem to forget now and then.
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indicates that there isn't enough hydrogen in the local neighborhood
I've heard this before, this and there are many other reasons that don't make a pure Bussard ramjet possible. A few years ago I came across these guys. [ibiblio.org] While I don't know how realistic their ship but one thing that did catch my eye was this. [ibiblio.org]
I was especially fascinated by how they address the fuel problem. They created something called an acceleration track. The idea is that fuel is launched before the ship is in packages. The ship would over take each fuel and supply package as it left the system.
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It seems to me that the faster a spacecraft travels, the more damage is done when a random chunk of whatever collides with the spacecraft. Outside of comet tails and meteor showers, I am not sure there are that many random chunks of matter in space but travelling at 1% of the speed of light when you hit a speck of sand, for example, must be a bad thing. Has this been quantified? Would a steel plate at the front of
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Obligatory Niven-Known Space reference (Score:3, Informative)
http://en.wikipedia.org/wiki/Bussard_ramjet [wikipedia.org]
http://en.wikipedia.org/wiki/Tales_of_Known_Space [wikipedia.org]
There are a lot of unanswered questions about this technology, but I'd quote AC Clarke at you if someone says it's impossible.
Re:Still just a curiosity... (Score:5, Interesting)
Also, the one problem I see with the idea of 'vacuuming' space, beyond the obvious engineering problems, is that in order to use them in a system you'd lose more momentum than you'd gain, at least using engine technology of this sort. Imagine it from the spacecrafts point of reference, all the very rarefied gas is coming towards it at the speed the spacecraft is traveling in the inertial frame. As it captures the gas, it has to slow it down to stationary, and then speed it up and send it back out; in doing so unless the exhaust velocity is faster than the spacecraft velocity, you're going to lose momentum rather than gain it.
Now if you could come up with a way to ionize the gas as it passes and use magnetic fields to accelerate it further (like a swimmer or an air-breathing engine) that would certainly be interesting.
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http://en.wikipedia.org/wiki/Bussard_ramjet
A system like that would take a helluva beating. I'm a bit skeptical.
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Moral of the story: you have to start somewhere.
Ooo (Score:3, Informative)
You mean it's NOT because .... (Score:5, Funny)
You mean it's NOT because they're shaped like bow ties?
Darn!
Re:You mean it's NOT because .... (Score:5, Funny)
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Evenly matched, of course.
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You mean it's NOT because they're shaped like bow ties?
Nah, NASCAR wasn't big enough back in '76 to be worth going after the "bowtie crowd".
No no no. Faraday effect! (Score:5, Insightful)
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.)
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The reason it creates an electric field is the same as the reason t
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Makes me pine for the old days... (Score:2, Interesting)
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The project was canceled because they had an intractable exhaust problem. The engine would actually emit huge amounts of uranium and carbon from the lining of the reactor during use, creating a pollution and service life problem. It's also true that nobody wanted to risk a nuclear explosion in the atmosphere over populated areas just to make space exploration cheaper, but there were plenty of other problems with the system.
That was 40 years ago, thought. Doesn't mean that with all the advances we've made in materials science and with applications from fusion research nuclear propulsion shouldn't be actively researched.
Good luck proposing nucular engine project for goverment funding, thought.
In any case, sans the cool orion nuclear impulse drive, I don't think any of these things would be appropriate to do heavy lifting in atmosphere. Would be good for a propulsion when you're in the orbit for much the same reasons as ion eng
Not news, and not impressive (Score:5, Informative)
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.
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As an EE engineer I could imagine limitless energy source would allow you to have several particle accelerator stages and you could use the waste heat to do all manner of useful stuff inside the space vehicle. Can't do that with batteries and/or solar panels.
There are also the more "radical" nuclear drives I've seen proposed without touching the good old nuclear impulse engine.
One proposal was something
why Xenon (Score:2)
Higher Efficiency? (Score:3, Interesting)
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Simply put, they dont have "fuel" in the conventional sense. They use electricity (which can be sources externally or generated oboard from a nuclear source) to exject a reaction mass at high speed. Over time this reaction mass will be consumed, but get far more benefit from it that they would with normal combustible fuel.
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Has anyone tried powering an ion engine vehicle with an external laser? How about a laser-driven interplanetary scoop that collects ions from interstellar space to drive as reaction mass powered by that remote laser?
