A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Space Science » Technology
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

Alternative to Rockets



 
 
Thread Tools Display Modes
  #11  
Old March 11th 04, 06:19 PM
Mark Foskey
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

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.


  #12  
Old March 11th 04, 08:59 PM
external usenet poster
 
Posts: n/a
Default 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.
  #13  
Old March 11th 04, 09:03 PM
Roger Stokes
external usenet poster
 
Posts: n/a
Default Alternative to Rockets


"Mike Miller" wrote in message
om...
"George Kinley" wrote in message

...

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.


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
be about 3 miles long. Also I seem to recall talk of a "supergun". The
astronauts could go by a small cheap rocket - I think 5 astronauts plus
short-term life support mass about one ton.

  #14  
Old March 11th 04, 10:41 PM
Igor
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

"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



Only if you can find a way around Newton's third law of motion.
  #15  
Old March 11th 04, 11:31 PM
Greg
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

"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


No. All the laws of physics forbid a reactionless system, ie there is
*always* some reaction mass. But there are reaction based propulsion
that give the same kinda results. Solar sails are my choice and we
don't need nano anything to make useful spacecraft.

Greg
  #16  
Old March 12th 04, 01:11 AM
Olli Wilkman
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

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.

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 it does need to carry some kind
of reaction mass, some easily produced and non-polluting substance
would be necessary (ice, if it was practical, would probably be superb
in terms of cost and ease of production).

Laser rockets have a kind of simplicity that appeals to me, but I lack
the knowledge of rocket or laser technology to form any concrete
opinion about their practicality.

--
Olli Wilkman
  #18  
Old March 12th 04, 04:13 AM
william mook
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

"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


No. What's wrong with throwing mass?

Check it out;

for non relativistic speeds;


F = mdot * Ve

where mdot = kg/sec = mass flow rate
Ve = m/sec = exhaust velocity (Isp*g0 = Ve)
F = N = force (in Newtons, N/g0 = kg, g0=9.82 m/s/s)

and the speed you can achieve is;

Vf = Ve * LN(1/(1-u))

where Vf = m/sec = final velocity
Ve = m/sec = exhaust velocity (Isp*g0 = Ve)
LN(...) = natural logarithm function
u = propellant fraction

the propellant fraction is a number between zero and 1. Its the
amount of propellant contained in the vehicle divided by the full up
vehicle weight.

Vf = a * t

Vf = m/sec = final velocity
a = m/sec/sec = vehicle acceleration
t = sec = time

D = 1/2 * a * t^2

D = m = distance
a = m/sec/sec = vehicle acceleration
t = sec = time

E = 1/2 * m * Ve^2

E = joules = total energy in the propellant mass
m = kg = mass of propellant
Ve = m/sec = exhaust velocity

P = 1/2 * mdot * Ve^2

P = watts = power of rocket engine
mdot = kg/sec = mass flow rate
Ve = m/sec = exhaust velocity

Now here are some interesting things to know;

ENERGY - this gives you Ve

CHEMICAL ROCKET: Hydrogen/Oxygen - 15.7 MJ/kg -- 5.6 km/sec
FISSION ROCKET: Pu -- Ac - 900 million MJ/kg -- 42,000 km/sec
FUSION ROCKET: 4H2 -- 2He - 9 billion MJ/kg -- 134,000 km/sec
ANTIMATTER ROCKET: Matter -- energy - 90 billion MJ/kg -- 300,000
km/s
LASER LIGHT SAIL: Reflect energy - limitless - 300,000 km/sec

Now, to achieve various missions requires that we achieve various
speeds. Here are some important ones;

This gives you Vf

Minimums
Earth Orbit: 7 km/sec
Earth Escape: 11 km/sec
Lunar Mission: 22 km/sec
Mars Mission: 30 km/sec
Solar System: 50 km/sec

Cool Interplanetary
1/10th gee constant - inner solar system - 150 km/sec
1 gee constant - inner solar system - 500 km/sec
1/10th gee constant - outer solar system - 1,000 km/sec
1 gee constant - outer solar system - 3,000 km/sec

Interstellar
Minimum - 15,000 km/sec - alpha centauri in 100 years
Reasonable - 90,000 km/sec - alpha centauri in 18 years
Cool - 180,000 km/sec - alpha centauri in 10 years

NOTE: 1 gee for one year equals the speed of light (300,000 km/sec)

So, using these formulae and figures we can see some interesting
things

How big is your gas tank;


Vf Chemic Fission Fusion Photon Laser
Ve 5.6 42000 134000 300000 infinity
MINIMUMS
Earth Orbit 7 0.71 0.00 0.00 0.00 0.00
Earth Escape 11 0.86 0.00 0.00 0.00 0.00
Lunar Mission 22 0.98 0.00 0.00 0.00 0.00
Mars Mission 30 1.00 0.00 0.00 0.00 0.00
Solar System 50 1.00 0.00 0.00 0.00 0.00
Cool Interplanetary
1/10th gee inner 150 1.00 0.00 0.00 0.00 0.00
1/10th gee outer 500 1.00 0.01 0.00 0.00 0.00
1 gee inner 1000 1.00 0.02 0.01 0.00 0.00
1 gee outer 3000 1.00 0.07 0.02 0.01 0.00
Interstellar Flyby
Minimum 1/20th c 15000 1.00 0.30 0.11 0.05 0.00
Reasonable 1/3 c 90000 1.00 0.88 0.49 0.26 0.00
Cool 2/3 c 180000 1.00 0.99 0.74 0.45 0.00
Interstellar One Way
Minimum 1/20th c 30000 1.00 0.51 0.20 0.10 0.00
Reasonable 1/3 c 180000 1.00 0.99 0.74 0.45 0.00
Cool 2/3 c 360000 1.00 1.00 0.93 0.70 0.00
Interstellar Round Trip
Minimum 1/20th c 60000 1.00 0.76 0.36 0.18 0.00
Reasonable 1/3 c 360000 1.00 1.00 0.93 0.70 0.00
Cool 2/3 c 720000 1.00 1.00 1.00 0.91 0.00


When the propellant fraction given in the table above exceeds 0.95,
then the rocket is generally impossible since you need 0.05 structural
fraction to build it. When the propellant fraction in the table above
falls below 0.05 then the propellant fraction approaches that of a
typical automobile.

