Fun_People Archive
8 Dec
Further Chunkin of Punkins

Date: Thu,  8 Dec 94 01:07:05 PST
To: Fun_People
Subject: Further Chunkin of Punkins

Forwarded-by: "pardo@cs.washington.edu" <pardo@cs.washington.edu>
TO:   Ballistic Pumpkin Review Board
FROM: DONALD A. DUNCAN              Internet: 62726080@eln.attmail.com
SUBJECT: Jumping to delusions

Dear Sirs:

Am in receipt of your communication 'Re: Ballistic Pumpkins.'
Thank you for doing calculations on muzzle velocity and acceleration.
Suggest you reference my communication 'Changing of the gourd'
[below --dp] for means of accelerating vegetables at rates exceeding
8g.  Would have thought the principles would be familiar to you,
as are used in pilot flight suits in high-performance aircraft.

Have you considered offering your services to the Republicans as a
commission on, say, pornography?  Or environmental impact?  [Those
punkin's sure have got impact! --dp]

-DAD



FROM: DONALD A. DUNCAN              Internet: 62726080@eln.attmail.com
SUBJECT: Changing of the gourd

The 1994 World Championship Punkin Chunkin is a wonderful example
of 'a fine madness', and I enjoyed the cleverly written account by
dylan@striper [actually, I think he copped it from somewhere else,
his was just the earliest name I have].  I get many a chuckle out
of envisioning the reaction of motorists to the 'Universal Soldier'
tooling down the highway to this year's contest...!

I showed it to a housemate, an elderly retired engineer, who also
found it hilarious.  Since we were both born with 'designer genes',
we immediately started considering ways to blow away the current
equipment.  As it happens, he (Milt) used to be a weapons designer,
and has generated such things as a 'water pistol' which breaks the
sound barrier (!), so his insights into the mechanisms were extremely
relevant.  For those who are interested, here is the gist of our
discussion.

Problem # 1:  How to get the pumpkin to survive the acceleration.
Solution:  Sabot.  A 'sabot' is a term used by weapons designers
  to refer to a shot carrier which is fired in the barrel but does
  not become part of the delivered load; it may be reused or not,
  and, with an additional round of engineering modifications, may
  be stopped in the barrel.

  In this case, the sabot would need to be filled with a fluid
  (water, although a fluid with the same density as the pumpkin
  would be better); the fluid, being virtually incompressible,
  serves to distribute the acceleration forces evenly around the
  pumpkin.  When it is fired, the water dissipates in the air while
  the projectile thunders on.

  It would be desirable for the pumpkin to be full of water, too,
  although it would increase the payload.  Is it allowed to punch
  a hole in it?

  *[The term 'sabot' comes from the French word for wooden shoes,
    and is the root of the term 'sabotage', from a tendency to drop
    them into machinery to disable it.]*

The Centrifuge:

For a centrifugal launcher, the rotating arms have some inherent
problems.  The arms themselves have to be balanced precisely, and
the mounting mechanism must be capable of withstanding an out-of-balance
condition when the pumpkin is released.  If it comes apart, it is
very dangerous.

Also, there is a limit to the speed which can be readily achieved,
not only because of the rapid increase in effects of the out-of-balance
condition, but because air resistance increases roughly as the cube
of velocity - increases in speed become ever more expensive of
power, the assembly has to keep being beefed up, and horsepower
requirements become prodigious well before optimal speeds are
produced.

A 30-foot arm turning at 1.5 rps achieves a perimeter speed of
roughly 140 feet per second, or 96 mph.  The optimal velocity would
be, according to Milt, just below the speed of sound - around 600
mph, or 880 fps.  Exceeding the speed of sound introduces other
complications, and won't gain much.

1. An effective solution is to build a disk, rather than rotating
   arms; this is more stable, can be streamlined, can be lighter,
   can achieve faster speeds with less hazard, and requires a lot
   less horsepower.  For a 10' disk, 880 fps is about 28 rps, or
   1680 rpm; increasing the disk size reduces the necessary rpm in
   linear proportion.

2. The payload must be offset with a precisely weighed dummy load
   (water alone), which is released in the opposite direction at
   the same time the payload is released.  Since this will be 45
   degrees downward toward the base of the machine, it will be
   necessary to clear people out of the area, and desirable to
   build a chute to catch and redirect the water upwards.  The
   chute will have to be a little stronger than sheet metal, and
   25 pounds of water striking it at just below the speed of sound
   should make a satisfactory 'boom' to corroborate firing and
   impress the natives!

