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'[OT]: SpaceShipOne - Blacksky!'
2004\10\13@144845 by rad0

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Alright rocket scientists.

Last year I suggested using compressed air for thrusters in space,
and someone here, I think everyone shot it down, handily I remember.
(I think we were discussing a small camera-vehicle for inspecting the shuttle
exterior)

Although I cannot remember the details.

Apparently SpaceShipOne uses compressed air for their RCS system.

So there!!

First is this true?  Do they use air?  And why was that reportedly  wrong last year?


Talk amongst yourselves...
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2004\10\13@194321 by Jim Korman

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rad0 wrote:

> Alright rocket scientists.
>
> Last year I suggested using compressed air for thrusters in space,
> and someone here, I think everyone shot it down, handily I remember.
> (I think we were discussing a small camera-vehicle for inspecting the shuttle
> exterior)
>
> Although I cannot remember the details.
>
> Apparently SpaceShipOne uses compressed air for their RCS system.
>
> So there!!
>
> First is this true?  Do they use air?  And why was that reportedly  wrong last year?
>
>
> Talk amongst yourselves...

According to the pilot, he used the compressed air thrusters to stop the
roll on the first "X" flight.

Jim


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2004\10\13@200936 by M. Adam Davis

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I suspect the problem may come from the difference between thrusters
meant to propel the ship and those meant for attitude control.  
Spaceship one did not use compressed air for propulsion - only for
attitude control.  I doubt compressed air would be a reasonable method
to propel a space vehicle (active thrust), especially compared to other
opportunities.

-Adam

rad0 wrote:

{Quote hidden}

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2004\10\14@010222 by Russell McMahon

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>> Last year I suggested using compressed air for thrusters in space,
>> and someone here, I think everyone shot it down, handily I remember.
>> (I think we were discussing a small camera-vehicle for inspecting the
>> shuttle
>> exterior)

I remember the general discussion but I don't remember anyone saying that
compressed air was unsuitable.

Gas thrusters are common enough BUT the "gas" is often synthesised on the
spot from a mono-propellant liquid because of the much higher energy density
available. Project Mercury capsules used Hydrogen Peroxide monopropellant
thrusters - the H2O2 being decomposed into steam and oxygen at the moment of
use. Modern thrusters usually use MMH (Mono Methyl Hydrazine) thrusters
which decompose the Hydrazine at the moment of use.

For a small autonomous craft OR for a craft with strictly limited duration,
air thrusters would work and have the advantage of simplicity and potential
reliability. Something like SpaceShipOne does not need much force to spin it
at an acceptable rate. Problems would occur if there were unexpected spin
forces that were larger than the RCS could handle - and this TENDED to occur
on the 1st of the 2 X Prize flights.  As the spin forces were apparently
main engine thrust related they were not a problem after MECO (main engine
cut off).

Air (or nitrogen etc) thrusters could use liquefied gas for storage which
would increase energy density but add complexity.



       Russell McMahon




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2004\10\14@014105 by Jake Anderson
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the literature i saw said "cold gas"
and their feather actuators are "penumatically opperated"
i'd guess its powered off CO2 for reasons of density and saftey

> {Original Message removed}

2004\10\14@102110 by No Religion

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by the way, is it possible to create thrust only by electricity?
If so, how?


At 20.09 2004.10.13 -0400, you wrote:
{Quote hidden}

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2004\10\14@103341 by Mike Hord

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We had an interesting discussion about this a few weeks ago.

See the following Wikipedia articles...

http://en.wikipedia.org/wiki/Lifter (2nd paragraph)
http://en.wikipedia.org/wiki/Ion_drive

Lifter technology MAY produce thrust with no reaction mass,
although it is also quite possible (some might say more, some
might say less) that it requires atmosphere to ionize, and the
flow of ions is the thrust mechanism.

Ion drives use very small amounts of reaction mass electrically
accelerated to very high speeds to produce thrust.  There are
several "flavors"; the article above links to all of them (i.e., Hall
effect drive, pulsed inductive, field effect electric, etc.).

In short, it might be possible, but if it is, typically small thrusts
are usually the rule.

Mike H.

On Thu, 14 Oct 2004 16:11:25 +0100, No Religion <obf> wrote:
>
> by the way, is it possible to create thrust only by electricity?
> If so, how?
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2004\10\14@110554 by Alan B. Pearce

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>by the way, is it possible to create thrust
>only by electricity? If so, how?

