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starship-design: LINAC efficiency

Johnny Thunderbird writes:
> . . .

You can woolgather and ramble on about ramscoops all you want, but if
you don't actually run the numbers, it doesn't matter what your verbal
arguments are.

For example, can you really ionize enough of the interstellar gas around
you to collect it with a ramscoop?  Is the energy you spend ionizing the
gas really going to be recouped when you put the gas through the fusion
reactor?  How do you get get around the problem of the ramscoop's drag
as the ship's velocity through the interstellar medium increases?  Is
there really proof for any figure you want to quote (or just make up)
for the density of interstellar gas, what does a properly-substantiated
figure mean for the effectiveness of any hypothetical ramscoop, and
what's the contingency plan if the ship runs through a local void?

Or, when you claim that we _have_ to sustain 1g acceleration of the ship
by thrust, have you really run the numbers to determine what the energy
cost to do that is for a journey of a given distance?  Once you've
figured out the energy cost, how do you intend to obtain the energy
needed to run the ship through a trip of a few tens of light-years?
Note that the energy cost in that case is many times the mass of the
payload, and that it's very untenable that any ramscoop could actually
provide usable thrust, let alone 1g thrust, at very high relativistic

Ultimately, engineering arguments aren't verbal; they're numerical.
Given some very real and frequently forbidding physical constraints, can
you actually make the numbers come out?