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

[Keith Henson]

Jeff, there is an immense literature on this topic.  Many people have
spent decades of time looking into ballistic launch using cannons,
high acceleration magnetic levitation or rail guns.  It a very well
explored area.  They have led to some very strange devices such as a
cannon that accelerated a huge block of concrete which in turn
compressed hydrogen and used that as the propellant gas.  (There are
sound engineering reasons to do this.)

But the problem is in reaching orbital or escape velocity in some
reasonable distance.  It's hard enough to try to do it on the moon.
Peak power is a problem as well.

v = at

s = 1/2 at^2

For 10,000 m/sec and one G the time required to get up to that speed
is ~1000 seconds, the distance is ~ 5,000km.

10 g, 100 sec, distance 50 km.

20 g 50 seconds distance 12.5 km.

Then you start looking into the power required.  The Ke in joules or
watt-seconds is 1/2mv^2.  If the projectile is 50 t, then the energy
is 2500,000,000,000J and the average power poured poured into the
launch over 50 seconds is 50 GW.  That's 50 large nuclear plants or
about 10 percent of US generation.

It turns out that going faster than sound causes drag problems as
well.  Spike is an expert on that subject and pointy end may not be
the best.  How much velocity you lose going through the atmosphere
depends on where you put the ejection end and the angle you can get.
Vertical and high up on a mountain is best.



On Sat, Feb 28, 2009 at 11:13 AM, Jeff Davis <jrd1415 at gmail.com> wrote:
> 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).  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.
>
> Fast forward, oh, eight or nine years, to the latest reprise of the
> space launch topic, and Paul Fernhout mentions "sky hooks" and a
> "space pier".  I asked Paul offlist about the "space pier", thinking
> he was joking, but no, he wasn't, and he provided me with a link(which
> I have since lost).
>
> On that website, I found a crude drawing depicting a horizontal
> launch/acceleration platform several tens of miles in length held
> aloft some tens of miles up by legs/struts.   Could have been fifty
> miles up, I don't remember exactly, but what I do remember is that
> there was no mention of no mention of "hypersonic acoustic shock" or
> whatever and it wasn't 100, 110, or 150 miles up, you know, up there
> in viable-orbit-land, which is to say thoroughly "out of the
> atmosphere".   There was, however, mention of post-"launch"
> rocket-mediated orbit "widening" to get it out of the upper reaches of
> the atmosphere.
>
> Which got me to thinking again about the gun launch idea.  And so my question.
>
> 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?

I don't understand what you are asking here.  Lowest orbit?

Keith

> Thoughts?
>
> Best, Jeff Davis
>
> "When I am working on a problem I never think about beauty. I only
> think about how to solve the problem. But when I have finished, if the
> solution is not beautiful, I know it is wrong."
>                      - Buckminster Fuller
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