Mass Drivers and All That
Keith Lofstrom
Electromagnetic Mass Drivers, Frozen Spinach Launchers, and All That
----------------------------------
1) Mass Drivers Cost Far Too Much
----------------------------------
There are frequent proposals to build electromagnetic launchers based on
coils, heaps of power switching electronics, and so forth, attempting to
beat the high cost of rockets. Proponents point to benchtop experiments
that push sub-kilogram masses to tens of meters per second, and extrapolate
to launch systems that can push multi-ton payloads into orbit.
It isn't that easy.
Power electronics cost can be measured in fractions of a dollar per watt
handled. Perhaps you can posit some super-duper new semiconductor
technology that somehow gets around power handling limits, but in
real life pushing large amounts of power through an electronic switching
device deposits heat in it. If you find a way around this, there is an
existing multi-billion dollar electronic power market just waiting for
you to take over. Miracles being unlikely, assume you are stuck with
what exists now.
Electronic systems are not 100% efficient - 90% is a more likely number.
If you are putting 10 watts into pushing the payload, about 1 watt will
be wasted as heat - mostly in the switching device. The system scales
to the amount of waste heat it has to absorb.
Magic materials aside, what is available for switching power in
semiconductors is silicon, and a number of exotic 3/5 compounds.
Silicon is a mediocre conductor of heat, but the other materials are
worse. A "nearly magic" device would consist of a thin silicon die
firmly welded to a chunk of copper. The thicker the silicon, the
longer it takes to get heat out. Let's assume a very thin but
remotely practical silicon die, perhaps 20 microns thick.
Silicon has a specific heat of 1.6 joules/cm3-C, and a thermal conductivity
of 1.5 watt/cm-C. These two numbers result in a "thermal diffusion
constant" of 1.1 seconds per cm2. A 1 cm thick slab of silicon will
reach thermal equilibrium in around a second, a 1 mm thick slab in
around 10msec, a 20 micron thick slab in about 40 nanoseconds. We
can treat time periods longer than equilibrium time as "steady state".
Mass drivers and such pulse their power through switches. How long are
the pulses? Well, a 1 meter payload nearing the exit end of an orbital
launch mass driver will be moving at about 10,000 meters per second, so
we can assume pulses of around 100 microseconds. Much longer than
40 nanoseconds - effectively, our silicon is in steady state.
The copper slab we are welded to doesn't reach equilibrium. Copper
has a specific heat of 0.8 joules/cm3-C and a thermal conductivity
of 4 watt/cm-C, so it diffuses heat about 5 times faster than silicon.
However, the block of copper is where the heat from the pulse ends up,
so it can be considered to be infinitely thick, and we are instead
worried about heat diffusion into copper. The temperature rise at the
surface of the copper is about 0.6 degrees Celsius times the power
in watts/cm2 and the square root of the time in seconds. Thus, for
a dissipation of 1000 watts per square cm, and a pulse time of 100
microseconds, we get a temperature rise of 6 degrees Celsius.
We can probably stand a 100 degree instantaneous rise on the silicon
surface - higher than that, and things will rapidly degrade. The silicon
device can thus take a pulse proportional to 170 watts/cm2, divided
by the square root of the pulse length. A cm2 of masked, processed
silicon costs about 2 dollars. Thus, for one dollar we can dissipate
around 100 watts * seconds^(1/2). A 100 microsecond pulse allows us to
dissipate 10,000 watts for a dollar. Assuming 10 percent efficiency,
we can deliver 100,000 watts into accelerating a payload, per dollar,
at this pulsewidth.
(Note: this is *incredibly* optimistic - typically, power electronics
costs more like $1/watt. I have a catalog with a microscope illuminator
power supply for $1500 that delivers 12 watts!)
Let's assume our mass driver is built up out of sections of length
LS, an exit velocity of VE, and we drive payloads of mass M with an
acceleration of A. The total length of the mass driver is
LT = VE^2 / (2 * A), and thus there are NT = LT / LS sections. Number
the sections N, from 0 to NT. The payload mass is M.
