7. THE NSWR AND INTERSTELLAR TRAVEL
The above engine analysis assumed a very low yield of 0.1%
and a modest enrichment of 20%. It is interesting to contemplate
what might be the potential ultimate performance of a
NSWR if more optimistic values are assumed.
Consider for example, an NSWR utilizing a 2% uranium
bromide solution with 90% enriched U233, and obtaining a 90%
fission yield. Assuming a nozzle efficiency of 0.9, the exhaust
velocity of this system will be 4725 km/s, or about 1.575% of
the speed of light (a specific impulse of 482,140 seconds). If the
300 tonne Titan mission spacecraft is endowed with 2700
tonnes of propellant (for a mass ratio of 10) a maximum
velocity of 3.63% of speed of light could be obtained, allowing
the ship to reach Alpha Centauri in about 120 years.
Deceleration could be accomplished without the use of
substantial amounts of rocket propellant by using a magnetic
sail [6] (or "magsail") to create drag against the interstellar
medium.
In a more ambitious approach, one could envisage a group of
interstellar emigrants selecting a small ice asteroid with a mass
of 30,000 tonnes and using it as propellant (together with 7,500
tonnes of uranium obtained elsewhere) for a 300 tonne spacecraft.
In this case the ship could obtain a final velocity of about
7.62% light speed, and reach Alpha Centauri in about 60 years.
While this might seem an excessive time for a mission, it would
allow some, at least, of a group of astronauts who initiated the
trip in their twenties to be alive a the conclusion of the journey,
while their first generation children would still be in their
prime. The original purpose of the mission could thus be
reasonably expected to be still understood by travellers, as there
would have been direct cultural transmission from the starting
crew to the actual arriving colonists