Not quite.

One of my colleagues has an instrument on a satellite called Smart-1 which
is currently on its way to the moon. The satellite is propelled by an ion
motor, which is about as near as you can get to propelling by electricity.

How it works is as follows - an atom that can be easily ionised by
electricity is injected between a set of electrodes which are arranged in a
manner similar to a CRT tube. The cathode is a ring, and the electric field
between the anode and cathode cause the gas to ionise. The nucleus is
accelerated up to a suitable speed before disappearing out through the hole
in the cathode, and this movement of mass is what propels the spacecraft.
The spacecraft can only be propelled while there is gas to feed the motor.

This particular spacecraft is being done as a demonstrator for Smart-2 which
is yet to be built for a planned mission to Mars.

Bear in mind that the propulsion force is quite small, this very small
spacecraft is going in ever larger orbits around the earth until eventually
it gets close enough to the moon to be captured by its gravity. This is
taking a number of months. Now compare that to the 3 days or so that the
Apollo astronauts spent going each way between earth and moon.

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2004\10\14@120113 by No Religion

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Thanks, but I knew the effect, and that it doesn't work in
vacuum (as in interstellar space), although it still works
at *near* vacuum.

No, my question was if it is possible to produce thrust just
by electricity. Maybe shooting a lot of photons? :D

Seriously, if we shoot n photons, we could measure how much
energy they had. Now, if we convert this energy into mass
(thanks to Albert's E=mc²), would (that very small equivalent
mass) it have produced the same thrust as if we shooted
mass instead of energy?

Just wondering..

Thrust is exhausted mass * exhaust velocity, right?

If so, that should be quite efficient, although it's quite
hard to imagine what a device one would need to build in
order to produce so much light. :D



At 09.33 2004.10.14 -0500, you wrote:
{Quote hidden}

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2004\10\14@121808 by M. Adam Davis

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Thrust can only be created when you are pushing against something else.

Most thrust engines (chemical, air, particle, etc) work by taking mass
from the spacecraft, and pushing it out the back - a rocket motor
expands solid, liquid, or gas elements and pushes them out the back of
the rocket.

In atmosphere you can press against the air with, for example, fan
blades (helicopter).

In space you don't have that luxury - there is very little to thrust
against.

A purely electric drive, from this standpoint, doesn't appear to have
anything it can thrust against, nor are you pushing against mass that
you are releasing from the spacecraft.

Ion motors (as mentioned by another poster) emit particles of material
and then thrust against them more strongly by charging them and thus use
electricity to enhance the thrust.

Solar sails work by particles emitted by the sun.

If the magnetic field of the earth were sufficiently strong then one
could thrust against that (and some satellites do use a teeny bit of
this for attitude control, but mostly out of scientific curiosity than
for practical purposes).

The only other field which, in threory, could be manipulated
electrically would be gravity.  So far no one has been able to do so.

Electricity is essentially movement of electrons.  It's hard to
efficiently spit out electrons without somehow getting them back through
a return line, which would negate any benefit of spitting them out in
the first place.  When you can spit them out without a return you'll
find that they have such a miniscule mass that it provide very little to
thrust against.

So, in short, there is currently no practical 'electricity only' method
of propulsion in space.  On the ground there are a few effects that can
be had with static fields (high voltage) but those are thrusting against
a charged surface and the effect has exponential decay with distance.

I haven't done extensive study in this area, so I'm sure there's more
out there than what I've mentioned, but I believe if there were a
practical method then it would be in use since attitude control on a
satellite is so expensive (weight, limited lifetime).

-Adam

No Religion wrote:

{Quote hidden}

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2004\10\14@125607 by Denny Esterline

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>
> by the way, is it possible to create thrust only by electricity?
> If so, how?
>
>

It's not quite electricity only, but I remember an article in NASA Tech
Briefs a few years back that had some micro thrusters made from a small
tube with a wire down the center and Teflon filler between them (think
coaxial hardline) A high voltage spark would flash over at the end and
cause a small amount of the Teflon and core wire to ablate producing a tiny
bit of thrust. IIRC they were purposing them for satellite positioning and
such.

