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This figure gives us a total payload fraction of 100 tons which is what the
ICAN ACMF engine is designed to put in Mars orbit. In other words we could
send a Pathfinder to Mars with first generation ACMF engines in about 45
days. This mission generates a total delta v of 120 km/sec or approximately
2,500 times less than what is necessary for a one way mission to a star or
5,000 times less than what would be necessary for a two way mission.
Clearly, we will need more thrust than even the ACMF engine is capable of
producing if we are going to reach the stars.

So what options are available? Is there hope of improving on these figures?
The following table summarizes the available technologies:

Propulsion Type 			Specific Impulse [sec] 		Thrust-to-Weight Ratio
Chemical Bipropellant 			200 - 410 				.01 - 100
Ion-Electrostatic 			1200 - 5000 			10-6 - 10-4
Nuclear Fission 				500 - 860 				.01 - 30
Nuclear Fusion 				10+4 - >10+5 			.01 - 10(?)
Antimatter Annihilation 		>10+7 				.01 - 10(?)

The ACMF engine is somewhere between fission and fusion in performance with
an Isp of 10+3. Obviously, we have room to improve here with follow on
generations of engines utilizing advances in fusion fuels. Simply upgrading
the engine to another fuel can produce significant increases in available
energy. The table below shows the energy available from various fusion
reactions.

Fusion Reactions Among Various Light Elements
D+D   -> T (1.01 MeV) + p (3.02 MeV) (50%)
      -> He3 (0.82 MeV) + n (2.45 MeV) (50%)  	<- most abundant fuel
      -> He4 + about 20 MeV of gamma rays 	(about 0.0001%; depends
                                           	somewhat on temperature.)
      (most other low-probability branches are omitted below)
D+T   -> He4 (3.5 MeV) + n (14.1 MeV)  		<-easiest to achieve
D+He3 -> He4 (3.6 MeV) + p (14.7 MeV)  		<-easiest aneutronic reaction
                                     		"aneutronic" is explained below.
T+T   -> He4 + 2n + 11.3 MeVHe3+T -> He4 + p + n + 12.1 MeV (51%)
      -> He4 (4.8) + D (9.5) (43%)
      -> He4 (0.5) + n (1.9) + p (11.9) (6%)  	<- via He5 decay

p+Li6 -> He4 (1.7) + He3 (2.3)      		<- another aneutronic reaction
p+Li7 -> 2 He4 + 17.3 MeV (20%)
      -> Be7 + n -1.6 MeV (80%)     		<- endothermic, not good.
D+Li6 -> 2He4 + 22.4 MeV            		<- also aneutronic, but you
                                              	get D-D reactions too.
p+B11 -> 3 He4 + 8.7 MeV 				<- harder to do, but more energy than p+Li6
n+Li6 -> He4 (2.1) + T (2.7)        		<- this can convert n's to T's
n+Li7 -> He4 + T + n - some energy

As you can see, utilizing either the D+He3 or D+Li6 reactions can
significantly improve the available energy and thereby increase the Isp from
second and third generation ACMF engines. Is it enough? No, even if you
assume that the additional energy is directly convertible into additional
Isp, at best we could only expect an Isp of around 50,000 seconds which
although good enough to get a smaller, unmanned payload there, isn't enough
to get our 400 ton Pathfinder there.

In short, even the ACMF engine is not going to get us to the stars in any
reasonable length of time. It seems we are going to have to wait for
antimatter engines or some exotic concept not yet invented. Sigh.

Lee Parker

                                                      (o o)
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Duct tape is like the Force. It has a light side, a dark side,
and it holds the universe together....        -- Carl Zwanzig