In article ,
wrote:
In sci.physics Gordon D. Pusch wrote:

2.) Anything that does _not_ "throw mass out the back" (or more precisely,
_momentum_) in order to accelerate would violate Newton's 3rd Law of Motion
(AKA, the conservation of Momentum). In 300 years, _NO ONE_ has observed
a replicatable violation of Conservation of Momentum.


-- Gordon D. Pusch

Ummm, how about "catching" momentum, i.e. a sail.

Yeah, I know, it is still conserved.

Homework problem:

Given:

A. A light sail.

B. A light sail that is also a "solar cell" and uses the electricity to
power an ion rocket.

Assume equal mass for A and B (at the start), that everything is 100%
efficient and your speed is nowhere near relativistic.

At the start, do you get more "go" from B or are they the same? Why?

For a given amount of energy in your exhaust stream, you'll get more
thrust when you're throwing out more mass. If nothing else, B could be
made to have more "go" by letting its specific impulse go to crap.

But an ion engine, the propellant, and solar cells all add weight. I know
there are solar panels either existing or in development that have organic
layers on a thin plastic sheet, but designing solar sails involves
engineering tradeoffs between the mass of aluminum deposited on the sail
and the transparancy! For some reasonable figures on payload weight and
sail weight and area (about a square kilometer) you're looking at about
0.5 mm/s^2 (recalling info from a book I'd read on the subject...). Solar
sails won't win any sprints. The advantage is over the long haul, with
an acceleration that never quits.

--
"When the fool walks through the street, in his lack of understanding he
calls everything foolish." -- Ecclesiastes 10:3, New American Bible