-Denny


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2004\10\14@132016 by David Minkler

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Not true.  It's an "equal and opposite reaction" thing.

If you were to throw rocks in free space, you would propel yourself in
the opposite direction to your rock shower.  It wouldn't make one bit of
difference if the rocks that you threw then hit rocks that you had
previously thrown.

Dave

M. Adam Davis wrote:

> Thrust can only be created when you are pushing against something else.




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2004\10\14@133212 by Dave VanHorn

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At 12:22 PM 10/14/2004, David Minkler wrote:

>Not true.  It's an "equal and opposite reaction" thing.
>
>If you were to throw rocks in free space, you would propel yourself in the
>opposite direction to your rock shower.  It wouldn't make one bit of
>difference if the rocks that you threw then hit rocks that you had
>previously thrown.

Absolutely.

And while solar wind is nice, light pressure also exists.
www.europhysicsnews.com/full/07/article4/article4.html
http://www.technovelgy.com/ct/content.asp?Bnum=349



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2004\10\14@143221 by Mike Hord

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> No, my question was if it is possible to produce thrust just
> by electricity. Maybe shooting a lot of photons? :D
>
> Seriously, if we shoot n photons, we could measure how much
> energy they had. Now, if we convert this energy into mass
> (thanks to Albert's E=mc²), would (that very small equivalent
> mass) it have produced the same thrust as if we shooted
> mass instead of energy?

That would work.  I recall seeing a model where a highly reflective
metal cone on a rod was propelled up the rod by very bright very
brief pulses of laser light.

That implies that laser light could be used as a propulsive force,
you just need something to push off of.  The question I have is,
was the "equal and opposite" force pushing the laser itself down,
or were the photons the ones losing the energy.  In the first case,
you could "zap" the Earth with a laser to push the spacecraft along
(seems unlikely), or hit the spacecraft with a laser.  In the second
case, a large sail with a laser emitter being towed along behind it
could push the whole works forward.

Mike H.

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2004\10\14@144306 by Dave VanHorn

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At 01:32 PM 10/14/2004, Mike Hord wrote:

> > No, my question was if it is possible to produce thrust just
> > by electricity. Maybe shooting a lot of photons? :D
> >
> > Seriously, if we shoot n photons, we could measure how much
> > energy they had. Now, if we convert this energy into mass
> > (thanks to Albert's E=mc²), would (that very small equivalent
> > mass) it have produced the same thrust as if we shooted
> > mass instead of energy?
>
>That would work.  I recall seeing a model where a highly reflective
>metal cone on a rod was propelled up the rod by very bright very
>brief pulses of laser light.

In that one, they were exploding the air within the cone.
They had to pulse it, to let the air come back.

Light pressure drive would be constant.


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2004\10\14@145609 by Wouter van Ooijen

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> > Thrust can only be created when you are pushing against
> something else.

> Not true.  It's an "equal and opposite reaction" thing.
>
> If you were to throw rocks in free space, you would propel
> yourself in
> the opposite direction to your rock shower.  It wouldn't make
> one bit of
> difference if the rocks that you threw then hit rocks that you had
> previously thrown.

So you do agree: it is the first rock you are pushing against.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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2004\10\14@160726 by David Minkler

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Yes.  Or the second rock.  I was trying to dispel the myth that there is
something more to be gained if the second rock hits the first.

Dave

Wouter van Ooijen wrote:

{Quote hidden}

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2004\10\14@191409 by M. Adam Davis

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I agree with you, and still stand by my statement.  In your example you
are pushing against the rock as you throw it.

I can see that my statement is not a great way to explain the principle,
but 'equal and opposite' is still at its core.

I'm not sure where rocks hitting other rocks enters the discussion -
once you are no longer 'pushing' against a rock it has no effect on you,
and I can't see how that might be construed from what I said..

But I believe it's semantics.  Perhaps there's more wrong with my
statement...

-Adam

David Minkler wrote:

{Quote hidden}

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2004\10\14@195953 by James Newton, Host

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If memory serves, that laser was heating the air under the cone, and using
the expanding gas as the propellant.

