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Alternative to Rockets



 
 
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  #21  
Old March 12th 04, 01:32 PM
Mike Miller
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Default Alternative to Rockets

"Roger Stokes" wrote in message ...

As was pointed out in an earlier post to this web site, well designed and
packed "cargo" can withstand hundreds to thousands of gees acceleration. A
1000G electromagnetic launcher would only


I'm well aware of the ability for cargo to survive 1000G (and higher)
launches. I was specifically addressing passengers in cannon-type
launch systems.

Mike Miller, Materials Engineer
  #22  
Old March 12th 04, 03:08 PM
william mook
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"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


I computed a table relating power to thrust, and was off by a factor
of g0 - that's the acceleration of gravity. In metric units this is
9.82 m/s/s. So the table posted parallel to this was WATTS PER
NEWTON, not WATTS PER KG as stated in the text.

Okay so the revised table for WATTS PER KG OF THRUST is;

HOW BIG IS YOUR ENGINE? IN WATTS



Auto Airline Supertanker
Type Ve W/kg HP/lb 1000 300000 500000000
Chemical5.6 27496 16.76 27.4E+6 8.25E+9 1.3748E+13
Fission 42000 206E+6 125702 2.1E+11 6.2E+13 1.0311E+17
Fusion 134000 657E+6 401050 6.6E+11 2.0E+14 3.2897E+17
Photon 300000 1.47E+9 897875 1.5E+12 4.4E+14 7.365E+17

Other interesting Numbers:

Average US Home: 833
200 HP Engine: 143,000
US Utility Grid: 800E+9
Humanity (all forms)4E+12
Solar Output: 3.8E+26

If humanity were space faring they'd have say, four billion fusion
rocket fliers the size of autos, along with say 4 million airliner
sized vehicles and 40,000 supertanker size freighters. At any one
time a quarter of the autosized vehicles and airliner sized vehicles
might be operating, three-quarters of the big guys would be operating,
so humanity's power consumption if spacefaring would be;

AUTO - FUSION - 1E9 * 6.6E11 = 6.6E20
AIRCRAFT - FUSION - 1E6 * 2.0E14 = 2.0E20
SUPERTANK- PHOTON - 3E4 * 7.4E17 = 2.2E22
INTERNAL USE - industrial use 2.0E13

Which all adds up to 2.2E22 watts of usage - about 1/17,000th the
sun's total output

At a 7% economic growth rate we could double usage every 10 years.
This lets us compute that it would take 323 years for humanity to
reach this level of consumption.

Of course, widespread use of laser beamed energy and nuclear fusion
power (once its available) would be a quantum leap that would not be
part and parcel of the natural growth of the existing energy
infrastructure. In that case, we might expect a doubling every year
rather than every ten years. Just as low cost computing caused a
doubling of computer use. That would mean we could transition to a
space faring species with a spaceship in every garage in a single
generation.

Sweet.
  #23  
Old March 12th 04, 04:35 PM
nafod40
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Mike Miller wrote:
2) Mass drivers/railguns/coilgun launchers. Unfortunately, the big
electromagnetic launchers that can fling a spaceship into orbit
without turning passengers to goo is really, really long, like 600-700
miles for a 3G launch.


Not that it would be a "make or break" factor, but they did centrifuge
tests way back where the subjects were immersed in water, then spun up.
The water pressure outside the body opposed the internal pressures, and
the subjects/victims were able to carry on conversations at 12+ G's.

I've thought that if you need to carry water on missions, for shielding
and all of the other reasons, might as well put it to work for you.

Just did a quick google...the Swiss have a suit called the Libelle that
is based on the principle, lets you chit chat at 12 G's sustained. It
would be higher laying back in a couch.

  #24  
Old March 12th 04, 07:14 PM
tom perkins
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2) Mass drivers/railguns/coilgun launchers. Unfortunately, the big
electromagnetic launchers that can fling a spaceship into orbit
without turning passengers to goo is really, really long, like 600-700
miles for a 3G launch.