The velocity at the Nth section is
VN = sqrt( 2 * A * LS * N )
The time pulsewidth at the Nth section is
TN = LS / VN = sqrt ( LS / ( 2 * A * N ) )
The power at the Nth section is
PN = Force * Velocity
= M * A * VN
= M * sqrt( 2 * A^3 * LS * N )
The power dissipation cost of the Nth section is
CN = PN * sqrt( TN ) / ( 100 / 10% )
= M * 2^(1/4) * A^(5/4) * LS^(3/4) * N^(1/4) / 1000
(M in kg , A in m/s2 , LS in m, results in dollars )
The total power dissipation cost is approximately
= M * 2^(1/4) * A^(5/4) * LS^(3/4) * NT^(5/4) / 1250
(M in kg , A in m/s2 , LS in m, results in dollars )
********************************
= M * sqrt( VE^5 / LS ) / 2500
********************************
( VE in meters/second, LS in meters, M in kg, results in dollars )
Note that the cost is independent of total length or acceleration. A
longer system uses less power but more sections.
A benchtop device with an exit velocity of 100 meters/second,
a section length of 0.1 meters, and a payload of 0.1 Kg should have
a silicon cost of about 13 dollars.
Assuming a 1 meter section, a 1000 Kg payload, an exit velocity of 10,000
meters per second, we get a resulting cost of about 4 billion dollars.
This is just silicon cost; everything else is assumed free, including
a power delivery system capable of producing many gigawatts.
Power source cost? The peak power at the muzzle is
Pmax = M * V^3 / 2 * LT. For a 100 Km long accellerator, 1000 Kg,
and 10Km/sec, that's 5 GW worth of power. A shorter accelerator is
correspondingly higher power. That's a *lot* of Die Hards.
---------------------------
A more conservative power cost estimate is based on just the peak watts,
not the pulse width. If one assumes a constant 200 controlled watts per
dollar, the equation turns into
Cost = M * VE^3 /( 600 * LS )
and our two examples become $830 and $830 billion dollars respectively.
---------------------------
Conclusion about mass driver costs:
The electronic cost of an electronically switched mass driver goes up as
something between V^2.5 and V^3, and is proportional to the mass. A
high velocity mass driver is far more expensive than a benchtop toy,
and real driver costs will be far higher than these incredibly optimistic
estimates, due to the cost of coils, structure, heat sinking, and assembly.
Low rep-rate pulsed systems are far more costly than steady state systems.
------------------------------------------------
2) How much "Raw Material" do you need, anyway?
------------------------------------------------
Any surface-based mass driver will have to punch through the atmosphere.
No matter how long and gentle the mass driver itself is, the payload
will undergo rapid decelleration when it hits a wall of air at mach 30+
at ground level. Unless the payload is of asteroidal proportions, the
G forces will destroy most everyday items, such as machinery and people.
Thus, I refer to such payload drivers as "frozen spinach" launchers, as
frozen spinach, unshaped metal, and artillery-grade mechanisms are about
all that will survive the trip.
Our 100Km mass driver produces accelerations of 50 G's (and much larger
deccelerations when the payload leaves vacuum and hits atmosphere) .
For reference, a long range naval cannon produces about 150 G's. Not
too many things are capable of being delivered by artillery. I'm not.
If raw materials were that important, we would all be stinking rich.
My wife and I share a house on a 20 x 30 meter lot. We own the mineral
rights, and thus a cone of rock and magma down to the Earth's core.
That is about 7 billion tons of material which we keep handy to
gravitationally hold down a nice column of air above our heads.
About 30km of that is crust, and the rest is silicon iron. That leaves
us with about 70 million tons of crust. Thus, I own 19 million tons
of silicon, 5 million tons of aluminum, 1.4 million tons of manganese,
300 thousand tons of titanium, etc. If I dug up just the titanium
and sold it at market rates, I would get about $15 billion, (and my
house would now be in a 100 meter deep hole! - I'll buy some fill dirt).
The net energy cost of delivering the raw materials would be
150 billion joules, 42 thousand watt-hours, or about $2500 worth of
electricity, though if I just drop the spill back in the hole rather
than extracting energy from it it will take 45 trillion joules and
almost a million dollars worth of electricity. Still, not bad out
of $15 billion ...
Okay, so the results are ridiculous. Unfortunately, this same sort
of silly thinking goes into the "raw materials in space" argument.
We don't need raw materials in space - there aren't any refineries,
metal shops, semiconductor fabs, or assembly lines up there. Even
liquids and gasses will probably have to be frozen solid to survive
the trip, and require heavy tankage. Not very cost effective.
-------------------------
3) So what's the point?
