Business Week reports on a nuclear electric battery (more like a reactor) that has 27 MW for 5 years worth of juice and it's "the size of a hot tub". It's patent pending (20040062340; search for "uranium hydride over at USPTO.gov). That's about 64 cubic feet. That's 1.2 terrawatt hours or 1.2 billion kwh. They say it's 70% cheaper than natural gas--maybe $30 million? If it's 54 cubic feet and pure uranium hydride (a high overestimate), it would weigh about 15 tons. Compare that with 15 tons of LOX and hydrogen with 66,000 kwh at 39 kwh per kg of hydrogen. Pretty good ISP. Thrust to weight not so good. Combine it with a reaction mass fill up on Mars?