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> {Original Message removed}

2004\10\15@052645 by Alan B. Pearce

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>No, my question was if it is possible to produce thrust just
>by electricity. Maybe shooting a lot of photons? :D

There have been schemes around to propel spaceships using light from the
sun, by spreading out a monstrous "kite" which would catch enough photon
energy in the suns radiation. However I suspect that the energy per square
unit is just too low to make it a viable proposition. For the effort
involved in hauling the necessary material into space to build the kite, it
is just as easy to carry fuel for the journey.

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2004\10\15@122302 by David Minkler

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I realized after my initial post that I had trimmed too much from your
post.  I'll try again.

You wrote (I've numbered the statements for convenient reference):

1. Thrust can only be created when you are pushing against something else.

2. Most thrust engines (chemical, air, particle, etc) work by taking
mass from the spacecraft, and pushing it out the back - a rocket motor
expands solid, liquid, or gas elements and pushes them out the back of
the rocket.

3. In atmosphere you can press against the air with, for example, fan
blades (helicopter).

4. In space you don't have that luxury - there is very little to thrust
against.

My comments

1. Or if something is pushing against you but what's the difference.  
Let's call that one true.

2. I don't know of any that don't work by this principle.  Magnetic
repulsion doesn't work by this principle but I understood what you
meant.  In any case, even magnetic repulsion pushes against some other
mass in order to produce the reaction.  Let's call this one true as well.

3. While true, a helicopter is more complicated than the inclined plane
model.  Yes, I'm aware of ground effect but that's a lot more like
throwing a rubber ball and having it return to push against you a second
time.

4. This is where I derived the multiple rock toss model from.  The
implication here is that the rocket, as a system, enjoys an advantage
when its thrust engines are pushing against air.  I'm suggesting that
this is not true.

Dave

M. Adam Davis wrote:

{Quote hidden}

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2004\10\15@123859 by Peter L. Peres

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On Wed, 13 Oct 2004, rad0 wrote:

> Alright rocket scientists.
>
> Last year I suggested using compressed air for thrusters in space,
> and someone here, I think everyone shot it down, handily I remember.
> (I think we were discussing a small camera-vehicle for inspecting the shuttle
> exterior)
>
> Although I cannot remember the details.
>
> Apparently SpaceShipOne uses compressed air for their RCS system.
>
> So there!!
>
> First is this true?  Do they use air?  And why was that reportedly  wrong last year?

Afaik the manpack maneuvering unit (I don't know what the proper name is)
on the American Apollo and Skylab programs used nitrogen jets for
maneuvering. I also proposed nitrogen jets for a small camera vehicle for
inspecting the shuttle exterior in a message on this list last year.

Peter
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2004\10\15@123900 by Peter L. Peres

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On Thu, 14 Oct 2004, No Religion wrote:

> by the way, is it possible to create thrust only by electricity?
> If so, how?

Electricity is a form of energy. If you throw it out of a vehicle at a
suitable rate and speed you can get thrust. Then there are other issues
(like compensating the charge you are throwing out). Look up ion engines.

Peter
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2004\10\15@133355 by M. Adam Davis

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I understand now, and your point is certianly valid.

-Adam

David Minkler wrote:

{Quote hidden}

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2004\10\15@134931 by Bill & Pookie

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I have used "Computer Power" on my skate board.  Just heave the desktop
while on the skate board and you will travel a little,

Pookie

{Original Message removed}

2004\10\15@150559 by Mike Hord

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On Fri, 15 Oct 2004 10:28:47 +0100, Alan B. Pearce <> wrote:
> >No, my question was if it is possible to produce thrust just
> >by electricity. Maybe shooting a lot of photons? :D
>
> There have been schemes around to propel spaceships using light from the
> sun, by spreading out a monstrous "kite" which would catch enough photon
> energy in the suns radiation. However I suspect that the energy per square
> unit is just too low to make it a viable proposition. For the effort
> involved in hauling the necessary material into space to build the kite, it
> is just as easy to carry fuel for the journey.

http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/sunlight_exerts_pressure.htm

The above website goes in depth about solar force.  Their estimate is that,
at 1 AU, .00000453 Newtons per square meter (.000000000656 PSI).

And thats at 1 AU.  Go out to Saturn or interstellar space and see what
kind of push you get.

Russell pointed out that harnessing the massive bursts of energy from
large cosmological events might work.

