Alternative to Rockets

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.

In sci.space.science Olli Wilkman wrote:

The website describes the technology thus:
"The back side of the craft is a large, highly polished parabolic
mirror that is designed to capture the laser beam projected at it from
the ground. The mirror focuses the beam, rapidly heating the air to 5
^^^^^^^^^^^^^^^
TIMES the temperature of the sun, creating a blast wave out the back
that pushes the vehicle upward. As the beam is rapidly pulsed, the
vehicle is continuously propelled forward, on its way to orbit."

This seems to imply that no propellant is used as such,

Except air. How's this supposed to reach orbit?

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"

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


Yes there is.

Sander Vesik wrote:
Strictly speaking, the solar sail too throws mass (photons) out in one
direction and moves in the other direction. Where the mass being
thrown out comes from is not too important.

I'm sure someone else will also point this out but photons are massless. A
solar sail basically redirects the momentum vector of some of the incident
photons. In order for momentum to be conserved the solar sail must acquire
the vector difference.

Jim Logajan wrote:

Sander Vesik wrote:
Strictly speaking, the solar sail too throws mass (photons) out in one
direction and moves in the other direction. Where the mass being
thrown out comes from is not too important.

I'm sure someone else will also point this out but photons are massless. A
solar sail basically redirects the momentum vector of some of the incident
photons. In order for momentum to be conserved the solar sail must acquire
the vector difference.

On another point, the sail itself may have low mass, but something is
going to have to hold it open and taught.


Richard

In article ,
Sander Vesik wrote:
In sci.space.tech Mark Foskey wrote:
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


If they don't work that way, then we don't call them rockets.

One other possibility is a solar sail that uses the pressure of
sunlight, but such a sail would have very low thrust.


Strictly speaking, the solar sail too throws mass (photons) out in one
direction and moves in the other direction. Where the mass being thrown
out comes from is not too important.

It also catches things, which is something that most rockets don't do.

--
"Suppose you were an idiot... And suppose you were a member of
Congress... But I repeat myself." - Mark Twain

Richard Lamb wrote:

Jim Logajan wrote:

Sander Vesik wrote:
Strictly speaking, the solar sail too throws mass (photons) out in one
direction and moves in the other direction. Where the mass being
thrown out comes from is not too important.

I'm sure someone else will also point this out but photons are massless. A
solar sail basically redirects the momentum vector of some of the incident
photons. In order for momentum to be conserved the solar sail must acquire
the vector difference.

On another point, the sail itself may have low mass, but something is
going to have to hold it open and taught.

Radiation pressure plus the solar wind will balloon it out no
problem. Two immediate problems come to mind,

1) Steering in general. How do you tack? If you water sail, what
will pulling on the sheets (lines, for the rest of you) mean in the
context of a solar sail? Where do you set your feet to do the pullng?

2) The curved solar sail - huge area - will have a focus or
caustics. Where do you put the ship? If it crosses the sweet spot
(or the sweet spot crosses it - billowing in solar storms) the ship is
literal toast.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!

In article ,
Richard Lamb wrote:
Jim Logajan wrote:

Sander Vesik wrote:
Strictly speaking, the solar sail too throws mass (photons) out in one
direction and moves in the other direction. Where the mass being
thrown out comes from is not too important.

I'm sure someone else will also point this out but photons are massless. A
solar sail basically redirects the momentum vector of some of the incident
photons. In order for momentum to be conserved the solar sail must acquire
the vector difference.

On another point, the sail itself may have low mass, but something is
going to have to hold it open and taught.


Richard


Possibly centrifugal forces. My favorite design involves unrolling sails
like windowshades from a rotating craft.

--
"The average person, during a single day, deposits in his or her underwear
an amount of fecal bacteria equal to the weight of a quarter of a peanut."
-- Dr. Robert Buckman, Human Wildlife, p119.

"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

At last - a segue to announce my goofy new interstellar travel system!

One can pull masses together rather than throwing them apart! The idea is
to use a lot of specially designed "chains" that are strung between star
systems - each end of each chain is anchored to some asteroid or small
planet. It will take some time to create all the links "the old fashioned
way": basically shoot a spool into space and let it unwind. The scheme is
called HTML (Hyper Thread Movement Links) that define the WWW (Wanking Wide
Web - cause it spans the galaxy with a LOT of links).

Anyway, to travel to another system a special vehicle called the HTTP
(Hyper Thread Transport Packet) is pulled along the wire. Basically the far
anchor is minutely pulled closer while the HTTP moves forward. The HTTP
doesn't actually touch the "chains". The "chain" is actually an alternating
link of steel and glass segments. The HTTP contains an electromagnet. As it
passes the halfway point of a glass segment it turns on the electromagnet -
and gets pulled toward the next closer steel segment. As it comes to the
steel segment it shuts off the electromagnet and coasts on to the next
glass segment, at which time it repeats the same cycle. As the HTTP vehicle
speeds up, the on-off frequency must increase. The mechanism to keep things
in sync is called the TCP (Transport Control Pull). TCP uses visual sensing
(via the luminiferous ether) to view the chain links as its feedback
mechanism. So TCP typically runs over the ether.

The scheme has a few minor flaws, though. But I may yet get DARPA
funding....

;-)