Project Bifrost: (Fission) Rockets of the Future?

Nuclear thermal rocket != nuclear pulse rocket. The latter is the classic "Project Orion" engine, utilizing super-critical explosions for propulsive force. The former is actually more akin to a traditional chemical rocket, in that it works by expelling reaction mass from thruster nozzles. However, the energy of the reaction mass is imparted by heat generated in critical or sub-critical (but not super-critical) nuclear reactions. You can use any number of materials for this reaction mass, though the popular ones are hydrogen and water. Neither is inherently harmful, nor is there any reason they would need to pick up radioactivity from the reactor (any more than the cooling water which cycles through the heat exchangers of nuclear electrical plants or naval vessels becomes radioactive).

The test ban treaty has nothing to do with this. Nuclear pulse rockets are certianly forbidden by the test ban treaty - after all, they are literally exploding nuclear bombs as part of the engine's normal operation - but there's no reason nuclear thermal rockets would be that I can see. The argument about a "dirty warhead" is potentially valid (in that some would claim it, not in that it would be a plausible danger when you consider we already have nuclear-tipped ICBMs). However, there's no law or treaty against launching radioactive material into space. In fact, quite a few of our space probes and planetary rovers use radioactive thermal generators.

Compared to chamical rockets, nuclear thermal rockets have a vastly higher specific impulse, which is to say that a given quantity of reaction mass (rocket fuel or hydrogen flowing past a reactor) can produce a greater thrust (simply put, higher efficiency). This is due to their (much) higher exhaust velocity. Remember, E (in Joules) = mass (in kg) * velocity (in meters/second) squared. If you divide both sides by kilos (fuel or reaction mass), your energy per unit of reaction mass becomes a function of v^2. In other words, doubling the speed of the reaction mass will get you four times as much energy for a given unit of reaction mass.

Since the amount of thrust you can get out of the quantity of reaction mass that can be placed on a spaceship is the current limit on spacecraft range, speed, and payload, increasing that efficiency has the potential to revolutionize space travel.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/2UB0vkB15jo/project-bifrost-fission-rockets-of-the-future

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