Mike H.
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2004\10\15@154137 by Howard Winter

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Mike,

On Fri, 15 Oct 2004 14:05:01 -0500, Mike Hord wrote:

>...<
> The above website goes in depth about solar force.  Their estimate is that,
> at 1 AU, .00000453 Newtons per square meter (.000000000656 PSI).
>
> And thats at 1 AU.  Go out to Saturn or interstellar space and see what
> kind of push you get.

Yes, and that's directly away from the Sun.  To travel in any other direction (coming back home, for example)
you'd have to tack like a conventional sailing boat, reducing the effective thrust considerably!

Cheers,


Howard Winter
St.Albans, England


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2004\10\15@155905 by Peter L. Peres

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On Thu, 14 Oct 2004, Mike Hord wrote:

> That implies that laser light could be used as a propulsive force,
> you just need something to push off of.  The question I have is,
> was the "equal and opposite" force pushing the laser itself down,
> or were the photons the ones losing the energy.  In the first case,
> you could "zap" the Earth with a laser to push the spacecraft along
> (seems unlikely), or hit the spacecraft with a laser.  In the second
> case, a large sail with a laser emitter being towed along behind it
> could push the whole works forward.

The trick with the laser and the mirror won't work as you think, but if
you lose the mirror and just use the laser it should work (you can use
the mirror for thrust vectoring).

Producing thrust is really simple: take something with mass and toss it
away from you. Do not look where it is going (you don't really care).
Photons have mass so it works. It works with anything that has mass. The
interesting part is that the efficiency is greatest when the speed of what
you toss is equal and opposite to your vehicle's speed. Iow, using a
rocket to start from standstill or fly at low speed in general is
extremely inefficient.

So if a photon powered spaceship would approach c then its ejecta would
approach standstill, i.e. frozen photons (think of it as extreme
redshift). Then, would they still have mass ? Imho this is an interesting
paradox (knowing that the ship would increase in mass as it would approach
c).

Peter
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2004\10\15@155929 by Peter L. Peres

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part 1 1228 bytes content-type:TEXT/PLAIN; charset=iso-8859-1; format=flowed (decoded quoted-printable)


On Thu, 14 Oct 2004, No Religion wrote:

{Quote hidden}

Yes that works but making photons from scratch is inefficient at present so you use the photons made by someone else (the sun), using a sun sail, which is a large mirror.

> Just wondering..
>
> Thrust is exhausted mass * exhaust velocity, right?

Thrust (force) is mass rate * exhaust velocity. Eg 1 kg thrust = 1 kg mass / second * 1 m/sec ejection speed.

> If so, that should be quite efficient, although it's quite
> hard to imagine what a device one would need to build in
> order to produce so much light. :D

Start with a small sun ;-)

Peter

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2004\10\15@161435 by David Minkler

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I'm not a sailor, so I can't say that I know but ...

Isn't this a possibility in a sailboat only because you have the ability
to take advantage of the boats motion through the water to steer it
somewhat into the wind?  With no water to work against, I can't see how
you could get inbound velocity from the outbound solar wind alone.  Help
me understand if I'm wrong.

Thanks,
Dave

Howard Winter wrote:

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2004\10\15@161842 by Dave VanHorn

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At 03:16 PM 10/15/2004, David Minkler wrote:

>I'm not a sailor, so I can't say that I know but ...
>
>Isn't this a possibility in a sailboat only because you have the ability
>to take advantage of the boats motion through the water to steer it
>somewhat into the wind?  With no water to work against, I can't see how
>you could get inbound velocity from the outbound solar wind alone.  Help
>me understand if I'm wrong.

Let Gravity be your rudder.

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2004\10\15@170749 by David Minkler

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Or, in this case, your only source of inbound acceleration.  Solar wind
doesn't help at all.

Dave

Dave VanHorn wrote:

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2004\10\15@171557 by Dave VanHorn

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At 04:09 PM 10/15/2004, David Minkler wrote:

>Or, in this case, your only source of inbound acceleration.  Solar wind
>doesn't help at all.

Well, it can at a price.
You detach your sail, and let it float outward, while reflecting the sun
back at you in the inward direction. You'll probably need active focusing.
 

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2004\10\15@183608 by David Minkler

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Uh yeah (LOL).  I'll use my sail as reaction mass thank you.

