Old RAIR drive system

[kgstar@most.fw.hac.com (Kelly Starks x7066 MS 10-39)]

Heres some more text to look over.  Attack at will.  This was what I
consider the baseline drive system for my Explorer class design.  Untill I
figured out it probably wouldn't work.

Kelly



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multi-cycle Ram Augmented Interstellar Ramjet (RAIR)

Spring 1995

A drive idea I came up with, and used as the assumed system for this ship,
is A multi-cycle Ram Augmented Interstellar Ramjet (RAIR). It would scoop
up reaction mass from interstellar space like a pure ram scoop, but it
would only use it as reaction mass, not fuel. But the scoop system could
simultaneously scoop up fuel thrown ahead of the ship by a fixed launcher
back in our solar system.

The system is basically an electromagnetic accelerator running through the
core of the ship, powered by onboard fusion reactors. The accelerator could
accelerate scooped-up interstellar matter (or ram flow), or reaction mass
carried in the ship. Again, this kind of system is different from a true
Bussard ramscoop drive in that the ram flow is not fused for power. Indeed,
it normally isn't even slowed down. The engine functions in various ways at
various speeds.



Interplanetary Flight Mode (Plasma Drive)

At low speed interplanetary runs, the drive works like a conventional ion
rocket or mass driver. Stored reaction mass is feed into the
electromagnetic or electro static accelerator core. Unlike normal thermal
rockets, an ion thruster works more efficiently with heavy atom ions. So
I'll assume we are storing iron for reaction mass. Specific impulse varies
depending on the exhaust velocity of the expended mass.




Acceleration To InterStellar Cruise

For acceleration out of the Sol system the ship will deploy its ramscoop
and switch to the externally fueled RAIR configuration I came up with. As
mentioned above, a normal RAIR doesn't try to fuse anything it scoops up.
It just ionizes it (with a laser), scoops it (interstellar gas and dust)
up, and accelerates it as reaction mass in a linear accelerator. This gets
around the Bussard Ramscoop problem of getting more drag trying to fuse the
interstellar matter than you get back as thrust (and the fact we can't find
enough in interstellar space to run it), and could put any interstellar
matter to best use. But you still need a lot of fuel to power the
accelerator.

The idea I came up with to get around this fuel problem is to launch the
fuel ahead of the ship with orbital accelerators (thus the externally
fueled part of the name). The accelerators back home throw out HUGE
quantities of frozen pellets of whatever fusion fuel is selected, in the
path of the ship (called the acceleration track). The fuel is launched at
measured and timed speeds, so that the ship will encounter a fairly steady
stream of fuel during its run down the fuel loaded acceleration track. As
the ship accelerates, it catches up with fuel going at increasing speeds,
but always at speeds slightly slower than the ship itself. This way the
ship doesn't have the drag a true ramscoop would have accelerating
interstellar fuel that is going much slower than the ship is. All incoming
mass is hit with a short length of electric or magnetic forward thrust once
it enters the ship. The fuel, being close to the speed of the ship, will be
accelerated to the ship's speed and can be transferred to temporary fuel
tanks. The interstellar mass, being at a far different speed and direction
than the ship, will blow past the magnetic catcher into the main thrust
accelerator (powered by the soon to be burned fuel).

If you can load a 1/4th light year track with enough fuel to keep the ship
accelerating at 1g, the ship will (after 6 months) exit the track at half
of light speed (0.5c). If a pure ramscoop can work at any speed, it should
work at this speed. But assuming we can't make a pure ramscoop work (as we
are) the ship can accelerate the external mass using power supplied by fuel
stored on-board, or the ship can coast to the target star. Assuming Alpha
Centauri, in the later case it will coast for about eight years.



Deceleration and system entry

Now for the bad news - you have to slow down. We can't pre-load the
deceleration course track with fuel at the target star because it would be
virtually impossible across interstellar distances. Carrying enough fuel /
reaction-mass to decelerate at the target star would be prohibitive at any
great speed. Fortunately, there may be a solution to this dilemma, from a
rather unlikely source.

G. E. Seger found a 1978 paper by Heppenheimer on ramjets. Heppenheimer
applies radiative gas dynamics to ramjet design and eventually proves that
radiative losses (via bremsstrahlung and other similar synchrotron
radiation-type mechanisms) from attempting to compress the ram flow for a
fusion burn would exceed the fusion energy generated by nine orders of
magnitude (IE. about a billion times!). By conservation of energy laws,
those energy losses have to be made up from somewhere. The only suitable
source around is the starships kinetic energy. So a bad Ram Scoop drive
makes a great starship brake.

