Richard Aiken wrote:
 > To my original question of "Is 5% of volume enough fuel for four
 > round trips to the typical 100d limit?"

The answer is no. Let's work through why.

It is not possible for that volume of hydrogen, as you need to have a
rocket whose exhaust travels faster than light with that fuel
fraction/implied mass ratio.

With 5% fuel mass (water 5% of volume), as previously calculated, one
round trip with 1G continuous acceleration requires an exhaust velocity
of ~9460 km/sec.

If we assume highly efficient fusion drives with an exhaust velocity of
10% of the speed of light:
delta-v = exhaust velocity x ln(initial mass/final mass)
delta-v = 0.1 x c x 0.051, using a fuel mass fraction of 5%
= 1530 km/sec

So our hypothetical water rockets do something more than fuse the
hydrogen and oxygen atoms to get thrust.

If we use the antimatter example:
delta-v = 0.58 x c x 0.051
= 8864 km/sec

So our water rockets likely violate conservation of mass-energy. You
can't get more energy out of something than mc^2 for a given mass.

If four round trips are the required level of performance, then the
required delta-v goes up to ~37840 km/sec.

Using the antimatter rocket example:
ln(initial mass/final mass) = 37840/(0.58 x c)
initial mass/final mass = 1.24

So if the fuel is 20% of the vehicle's initial mass, then you can do
four round trips at 1G constant boost with our hypothetical antimatter
rocket.

The exhaust is a multi terawatt stream of gamma rays and neutral
particles produced by annihilating the fuel.

This has been a small sampler of 'reasons why drives that are *not*
rockets are popular in space operas'.

Rob O'Connor