[RightWhale]

Specific Impulse, I_spc, the number one factor in the rocket charts, is highest for hydrogen/oxygen among strictly chemical reactors. Something like 200 seconds. KIWI was something like 400 seconds, which makes it possible to include most of the structure as payload, and they need it because the reactor is fairly heavy itself.

[tricky_k_1972]

I always thought DeltaV was the most important factor.?