By reconfiguring the RAIR fields to attempt to compress and initiate a
fusion burn in the ram flow, the starship instead will be generating a
tremendous braking force. The resulting plasma (fused or unfused) can then
be run through the main accelerator (rerigged as a generator) and used to
generate electricity. Which in itself will cause further drag on the ship.
Or, if we power the accelerator in reverse and force the pre-fusion plasma
back down the throat of the oncoming ram flow, it will serve as a reversed
plasma drive.



Insystem shuttling

Once in the target star system the ship once again becomes a fast Plasma or
Ion drive ship. Its first target will no doubt be the outer star system and
a asteroid, moon, or comet rich in fuel and reaction mass. After that it
can shuttle back and forth in the star system. Though it will be crawling
compared to its high speed interstellar run. Its 1g thrust capacity will
allow it to rapidly reach any part of the star system.



Accelerating out of the target system

If we drop most of the exploration equipment and eat the consumables, the
ship should be a lot lighter on the return leg. This should make it easier
for it to accelerate without an externally loaded acceleration fuel track.
The ship may be able to load itself with massive amounts of extra fuel,
burning all the fuel, even most of the fuel that would have gone into
decelerating the ship, into accelerating it. But this must be worked out in
more detail. (See Internally fueled fusion rockets.)




Otherwise, the crew could be forced to construct an automated fuel launcher
system at the target star. This seems a clumsy and unreliable method, but I
haven't thought of another.



Reentry to the home starsystem

The ship can not only use its ramscoop brake to slow down, it can use the
Externally fueled RAIR scooping up fuel from a deceleration track at the
edge of Sol, preloaded with fuel. This works like when we first left the
Sol system, except that the fuel is blasting right down the throat of the
ship at high speed. The fuel will be the mass that's the hardest to
accelerate to the ships speed, causing very high drag, further slowing down
the ship.

Hopefully between the high fuel drag, ram drag, and the engines in full
reverse, the Explorer class ship will reenter our home star system Sol.
Decades older, probably worn out, and definitely antiquated, but loaded
with data banks full of information.



Ram Scoop collector
 and the bad news

The problem with using an interstellar scoop, is their isn't much in
interstellar space to scoop up. We found papers that proposed1000 km
diameter scoops that only weighed 200 tons. Assuming your moving at 1/3rd
the speed of light (100,000 kilometers per second) with a scoop area of
1000 km across (pi*R^2=pi(50,000,000cm)^2 = 7.854E15 cm^2). You'd be
scooping up the mass in 7.854 E25 cubic centimeters of space.

A big question is the composition of interstellar space. A classic
assumption is that there is nothing out there but about 1 atom of hydrogen
per each cubic centimeter of space. More recently, people guess it might as
little as .054 atoms per cubic centimeter, or as much as 10. Even more
recently than that (say the last few months) it has been proposed that
there may be a lot of long-chain carbon molecules in space. Perhaps 60-200
atoms / molecules. These small, dark, heavy molecules might be the missing
90-99% of the mass of the galaxy (euphemistically called "dark matter").

So far, no one really knows. This is unfortunate, because the composition
of the interstellar medium makes a hell of a difference in the design and
feasibility of a Ramscoop or RAIR-based starship. If we assume one hydrogen
atom per cubic centimeter (at a proton mass of 1.673 E-27 Kg), and assume
we're running at 1/3rd light speed (0.333c). Our 1000 km in diameter scoop,
would scoop up a ram flow of 131.4 grams per second. That's about 345 tons
a month. That doesn't sound to bad unless you realize how much fuel our
ship would need, and remember that the scoop weighs 200 tons (and you
really want the mass at slower speeds). Given the amount of time the scoop
could be used it couldn't scoop up enough to be very helpful. So this
stardrive would go into the impractical bin.

If on the other hand, you assume that each cubic centimeter had a hundred
atom carbon molecule in it, the same scoop would give us over a thousand
times as much mass to work with. Making a RAIR drive powered from stored
fuel far more attractive. But since we don't know. Its hard to recommend
this drive.





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Kelly Starks                       Internet: kgstar@most.fw.hac.com
Sr. Systems Engineer
Magnavox Electronic Systems Company
(Magnavox URL: http://www.fw.hac.com/external.html)

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