Space Forum / Space Flight / November 2005

Conventional wisdom is that fission can't produce fast
interstellar velocities, except by powering a propulsion beam.
Here, I show how bombtrack propulsion can be used to acheive
45%c.  In particular, this propulsion method may be used to
decelerate at a destination star system and it may be used to
return from the destination star system.

First, the basic starship design:

    Front view           Side view

    o---------o          o-._
   / \_______/ \         | | "-._
  /  /       \  \        | | | | |
 /  /         \  \       | | | | |
/  /           \  \      | | | | |
o--<             >--o     o-+-+-+-|
\  \           /  /      | | | | |
 \  \         /  /       | | | | |
  \  \_______/  /        | | | |_|
   \ /       \ /         | |_.-"
    o---------o          o-"

This starship is huge magsail shaped like a basketball net,
with the main payload strung around the rearmost magloop.
This payload is dominated by stored nuclear bombbots which
will be formed into a deceleration bombtrack during the
journey.

The starship is accelerated by traveling along a long
track of nuclear bombbots.  As the fast protium products
of the explosions expand outward from the bombtrack,
they hit the conical inner face of the magsail and propel
the sail forward.  Assuming protium products traveling
at ~30%c, and a conical taper ratio of, say, 1:2, an
ultimate starship velocity of maybe 45%c is plausible.

Note that the bomb products expand sideways from the
bombtrack, rather than forward/back.  This conical magsail
uses the same principle that a sailboat uses when it is
sailing sideways to the wind.  Its angled surface deflects
sideways moving particles rearward.  With this sort of
sail, it's possible to sail faster than "wind velocity".

Bomb design:

           ---=============-   <--tertiary
          /   _____________ |
primary-->( o (_____________||  <--secondary
          \                 |
           ---=============-   <--tertiary

Each pulse unit is an elongated three stage nuclear bomb.
The final stage is an outer fission layer with protium
hydrogen embedded in the outer surface.  Upon detonation,
the final stage is heated by its own efficient fission
and radiation from the previous stages, heating up the
hydrogen in the process.  Because the hydrogen has such
low atomic weight compared to the fission fragments, it
gets blown outward at up to 30%c instead of a mere 3%c
for fission fragments.

With an efficient reaction, the fission fragments are
created with average energy of 84MeV.  If this is thermally
mixed with protium at a 1:1 ratio, the protons receive
42MeV of energy, for a velocity of around 29%c.  If you use
a protium to fragment ratio of 3:1, it gives you 20%c at
three times the mass ratio 1:30.  If you use a protium to
fragment ratio of 9:1, it gives you 13%c at a mass ratio
of 1:14.  To get the ultimate mass ratio, multiply these
ratios by half the mass of uranium (plus the hydrogen):

Proton Mass Ratio  | Proton Velocity
-------------------+-----------------
     1:120        |    29%c
     1:30         |    20%c
     1:14         |    13%c

For maximum efficiency, the bombtrack starts off with bombs
relatively rich in protium and the protium levels are
gradually decreased to increase proton velocity.

There are, of course, going to losses all around for various
factors.  For example, the sideways "wind" generated by the
outer cylinder of each bomb unit won't be perfectly sideways,
and it won't be perfectly deflected by the magsail.  Still,
my gut feeling is that a mass ratio in the hundreds will get
the starship to 45%c.

Let's assume a mass ratio of 300:1 is required.  Assuming
acceleration is done with a bombtrack, this implies a total
mass ratio of 90,000:1.  For a return journey, a third
bombtrack is launched shortly after the main starship at
a slower velocity.  For example:

Primary Starship payload mass = 1 megaton
Primary Deceleration bombtrack = 300 megatons
Primary Acceleration bombtrack = 90,000 megatons

Secondary Starship payload mass = little more than magsail
Return Acceleration bombtrack = 300 megatons
Secondary Acceleration bombtrack = 6,000 megatons

Mission profile:

Year  0 - prepare 90,000 megaton acceleration bombtrack
Year  1 - Launch 301 megaton Primary Starship to 45%c
Year  1 - prepare 6,000 megaton acceleration bombtrack
Year  1 - launch 300 megaton Secondary Starship to 20%c

Years 1-11 Cruise Phase
   Sun    2-->       1---->             Alpha

Year 10 - First starship prepares deceleration bombtrack
Year 11 - First starship decelerates at destination star

Years 11-20 Research Phase
   Sun                 2-->            1 Alpha

Year 20 - Second starship prepares return bombtrack
Year 21 - First starship accelerates to 25%c back home

Years 21-38 Return Cruise Phase
   Sun                   <---1           Alpha     2-->

Year 37 - Home system prepares braking pellet track
Year 38 - First starship returns home

The first starship is the manned research vessel.
The second starship is unmanned and its only purpose
is to prepare the return bombtrack.  Since the return
bombtrack is traveling at 20%c, the acheivable return
speed is only 25%c.

Isaac Kuo