Dave

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2004\10\15@184741 by Dave VanHorn

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At 05:37 PM 10/15/2004, David Minkler wrote:

>Uh yeah (LOL).  I'll use my sail as reaction mass thank you.

Several SF writers (the good kind) have proposed this mechanism, IIRC Bob
Forward was one.

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2004\10\15@194703 by David Minkler

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Which mechanism?

1.  Solar sail, standard configuration.
2.  Solar sail, released and used in reverse
3.  Using the sail as reaction mass

Obviously, the effectiveness of 2 vs.. 3 depends upon the mass to area
ratio of the sail (and its reflectivity).  There may be a sweet spot (or
curve).  A ball of tungsten would probably favor 3.  A sail with a mass
to area ratio of zero would accelerate away from the sun so fast you'd
get no benefit from the reflection.  Somewhere in the middle would
presumably be an optimum.  This optimum would probably depend upon
reflectivity (hence the curve).

Dave

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2004\10\15@201616 by Dave VanHorn

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At 06:48 PM 10/15/2004, David Minkler wrote:

>Which mechanism?
>
>1.  Solar sail, standard configuration.
>2.  Solar sail, released and used in reverse
>3.  Using the sail as reaction mass

#2. IIRC they were illuminating it with a kick ass laser.

>Obviously, the effectiveness of 2 vs.. 3 depends upon the mass to area
>ratio of the sail (and its reflectivity).  There may be a sweet spot (or
>curve).  A ball of tungsten would probably favor 3.  A sail with a mass to
>area ratio of zero would accelerate away from the sun so fast you'd get no
>benefit from the reflection.

Careful, light still moves at C, so the only thing that happens is the
wavelength redshifts downward, twice the shift caused by the recession speed.


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2004\10\16@034541 by No Religion

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At 13.51 2004.10.15 -0400, you wrote:

>On Thu, 14 Oct 2004, Mike Hord wrote:
>
>>That implies that laser light could be used as a propulsive force,
>>you just need something to push off of.  The question I have is,
>>was the "equal and opposite" force pushing the laser itself down,
>>or were the photons the ones losing the energy.  In the first case,
>>you could "zap" the Earth with a laser to push the spacecraft along
>>(seems unlikely), or hit the spacecraft with a laser.  In the second
>>case, a large sail with a laser emitter being towed along behind it
>>could push the whole works forward.
>
>The trick with the laser and the mirror won't work as you think, but if you lose the mirror and just use the laser it should work (you can use the mirror for thrust vectoring).
>
>Producing thrust is really simple: take something with mass and toss it away from you. Do not look where it is going (you don't really care). Photons have mass so it works.

No, photons don't have "mass", if we give to the word "mass" the meaning we are giving it in this context..

Since photons do have energy and E=mc² one can assign to a photon a "relativistic mass", i.e. a mass equivalent to the energy it carries, but photons don't have "mass" is the meaning we're using here. Check this URL for more details: http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html


>It works with anything that has mass. The interesting part is that the efficiency is greatest when the speed of what you toss is equal and opposite to your vehicle's speed.

Ehm.. I'm in dubt of this statement too..


>Iow, using a rocket to start from standstill or fly at low speed in general is extremely inefficient.

That may well be for other reasons..


>So if a photon powered spaceship would approach c then its ejecta would approach standstill, i.e. frozen photons

Albert's relativity theory wouldn't agree with you at all.


>(think of it as extreme redshift). Then, would they still have mass ? Imho this is an interesting paradox (knowing that the ship would increase in mass as it would approach c).

I think there's much confusion and too many assumptions based on imagination here.. but I admit I'm no physician, so.. anybody out there that can really clarify? :D


>Peter
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2004\10\16@053944 by Russell McMahon

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>>It works with anything that has mass. The interesting part is that the
>>efficiency is greatest when the speed of what you toss is equal and
>>opposite to your vehicle's speed.

This is correct for non relatavistic cases. Think of it as a nother example
of the maximum power transfer theorum. Once relativity becomes significant
there are other considerations.

>>Iow, using a rocket to start from standstill or fly at low speed in
>>general is extremely inefficient.

> That may well be for other reasons..

