Alternative to Rockets

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?

--
Jim Pennino

Remove -spam-sux to reply.

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?

My guess is A:
The solar cell is much heavier than a reflective sail. This effect totally
dominates the more interesting effects, probably even if a reflective
solar sail concentrates light on a much smaller solar cell.

My guess is that B has better acceleration: you lose energy in the thrust
stream, maybe 95% of it. But you put that energy into a much larger
amount of momentum, supposing Ve C. But B will run out of "go"
sooner than A, so A has more total impulse.

Here's an interesting question of perhaps more relevance in the shorter
term: is there a limit to the amount of velocity that can be picked up
from gravitational slingshot maneovers, and if so, what sets that limit?

I can see one kind of limit, which is that the perigee during a planet
pass has a practical minimum, and so objects approaching with more velocity
get less directional change. At some speed no alignment of the planets
will bend the trajectory back into the solar system and the craft is on
its way.

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

In article , (Gregory L. Hansen) writes:
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.

While it never quits, it goes asymptotically to zero (1/r^2) with
growing distance to the light source. It is a rather simple exercise
to estimate to limiting velocity a solar sail with a given mass/area
ratio will achieve, upon being launched from a given distance from the
sun. While it'll beat chemical rockets, in the long haul, it is still
far from adequate for really long trips.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

wrote:

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?


The answer is clearly "B", right up until you run out of propellant.
Then it's "A", because you don't have to carry any propellant.

Choosing an exhaust velocity (which implies a subset of possible engine
designs) is fundamentally a trade off between the costs of launching (1)
propellant mass and (2) an energy source. For chemical rockets, the
mass/energy ratio and hence the ideal exhaust velocity is implied by the
propellant you choose because it is also the energy source.

But if you've got a lot of energy available, as from a nuclear reactor
or big solar array, then your propellant doesn't have to be its own
energy source. By using the reactor or solar array as the energy source,
you can eject your propellant at much higher velocities with something
like an ion engine and get that much more impulse from each unit of
propellant mass.

But if you're trying to save energy, then you want a *low* exhaust
velocity. The ultimate example of this is a car, which uses the entire
earth as its reaction mass. That's why few cars are rocket powered;
aside from the noise and lack of safety, they just aren't very
fuel-efficient. Unfortunately, spacecraft can't carry the earth with
them to push on once they're in space. A space elevator would come
close, though.

Phil

wrote in message ...
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?

The answer is secret answer (C), not enough information and/or
not exactly comparable situations.

In B you expend reaction mass, in A you do not expend
reaction mass. If there were no limits to reaction mass
then B would be better because you get more thrust for
the same energy. However, reaction mass is never not
limited, and the limitations on reaction mass are almost
always the foremost concerns for any propulsion system in
vacuum.

Anway, solar cells are way more massive than solar sails,
by orders of magnitude. So you don't get any great
advantage from a combo, and separate systems in use in
parallel would almost certainly be vastly superior.

On 11 Mar 2004 04:23:50 -0600, (Gordon
D. Pusch) wrote:

"George Kinley" writes:

Are there any way for rockets to fly in space , other then throwing mass
out in one direction and moving in other

1.) The _definition_ of a "rocket" is "something that throws mass
(or more precisely, _momentum_) out in one direction in order to
accelerate in the opposite direction." If it _doesn't_ "throw mass
out the back," it _ISN'T A ROCKET_.

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

How about assembling a long tapered fuel rod in space. Really long fuel rod
and reasonably straight to boot. Make an entry hole up front where the
useless pointy end would have been and have the "rocket" ingest the fuel
instead of accelerating it. The devil is in the details, yes I know.

--

Boris Mohar

In article , George Kinley
writes:
Are there any way for rockets to fly in space , other then throwing mass
out in one direction and moving in other


Most rockets and other stuff accomplish space flight by coasting; going with
the net flow of gravitation: It's only when they want to steer; accelerate or
decelerate relative to that, that they have to throw off mass.

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George Kinley wrote:

Are there any way for rockets to fly in space , other then throwing mass
out in one direction and moving in other

o solar sails
o radiation

Conservation of Momentum applies
http://scienceworld.wolfram.com/phys...fMomentum.html

In sci.physics George Kinley wrote:
Are there any way for rockets to fly in space , other then throwing mass
out in one direction and moving in other

Sails.

--
Jim Pennino

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