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Re: starship-design: Genuine STR question

Hi Johnny & SSD,

Linacs have certain constraints which I'll discuss. To achieve significant
velocities they really need beamed power. You probably already propose such
a system, but here's my discussion.

----- Original Message -----
From: Johnny Thunderbird <jthunderbird@nternet.com>
To: AJ & AJ Crowl <ajcrowlx2@ozemail.com.au>
Cc: starship-design <starship-design@lists.uoregon.edu>
Sent: Tuesday, February 23, 1999 2:40 AM
Subject: Re: starship-design: Genuine STR question

>AJ & AJ Crowl wrote:
>> Hi SSD,
>> Johnny, in a word, yes.
>> <....>
>> The real problem for relativistic starships is the energy of the
>> 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
>> easily vaporise the engine, hence why you don't run that high. Reactions
>> that involve "combustion" away from the drive, confined by fields, are
>> highest thrust systems - pulse drives being the most studied.
>> Personally I think truly advanced spacedrives would involve beamed power
>> 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.
In which case you have the problem of handling the power transfer. How?
We're talking about multi-terawatt power levels and that sort of power
couldn't be handled by any known cabling. Plasma conduits maybe?

>I may choose to stick a continuous fusion reaction out in front of the
>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.
Good point, but perhaps impractical.

>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
>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.

The most glaring problem is : How do you power your LINAC? Some sort of
reactor which will, perhaps, use some sort of heat engine to transfer energy
from the reaction to the power system. I suppose is we got really smart we
could use a Bussard Fusor which converted most of its reaction energy to
electrical power [cabling problem again] to power the LINAC, but even with
very high efficiency conversion there will be heat loses and we'll need to
dump waste helium, which at substantially high power levels will be a pretty
respectable jet itself. At most the energy we can transfer from reactor to
LINAC exhaust  is directly related to the mass fraction of energy liberated
in our power reaction. We can use way more reactor fuel to beef up the beam
energy but then thrust from the reactor exhaust would be MUCH higher than
from the LINAC beam.

You can probably see that only beamed power really makes sense when using a
LINAC to achieve close to c velocities. Then the advantages are high for
being propelled by an ultrarelativistic beam.

>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
>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.
All true. and ditto for the power source.

>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.
I think the ultimate sub-light drive system [not involving weird gauge field
reactions that I've speculated on] would be a network of "starbases" across
the Galaxy that push starships around via mass-beams. Nano-probes could be
launched at near c , powered by convential systems, to arrive at one's
destination to set up a deccelerator. Ultimately it would have an expansion
rate of between 1/3 and 1/2c, and the Galaxy could be networked in a few
hundred thousand years. Very high TDFs could be achieved if acceleration
effects could be minimised by "inertial dampers" or some other system, even
immersion in oxygenated flurocarbons if we were desparate.

Perhaps at some stage after the wave is spreading they'll develop worm-hole
stargates, which will then spread out just a bit slower than c on the
network. So some time hundreds of kiloyears from now an Empire could be
set-up, linked by stargates... kind of makes me wonder if it hasn't already
been done. Do we have a stargate waiting in near-space? Perhaps guardian
systems have been set-up to shoo visitors away from a developing species of
sophonts, still too primitive to detect the space-time distortion drifting
around in their inner comet cloud.