Another interesting table is how powerful is your engine?

VEHICLE SIZE
Auto Airline Supertanker
Type Ve W/kg HP/lb 1000kg 3E+5kg 500E+6
Chemical 5.6 2800 1.71 2.8E+6 840E+6 1.4E+12
Fission 42000 21E+6 12800 21E+9 6.3E+12 1.05E+16
Fusion 134000 67E+6 40840 67E+9 2.01E+13 3.35E+16
Photon 300000 150E+6 91433 1.5E+11 4.5E+13 7.5E+16

So, the interesting thing is that a chemical rocket needs 2.8 MWatts
to move an automobile sized rocket around but to apply the same forces
with a fusion rocket requires over 30,000 times as much energy!

To compare these usage rates (in Watts here - there are 745.7 watts
per horsepower if you wish to convert them) to typical rates here are
a few interesting numbers;

US Household - 833 watts
200 hp engine - 149kW - 149e+3
Total US utlity net - 800GW - 8e+11
Total humanity - 4TW - 4e+12

So, a photon rocket adapted for use in a home automobile hovering in
place would consume as much energy as about 1 billion people! Such a
rocket would have to be pretty damn efficient not to be dangerous.
One way to achieve that - since we're dreaming and not building it
right now - is to use something that transfers momentum and nothing
else - something like neutrinos for exhaust, or maybe a graviton
rocket - which are both versions of a photon rocket since both
gravitons and neutrinos are massless.

Industrial humanity with all of its fuel fired technology when
compared to a truly space faring species would seem as a tribe huddled
around a rather small campfire in the wilderness. Our fighting over
oil and such would be akin to people huddled around that fire arguing
over this or that ember in the campfire.

Laser sustained rockets and laser light sails are an interesting
innovation as are microrockets. Micro nuclear rockets might also be
interesting. Or solar pumped lasers in space (the sun is a fusion
reactor) driving laser rockets and laser sails around would also be
interesting.

http://clifton.mech.northwestern.edu...crorockets.gif
http://www.grc.nasa.gov/WWW/RT1998/i...schneider2.jpg

Basically, you have modified inkjet printheads so that they blow
rocket propellants through shaped holes rather than ink onto paper.
With this technology you can create utterly reliable, utterly safe,
quiet, highly controllable propulsive skins for vehicles.

With a laser powered, or nuclear fusion powered, microrocket skinned
spaceship you could hop in a vehicle and fly to mars in a few days,
while feeling 1 gee acceleration all the way. You might meet up and
dock with larger hotel like vehicles to while away your time as you
traverse the expanse between worlds.

In short, with the right rocket technology, interplanetary space is as
accessible to humanity as ocean travel by ship makes the continents
available. You can cross the solar system in a week at 1 gee. That's
not too shabby.

It takes a lot of power to sustain this lifestyle. But, there is lots
of power in the sun, and there are lots of nuclear fuels available in
the gas giants. So, what are we waiting for?

Is the squabbling over dwindling resources here really stopping us
from the greatest adventure, and greatest advance of all time? Yes,
yes it really is.

And that's a damn shame - and a failure of vision of those who believe
they lead us.

Phht!
  #19  
Old March 12th 04, 05:52 AM
Jim McCauley
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

One interesting alternative might be what could be called a "recursive
trebuchet": essentially, a highly damped and well-controlled whip. On the
surface of a planet, anchor a catapult with a very short moment (swing),
perhaps less than one degree. On top of that, put a smaller catapult, and
another on that, and so on.

If kept static, such a structure could not exceed 10-12 km in height because
of limitations of structural strength, but if it were kept in continuous
motion, the sum of centrifugal forces balanced against the pull of gravity
might allow something of considerable length and enormous flexibility and
strength. If it could be made long enough to allow its tip to describe an
arc of, say, 1000 km, acceleration of the tip to orbital velocity might be
kept down to the vicinity of three gravities.

This would be a very large structure, and its mass would probably allow the
launching of enormous projectiles. Think of capsules the size of cruise
ships being plucked from the ocean and flung into space; that will give you
a notion of the scale.


Jim McCauley

  #20  
Old March 12th 04, 07:15 AM
Mark Folsom
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

"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


Throwing planets often helps a lot.

Mark Folsom


 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Alternative to Rockets George Kinley Science 53 March 31st 04 02:45 AM
Pressure fed versus pump fed rockets Larry Gales Technology 16 November 20th 03 12:18 AM
Rockets not carrying fuel. Robert Clark Technology 3 August 7th 03 01:22 PM
"Why I won't invest in rockets for space tourism ... yet" RAILROAD SPIKE Space Station 0 July 30th 03 12:06 AM
Rockets George Kinley Technology 37 July 17th 03 01:18 AM


All times are GMT +1. The time now is 09:20 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.