-------------------------
A space launch system must (1) provide moderate accelerations and
(2) handle power as continuously as possible. A rocket may be silly
and inefficient, but at least it runs for minutes rather than the
microseconds of a mass-driver pulser, and is capable of operating
at moderate accelerations. There are many alternatives to the rocket
that meet the two criteria above; ground-based mass drivers meet
neither. A quest for alternatives is a good thing, but such quests
should be tempered with a healthy skepticism and some detailed
analysis. Positing extraordinary advances in mature technologies
in order to make a bad idea work is a pointless waste of time.
Keith
--
Keith Lofstrom kei...@klic.rain.com Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Power ICs
Henry Spencer
saying, but he's going a little overboard in a few places...)
In article <4bevhi$3...@chip.klic.rain.com> kei...@chip.klic.rain.com (Keith Lofstrom) writes:
>Our 100Km mass driver produces accelerations of 50 G's (and much larger
>deccelerations when the payload leaves vacuum and hits atmosphere) .
>For reference, a long range naval cannon produces about 150 G's. Not
>too many things are capable of being delivered by artillery. I'm not.
Actually, a remarkably large number of things are capable of being
delivered that way, especially if one is allowed to modify the design
slightly for greater robustness. The electronics in a smart artillery
shell look surprisingly ordinary in terms of physical construction; if
you keep the circuit boards small, anchor them solidly, and make sure all
parts are firmly secured to the boards, your electronic device has a high
probability of surviving an artillery launch.
The most fundamental limitation of such launch systems is not the high
acceleration, but the small payloads. The launcher hardware scales almost
entirely with the size of a single payload, and only minimally with the
launch frequency, so it's obvious at first glance that the way to maximize
mass throughput for a given capital investment is lots of small payloads,
as small as practical. (The lower limit for a gun system is probably set
by the requirement of surviving passage through the atmosphere.)
>We don't need raw materials in space - there aren't any refineries,
>metal shops, semiconductor fabs, or assembly lines up there...
Raw materials don't necessarily have to be inputs to sophisticated
manufacturing processes. For example, about half the mass currently
lifted into LEO is rocket fuel.
>Even
>liquids and gasses will probably have to be frozen solid to survive
>the trip...
Uh, why? Liquid-filled artillery shells were made by the millions
80 years ago. No particularly remarkable problems were involved (apart
from those caused by the nasty nature of most of those liquids).
>...and require heavy tankage...
If half the mass goes for tankage, and the launch cost per kilogram is
half that of a rocket, you're still ahead of the game (even if the only
use for the empty tanks is ballast mass to reduce the frequency of reboost
burns for LEO stations).
--
Look, look, see Windows 95. Buy, lemmings, buy! | Henry Spencer
Pay no attention to that cliff ahead... | he...@zoo.toronto.edu
George Herbert
Henry Spencer <he...@zoo.toronto.edu> wrote:
>(Prefatory note: I actually agree with some of the things Keith is
>saying, but he's going a little overboard in a few places...)
>
>In article <4bevhi$3...@chip.klic.rain.com> kei...@chip.klic.rain.com (Keith Lofstrom) writes:
>>deccelerations when the payload leaves vacuum and hits atmosphere) .
>>For reference, a long range naval cannon produces about 150 G's. Not
>>too many things are capable of being delivered by artillery. I'm not.
>
>delivered that way, especially if one is allowed to modify the design
>slightly for greater robustness. The electronics in a smart artillery
>shell look surprisingly ordinary in terms of physical construction; if
>you keep the circuit boards small, anchor them solidly, and make sure all
>parts are firmly secured to the boards, your electronic device has a high
>probability of surviving an artillery launch.
Hmm... 150 G's sounds low.
The 16" cannon are about 40 caliber weapons (I think), which would
be about 53 feet long, or about 17 meters. The MV is about 2400 ft/sec
which is about 720 m/s, so...
s = 1/2 a t^2 v = a * t = 720
17 = 1/2 a t^2 = 1/2 t (at) = 1/2 t (720) ~= 360t
t ~= 0.047 second, 720 = 0.047 * a
a = 15319 m/s^2
That's about 1500 G's not 150. I remember from documents on the
Copperhead CLGP weapon which is fired from on-land 155mm artillery
that it was rated to 100,000G's for the highest stress during firing.
