From: pksuk@mothra.syr.EDU (Peter K. Suk)
Subject: "Reactionless" Drive (Was: Re: Weapons systems?)
Followup-To: rec.arts.sf.science
Summary: Highest theoretical mass / delta-V.  Can build w/present tech.
Keywords: high efficiency drives
Organization: Syracuse University, Syracuse, NY
Date: Thu, 19 Nov 92 02:59:16 EST

In article <u894793.721933895@bruny> u894793@bruny.cc.utas.edu.au 
(T. Little) writes:

>pksuk@mothra.syr.EDU (Peter K. Suk) writes:
>
>>Of course, the more efficient your drive is, the less mass you have to
>>carry around, and the more it seems like a reactionless drive.  One the
>>most efficient drives would be a mass-driver or railgun capable of
>>hurling specks of stuff at relativistic speeds.
>
>If you have significant quantities of antimatter, you could use the photons
>produced by annihilation for thrust.  You *can't* get any better delta-V
>from such a rocket (by known physics).  The acceleration should probably be
>very low (unless you have 100% reflection of MeV gamma-rays)...

At first, I thought an anti-matter fueled craft was just too far-fetched.
But after thinking awhile, and reading through old issues of _Analog_,
I realized that we could build one today if we really wanted to :)

In the March 1988 _Analog's_ "Science Fact" column, Thomas Donaldson put
together information on schemes for converting energy from fusion
reactions.  One of these devices he calls a "Taussig Generator" after
its inventor.  Since "neutron and gamma radiation pass easily through
walls made of low atomic weight materials," we can build collectors for
this radiation around a fusion reactor, and extract the energy using a piston:

	+------------------------+             +---------------------+
	|       [heavy gas]      |             |   ||                |
	| ********************** +-------------+   ||                |
	| * Radiation Absorber *                   ||  [light gas]   |
	| ********************** +-------------+   ||                |
	|       [heavy gas]      |             |   ||                |
	+------------------------+             +---------------------+
	+------------------------+
	|         Reactor        |
	+------------------------+
	+------------------------+             +---------------------+
	|       [heavy gas]      |             |   ||                |
	| ********************** +-------------+   ||                |
	| * Radiation Absorber *                   ||<--- Piston     |
	| ********************** +-------------+   ||                |
	|       [heavy gas]      |             |   ||                |
	+------------------------+             +---------------------+

The claimed efficiency of this device is 75%.  It could also extract energy
from a matter/antimatter reaction.  We could use this device to power a 
free electron laser.  These have a theoretical efficiency of 65%.
(Scientific American, forget when.)  This gives us 48.75% of the energy
in our fuel expressed as photons all moving in the same direction.

We could even get efficiencies approaching 100% if we forget about the pistons
and the laser.  We could simply have the gamma-rays from the reaction heat 
the absorber.  The reactor/absorber would be placed at the focus of a 
parabolic mirror like so:

                                   /-----------------------------------------
                  /-----------------/
          /--------/
	 //
	/
	|==[]       <--- Reactor
	\
	 \\
          \--------\
                  \-----------------\
                                   \-----------------------------------------

The reactor can only lose heat through radiation.  Given a thick enough
absorber, all of the energy from the reaction is expressed this way.  
By choosing the right size for the reactor and the rate of reaction, we can
insure that almost all of the radiation either escapes directly through the 
opening or is reflected off the mirror.  We get the vast majority of the
photons moving to aft.  Conservation of momentum.  Et voila!
(No, not reactionless, but efficient enough to seem it.  Read the subquotes.)

Of course, as Messr. Little predicted, the maximum acceleration would have to
be very low, but not because we have to worry about being fried by gamma-rays.
Rather, we could only get so hot before we'd melt down.

	--Peter Kwangjun Suk

(P.S.  Does anyone have any schemes with an efficiency within an order of 
magnitude of this that have the potential for high-G?  By high, I mean 
1000 G and thereabouts.)