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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."
Sigs are too short to say anything truly profound so read the above post instead.
Xenon is apparently plentiful enough to be in most of many so-called "neon" signs: The gas that's in "neon" signage isn't always neon -- different gases are used, including argon, krypton and xenon. Neon gives a reddish-orange glow. If it's more blueish, it's probably krypton or xenon.
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Xenon is used because it is the heaviest of noble gases.
You'd best bone up on your Newtonian physics.
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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).
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.
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.
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.
for bluish signs it is actually argon with a touch of mercury. argon on its own is a dim purple color which is too dim to see with other lighting, but is really neat in a very dim room. the added drop of mercury causes the chemical to fluoresce bright blue. All other colors are by putting a phosphor coating on the inside of the tubes, which emits different colors when excited by the argon-mercury mixture. Neon is only used for the classic tomato orange color, or the deep red or purple which is done with different colored glass tubing. Krypton and xenon can also be excited to emit light, but they require more energy than is commercially viable, and are rather dim.
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.