Re: Engineering Newsletter

[Steve VanDevender <stevev@efn.org>]

rddesign@wolfenet.com writes:
 > >We can't build a dragon-fly system? I don't agree with that. Just make a big
 > >(heavy) mirror and detach just before the Asimov is going to decelerate and
 > >the mirror is going to accelerate.
 > 
 > Lookout, here comes the novice again. :-)
 > If yoyu are thinking about dicarding the mirror or anything else, think
 > about dumping it as Reaction Mass into the drive and use it to help slow
 > this puppy down. Along with mass brought along just for this purpose a dent
 > could be made in slowing the ship. Deploy a second "sail/mirror" behind the
 > ship to feed solar  energy from TC into the drive to assist also.
 > think about simpler soultions. 
 > The best Beads come from RD Designs.
 > Ric & Denisse Hedman

To quickly summarize an earlier discussion:

The novel _Flight of the Dragonfly_ by physicist Robert
L. Forward describes a starship, called the Dragonfly, which uses
a unique lightsail design.

During the boost phase of the trip, the ship uses a single large
lightsail to accelerate, powered by lasers from the Solar
system.

At the end of the boost phase, the large sail is detached but
left intact.  A smaller forward-facing sail is deployed on
the ship.

During the deceleration phase, the lasers remain aimed towards
the large sail (with ship ship and small sail shadowing part of
it), which is designed to carefully focus the incident light back
towards the small sail.

With this design, an ship can be accelerated and decelerated with
lightsails powered from the system of origin.  It's also pretty
clean in terms of its physical behavior, although the task of
focusing light from the forward sail to the retro sail is not
trivial as the distances involved are very large.  It is likely
to be more efficient than using the sail material as reaction
mass since it uses photons (already demonstrated to be a better
form of "reaction mass" than any other material) exclusively for
transferring momentum.

The design still has some difficulties, the biggest of which is
the focusing problem.  Heavy use of such spacecraft would leave
drifting relativistic-speed light sails zooming around the
universe.  These would gradually disintegrate from interactions
with interstellar gas, if of course they didn't disintegrate
while in use.  The power coming from the forward sail gets pretty
red-shifted near the end of the trip when the ship is nearly
stopped; like most relativistic light-sail designs you have to
turn up the power over the course of the trip.

In some ways I think the Dragonfly sail (if I was sure that
Forward invented it, I would call it the Forward sail both in his
honor and for the mnemonic appropriateness) is a much cleaner and
possibly more effective design than any of the other
externally-powered ship designs that have been considered so
far.