Mike Miller, Materials Engineer


AFAIK, if a human being is suspended in a fluid (easily deformable
medium) which has the same specific gravity as the mean specific
gravity of human tissue, then g-loadings on face down prone
individuals can be very high for periods of several seconds and even
minutes before significant short term ill effects are seen. I cannot
recall the source, but believe the Air Force looked at "wet" cockpits
to permit pilots to sustain maneuvers of many tens of g's, and
determined that while the costs outweighed the benefits, no plane
could be built which would overstress the pilot during the duration of
the ACM that might be required.

Perhaps 100g's for very short times would be tolerable. Certainly
blood pooling of short durations is an acceptable risk for many when
the destination is considered.

Can anyone say how seconds a human being so supported can tolerate
being accelerated at 10, 100, and (doubtfully at all) 1000gs.

If I remember it ok, 10 seconds is little trouble indefinitely, 100 is
barely doable for tens of seconds, and 1000 is out of the question
(bones falling through soft tissue, etc.).

Thank you, Tom Perkins
  #25  
Old March 12th 04, 09:36 PM
Iain McClatchie
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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.
  #26  
Old March 13th 04, 01:03 AM
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In article , Olli Wilkman writes:
On Mon, 08 Mar 2004 16:30:28 GMT, "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


This isn't exactly in response to the question, but what is the
current opinion on laser rockets? The idea is fairly simple, but I get
the impression that currently it is not a very strong candidate.


It is excellent grantmanship. Not much else.

The latest news I found (and that was from November 2000) is that
they've lifted a 51g craft to an altitude of 71 meters using a 10kW
laser.
"The 51 g, 12 cm diameter Lightcraft is propelled skyward when the
laser beam hits a parabolic condensing reflector on its underside.
This ablates a thin plastic coating, sending the craft upwards."
(http://optics.org/articles/news/6/11/9/1)
This thing is developed by a company called Lightcraft Technologies,
Inc. (http://www.lightcrafttechnologies.com/news.html) as a method of
launching micro-satellites. I wonder how they are currently doing -
the "Latest Developements" section of their website is last updated in
Dec 2000.

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, but the news
article mentions a platic coating.


If all you rely on is the air that's being heated up in the mirror's
focus, there is precious little of that. If you introduce some solid
to be ablated, this serves as propellant. Again, there is pretty
little of this in the focal spot.



Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
  #27  
Old March 13th 04, 02:09 AM
Sander Vesik
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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.

--
Sander

+++ Out of cheese error +++
  #28  
Old March 13th 04, 02:48 AM
Gregory L. Hansen
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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
  #29  
Old March 13th 04, 02:58 AM
Gregory L. Hansen
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Default Alternative to Rockets

In article ,
nafod40 wrote:
Mike Miller wrote:
2) Mass drivers/railguns/coilgun launchers. Unfortunately, the big
electromagnetic launchers that can fling a spaceship into orbit
without turning passengers to goo is really, really long, like 600-700
miles for a 3G launch.


Not that it would be a "make or break" factor, but they did centrifuge
tests way back where the subjects were immersed in water, then spun up.
The water pressure outside the body opposed the internal pressures, and
the subjects/victims were able to carry on conversations at 12+ G's.

I've thought that if you need to carry water on missions, for shielding
and all of the other reasons, might as well put it to work for you.

Just did a quick google...the Swiss have a suit called the Libelle that
is based on the principle, lets you chit chat at 12 G's sustained. It
would be higher laying back in a couch.



Cool! If they're chit chatting and, well, breathing, I assume the head
doesn't need additional protection at 12 G's?

I think the standard flight suit applies regular pressure on the legs and
torso, but as I recall a pilot can only go to about 8 G's before blacking
out in a turn. What's different about that?


--
"Things should be made as simple as possible -- but no simpler."
-- Albert Einstein
  #30  
Old March 13th 04, 07:46 AM
Phil Karn
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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
 




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