Futuristic propulsion of spacecraft

 P: 119 I read about ion thrusters. If they had enough energy, is it possible, that the descendants of the current engines fires the ions with relativistic speeds, and a small yacht could go from LEO to Moon and back, with small amount of propellant, in hours?
 P: 1,781 They already fire them with relativistic speeds. So I think the answer to your question is no.
 P: 119 i have found this one. http://www.physicsforums.com/showthread.php?t=108685 210 km/s is far from relativistic.
 Mentor P: 12,081 Futuristic propulsion of spacecraft LEO to Moon and back is not limited by fuel even for current ion drives. The limit is the maximal thrust (limited by the design, and to a lesser extent the power), which is quite low for current engines. Relativistic speeds would require large accelerators structures (>10m) and probably a small fission reactor for the necessary power.
 P: 119 Bigger than 10m? That fits to my image of small yacht or ship. Although i dont know, what should be the minimum size of the fission reactor.
 Mentor P: 5,490 To get to the Moon and back to Earth in just under a day would require a constant acceleration of 0.1G (with two periods of acceleration/deceleration). That's quite a high level of thrust, this is beyond any current ion drives. You may have come accross the proposal for VASIMR which is in the early stages of development and is intended to be operational at higher thrust but even that has (IIRC) a proposed specific impulse of ~3000 seconds. You'd have to make your ship 30 parts fuel for every 1 part ship for this venture and that's ignoring the mass of the fuel itself, the strain this would put on the engine and the fact that you'd need a hefty power source to run the whole thing.
 Mentor P: 12,081 Well, high-tech cavities reach ~30MeV/m for singly charged particles. If you accelerate protons (which is a bad idea for current ion drives, but probably unimportant for relativistic speeds), this would need ~10m for some significant fraction of c. This does not account for the ion source, the issue that the cavities need pulses and not constant beams, and other problems. In addition, the fission reactor needs space, mass, a cooling cycle and large radiators. Let's see: Assume a ship with a mass of 10 tons (probably too low, but whatever), about 10.8km/s (~10h to the moon) and ~1 hour acceleration/deceleration at the moon. There, I neglect its gravity and details of orbits - which is a good approximation at the moon (but not at earth). This requires an acceleration of 3m/s and therefore a thrust of 30kN. With E=300MeV per proton, the momentum is ~800MeV/c, which requires 7*10^22 protons per second and the nice power of ~3300 GW, which is a bit more than the total output of all power plants on earth ;). However, with non-relativistic speeds, the thrust to power ratio is better. There, 2P=Fv. With 1GW, v~100km/s and the journey would require some tons (~2 per direction) of reaction mass. With 100MW and 1/10 of the acceleration, you get the same numbers (apart from the time to reach the moon) and the power becomes more realistic. However, packing all this stuff in 10 tons of total ship mass is still unrealistic.
 P: 119 I see. So a 100 ton ship and a voyage of a week would be more realistic?
 Mentor P: 12,081 Longer journey times need a smaller acceleration and therefore a smaller power supply. Alternatively, they can use the same power and less reaction mass. "Realistic"... well, up to now, no ion drive could generate a thrust of this order of magnitude. So it is science fiction anyway, but at least it has some science in it.
Mentor
P: 5,490
I've just found an article that fits well with this thread, it outlines a slow (6 month) but relatively cheap Orbital Transfer Vehicle that continually can shuttle payloads to and from the Moon:

Projected Lunar Cargo Capabilities of High-Power VASIMR Propulsion
Tim W. Glover, Franklin R. Chang Díaz et al
Presented at the 30th International Electric Propulsion Conference, Florence, Italy
September 17-20, 2007
 Quote by Abstract A lunar cargo architecture based on solar-powered VASIMR plasma propulsion is considered. Performance in terms of the mass of cargo delivered to the lunar surface is presented as a function of specific impulse. A principal advantage of the VASIMR over other electric propulsion technologies for this application is its use of abundant and inexpensive argon as propellant. While it is generally believed that solar electric propulsion offers significant economic advantages over chemical propulsion to a large-scale lunar exploration program, the cost of solar photovoltaic power will be a critical factor in achieving real cost savings. Solar electric power cost will strongly affect the choice of thruster technology and optimal specific impulse.
 P: 119 Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?
 Mentor P: 12,081 With antimatter, the stored energy density is enough for all needs in the solar system. The issue is just to store and extract this energy in an efficient way. Oh, and you have to produce the antimatter first, of course.
P: 21,915
 Quote by GTOM Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?
 Quote by mfb With antimatter, the stored energy density is enough for all needs in the solar system. The issue is just to store and extract this energy in an efficient way. Oh, and you have to produce the antimatter first, of course.
Antimatter production is the biggest impediment.

Even with a source of antimatter, one still has to have a chamber in which to allow the anihilation reaction, which then has to transfer that energy to propellant, which is usually hydrogen. The thruster chamber (usually assumed to be a magnetic confinement system) pressure is usually the limiting factor.

The downside is the loss of about half the energy to neutral pions and (e+e-) to gammas.

The current problem with VASIMR is the low thrust and relatively low specific energy.
Mentor
P: 5,490
 Quote by GTOM Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?
An antimatter battery as if we didn't need more dangerous technology.
 P: 119 Well, if we want to rescue a damaged ship for example, it is worth the risk, to get there fast... get near to it fast, but outside the radius of a possible anti matter explosion or whatever. Basically the best futuristic way would be convert stabil matter into energy, but as far as i know, currently there is no way to do that.
Mentor
P: 5,490
 Quote by GTOM Well, if we want to rescue a damaged ship for example, it is worth the risk, to get there fast... get near to it fast, but outside the radius of a possible anti matter explosion or whatever. Basically the best futuristic way would be convert stabil matter into energy, but as far as i know, currently there is no way to do that.
The point is you have to keep antimatter caged up and hope that the storage mechanism never fails. The energy we are potentially talking about here is horrendous, if you manage to bring antimatter production down to reasonable prices then you've proposed a system whereby biosphere destroying devices are available for an unreasonable but possible price. Think of it this way: any vehicle fitted with a few grams of antimatter will release a Hiroshima scale explosion when damaged. A few kilograms and you've got the release of >Tsa Bomba scale explosion when damaged.

The potential harm of what you are proposing more than outweighs it's uses IMO.
 P: 1 FTL drive :P
Mentor
P: 5,490
 Quote by FTL FTL drive :P
There is no good reason to think this is possible.

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