Henry is right, it's easy to make high-G electronics. Worst comes to
worst you pot the whole assembly in epoxy, then it can deal with "a lot"
of G's (>> 100,000) if need be.
-george william herbert
Retro Aerospace
gher...@crl.com
Allen Thomson
>(Prefatory note: I actually agree with some of the things Keith is
>saying, but he's going a little overboard in a few places...)
>
>In article <4bevhi$3...@chip.klic.rain.com> kei...@chip.klic.rain.com (Keith Lofstrom) writes:
>>deccelerations when the payload leaves vacuum and hits atmosphere) .
>>For reference, a long range naval cannon produces about 150 G's. Not
>>too many things are capable of being delivered by artillery. I'm not.
>
>delivered that way, especially if one is allowed to modify the design
>slightly for greater robustness. The electronics in a smart artillery
>shell look surprisingly ordinary in terms of physical construction; if
>you keep the circuit boards small, anchor them solidly, and make sure all
>parts are firmly secured to the boards, your electronic device has a high
>probability of surviving an artillery launch.
[snip]
I was talking about this with some people working on gas guns at
LLNL, and they said electronics, optics, etc. capable of withstanding
30,000 gravities are pretty much state of the art. They thought that
100,000 would be possible with a bit of R&D.
Remember that proximity ( = radar) fuzed antiaircraft shells were used
in WW II, somewhat pre-transitor.
Doug Weathers
In article <4bevhi$3...@chip.klic.rain.com>, kei...@chip.klic.rain.com
(Keith Lofstrom) wrote:
>Magic materials aside, what is available for switching power in
>semiconductors is silicon, and a number of exotic 3/5 compounds.
The distributor in my car switches power around without any electronics.
I bet the efficiency is higher than 90%, too.
This is probably hopelessly naive, but isn't it possible that a mechanical
switching system would work better than an electronic one? I can picture
a long camshaft with the lobes spaced just right to trigger the field
coils in sequence at the right times. Since you don't need
semiconductors, you could use superconductors to help with the power
dissipation problems.
If timing the pulses is a problem, you could use electronic sensors to
close relays.
>Our 100Km mass driver produces accelerations of 50 G's (and much larger
>deccelerations when the payload leaves vacuum and hits atmosphere) .
>For reference, a long range naval cannon produces about 150 G's. Not
>too many things are capable of being delivered by artillery. I'm not.
Nuclear warheads can be delivered by artillery. Nukes are reasonably
complicated devices.
>Okay, so the results are ridiculous. Unfortunately, this same sort
>of silly thinking goes into the "raw materials in space" argument.
>We don't need raw materials in space - there aren't any refineries,
>metal shops, semiconductor fabs, or assembly lines up there.
We sure could use propellant up there! Even if we ship up prebuilt
modules for a space station (so we don't need aluminum delivered onsite)
we could still use water, oxygen, and yes, frozen spinach to eat.
Even
>liquids and gasses will probably have to be frozen solid to survive
>the trip, and require heavy tankage. Not very cost effective.
This doesn't sound at all reasonable. Why would, for example, water be
damaged by high accelerations? "Drat, this shipment of water arrived all
crushed. Now we'll have to put it in the stretcher and pull it back into
shape."
>--
>Keith Lofstrom kei...@klic.rain.com Voice (503)-520-1993
>KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
>Design Contracting in Bipolar and CMOS - Analog, Digital, and Power ICs
--
Doug Weathers, NetWare Administrator | I do not speak | On a clear
weat...@metro.or.gov | for Metro, only | disk, you can
Metro, Portland, OR USA | for myself. | seek forever
Jordin Kare
>Electromagnetic Mass Drivers, Frozen Spinach Launchers, and All That
Hmm, in the line of "what's Baby Oil made from?" if a laser launcher
uses a laser to produce thrust, and an electromagnetic launcher
uses electromagnetic fields, what does a Frozen Spinach launcher use??
>----------------------------------
>1) Mass Drivers Cost Far Too Much
>----------------------------------
>
>Power electronics cost can be measured in fractions of a dollar per watt
>handled. Perhaps you can posit some super-duper new semiconductor
>technology that somehow gets around power handling limits, but in
>real life pushing large amounts of power through an electronic switching
>device deposits heat in it.
