Re: starship-design: Genuine STR question

[Johnny Thunderbird <jthunderbird@nternet.com>]

AJ & AJ Crowl wrote:

> Hi SSD,
>
> Johnny, in a word, yes.
> <....>
> The real problem for relativistic starships is the energy of the reactions
> powering them, and the systems handling those energies can only handle so
> much... As power requirements [and jet-speed] go up, the possible thrust
> goes down. Realistic fusion rockets would run at very low thrust levels,
> unless they used fusion pulse drives, because of the extreme power levels
> involved. A small percentage heat-loss at the thousand terawatt level would
> easily vaporise the engine, hence why you don't run that high. Reactions
> that involve "combustion" away from the drive, confined by fields, are the
> highest thrust systems - pulse drives being the most studied.
>
> Personally I think truly advanced spacedrives would involve beamed power or
> mass-beams.
>
> Adam

I'm in agreement with all your statements, with a couple caveats for some
of my pet notions. Let's be aware that some of the parts of a drive system
can be decoupled for our convenience: the energy producing reactions do
not necessarily have to happen exactly where the energy is used, etc.

I may choose to stick a continuous fusion reaction out in front of the ship,
for example, just to help with the shielding against impingent cosmic rays
and gamma rays! This design might be "fusion powered" but not fusion
driven, if the mechanism of how the reaction mass is propelled rearward
actually involves linacs, as I was proposing. Ships may be designed to
have two hot zones, one where the energy is produced, and another where
the energy is used to produce thrust. I'm not necessarily advocating that
an energy producing fusion be decoupled from direct contact with the
drive mechanism, just pointing out we mustn't oversimplify.

Thank for the backup on the relativistic mass enhancement of the jet.
If this holds up, it implies that every feasible star drive system must use
linacs to throw their jet, for every gram of reaction mass that leaves the
ship at less than relativistic velocity is "wasted" by not going through the
linac to get its mass boosted. The linac would make that gram into a
dozen grams, or a hundred grams, or a kilo. If it flies back at 0.7 C or
0.8 C it's just a gram, so to us it means a leak, but if it flies at 0.99 C
it's a bunch of grams, and saves us from having to carry that much
reaction mass.

This means a strictly thermal mechanism can never compete with the
accelerator to drive the jet. However you get the energy to drive your
starship ( fusion, beamed power, antimatter ), your ship must be dozens
of times heavier with reaction mass, if you don't feed every particle of
that reaction mass through the linac so its mass gets multiplied. Ship
designs which require dozens of times more fuel than others will not get
built. So we need to quietly shelve all the plans which would give a
blackbody thermal spectrum to the jet, which includes all simple heat
engines however they're powered. Heat engine exhaust is speedy but
not relativistic, so it's all "wasted" by the above criterion. I'm using the
term "heat engine" to refer to the mechanism by which the energy is
applied to push the reaction mass away, inclusive of fusion and anti-matter
production of energy and every other means. If the exhaust has a thermal
spectrum, or any other spectrum other than purely relativistic particles,
it isn't good enough.

I too, by the way, favor beams to deliver power, and conceivably reaction
mass, to starships en route. The interstellar transport problem has a high
order of difficulty, so every feasible approach must be considered.

Johnny Thunderbird
http://www.dejanews.com/~liteage