Re: Cannon in space, how much more propellent efficent are they than missiles?

One regime where missiles are useful is when you have a choice between
high exhaust velocity but low thrust drives (plasma or ion thrusters,
realistic fusion drives, nuclear thermal drives) and low exhaust
velocity but high thrust chemfuel rockets.

Let's play it through. Question is, how big do you want to scale the
chem rockets, and what burn time are they going to have? IIRC the max
V_e for chem rockets is 4,5km/s so we are generous and use that. A
missile with 500kg dry mass and 2500kg fuel could achieve good
accelerations at 20kg/s mass flow: 3G initial acceleration and 18G
final acceleration, at an effective range of about 360km. (Of course
you need to take the two ships' relative velocities into account). It
will have a delta-V of 8km/s, and depending on the relative
velocities, the impact will be worth a few tons of TNT equivalent.

So this will be a useful weapon in scenarios where ships have rather
low accelerations, limited delta-Vs (in the 10s of km/s range at
best), and no access to lasers (high range, suitability as point
defense, etc.)

There is no inherent reason FELs couldn't be 99.9% or better efficient
with good energy scavenging and recycling.

Really? I read somewhere (ProjectRho I believe) that the theoretical
maximum is 65% for FELs, but maybe that was just poorly phrased.

 The biggest energy losses in today's FELs are in the refrigeration to keep the superconductors
at liquid helium temperatures,

Heavens, why don't they take HT-superconductors that are cooled with
liquid nitrogen and be done with it? ^^

Either way - if you take Lithium as expendable coolant, 1t of Li can
absorb about 4.7GJ of waste heat, if I got my numbers right. That of
course makes high efficiencies all the more precious.
.

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