starship-design: My two cents

[wharton@physics.ucla.edu (Ken Wharton)]

Steve and Timothy have had this argument before.  I think the misunderstanding
is stemming from what they mean by minimizing "Energy".

If you try to minimize TOTAL ship-energy = Kinetic Energy + Rest Mass Energy
then Steve is right; photons are best.

If you try to minimize the Kinetic Energy you need (i.e. assume that mass is
"free"), then Timothy is right: there is an optimum exhaust speed.

So which is correct?  That depends on the technology.  If it's "easy" to
turn mass into energy, then they're basically equivalent, and Steve is right.

If it's very difficult to turn mass into energy, then rest mass is much
easier to come by than the kinetic energy, so you try to minimize Kinetic
Energy alone, and Timothy is right.

So, as I understand it, right now Timothy is right about there being an
"optimum" exhaust speed because it's tough to turn mass into energy.  But
given that antimatter might make this easy in the future, Steve's slant
could very well be correct by the time we build the spaceship.

Ken