Space Forum / Space Flight / January 2006

For a given amount of energy input, the absolute limit on
the speed acheivable is set by dumping 100% of that energy
into the kinetic energy of a spacecraft.  In principle, a
rocket can acheive this up for speeds up to the maximum
exhaust velocity by matching exhaust velocity with the
spacecraft's speed.  This leaves behind a trail of
non-moving exhaust and a spacecraft moving at roughly
sqrt(2*E/m).  However, for speeds much higher than the
exhaust velocity, a rocket becomes exponentially less
efficient.

High efficiency for speeds much higher than the exhaust
velocity can be acheived with a "runway" style of propulsion.
The basic idea is to have a "runway" of pulse units which
produce annular blasts of plasma sideways to the runway.
A toroidal magnetic sail deflects the blasts rearward,
effectively transfering most of the kinetic energy from
the plasma to the sailship.  The end result is a sailship
moving at many times the "wind" velocity with most of
the kinetic energy input, along with slow rearward moving
plasma around the runway.

A diagram explaining the torusail concept is here:

http://www.geocities.com/mechdan/torusail/index.html

Using existing nuclear bomb designs, an annular blast of
fission fragments at .04c can be efficiently generated.
These bombs can launch a probe to Alpha Centauri at .25c
using current technology.  The main expense and R&D effort
for a near term interstellar mission would revolve around
the interplanetary launch system to pre-position the runway,
rather than issues around the stardrive itself.

Compared to my previous proposals for runway style
interstellar propulsion, torusail propulsion has several
advantages:

1. Very efficient--a large fraction of the bomb energy
  is converted into the starship's kinetic energy.

2. Uses cheap off-the-shelf nuclear bomb design.

3. No R&D into high power magnetic fields or dense plasma
  is required.  The technology risk is low.

4. Cost of the pulse device is minimized--no fancy magnetic
  coils or any other clever "puff" mechanism is required.
  Each bomb-bot is little more than a spin-stabalized
  cylindrical nuclear bomb with a couple stationkeeping
  thrusters.

Isaac Kuo