wacho! (Notes on cannon launch)

> i am very discouraged, by the state of the world's capacity to send
> material into low earth orbit. this may be seasonal depression also
> ;')
>
> i have been having pipe dreams about having a private railgun,
> combustion gun, linear motor.
>
> heres my train of thought.
>
> how small a payload and have it still useful?
>
> maximum g forces during launch?
>
> can these low earth sats be able to dock with each other and form a
> complex machine. say a powersat?
>
> ideally have bands of small powersats, beaming energy to a spot below,
> so each ground reciever will be in the path of powersats?
>
> can a small middleclass group of people invest a small amount of
> capital and a lot of sweat eguity in such a life goal?
> how much technical expertise would you need?

I think the minimum useful payload size is pretty small. CanSats
(http://www.cansatcompetition.com/) have a mass of 370 grams. I haven't
done the math, but I think that if you want to minimize launch energy
to a given high altitude (above 100 km) and take into account the
effect of typical electronics form factors on the missile diameter, the
optimum total mass is probably one or two kg. This leaves plenty of
mass for a Gumstix computer (http://www.gumstix.com/), plus some
sensors, photovoltaics, etc.

The maximum gee force depends on the length of the launcher, so you can
spend more money to build a longer launcher and decrease the force.
There is some length that minimizes the total system cost, because, for
all EM launch methods (except quenchguns?), shortening the launcher
proper decreases the efficiency, making the power supply more
expensive. The optimum depends on too many details to make a general
statement, but if you want to estimate the accelerating force, about 10
m is probably the shortest length for launch to orbit, and most
off-the-shelf solid state devices and carefully designed mechanisms
(http://www.edn.com/article/CA341454.html) should survive.

Regarding docking, I don't see a problem with it, although a related
NASA experiment recently failed
(http://www.armscontrolwonk.com/559/thunk-dart-meet-mublcom)

The final question, "can it be done," requires an evaluation of the
available technology. From my vantage point, launcher technology
presents a more fundamental difficulty than docking, beaming power,
etc. As you said, the possibilities are traditional pressure-driven
guns, railguns, or other linear motors. First, conventional guns:
Conventional guns have been able to launch payloads above 100 km since
1918 (see Paris Kanonen by Bull). The stated goal of the SHARP project
was to achieve speeds of 7 km/s, but, as far as I know, no conventional
gun has achieved a speed greater than 2 km/s. This is still pretty
useful, although it would be nice if the speed were higher to prevent
the circularizing rocket motor from eating most of the payload mass.

Next, the railgun. More than one group has demonstrated railgun speeds
of 6 km/s, so this is an excellent start. Standard railguns have
marginally acceptable efficiency at speeds below 3 km/s, and there are
known methods of improving efficiency in this regime. Above 3 km/s,
efficiency is pretty bad. Above 6 km/s, efficiency is zero, due to
fundamental physical reasons that appear insurmountable.

Other linear motors: The mass drivers that O'Neill's group demonstrated
in the seventies produced high accelerations, but were never scaled up
to high speeds by O'Neill. There was a well-funded effort during the
nineties to build a similar launcher at Sandia, but they never exceeded
1 km/s. This motor topology seems like a great idea, but there are
practical problems with switching voltage and timing and who knows what
else. I gather that the failure of this project discouraged funding of
other EM launch research.

Quench guns: Are a nice idea. As far as I know, no one has actually
built one. (Please correct me if you know differently!) The proposal
was to store energy in the magnetic field of a series of
superconducting solenoids. These magnets attract another solenoid,
probably also superconducting. Then (deviating from what the proposers
originally said to what I think they meant) as the accelerated magnet
passes the stationary magnets, it somehow drives a section of each
stationary magnet from superconducting to normal to prevent the
stationary magnets from decelerating the moving magnet after it passes.
In order to drive a superconductor normal, it must go outside of the
current-temperature-field envelope within which it is superconducting.
So there could be a small, high field magnet mounted on the projectile
that passes near a section of the stationary windings and drives that
section normal. (That section could be tailored to be poorer quality
than the rest of the winding. It would be nice if it were away from the
main field.) For best efficiency, the accelerated and stationary
magnets' self and mutual inductances should be chosen so that the
accelerated magnet drives the current in the stationary magnet to zero
as it passes. However, if the current is zero when the magnet that's
intended to cause the superconducting-normal transition passes, the
section will return to the superconducting state as soon as the applied
field passes. So there has to be enough current flowing in the
stationary magnet to heat the normal section to a temperature
sufficient to prevent the return to superconducting until the
accelerated magnet is out of range. (Wow, that was a convoluted
explanation, but I doubt if it can be done succinctly without
additional specialized jargon.)

It would be tricky to match the critical field values properly to make
a quench gun work; the resistance of a typical normal superconductor is
pretty low, which means there will be some deceleration; and I'm having
a hard time imagining a synchronization mechanism based on heating
functioning properly. Still, when I get a chance, I want to look at the
idea again and do some rough calculations...

The closest thing to a quench gun that has actually been built is an
inductively-commutated coilgun, built by a group at the University of
Texas. It used normal conductors, and I don't know of any specific
problems that it had. For some reason it is no longer being pursued. As
far as I know it never exceeded 1 km/s.

The development of small, high field superconducting magnets makes
other topologies possible. I'm doing research on one of these. I can't
predict its maximum speed. The plan is to gradually increase the speed
while measuring energy conversion efficiency. If there's an unpredicted
decrease, as was the case for railguns, then I'll begin a study of the
responsible mechanism. In the process, I expect to demonstrate launch
of payloads above 100 km, which should attract more funding.

I think the best course of action is to continue searching for a launch
method that doesn't have the wear, efficiency, and speed problems of
railguns. If one can be found, the cost and effort of constructing a
system will be closer to the reach of small groups.

I encourage interested people to work on novel linear motor topologies.
There are many possibilities. It wouldn't hurt to kick in some money
that will be redistributed to groups that have the most promising ideas
- The Lifeboat Foundation is accepting donations for this purpose
(http://lifeboat.com/ex/grant1).

Oh, uh, one more URL:
http://www.urbandictionary.com/define.php?term=wacho

Best regards,

Phil