Naked Science Forum
Non Life Sciences => Technology => Topic started by: Expectant_Philosopher on 14/10/2013 10:22:12
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As I see it the technology exists for this possibility, it just hasn't been combined/integrated into a single system. The simplistic designs of current ion thrusters merely generate a stream of ions from a source and minimally accelerate them. Ion plasma engines promise to ramp up the power by a factor of 100 over the best NASA design, but are we missing something better? Particle acceleration has been used for a number of years for fundamental particle research and towards fusion ignition. Accelerating particles has demonstrated increase mass of particles with increasing velocities. Increasing mass of a particle by 1% needs 14% the speed of light (C). Increasing mass by a factor of ten needs only three times the velocity. If we can take the VASIMR ion plasma design, and run the ion plasma through an accelerator ring accelerating the plasma to near C, we could vastly improve the thrust output of an ion engine. All it takes is power, power that we could generate from a fusion reactor. Now it seems to me that once an accelerator ring has achieved fusion ignition, the ring could then do double duty as a plasma accelerator to create amplified ion thrust. The plasma could be channeled to several thruster nozzles to provide even lift and/or directional control.
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Ion thrusters have demonstrated very high specific impulse (http://en.wikipedia.org/wiki/Specific_impulse#Examples), even with straight-through designs.
An accelerator ring requires very large magnets (probably superconducting) to bend the charged particles around in a ring. Bending the path of charged particles wastes energy by electromagnetic radiation. The higher the energy desired, the larger the diameter of the ring with practical magnets (see the larger rings deployed at CERN (http://en.wikipedia.org/wiki/File:Cern-accelerator-complex.svg) for successively higher particle energies). Unfortunately, heavy magnets and large-diameter rings are not compatible with small, light spacecraft.
Perhaps a better approach to an ion drive may be the "Accelerator on a chip (http://news.stanford.edu/news/2013/september/slac-chip-accelerator-092713.html)" demonstrated at Stanford? This uses the electric field of laser light to accelerate bunches of charged particles. This is potentially more compact than traditional linear accelerators, and avoids the need for large magnets in accelerator rings.
Controlled Fusion power has been a dream for many years, and it would certainly assist space exploration (and many other applications), if it can be accomplished in a small and light package.
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Ion thrusters have been in space since the 1950's. Certainly more power = more thrust but there's no magic involved and you still need a propellant.
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Ion thrusters have been in space since the 1950's. Certainly more power = more thrust but there's no magic involved and you still need a propellant.
I don't really know but I'd imagine that the specific thrust obtainable from a given mass of propellant is much greater from an ion engine than from say a rocket engine. I know that in an electron beam gun with 200 kV anode potential the electrons' mass increased by a factor of 2. This sort of effect would happen in an ion engine, wouldn't it?
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The virtue of an ion thruster is its ability to deliver a very small amount of thrust controllably, efficiently, continuously and up to very high speeds, for as long as the sun shines - or whatever energy source you are using - and you have some propellant. So if you are content with continuous low acceleration you can travel very fast in zero gravity. But to escape from a strong gravitational field you need enough thrust to lift the mass of the rocket off the deck in the first place, and this is where an electron microscope tends to fail as a flying machine. The thrust per unit mass of propellant is high, but the thrust per unit mass of engine is very low.
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How fast are the rocket engine's exhaust gases moving, do you know? In the 200 kV electron gun the electrons move at about 0.9 c. The faster they move the greater the thrust obtained. Thrust isn't the object in an EB welder but it's there anyway. The gun current in the ones I'm familiar with was only 0.6 amps so the thrust was small. I don't know how small, unfortunately. But the total mass of electrons fired from it per second was tiny. I wasn't wondering if a much bigger electron gun could be made so as to produce greater thrust. No. All I'm saying is that such guns accelerate particles to such high speeds. Perhaps an ion engine could use similar acceleration technology using much heavier, charges particles. Maybe they do so already? How are the ions accelerated, do you know?
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Ion engines typically use a massive atom like Xenon, ionised and accelerated through a high voltage, but there are many varieties that have been built or proposed. See: http://en.wikipedia.org/wiki/Ion_thruster
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Consider the electron beam welder
electron mass is about 10-30 kg
electron charge is about 1.6 x 10-19 coulomb
so 1 amp = 10-30/1.6 x 10-19 = 6.25 x 10-10 kg/s
Speed of light = 3 x 108 m/s
so the thrust of a 1 amp electron beam at 0.9c is about 0.9 x 3 x 108 x 6.25 x 10-10 =0.17 newton
Interestingly, this should be measurable, but it isn't going to lift a 2 tonne welder off the deck, for which you need 20,000 newton, never mind putting it into orbit. There is an additional technical problem of getting the electrons out of the welder gun: you can only accelerate them in a vacuum so you need one hell of a pump to maintain a suitably low air pressure at the exhaust!
Things get better if you use a heavy ion. Since the proton mass is about 1000 times that of an electron, an ion source with atomic weight around 100 will deliver 10 kN if you can wind the ions up to 0.9c. You still have the problem of getting the ions out of an evacuated tube.
The real joy of chemical rockets is that you can store a lot of energy in a small mass, most of which is burned as the flight progresses, and the rate of energy release can be enormous at the beginning of the flight. Ion thrusters are almost the opposite in every respect - significant deadweight and only a constant slow burn. But once you are in space, there's no problem of evacuating the tube and no need for a large initial thrust.
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It turns out that the thrust/weight ratio of an ion drive depends mostly on the power/weight ratio of the power source.
Ion drives need a certain amount of power to ionise the propellant, and more to accelerate it.
If you work it out, it's a hell of a lot of power, and no existent power source can provide sufficient power to an ion drive for its exhaust speed of around 15,000-30,000 m/s, not if it's to give enough thrust to also lift the power source.
Off hand I don't even expect that fission or fusion power would be able to do that either; their energy/weight ratios are fantastic, but their power/weight ratios are not that great.