Richard Aiken wrote:
 > The fuel is liquid water, which IIRC actually works out to be a more
 > efficient reaction mass (given the relatively uncomplicated storage
 > requirements for water versus liquid hydrogen).

Yes, none of the design rules account for the insulation and cooler
requirements associated with storage of cryogens. It disappears into the
hull and structure numbers.

If we assume water, mass goes to 5% (assuming the rest of the ship has
an average density equal to water[!]), so ln(1/0.95) = 0.051.

This helps a bit.

A round-trip to 100 diameters and back at 1G continuous acceleration
requires 13.44 G-hours or a delta-v of about 482 km/sec.

So exhaust velocity = 482/0.051 ~ 9460 km/sec, from the rocket equation.

 >> Direct conversion of most of the fuel's mass-energy to kinetic energy.
 > Would running the water through a fusion reactor core accomplish that?

No. You would produce a plasma of hydrogen and oxygen, which could be
ejected at speed - but not at the required level of performance.

Fusion implies conversion of about 0.7% of the fuel's mass to energy.
Maximum exhaust velocity is about 12% of the speed of light (100%
efficient, all the fuel fused and ejected out the back of the rocket).

Fission implies conversion of about 0.1% of the fuel's mass to energy.
Maximum exhaust velocity is about 4% of the speed of light.

 > If not, would adding an antimatter afterburner (spraying antiprotons
 > into a secondary exhaust chamber) help?

It will allow you to get a hotter, faster exhaust. The upper bound on
exhaust velocity with antiprotons annihilating protons is thought to be
58% of the speed of light.

 > Not sure what the exhaust from my Fireflyesque rockets would consist
 > of materially, but I have serious doubts it would be harmless.

As above, the exhaust would be a hydrogen - oxygen plasma at terawatt
power levels.

Rockets emit a lot of energy in the exhaust.
Recall that the Space Shuttle main engines had gigawatt power outputs.
The F-1 engines on the Saturn V burned for ~2 minutes for about the
equivalent of 6-8 G minutes of thrust. Each F-1 was equivalent to half a
dozen Space Shuttle main engines.

I chose neutrinos as they do not readily interact with matter (weak
force and gravity only); 65 billion per second per square centimetre is
a ballpark value for solar neutrinos passing through the earth.

You get 1 interaction for every 10^36 (1 trillion trillion trillion)
target atoms with this level of flux.

Multi-terawatt level streams of neutrinos are going to be hard to detect.

Robert O'Connor