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Magnetoplasmadynamic thrusters have the advantage that they can be
scaled up to produce large amounts of thrust, while still maintaining
the high ISP of ion drives:
Magnetoplasmadynamic Thrusters.
"Testing for these thrusters has demonstrated exhaust velocities of
100,000 meters per second (over 200,000 mph) and thrust levels of 100
Newtons (22.5 pounds) at power levels of 1 megawatt. For perspective,
this exhaust velocity will allow a spacecraft to travel roughly 11
times the top speed of the space shuttle (18,000 mph)."
http://www.nasa.gov/centers/glenn/about/fs22grc.html
MY ELECTRIC ROCKET ENGINE.
http://www.waynesthisandthat.com/mpd.htm
The problem is the high amount of power required. However high
electrical power has been delivered up to hundreds of kilometers on
Earth over power lines. Then this could be used to deliver the
required electrical power to the thrusters from the ground.
Bob Clark
c.f.,
Newsgroups: sci.astro, sci.space.policy, sci.physics
From: "Robert Clark" <rgregorycl...@yahoo.com>
Date: 20 Mar 2006 20:23:18 -0800
Local: Mon, Mar 20 2006 11:23 pm
Subject: Long cables to power arcjet rockets to orbit?
http://groups.google.com/group/sci.phys … a4a33a6d13
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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Power transmission cable typically masses 10-20 kg/meter. Say you can get that down to 1 kg/meter - 300 kms of cable is still going to mass 300 tonnes. That seems like some pretty serious overhead.
Maybe space elevator ribbon research will help out with this issue?
Fan of [url=http://www.red-oasis.com/]Red Oasis[/url]
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And how are these thrusters resupplied with fuel?
[color=darkred]Let's go to Mars and far beyond - triple NASA's budget ![/color] [url=irc://freenode#space] #space channel !! [/url] [url=http://www.youtube.com/user/c1cl0ps] - videos !!![/url]
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The impetus for this was this proposal by Launchpoint Technologies
to launch small satellites by magnetic fields:
Huge 'launch ring' to fling satellites into orbit
http://technology.newscientist.com/article/dn10180
However, there are many difficulties with getting large mass objects
up to orbital velocity with EM fields alone, discussed in this thread on sci.astro:
Subject: Coilguns and EM launchers.
http://groups.google.com/group/sci.spac … c6417ca8a/
And this article describes research dating back from 1977 able to
get a 3 gm object up to about 6000 m/s, and that record still hasn't
been exceeded for larger mass objects:
For Love of a Gun By Carolyn Meinel
First Published July 2007
The tumultuous history of electromagnetic launch.
http://www.spectrum.ieee.org/jul07/5296
If the launch system is to stay on the ground and for low mass
payloads you can just as well use reaction mass methods, i.e, rockets,
at high ISP to get the craft up to orbit velocity at short distances.
You wouldn't need to have hundreds of kilometers of cable extending
into air trailing from the craft. You could have a cable lying on the
ground and a short length of cable extending from the craft to the
cable on the ground, say 10 to 100 meters long. Keep in mind, just as
for the magnetic launch proposal, the main thing is getting that
horizontal velocity component required for orbit. To get to the
altitude for LEO is just a small proportion of extra velocity and
energy of that required for orbital velocity.
Note that for large launch systems such as the space shuttle a large
amount of thrust is needed just to accelerate that huge mass of fuel
that needs to be carried along. But when the exhaust velocity is much
larger than the ending velocity, say 100,000 m/s compared to 8,000 m/s
then by the rocket equation the mass of the fuel will be about the
same small proportion to the mass of the rocket, 8/100. (The exhaust
velocity being 100,000 m/s for this MPD thruster means the ISP,
specific impulse, actually is a quite high 10,000 s.)
The Launchpoint magnetic launch proposal only talked about launching
small satellites, 10 kilograms or so. Only one of the NASA Glenn
magnetoplasmadynamic (MPD) thrusters would be needed to accelerate a
10 kg mass to 1 g. Five of them could accelerate it to 5 g's at 5
MWatts power.
However, I should say key for this proposal is the idea the MPD
thrusters could be made lightweight. From the descriptions of the mode
of operation, essentially only requiring two electrodes, I'm assuming
this is the case. The images of them shown also suggest they would be
small and light weight.
Assuming that it is indeed the case the weight of the thrusters would
stay low when the thrust is scaled up, this might be used to launch
most satellites and also astronaut passengers. Most satellites are
less than around 1,000 kg. A 1 Gwatt power plant assuming power to
thrust scales up could accelerate this at 10 g's. Transportable gas
turbine electric generators at the 100's of megawatts scale can be
bought in the 10's of millions of dollars range. So 1 Gwatt total
would cost in the range of 100's of millions of dollars.
NASA documents give the human endurance level for acceleration
according to duration, as described here:
G tolerance (Dani Eder; Henry Spencer; Jordin Kare; James Oberg)
http://yarchive.net/space/science/g_tolerance.html
At 9 g's it's about 3 minutes for astronauts lying down in
acceleration seats. The formula for speed v attained at an
acceleration a over distance d is v^2 = 2ad. So for v = 8,000 m/s and
a = 10 g's = 100 m/s^2, d is 320 km. They would have to undergo this
for t =v/a = 80 s.
You could have the craft go in a circle at a smaller radius to reduce
the scale of the distance covered by the cable on the ground, but this
would result in a higher acceleration according to the formula a = v^2/
r. For a radial distances of a few km's you get accelerations at the
1,000's of g's scale, which would greatly reduce the payload and make
it impossible for human passengers.
However, for small satellites, a few kilos, it might be easier to use
such small linear or radial distances of just a few kilometers.
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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