This is why most pulsed-power systems do not use conventional
semiconductor
switches. They tend to use spark gaps, hydrogen thyratrons,
or avalanche-mode
semiconductors (in recent years) which have substantially higher peak
power
levels per dollar than conventional semiconductors. They're generally
make-only switches (finding a good "opening switch" is the holy grail of
pulsed power) and don't have high rep rates, but they can readily handle
10^4 to 10^6 watts per dollar.
> If you find a way around this, there is an
>existing multi-billion dollar electronic power market just waiting for
>you to take over.
As noted, the limits on high-power switches make them unsuitable for most
conventional power-electronics applications. If a thyratron has a
life of 100,000 cycles, that's fine for a launcher, but no
good for an electric utility switching things at 60 Hz.
>Miracles being unlikely, assume you are stuck with
>what exists now.
Aww, that takes all the fun out of it :-)
>Electronic systems are not 100% efficient - 90% is a more likely number.
>If you are putting 10 watts into pushing the payload, about 1 watt will
>be wasted as heat - mostly in the switching device.
Actually, that's probably vastly optimistic and pessemistic at the
same time (!). You're unlikely to get anything like 90% overall
efficiency with any propulsion system (although Dani Eder's
rope-and-pulley
system might come close at low velocity :) but much of the waste power
is dumped into bulk losses -- ohmic losses in conductors in a non-
superconducting system; things like eddy current losses into the
support structure in superconducting
systems. Pulsed power switches can have very, very low losses
(e.g., 1 volt drop in switching 10,000 volts at 10,000 A).
>The system scales to the amount of waste heat it has to absorb.
Very true for some systems, notably lasers. Rarely true for pulsed
power systems, which tend to be driven by things like mechanical stress
due to J x B force on conductors and allowable inductance.
>...A "nearly magic" device would consist of a thin silicon die
>firmly welded to a chunk of copper. ...Let's assume a very thin but
>remotely practical silicon die, perhaps 20 microns thick...
[Calculation of diffusion time constant]
>...effectively, our silicon is in steady state.
>
>The copper slab we are welded to doesn't reach equilibrium. ...The
> temperature rise at the
>surface of the copper is about 0.6 degrees Celsius times the power
>in watts/cm2 and the square root of the time in seconds.
Now here you're getting pessemistic, even for your chosen technology.
To give you several possibilities:
Microchannel heat sinks:
Etched-channel laminar-flow heat sinks can transfer heat to
flowing coolant at up to ~10 kW/cm^2 with small (<<100 C) temp. rise;
LLNL uses this approach to cool diode laser arrays. Not optimum for
sub-millisecond pulsed systems, but beats your "nearly magic" device
handily for >10 millisec time scales.
Diamond-copper heat sinks:
Use industrial diamond dust (which is
not very expensive) in a copper matrix to give significantly better
conductivity than solid copper. Howie Davidson is working on this stuff.
Impingement cooling:
An alternative to microchannels; cool the back surface of
the device with a spray of liquid droplets. The heat of
vaporization is much greater than any solid heat capacity, and
it works even for short time scales.
Thus, for one dollar we can dissipate
>around 100 watts * seconds^(1/2). A 100 microsecond pulse allows us to
>dissipate 10,000 watts for a dollar.
Another standard "trick" in pulsed power systems is pulse compression,
in which all-passive components (basically L, C, and diodes)
are used to shorten the duration of a
pulse without active switching. At the
scale needed for a launcher, one might very well do pulse compression
from several-millisecond pulses down to 100 microsec. or shorter.
>
>(Note: this is *incredibly* optimistic - typically, power electronics
>costs more like $1/watt.
But that's valid only for linear systems. Probably every person reading
this is sitting next to a high-precision, high-efficiency power system
that cost less than 50 cents per output watt -- the switching supply in
their computer. True pulsed power systems can be vastly cheaper.
>
>Let's assume our mass driver is built up out of sections of length
>LS, an exit velocity of VE, and we drive payloads of mass M with an
>acceleration of A....
>
> = M * sqrt( VE^5 / LS ) / 2500
> ( VE in meters/second, LS in meters, M in kg, results in dollars )
>
>Note that the cost is independent of total length or acceleration. A
>longer system uses less power but more sections.
That is an interesting result!
>
>Assuming a 1 meter section, a 1000 Kg payload, an exit velocity of 10,000
>meters per second, we get a resulting cost of about 4 billion dollars.
>This is just silicon cost; everything else is assumed free.
