Self-contained interstellar vehicle
- From: "Logan Kearsley" <chrono.surfer@xxxxxxxxxxx>
- Date: Sat, 05 Nov 2005 18:40:28 GMT
Despite the advantages of bombtracks, magsails, lasersails, and other
proposals that require home-system support or otherwise are only good for
one trip before you need a whole industrial complex to relaunch... I don't
like the fact that you need a whole industrial complex to re-launch. And so
I have been pondering self-contained propulsion ideas that could still go
reasonably fast.
The biggest problem I know of with a standard bussard ramjet is that the
maximum compression of the incoming protons is limited by the kinetic energy
of the ship, or if we take the ship as the rest frame the kinetic energy of
the incoming protons.
The gas heats up as it is compressed and that energy has to come out of the
kinetic energy of the incoming plasma, or else you have to have another
power source on-board which can be used to provide the energy to compress
the plasma up to fusionable levels. Or, you use another power source to not
compress the plasma up to fusionable levels, but rather to simply accelerate
the plasma out the back, using it as a propellant but not as a fuel, which
is still a great help.
Without an on-board power source (or even with one, I suppose), we're going
to want to cool (or otherwise remove energy from) the plasma a bit so that
it can be further compressed to the point where it can fuse. Since
compressing/heating the plasma automatically means slowing it down relative
to the ship, this is where we hit the 'terminal velocity problem', that the
ship can't go faster than the speed at which the speed of incoming hydrogen
equals the speed of the exhaust. But, that only applies if the extra energy
is just radiated away and wasted.
There's going to be some cooling of the incoming plasma due to cyclotron
radiation- some of that can probably be captured by rectennas. If the
collection efficiency can be made high enough, maybe it would be best to
maximize the amount of cooling that occurs because of cyclotron radiation
before the hydrogen gets into the ship, or maybe incoming protons and
electrons can be diverted into circular particle accelerators running in
reverse (circular rather than linear so as to greatly increase the distance
and time available for the particles to lose energy without making the ship
much longer), such that the particles dump their kinetic energy into the
magnetic fields of the decelerators, converting it into electrical energy
before the particles are then dumped.
Otherwise, maybe we can allow the plasma to be compressed a little bit by
moving through the ramscoop, converting kinetic energy into heat as usual,
up to the point where there is just a enough heat energy left in the plasma
to make it fuse if it can be made dense enough, but then run it through a
magnetohydrodynamic generator to get rid of the excess. The energy collected
via the MHD generator / rectennas / decelerators can then be used in the
same manner as an on-board power source to compress the plasma until it
fuses and to further magnetically accelerate the exhaust products such that
the exhaust ends up with some fraction of the incoming plasma's original
kinetic energy as well as the energy released by fusion, thus getting around
the terminal velocity.
Anybody see any problems with that (aside from protium fusion being really
hard)? It looks to me like, if there are no problems that I haven't seen in
the above, the bussard ramjets ought to be feasible IF we can create a
containment system for the conditions necessary for protium fusion (maybe a
pulsed ramjet?), and there is some boosting system available to raise the
kinetic energy of the incoming plasma relative to the ship to the point that
it is equal to or greater than the energy needed to heat and compress the
plasma to fusionable levels.
Now, since we already know that building such a containment system is Very
Difficult, I'm going to move on to the on-board power source option. If we
use antimatter as the power source, we get twice the effective energy
density of a total mass-energy conversion power source (such as an
antimatter rocket that carries hydrogen fuel as well as antihydrogen fuel)
by using the ISM as the second half of the fuel and the propellant. To
maximize the interaction of the antimatter fuel and the collected plasma,
and to maximize the conduction of the heat produced through all of the
plasma, and to make sure that as much of the energy produced does go into
heating the plasma rather than escaping from the plasma and heating the
walls, we're probably still going to want to compress the plasma. We could
use all of the methods above, but maybe the unassisted ram-compression would
be enough for those purposes. Mewonders, though, even if ram-compression is
enough, if the efficiency of the rocket engine could be improved by still
converting the plasma's initial energy into electricity, then reacting it
with the antimatter, then using that electricity to power some sort of
linear accelerator (maybe a magnetohydrodynamic engine) to make sure that as
much of that initial energy goes into giving the exhaust velocity directly
in the line of thrust, with minimal thermal bloom. Additionally, I wonder if
it might be possible for the energy provided by antimatter annihilation to
trigger some fusion of the protium plasma.
So, here's my proposal for a self-contained interstellar vehicle:
Use antiprotons as fuel, and a (magnetic? electrostatic? probably magnetic)
bussard ramscoop to collect ISM as propellant and the second half of the
fuel. A large amount of stored antiproton gas will give the vehicle a slight
negative charge, which will help to attract the protons in the ISM, but that
probably shouldn't be designed in as a major contributor to the ramscoop
field, as the charge will be diminishing with time as the fuel is used up.
Possibly place a rectenna array around the front of the ship to reclaim some
of the energy lost to cyclotron radiation. I'm hoping that the ramscoop
field would be shielding enough, but if it turns out that the rectennas (or
the front of the ship regardless of the presence of rectennas) require some
sort of physical shielding (a few tens of meters of ice, for example, ought
to do very nicely for both radiation and small solid bits), it may not be
worth it, as the shielding would be blocking the radiation that they would
be trying to collect. Run the incoming protons and electrons through
particle decelerator rings to cool them, then inject them into the engine
where the plasma (it ought to be plasma, from the energy released by
combining the electrons and protons, right? if not, it will be soon)
compressed before antiprotons are injected. Maybe fusion occurs here as
well, maybe not. I purposely left out positrons in the fuel both to reduce
the complexity of the storage facilities and to leave the exhaust plasma
with a negative charge to make it easier to handle, although I don't know if
that would actually make a significant difference in the ease of handling.
The exhaust is then further accelerated out the back of the engine by a
magnetohydrodynamic engine. The compression mechanism, magnetohydrodynamic
engine, and the rest of the ship would be powered by the energy collected
from the particle decelerators and heat engines coupled to the engine walls.
Obviously, we have to be moving fairly fast before the ramscoop is
delivering enough material to be useful. It would be really cool if some
mechanism could be devised to very slowly accelerate the ship by pulling on
the ISM, powered by reacting antimatter with the very small amount of
hydrogen coming through, but even if it could be, I expect it would be far
*too* slow to be convenient. So, we start out with large on-board tanks of
hydrogen to accelerate the ship up to ram speeds like a regular rocket. Once
they're empty, though, we don't want to throw them away, because we'll use
them again later, so make them useful during the journey by using them as
living space. When nearing the destination, turn off the engine and use the
ramscoop as a brake, storing the hydrogen it collects in the fuel tanks /
auxiliary living space. The loss of living space to fuel storage at the end
of the journey helps avoid the problem of a generation ship society that
reaches its destination and then says 'why bother leaving the ship?' Now, to
launch again, the ship just needs to be supplied with antimatter. Since one
can produce antimatter with just a particle accelerator and a dense target,
which can easily be included on the ship, although it would take a *very*
long time the ship could re-fuel itself via solar power. So, an industrial
complex is very convenient for re-launching, but not strictly necessary, and
since the fuel mass is low, it might be possible to design the ship to go
for two or three interstellar voyages before it needs refueling. And it
doesn't require any home-system support.
-l.
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