3. The area in front of the payload(s) must have a wind-breaking
   shell, like the fuselage fairing in front of a cockpit, in order
   to allow the pumpkin to get free of the mechanism before being
   buffeted by the full velocity of the wind.  This adds horsepower,
   but you'll never get the pumpkin out intact without it.

4. At the peripheral speeds aimed for, timing of the release is
   critical.  My choice would be an encoder on the shaft which
   tracks the rotational angle and velocity, and a
   microprocessor-controlled release mechanism which automatically
   triggers when a given critical speed is achieved.  This could
   be started low and gradually increased during development.  The
   length of time it takes for the release at different speeds
   would have to be determined experimentally and factored in.

5. The release mechanism needs some thought, since - unlike a
   pneumatic projectile - the release occurs at the surface which
   has to withstand the highest (centrifugal) forces, and has to
   be watertight in the closed position.

6. The power should be an auto engine with an automatic transmission
   and remote accelerator rig (and tach?) - nobody wants to be near
   this thing when it's cranking up!  Milt estimates that it will
   require 120-150 hp, so it won't need a pony car engine.

The Cannon:

The cannon will need a captive sabot - you need something to control
the gas leakage, since pumpkins don't come in fixed caliber, as
well as to hold the water bath, and you don't want it flying
downrange among the spectators.  If it's long enough to hold the
requisite water at a 45 degree angle, it won't be necessary to
develop a rupturable release cover - the water will continue on as
the sabot slows.

To capture the sabot, one approach might be a three-point tapered
brake with a spill to release remaining gas pressure.  It will be
caught fairly tight, so a means of releasing it would be required.
I haven't done any calculations to determine how fast it would be
going, or how much energy would have to be dissipated.

The sabot/barrel design, the length of the barrel, and the gas
(steam) supply determine the range.  The sabot needs to have the
Minie ball rear design - a hollowed, somewhat flexible area which,
under gas pressure, expands outward against the barrel to provide
a seal.  One trick which has been used is to make the barrel larger
in diameter near the breech and reducing the diameter partway down
the barrel; the goal is to maximize the acceleration at the point
at which the gas pressure is the highest.  This is possible if the
sabot skirt has enough flexibility/rigidity to accomodate both
diameters without inverting (large dia.) or dragging excessively
(lower diameter).

Note that the guy who built the 'Universal Soldier' probably knew
a lot of this - Milt interprets the 'cloud of vapor' at the muzzle
to be the contents of the sabot, rather than escaping steam.  It's
hard to imagine being able to fire a pumpkin half a mile without
it.

Logistical Considerations:

Building the contraption is only one aspect of the project, though!
Consider:

1. Where do you get dummy loads?  What is the range in size of
   10-lb.  pumpkins anyway?  You can hardly develop this only at
   Halloween!

2. Where can you test fire such a machine?  You'll need a mile or
   so of clear space, and a string of observers, connected by
   walkie-talkie, who are willing to risk a high-speed gourd plunking
   their gourd.

3. The sabot solves the problem of high spin, but the pumpkins are
   hardly aerodynamic, and cannot be expected to fly true - and
   the farther you 'chunk' them, the wider the target range needs
   to be.

4. Can you envision calling the Lewes Chamber of Commerce and
   negotiating the contest location based on your anonymous assurance
   that you can throw a pumpkin a mile?

Milt suggests firing it out to sea, or over a big lake.  This would
require observers in boats; they'd have to be confident that they'd
see incoming projectiles in time to get out of the way!

Of course, the increase in distance won't be proportional.  It
should be possible to determine the rough range by using 1200 feet
for 100 mph, calculating the ratio of deceleration by wind resistance,
and plugging it into a ballistic curve.  I haven't used those
equations in 20 years, and I'm too lazy to haul them out.  However,
if I was going to build something, I'd certainly do the calculations
first.  It may not be practical to do much better than the 'Soldier';
a .22 bullet - rifled to spin,  aerodynamic, and exiting at well
above the speed of sound - only has a range of about a mile.

-DAD

© 1994 Peter Langston