This seems high, but not silly. Even EM launcher advocates would
probably price a 10 km/s, 1000 kg launcher up in the $10 billion
neighborhood.
>
> Pmax = M * V^3 / 2 * LT. For a 100 Km long accellerator, 1000
Kg,
>and 10Km/sec, that's 5 GW worth of power. A shorter accelerator is
>correspondingly higher power. That's a *lot* of Die Hards.
At 10 kW per DieHard (they're actually good for >20 kW) it's 500,000 of
them.
At $100 each, that's $50 million. Not bad at all. Next question?
>Conclusion about mass driver costs:
>
>The electronic cost of an electronically switched mass driver goes up as
>something between V^2.5 and V^3, and is proportional to the mass.
No argument. Mostly, this makes clear the value of lowering the exit
velocity as much as possible, perhaps by using an on-board rocket (which
is needed anyway, for circularization).
>A high velocity mass driver is far more expensive than a benchtop toy,
But not necessarily in proportion to your formula; the scaling is unlikely
to have constant cost per watt (or per joule, or any other simple measure)
over that large a range.
>and real driver costs will be far higher than these incredibly optimistic
>estimates, due to the cost of coils, structure, heat sinking, and
assembly.
I disagree with "incredibly optimistic"...
>Low rep-rate pulsed systems are far more costly than steady state
systems.
True only if they have the same average power, and sometimes even then
only
if the pulsed system is designed using steady-state approaches.
Piston engines are cheaper than turbines!
>------------------------------------------------
>2) How much "Raw Material" do you need, anyway?
>------------------------------------------------
>Any surface-based mass driver will have to punch through the atmosphere.
>No matter how long and gentle the mass driver itself is, the payload
>will undergo rapid decelleration when it hits a wall of air at mach 30+
>at ground level. Unless the payload is of asteroidal proportions, the
>G forces will destroy most everyday items, such as machinery and people.
Lessee, F/A = 1/2 rho CD V^2, where CD is the drag coefficient.
rho is about 1 kg/m^3, and V = 10^4 m/s. CD for far hypersonics can
get down to 0.1 or so. So F/A = 5E6 Newtons/m^2. Deceleration
a = F/m = F/A * 1/beta, where beta is mass per unit area.
A 1000 kg projectile with a 40 cm diam. (1/10 m^2) has beta=1E4 and
would decelerate at 500 m/s^2 or about 50 G's. A bit much for people,
even for <1 second. But even 3 or 4 times lower -- say, a 10,000 kg
projectile with a 1/3 m^2 cross section) and you'd have something
people could potentially ride. 50 G's for <1 second is tolerable
for almost anything even modestly ruggedized. No, it's the launcher
acceleration that's the problem, not the deceleration.
>For reference, a long range naval cannon produces about 150 G's.
Much more, as several folks have noted. 150 G's is peanuts. Heck,
-disk drives- are rated for 50 G's non-operating these days (and
before someone yells at me, that is a shock rating, not sustained,
I know....)
>Not
>too many things are capable of being delivered by artillery. I'm not.
Sure you are. You just won't be doing much afterward :-)
>
>If raw materials were that important, we would all be stinking rich....
No argument; there -is- a real problem justifying large-scale space
activity.
>
>-------------------------
>3) So what's the point?
>-------------------------
>
>A space launch system must (1) provide moderate accelerations and
>(2) handle power as continuously as possible. ... There are many
alternatives to the rocket
>that meet the two criteria above; ground-based mass drivers meet
>neither.
So what's the point? Neither of your criteria are requirements, although
both are desirable. And, as far as I know, there are few
serious proposals for pure coilgun/mass driver launchers around.
Electromagnetic catapults for boosting SSTO's, yes. Gas guns,
ram cannons (excuse me, scramaccelerators), even rail guns.
Lunar or in-space mass drivers. But I haven't seen a
ground-to-orbit mass driver design for years.
>A quest for alternatives is a good thing, but such quests
>should be tempered with a healthy skepticism and some detailed
>analysis.
>Keith Lofstrom kei...@klic.rain.com Voice (503)-520-1993
"It is good to keep an open mind, but not one open at both ends."
It's also dangerous to assume that there are not ways around what
appear to be fundamental limitations. Recall countless examples,
including "proof" that rockets could not reach orbit because no
chemical fuel contained enough energy to raise its own mass to
orbital velocity....
Jordin (All Power Corrupts, but we need Electricity) Kare