Consider - a rocket has essentially constant thrust (while some can be
varied over a limited range and some very special ones over a wide range
they generally work at approximately constant thrust). Work done = force x
distance. At the moment of liftoff the rocket has most mass, and is moving
at essentially zero speed. Work done is approximately zero!. As speed
increases force x distance per unit time increases and work done on the
rocket increases. Energy input to the motor per unit time is approximately
constant. Energy actually imparted to the rocket increases as its velocity
increases so efficiency rises with velocity. Rockets are not usually an
expecially efficient way to go places. When it comes to going to space they
are, so far, the only way to go places :-(.   The market value of the
potential energy of a kilogram at geosynchronous orbit altitude is under
$US10 per kg! The current cost of putting it there is in excess of $10,000

>>So if a photon powered spaceship would approach c then its ejecta would
>>approach standstill, i.e. frozen photons
>
> Albert's relativity theory wouldn't agree with you at all.

Correct. but a laser could be used as a "light drive" simply by "throwing
photons out the back".



       Russell McMahon

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2004\10\16@152503 by Peter L. Peres

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On Sat, 16 Oct 2004, Russell McMahon wrote:

>> Albert's relativity theory wouldn't agree with you at all.
>
> Correct. but a laser could be used as a "light drive" simply by "throwing
> photons out the back".

Still, what would an observer looking at the ship from behind see ?
Nothing ? Frozen photons ? Bits of mass ? Also if the photons would be
redshifted seriously their energy would be near zero. Wp = h*mu with mu
tending to zero for this case. So *what* is a slow (extremely redshifted)
photon anyway. Radio frequency ? Audio ? Vlf ? DC ?!!!

Peter
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2004\10\16@152511 by Peter L. Peres

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On Fri, 15 Oct 2004, Bill & Pookie wrote:

> I have used "Computer Power" on my skate board.  Just heave the desktop
> while on the skate board and you will travel a little,
>
> Pookie
>
> {Original Message removed}

2004\10\16@152525 by Peter L. Peres

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On Fri, 15 Oct 2004, David Minkler wrote:

> I'm not a sailor, so I can't say that I know but ...
>
> Isn't this a possibility in a sailboat only because you have the ability to
> take advantage of the boats motion through the water to steer it somewhat
> into the wind?  With no water to work against, I can't see how you could get
> inbound velocity from the outbound solar wind alone.  Help me understand if
> I'm wrong.

The wing is released from the ship and starts drifting away from the sun,
slowly, pushed by its radiation pressure. But it's a mirror so it can
reverse the pressure vector and if aimed right this reversed and *focused*
beam can push the ship *towards* the sun. This is all theory, nobody has
done this yet.

Peter
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2004\10\16@153307 by Dave VanHorn

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At 02:14 PM 10/16/2004, Peter L. Peres wrote:


>On Sat, 16 Oct 2004, Russell McMahon wrote:
>
>>>Albert's relativity theory wouldn't agree with you at all.
>>
>>Correct. but a laser could be used as a "light drive" simply by "throwing
>>photons out the back".
>
>Still, what would an observer looking at the ship from behind see ?
>Nothing ? Frozen photons ? Bits of mass ? Also if the photons would be
>redshifted seriously their energy would be near zero. Wp = h*mu with mu
>tending to zero for this case. So *what* is a slow (extremely redshifted)
>photon anyway. Radio frequency ? Audio ? Vlf ? DC ?!!!

As the ship approaches C, the light to the stationary observer would
redshift drastically, but it would still be moving at C, just like the
light from distant receding stars does.
To the observer on the spacecraft, nothing changes, but his time is slowing
down so from our view, each increment of acceleration would take longer and
longer, which matches with the lowering of the energy in the photons. The
last bit of acceleration would take forever.

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2004\10\17@064430 by Jake Anderson

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if you have the time (or are under escape velocity)
couldn't you angle your sail such that it was acting more to slow you down
rather than directly accelerate you away?
I'd imagine that most of the time most solar sailors would be tuned to
increasing their angular velocity rather than trying to sail directly away
from the sun.
if you did try to sail directly away from the sun unless your thrust to
weight was > 1 you would just wind up in a slightly higher orbit, IE the
effect would be similar to slightly reducing gravity.

most schemes I have seen involve putting the sail initially into a highly
elliptical orbit and having it point towards the sun while it is on the
outbound part of the orbit then rotate the sail so it is edge on as it falls
back towards the sun. Rinse Lather and repeat... often.



> {Original Message removed}

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