Prediction: by year 2100 earth to mars in 19 days

That's not really the hitch: you don't care about power consumption, since you can always haul your own fission plant along with you. The reason we don't have a drive is that we're never able to keep the reaction going for more than a few seconds -- I think the record *might* be a minute, but I'm not sure.

As far as I know, all past reactions have been done with deuterium and tritium, because they react at a lower temperature. This is also the reason helium3 is desirable. In the few minutes of googling I have just done, the reason usually given for preferring decaborane as a fuel is that most of the energy comes out in charged particles and not neutrons. However, it ignites at a higher temperature, so I don't believe any sustained reaction using that fuel has been achieved yet.

This implies that your ship will have a thrust to weight of 1 (or thrust to mass of 9.8N/kg if you prefer.) If your whole ship masses a ton you will need to produce 9800N thrust at a Ve of 11,800,000 which implies 0.8g/s of propellant. Your exhaust will be helium at a temperature of about 100,000,000K. Now, I don't know if the ideal gas law extends to those temperatures, but if it does 0.8g of helium at that temperature would have a volume of ~1600m^3 at one atmosphere pressure. Your engine will have to be huge or high pressure, or both. It will not be light. I guess I'm comfortable with my prediction of 19 days, although the method might be fusion drives at less than 1g acceleration.

I looked around on the web and found this lecture that talks about D-He3 fusion.

http://fti.neep.wisc.edu/neep533/SPRING ... ture28.pdf

Looking at the chart on page 4 you get T/W=0.001 at Ve=10^6 m/s. Even my low performance engine with Ve=10^5 and T/W=50 looks iffy.

what about accelerating and decelerating at 3Gs? How long can the body withstand such loading without harm?

Who said anything about 3g accelerations? I, at least, was talking about a 1g constant burn -- did I miss something?

Edit: here is an interesting article.