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."
Re:bad idea (Score:5, Informative)
Cool but... (Score:1, Informative)
Ooo (Score:3, Informative)
Re:why xenon? (Score:3, Informative)
Re:bad idea (Score:5, Informative)
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.
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:why xenon? (Score:3, Informative)
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.
Re:bad idea (Score:4, Informative)
You'd best bone up on your Newtonian physics.
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:T.I.E. (Score:2, Informative)
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.
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: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.
Re:No no no. Faraday effect! (Score:3, Informative)
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.
Re:why xenon? (Score:4, Informative)
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.
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.
Re:bad idea (Score:3, Informative)
Re:bad idea (Score:5, Informative)
Re:Higher Efficiency? (Score:3, Informative)
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.
Qinetiq (Score:5, Informative)
Re:why xenon? (Score:2, Informative)
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.