Re: Gun launched orbital systems

From: Doug Jones (
Date: Sun Oct 21 2001 - 15:03:54 MDT

jeff davis wrote:
> The threads "BIOTERRORISM: Our heads in the sand...",
> and "Humans doomed without space colonies, says
> Hawking" give birth to this one.
> A google search for babylon gun, from the
> "BIOTERRORISM" thread gives
> At
> and
> 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.

Beware of any cost to orbit estimates made by anyone- *none* have ever
survived contact with the real world. Not one.

Bull's superguns would have eliminated only the first stage of a launch
system; the net energy imparted to the projectile would give about 2-3
km/s velocity at apogee, where several stages of solid rockets would
have to fire for the remaining 4-5 km/s needed to get into orbit. These
rocket stages would have to be able to take about +2000 and -50 gee
accelerations, and would need an excellent ablative protection for the
ascent at mach 8 or more. Getting _any_ payload into orbit out of a
2000 kg gun projectile would have been challenging.

First stage boosters are relatively cheap, and can be scaled as needed
to support an upper stage design. With optimal acceleration a "soft"
vehicle can use cheap propellant like LOX/kerosene and high propellant
fraction to reach orbit with just two stages, as opposed to the heavy
structures and low performance of solid motors which would need three
stages *after* the gun launch.

Gerry Bull sold the Iraqis a line of "Bull" that never could have been
an economical space launch system... which was fine by them because they
just wanted to drop warheads on Tel Aviv and Tehran.

> 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--the economic advantages of
> gun launch present a striking commercial opportunity.
> Additionally, and logically, for anyone interested in
> more human activity in space, a dual-mode launch
> strategy, based on the complementarity of gun and
> rocket launch methods, would seem a promising means to
> that end.

Far more promising is a fully reusable small space transport with a high
flight rate- at least 2-3 three sorties per week per vehicle- ideally
daily. Low utilization will make *any* vehicle uneconomical; fly a 747
once a month rather than daily and the fixed costs will eat you alive.

Any payload larger than a human being can be spread across many
launches, habitats can use inflatable structures to give comfortable
room sizes, and routine access to space will drop the cost of labor in
space (the current reason for NOT doing major orbital outfitting).

> Also, sometime back I read that the PV solar panels
> for the Internation Space Station generate 65 kw and
> cost $500,000,000. Now I can't verify the accuracy of
> that number--I no longer remember where I read it--but
> I recall thinking that half a billion dollars for 65
> kw seemed a bit much, and it got me to thinking about
> low-tech power generation in space. Specifically, I
> envisioned, as the basis for such a system, a
> parabolic dish made of minimum thickness styrofoam
> and coated with a film of appropriate reflective
> material.

The high cost is due to several factors- the first being that the panels
like most gov't aerospace products are purchased on a cost-plus
contract. The insidious nature of this process is stunning- if some
clever engineer points out that making the panels twice as big and only
half as efficient will cut their price by ten while doubling the launch
cost, he'll be unemployed in a heartbeat because the contractor profits
more by making the product *more* expensive. If your profit is five
percent of "x", the tendency is to mark up "x" as far as the customer
will take without screaming. This completely perverts any market

Secondary effects, like non recurring engineering costs, the demand for
absurd reliability, and the lack of line-servicability, just help the
contractor inflate the prices.
> 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.)

Solar dynamic systems were proposed way back when, but got lost in the
design reviews in the early 90s, largely due to NIH. By using molten
salt thermal storage at the collector, they could have maintained
constant power output throughout the orbit, without need of massive deep
cycling batteries during eclipse.
> Two immediate obsevations. Weightlessness would seem
> to allow an EXTREMELY large dish, and the hard vacuum
> of space seems to suggest that the reflective surface
> would not be subject to oxidative degradation.

In fairness, the atomic oxygen environment in LEO is pretty nasty- I'd
recommend a good strong anodized coating on any aluminum reflectors.
Weightlessness does not eliminate all forces on a large structure, and
in the case of ISS, the plumes from the space shuttle maneuvering
engines exert large forces on solar arrays- they are turned on edge
during dockings to prevent ripping them loose.
> As to size, anyone care to suggest how big is too big
> and why?

Drag. Stations in LEO must make up drag from the upper atmosphere. On
the other hand, solar dynamic systems do appear to need less area than
photovoltaics, and on the gripping hand excess power could be used to
run an electrodynamic tether to do drag makeup without propellant. Joe
Carroll at Tether Applications built a tether that could have kept Mir
aloft, but it was classed as a munition and was blocked from export by
the State Department in 2000. Yet another example of George Abbey's JSC
mafia calling in favors to block the competition.
> I envision the dish as spin stabilized to keep it
> pointed at the sun, with steam generator, and dual,
> counter-rotating turbine/generator units axially
> mounted, and the waste heat radiator located in the
> shadow behind the dish.

Dual-spin devices are a pain, and if they are counter rotating there is
no net angular momentum to maintain pointing. Spin stabilizing doesn't
work for sun tracking, anyway, since the sun line moves about a degree a
day which would require torquing the spun vehicle. Use three-axis
active stabilization, and arrange the rotating machinery to have zero
net momentum.
> I have a nifty little idea for the design of that
> radiator. A conical 'balloon', perhaps
> stiffened/stabilized with an axial mast, into which
> the exhaust steam is vented. As it condenses, the
> spin of the system will send the droplets toward the
> inner surface of the balloon where the 'slope' will
> cause the liquid to flow back to the rim of the dish
> for reinjection to the steam generator.

Radiators are difficult to make low-mass, efficient, *and* long life,
because micrometeorites will puncture them leading to coolant loss.
Multiple heat pipes with whipple shields are the order of the day, and
you'll still need to replace a fraction every year as they lose their
fluid. Design for maintainability and use cheap manpower to lower
overall system costs.

Stirling or brayton cycle engines are simpler and more reliable than
rankine engines, and don't have phase seperation issues to deal with in
zero gee.
> My engineering background always makes low-tech and
> cheap more appealing than high-tech and expensive.

That's a useful bias, but sometimes it's hard to tell which option
really is low-tech and cheap- the devil is in the details.

Doug Jones, Rocket Plumber
XCOR Aerospace

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