[ExI] question re "honkin' big cannon" space launch

spike spike66 at att.net
Thu Mar 5 21:53:21 UTC 2009


> -----Original Message-----
> From: extropy-chat-bounces at lists.extropy.org 
> [mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of 
> Jeff Davis
> Sent: Saturday, February 28, 2009 10:14 AM
> To: ExI chat list
> Subject: [ExI] question re "honkin' big cannon" space launch
> Sometime back I was chatting with spike about various nerd-boy (or
> girl) topics, when I touched on the ancient(Verne/Wells?) 
> idea of space launch by cannon.  Spike, (in a manner far too 
> conventional and dismissive for one of his imagination and 
> flagrant sense of humor) naysayed the notion, mentioning some 
> problem (hypersonic acoustic shock, or some such 
> techno-jargonaut obscurantism)...

Jeff I just returned from mopping up the remains of my family's failed
agriculture venture, oy.  We went bust on it, but fortunately we stopped it
before it busted us.  {8-|  And of course I still have my 9 to 5 which I use
to support my farming habit.

Your post brings back fond memories from the 1980s when I was doing a bunch
of these calculations.  I still have those calcs somewhere, for I did my
BOTECs in bound notebooks and kept them all.  I have some that go back to my
college days, so tragically many years ago.  

>  I was not impressed.  I 
> figured if the nose cone was pointy enough, then the 
> acceleration of the surrounding air molecules could be kept 
> low enough to get around those difficulties to which spike 
> and his overqualified "sources" were referring...

At the time I mighta mentioned something about hitting max Q (maximum
dynamic pressure) at the exit from the barrel.  In that case it doesn't much
matter how pointy is the nose, for friction with the atmosphere at those
velocities will change that shape immediately.  

After I started working the equations, I found that the projectile actually
hits max Q before it reaches the end of the tube, since the tube has air in
it.  But I had an idea for that.  The notion is to put a cap on the end of
the tube, then evacuate the tube to about .01 atm, then open the cap just
before the projectile gets there.

At the time (around 1985 I think because it predated my first mac, and I was
doing the calculations on a creaky Commodore with an ancient spreadsheet
program) I had a scheme to accelerate air in the barrel ahead of the
projectile, hoping to get a couple thousand more meters before reaching max
Q.  The notion is that the wind blowing ahead of the projectile forms a
transition to full shock wave.  That was in the days before the web, so
information was much harder to find back then.  I claim independently
thinking of the idea of putting charges ahead of the projectile to push a
column of air forward, altho I imagine plenty of amateur rocket scientists
have thought of the notion before I did and since then.

My model at the time was to find a really high mountain, such as Everest,
drill a long straight hole about a meter in diameter and about 10 km long,
line it with polished steel for a barrel.  The projectile would be about ten
meters long, a meter in diameter.  There would be intermediate charges along
the barrel to keep the pressure high behind the projectile.  The barrel
would be angled about 45 degrees from horizontal, although a lower angle is
better if one can work out a way to do it.

As I recall, the atmospheric losses going out of the atmosphere required
reaching a muzzle velocity of about 20 km/sec, so the time in the tube was
about 1 second, acceleration about 2000 G, mass of the projectile was about
100,000 kg, (it was about 7 cubic meters of alternating layers of enriched
uranium and steel, the steel preventing the uranium from forming a critical
mass) so the pressure on the back of the projectile needed to be about 3
billion newtons per square meter or 30,000 atmospheres.

So at the time I realized it is better if we could drill a much longer tube,
but that introduces more complications, since really high mountains often
have magma below them.  Then there is the problem of the yahoos wanting to
climb the tallest mountains, and other yahoos thinking the tallest mountains
are sacred, that kinda stuff.  These kinds of people would need to be
eliminated by some means, or at least the climbing/worshipping memes.  Or we
could find a really tall mountain that cannot be climbed, and one that
everyone considers secular.

I looked at hauling the cannon to really high altitudes with balloons.  That
makes some of the problems simpler, such as reducing the delta vee needed to
round out the orbit after the projectile leaves the barrel.  But I found it
requires one to haul the thing waaaay high up to do much good, which is
difficult, and it introduces new difficulties, such as aiming and
stabilizing the barrel that is suspended from ballons.  Not impossible, but
challenging.  Of course one can also hoist conventional rockets with
balloons, and save a lotta payload by reducing the weight and strength of
the payload fairing, and by enabling higher expansion ratio nozzles on the
rocket motors, so they are more efficient.

Keith has some really interesting notions of using orbiting lasers with
ablative propellant to round out the orbits of projectiles.

So if one decides to use balloons, as I recall, the rockets ended up scoring
better than the big cannon.  So the cannon is back down on the ground, and
the challenge is seeing how deep the hole can be made.  Any reasonable hole
requires very high accelerations, so only very robust payloads need apply.
The debate back in the 80s caused me to wonder if biology units could be
launched if submerged in water.  I put a bunch of fish in a thermos bottle
and put that in the centrifuge, ran them up to 100 g for ten minutes.  All
survived.  But I concluded they wouldn't survive 2000 g even for a second,
because slight differential in tissue density would be harmful or extremely

Turns out, big ground based rockets scored better than the balloon-hoisted
rockets, which explains why we keep enduring all the disadvantages of
rockets, while we still have not seen balloon-carried rockets, balloon
carried cannons or ground-based cannons.

> Let's get specific.  What are the limitations -- boundaries 
> if you will -- of the gun launch -- or similar 
> "emerges-from-launch-tube-at-orbital-or-greater velocity" 
> launch schemes?  And what features -- such as the pointy nose 
> cone --  can
> help?   In particular, what's the altitude lower bound?  Yeah, that's
> what I want to know: lowest possible altitude?
> Thoughts?
> Best, Jeff Davis

Jeff, I don't know if the previous was helpful, and it is going strictly on
old memories.  Today is my 20 year service date with Lockheeed, and I know I
didn't do much more with these notions after I joined this company, so the
23+ year old memories could be inaccurate.

Do let me see if I can find those old BOTECs, although I suspect they may
have been moved when we were setting up a room for the arrival of Isaac in
2006.  Good chance my old notebooks are in a storage container up at the
failed agriculture business in Oregon.


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