# starship-design: further debate

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Steve,

There IS a trade-off here.  You are right that you can always get more
momentum out of a given amount of mass/energy by converting as much of
it to energy as possible.  But you're automatically assuming we would
WANT get the highest momentum/fuel mass ratio.

Here's an example.  We have an engine capable of coverting 100% of the
fuel mass into pure energy (photons).  But Timothy thinks it's optimal
to not convert that much and decides to only convert a small percentage
to pure energy, and use that percentage to kick out the rest of the fuel
(at 0.1c, I think he derived).  I'm not sure of the exact number, but I
think this corresponds to converting only 1% of the fuel mass into
energy, and using the other 99% as reaction mass.  This, he says, is the
optimum design.

Not so!  You respond.  You can get more momentum per fuel mass if you
convert it all to photons.  (and you are correct).  So you design a ship
that beams only photons out the back, and you find it needs to carry
less mass in fuel.  Because you have a smaller mass ship, it also needs
less thrust.  Ah HA, you say: this is the true optimal design.

But Timothy's point is that *The amount of fuel you save is less than a
factor of 100*

Why does that matter?  Because your ship converts 100 times more fuel to
energy than Timothy's ship; he only had to convert 1%, you convert 100%.
And let's guess you can do it with twenty times less fuel; you still
need 5 times as much ENERGY (this is Kinetic energy, that you MAKE
onship) than Timothy did.  Timothy has optimized his design to minimize
*Kinetic Energy*, not total fuel mass.

So although you've minimized the mass of the ship, you still need an
engine that can handle 5 times the energy as Timothy's design.  You need
to beam five times the energy out the back.  Timothy's point is that if
you try to minimize the amount of Energy your onboard reactors need to
produce (from your fuel), you don't want to use photons as reaction
mass.

Now, maybe one day antimatter will be so cheap that it won't matter, and
maybe one day we won't care about how much energy we're processing, in
which case we'll go your route.  But as long as matter is at least five
times cheaper than antimatter, Timothy's design has the lowest cost.
And five times lower power-production requirements.

As I said before, it's all a question of what you're trying to minimize.

Ken

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