both this and the previous are wrong.<br /><br />Nuclear also approximates the 'instantaneous' dv change when you look at how long most nuclear systems are proposed to be operating. I generally see things along the lines of 5-10 hours of thrust somewhere in the region of 0.10 G acceleration. When you see a trip to Mars taking hundreds of days, acceleration periods of only a few hours amount to a fraction of a percent of the whole trip time, and thus should be considered for practical purposes 'instantaneous', though you are correct that this won't do for a navigational computer.....<br /><br />And while I agree that it is certainly POSSIBLE to accelerate longer with NTR, using more fuel, to obtain a more direct trajectory, requiring a deceleration period near the destination, just as electric propulsion accelerates and decellerates, the poor Isp of NTR compared to electric propulsion means, watt for watt, NTR takes 15-30 times more fuel than electric for such high speed transits. NTR only gains a bit on that ratio because it can put more watts into propulsion fuel thermally than it can generate electrically (typically 5-10 watts of heat for every watt of electricity produced using Brayton cycle), but much of that heat is also wasted in NTR propulsion from stagnation temperature losses, etc. I do not consider 800 sec Isp to be sufficient for interplanetary travel with anything but hohman-style transfers. Expect to pack a lunch and a few farm animals.<br /><br />In my trip calculations, I showed that VASIMR can do the trip in 90 days at high acceleration max dv with less fuel than NTR requires to even acheive the minimum dv Hohman transfer. If NTR were to attempt a similar high accel high speed trip, it would require many times more fuel than the VASIMR system requires. I never said it couldn't do it. Given the political will to launch millions of lbs of fuel into orbit, you can go anywhere, but the will is what is lacking, which is why we haven't launched any interstellar