1. Fission propulsion. typically, fission propulsion systems are not well suited for cost-effective missions because of the large amount of radiation being produced requires a lot of radiation shielding for living occupants and a large pusher plate. Secondly, because a fuel like plutonium has a critical mass of around 11 kilograms (ignoring compression devices), the detonation is quite large and somewhat unsafe. Thirdly, is that while it is far more energetic than chemical reaction, it is nowhere near as energetic as the next two systems.
2. Fusion. Fusion does not produce as much radiation as fission and is the cleaner alternative, and is around 3 times more energetic. Fusion is however extremely difficult to get started - in fusion bombs a fission fuse ignites the fusion burn which essentially means that a similar system in space would be a fission propulsion system with a big double-hit. When not using fission fuses, as in the Daedalus system, it is, again, very difficult to initiate. We still can't do it here on Earth.
3. Antimatter. The problem with antimatter, as you'd have guest is that dirty little four letter 'C' word. Cost. Antimatter is the most expensive substance known to science. Pity. It is around 100 times more energetic than fission and does not have the same downsides. However, it does produce sizeable amounts of gama and is more difficult to store.
Now, antimatter catalyzed micro-fission/fusion drives work by using a minuscule amount of antimatter (around 140 nanograms for a journey to Mars and back in 120 days, that includes a 30 day stay on the surface) to annihilate with a fissile material. The annihilation forces the nucleus to fission, producing a very small fission explosion which ignites a fusion fuel. Most of the energy comes from that fusion. It overcomes the cost burdon since only a very small amount of antimatter is used, it overcomes the dirty problem since the critical mass for fission essentially becomes a single atom, and it overcomes the difficult problem since fusion is initiated by a fission fuse. All up, it makes for a great little propulsion system. For longer ranged missions (interstellar) the fission cycle can be excluded and antimatter simply acts as the fuse to initiate a fusion burn. the AIMstar design uses this principle.
The documentary, 'Alien Planet" uses this propulsion method to accelerate the Von Braun up to 20% c. for a 42 year journey to a habitable planet 6.5ly away. Awesome documentary (it's all on youtube) and an intriguing propulsion system. Penn state has done some great work on this concept and shown through experimentation that antiprotons causing a uranium target to undergo fission produces sufficient energy to initiate a fusion burn.
Plenty of information, very meaty but the first page sums it up.
So... What do you all think? what are your favourite concepts?
Edited by eacao, 23 June 2012 - 02:56 PM.