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The problem is how fast they can convert the energy of the fuel into acceleration. I think we're still a long way from an ion engine that could lift even it's own weight on Earth surface, let alone weight of an entire spacecraft of any kind. A current or foreseeable technology ion engine on the surface of Earth will just sit there, even on full power.
So you still need something with a lot of thrust
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So you'd have to make a lot of assumptions (type and mass of energy source, amount of fuel on board, amount of propellant on board) to calculate efficiency like you say.
But for any sensible assumptions, the efficiency will be much much higher than wi
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This new engine has a power source that has its own efficiency converting its fuel to electricity, which is the starting point. Then there is the efficiency of the ion engine itself converting that electricity to the kinetic energy of the moving vehicle. Multiply those two (fractional) efficiencies together for the total fuel efficiency of this vehicle. That's what I want to know.
Th
say what now? (Score:2)
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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
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Qinetiq (Score:5, Informative)
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GOCE satellite (Score:5, Informative)
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.
UK chucking money around like there's no tomorrow (Score:2)
"Yet despite this humble appearance, it took 20 to 30 years to develop, at a cost of tens of millions of pounds."
Divide tens of millions of pounds by 20-30 years, and you get an annual cost of some UK engineers and their equipment. Unless, of course the figures aren't right, but I have a feeling that they are not too far out.
I say, dash it all, buck it up you fellows! I know chaps in the RFC who are just itching to slap a couple of ion drives on their kites, what?
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Wouldn't a heavier material like lead be more efficient at mass transfere and therefor thrust?
Mass has nothing to do with it. The only thing that matters is charge. If you put H+ or Xe+ in a constant electric field they will will both have the same kinetic energy once they exit (hence, something that has a 1 e charge put across a 1 V potential will have 1 eV of kinetic energy on exit). Xenon is useful because you can strip a lot of electrons off of an atom to make a highly charged ion. It is also easy to store and ionize.
Re:why xenon? (Score:5, Insightful)
Re:why xenon? (Score:5, Informative)
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.
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They don't want to pollute space?
Re:why xenon? (Score:5, Informative)
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).
Re:why xenon? (Score:5, Funny)
Re:why xenon? (Score:5, Insightful)
Re:why xenon? (Score:5, Funny)
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(emphasis mine)
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Yer both wrong/right (Score:5, Interesting)
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.
Re:Yer both wrong/right (Score:5, Informative)
That's ln(m1/m2); units analysis is sufficient to show your version is wrong (you can't take the log of a quantity with units in it).
The problem is that in chemical rocketry, Isp and density Isp matter, but in ion engines energy efficiency matters too. Raising the Isp raises the mass efficiency, but at high Isp the energy efficiency drops. Since the solar cells and power electronics are heavy, energy efficiency matters. For most current applications, ion engines have more Isp than they need, even with xenon. Besides, excessively long burn times add a delta-v penalty for doing too much of the burn high in the gravity well.
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What about photons?
there are many kinds of efficiency. You must always remember to be most efficient with the thing you have the least of.
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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 a
Re:why xenon? (Score:5, Informative)
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.
Re:why xenon? (Score:4, Informative)
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Re:Cool but... (Score:4, Informative)
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.
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Alas, the energy requirements are tough. About the only option is some form of nuclear propulsion -- though there are a number of interesting varieties of nuclear. The original Orion [wikipedia.org] concept is an interesting one -- I've been reading some of the original cost estimates (pdf [nasa.gov]), and they get quite interesting -- $3.30 per kg for a Jupiter mission, assuming reasonable costs for the plutonium. (Not 2008 dollars, and I don't know what the actual price of plutonium is these days.)
More modern interesting propo
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No, that's not what I want. Current estimates put maximum Isp for closed cycle gas core rockets low -- perhaps 2000s or so, maximum. That's far better than chemicals, certainly, but not anywhere near what the other designs are capable of. The open cycle designs also spew radioactive exhaust.
For the first stage, chemical rockets will do just fine. Once you're out of the atmosphere, the high-Isp nuclear designs are fine -- just make sure the flamey stuff misses the Earth, which is a relatively trivial t
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Please feel free to correct me. I am not a physicist, and I'm probably completely wrong about this.
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Mach 1 in space... (Score:2, Insightful)
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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