Re: Gun launched orbital systems

From: Spike Jones (
Date: Sun Oct 21 2001 - 11:43:29 MDT

jeff davis wrote:

> ...wherein we find launch costs estimated at $600/Kg (in
> 1990 dollars) for a 200 kg payload. I
> believe--someone care to confirm?--that this is
> substantially cheaper than than current launch costs
> using conventional, ie multistage rockets, methods.

Yes however there are still substantial problems. In theory
we should be able to find a way to round out the orbit, but
I haven't seen anything really convincing. Mike Lorrey pointed
out that it is possible to harden electronics to 1400G. We
still need to carry some fuel and some means of keeping the
orbitter pointy end first while we burn that orbit adjust
fuel, which would need to be a substantial fraction of the payload,
depending on the angle we left the atmosphere. Let me get
off my lazy ass and spin up a spreadsheet of necessary
fuel wt vs angle of exit from atmosphere. I did this once
about 10 yrs ago and was disappointed by how much
fuel was still needed and how low an angle outbound
was needed.

> It would seem to me that for any payload capable of
> withstanding the rigors of such a launch method--bulk
> payloads of food, water, fuel, and building materials
> immediately come to mind--

Nuclear fuel would be good perhaps, since it is so
dense. Water might be a bit problematic since its
density is low enough that the payload ballistic
coefficient of the shell goes down.

I was once interested in a rail launch where a
human is sent up, submerged in water. I got
into this after putting a number of minnows in
a centrifuge at 100 G for 10 minutes. They all
survived. {8^D

My design for a rail launched
human capsule didn't work out with a reasonable
rail length, unless you can go up the side of one of
the worlds tallest mountains. This can be problematic,
for as soon as you propose it, some silly prole will
argue that mountains are sacred, yakkity yak and
bla bla.

> the economic advantages of
> gun launch present a striking commercial opportunity.

Roger that.

> ...would seem a promising means to that end.

Roger that too.

> So I have to ask, "Where's the problem?. Why hasn't
> anyone done this?"

Its hard. Even after ya know how to do it. {8-]

> Also, sometime back I read that the PV solar panels
> for the Internation Space Station generate 65 kw and
> cost $500,000,000.

Ja, those things are pricey. They are specialty PV
panels, able to withstand launch environment, orbit
environment and still demonstrate 3 nines reliability
after 5 yrs in orbit.

> parabolic dish made of minimum thickness styrofoam
> and coated with a film of appropriate reflective
> material.

Styrofoam wont hold up in earth orbit for even a few
days. Atomic oxygen and UV will eat it up. But I
hear what you are suggesting. The convincing studies
to date have been on mylar balloons which are metallized
by vaporized aluminum on orbit. Then the reflective surface
holds up after the mylar surface is oxydized away. Ill see if
I can find URLs that refer to it.

> For comparison purposes, at 10% conversion efficiency,
> and 1300 watts/sq meter, you can get 65 kw out of a
> dish a mere 13m in radius. (And a dish of 100m radius
> delivers a healthy 4 mega watts.)

The problem with the biggie dish is the conversion at the
focal point. Since we are materials limited to max temperatures
below about 4000K, there might not be any big advantages
to making one huge reflector.

> Two immediate obsevations. Weightlessness would seem
> to allow an EXTREMELY large dish,

Right, it does. So how do we convert the energy once we
concentrate that much of it?

> and the hard vacuum
> of space seems to suggest that the reflective surface
> would not be subject to oxidative degradation.

Only if we get waaaay up there. Unfortunately the
leo environment has atomic oxygen, highly reactive
and at high speed relative to your orbitter. Its pressure
is very low, of course, but there are still jillions of
collisions per square cm per second, and materials
break down after a while. If we can lift the stuff
a couple thousand km, this problem goes away,
and then we merely need to contend with radiation.

> I envision the dish as spin stabilized to keep it
> pointed at the sun,

Sure but you still need to adjust the axis of rotation
a degree a day to account for its orbit about the
sun. {Assuming you have it orbitting in the earth's
neighborhood.} So spin stabilization doesnt solve
everything. But a degree a day isn't that hard to do,
and the propellant required is estimated thus: it takes
as much angular momentum to change a rotating body
pi/2 as it had to start with, so you need to turn 2 pi per
year, so estimate the moment of inertia of the dish and
the necessary rotation rate, multiply to get the angular
momentum and you need 4 times that much per year.

Perhaps that argues against getting *too* carried away
with the size of the dish, since moment of inertia increases
as the 4th power of the diameter of a disk in general.

More later on that, gotta go.

> Maybe I'm just talking out my asteroid, but hey, I'm
> havin' fun.

Me too. Speaking of which, shall we start a Lorrey-esque
subgroup to discuss this, or will the extropians indulge us
by allowing us to post orbit-gun and inflatable dish stuff
on the main list? spike

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