Peter Fairbrother wrote in message ...
Andrew Higgins wrote

You can prove on gasdynamics grounds that a powder
gun can *never* exceed 3 km/s muzzle velocity.

The "gas speed" limit in a "gun" is not the speed of sound
in the gas, assuming no projectile.


Correct: maximum velocity is *not* equal sound speed in propellant
gas, but they *are* directly related.

In fact, as can be shown from the Method of Characteristics for
unsteady gasdynamics, the maximum velocity of a gas expansion is:

Vmax = 2 a0/(g-1)

....where "g" is the ratio of specific heats and a0 is the *initial*
speed of sound of the propellant. For g = 1.4, for example, Vmax = 5
a0. This is for a complete expansion to vacuum, only realizable with
a zero mass projectile.

In practice, actual guns get nowhere near this limit. The base
pressure drops off exponentially as the projectile accelerates,
approximately as:

p/p0 ~ exp(-g * V/a0)

....where p0 is the initial pressure (before projectile started
moving). Once the projectile has reached twice a0, the base pressure
is only about 5% its initial value, and guns can rarely accelerate
projectiles any faster than this.

For powder guns, the gaseous propellant is the combustion products of
the powder, which typically have an initial sound speed around 1 km/s.
Hence, the speed limit of about 2 km/s for powder guns. I am very
skeptical of claims of a 3+ km/s powder gun.

Experimental railguns have already achieved 30,000+ fps,
well over orbital velocity,


References, please. I am not aware of any railguns
exceeding 6 km/s.

The 30,000 fps plus figure came from a USN press report, with no details. I
think it slipped out. I don't know if it's right, but for sure some research
and development on railguns is classified. The same is even more true of
travelling charge guns (which, btw, the 5 kps gun is not).

Traveling charge guns, as far as I am aware, have *never* demonstrated
a boost in muzzle velocity over conventional, breech-fed guns. The
last review I've seen was:

Baer, P.G., and I.W. May, "Traveling-Charge Effect" in "Gun
Propulsion Technology" ed. L. Stiefel, Vol 109, AIAA Progress in
Astronautics and Aeronautics, 1988, pp. 499-536.

....and I am not aware of any new experimental results since then.


However, these guys reportedly got 7.1 km/s:
http://www.ioffe.rssi.ru/PAPERS/94w06.pdf


Thanks for the reference; this claim, if true, would be a new velocity
record for a rail gun.

However, I would need to see more data from the actual test before
being convinced; some claims at record muzzle velocities from rail
guns have actually been jets of plasma squirting from the end of the
rails, not coherent projectiles.


I have the impression that considerable unannounced progress has been made.
This looks interesting:
http://library.kmitnb.ac.th/df/ieee40/370192.pdf


I think "Promising but unconfirmed theoretical study" is more
appropriate description of this paper, rather than "considerable
unannounced progress."


By the way, does anyone know anything about an "electric gun" at LLNL? Not a
railgun, I'd guess, they were talking about 15-18 kps?


I am not aware of a 15-18 km/s gun at LLNL, although certain there is
interest there (an elsewhere) in such a device.

You may be thinking of two different, recent developments:

One is the use of the Sandia Z machine (basically, a very, very large
pulsed power supply) to launch thin metal foils to velocities of 15-20
km/s via magnetic loading:

http://www.sandia.gov/media/NewsRel/NR2001/flyer.htm

The other is a 3-stage gun at Sandia that has also accelerated small
flyer plates ( 1g) to 15 km/s:

http://www.cs.sandia.gov/HPCCIT/hyprvel.html

Note that the third stage of this 3-stage gun is basically the
2nd-stage projectile impacting a special, density-gradient material
that "smoothly" shocks the smaller flyer to the final velocity.

In both of these devices, there are still concerns if the projectile
is actually intact, or has been "spalled" by the acceleration process.

Note that all of this work uses projectiles smaller than 1 g, and is
used exclusively for fundamental equation of state studies motivated
by inertial confinement fusion studies, H-bomb physics, and
fundamental planetary physics.

Other than as a possible simulator for micrometeoroid and orbital
debris impact, these devices have no relevance to the subject of this
thread ("Cheaper way to space!? A supergun.") because they cannot
scale and because they violently shock their projectiles to their
yield strength.

As for concepts that are scalable for "soft" direct space launch, the
two most interesting concepts (the Ram Accelerator and, more
speculatively, the Slingatron) remain too underdeveloped to make
definitively assessments of feasibility. Neither of these concepts
have ever had the funding support necessary to determine what the
maximum velocities achievable are, although a small amount of basic
research is continuing on both.
--
Andrew J. Higgins Mechanical Engineering Dept.
Assistant Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/