I am trying to design a device to throw a tomato a distance of 50
metres.  The device is to be pneumatic and use a captive piston within
a barrel to actually fire the tomato.  The barrel is to be made from
steel pipe and a suitable diameter to carry the tomato and a common
size to carry the piston.  The piston is to be pressurised by a 58
litre beer keg connected by as large a valve as we can find off the
shelf.  The keg acts as a pressure receiver and will be pressurised by
a yet unsized compressor.

Can anybody provide me with the following:

=> Comments on our overall design proposal, strengths weaknesses etc.
=> The muzzle velocity required to carry the tomato over that distance
and consequentially the component dimensions and pressures required to
achieve this.

What a great project. My friends and I spent many joyful hours
blasting things into orbit with an oxyacetylene cannon that I made.
Although we used many different objects for ammo, our favorite was
green apples. We used rags for wadding and blasted the apples about
200/300 yards. I have no idea what the muzzle velocity was, but if you
didn't watch very closely you would lose sight of the apple. Sometimes
we made applesauce in midair. I have a small air cannon that I
recently built, but it just isn't as much fun. I doubt that you need
to use steel pipe for your barrel. The captive piston idea might work
if you use something very light. You should be able to launch tomatoes
150 feet without making juice (until they land).

http://platinumchromatography.com/potato.htm

http://www.spudtech.com/content.asp?id=29

About 18 m/s at 45 degrees is needed to travel 50m horizontally.

To get the necessary length of powered travel calulate the K.E. (1/2 m
v**2) of tomato plus piston at muzzle velocity and divide by the force
used (pressure * area).

When is the next election campaign in your area?  I won't need one
till next year. Are you planning to sell them?  ;-}

Redhoss - We had some great ideas about using propane gas until we
were warned about the possibilities of blowing ourselves up - I must
say high pressure pneumatics aren't much better so who knows!!  We
were planning on using an ally piston from a small petrol engine so
that it would be a reasonable sliding fit in the barrel.  An exhaust
port was to be added at the end of its travel to allow the pressure to
discharge

Myoarin - Good point! ideally you would want the flexibility to switch
loads to rotten eggs as well as tomatos for politicians.  Something
tells me they wouldn't see the funny side of it these days though {:-(

You'll need to know the maximum acceleration you can apply to a tomato
so as not to liquidise it in the barrel - should be a fun experiment! 
Does it have to be a ripe tomato?  It might be worth seeing if you
could make a sabot to give some support.

Ian G.

The trick is going to be getting the optimum paintball effect on
target and not when you let the thing fly. If your tomatoes are very
much the same size, they could fit in a cup on the piston  (I don't
want any wadding or the like flying around when I playing Jackal. 
Yes, I would need very ripe tomatoes; the firm ones don't provide the
desired effect. But Brummiebloke may have other targets in mind).
One way to avoid this problem would be to have something that
accelerated during the projection  - instead of just slapping the
tomato with full force.
I think that those hunting bows that have pulleys do this.  Rig one up
with a tube - slotted for the bowstring - and a lightweight cup
(pieces from an eggcarton or the like, maybe on a string to keep it
from it from going astray  - would have to be reinforced where the
string bears on it), and off it goes.  Yeah, I bet you could achiever
18m/sec, but of course a tomato would slow down rather soon.

And then you have the problem of aiming and dealing with tomatoes of
different weight.  I could see you doing a lot of target practice with
schedules of weight, diameter ...  But that will keep you off the
street for a while.
But then ...!  ,-)

50 meters is nothing with a pneumatic gun.  bozo99 appears to have
underestimated the necessary muzzle velocity though.  With no air
resistance, it takes a muzzle velocity of 22 m/s to shoot 50 m with
gun elevation at 45 degrees.  Muzzle velocity depends on mass of the
tomato+piston, pressure, barrel diameter, barrel length, and amount of
friction.  If friction is low, you only need about 10 psi of pressure
to shoot 50 meters, assuming a tomato+piston mass of half a pound,
barrel length of 4 feet, and barrel diameter of 2.5 inches.

Propane is not a good idea though, not because of any risk of blowing
yourself up, but because it just doesn't work.  It makes a very weak
'fwoop' sound and a flame shoots out the end of the barrel, but it
doesn't burn fast enough to shoot anything very well.  If you want to
use explosive gas instead of pressurized air, hairspray or starter
fluid (ether) work much better, or, if you want to get dangerous,
oxygen and acetyene as mentioned by redhoss.

I think your biggest design problem is going to be the piston.  If it
stops abruptly at the end of the barrel, finding a design that won't
bang itself to pieces will be a challenge.  One possibility is to use
a lightweight cup for the piston, and tether it to the gun with a
longish bit of stretchy string.  The cup and tomato will leave the
barrel together, but the string will soon stop the cup.  You may have
some misfires if the cup tends to turn around backward before the
string goes tight though.

A gun by any definition is dangerous. One of my friends used to say,
"If you're going to be a bear be a Grizzly bear". Of course what he
meant is, if you want to do something go for the gusto. If you want to
have fun and launch something, why not do a proper job. Yes, propane
and oxyacetylene might be considered dangerous by some of the faint
hearted. However, I wouldn't trade any of the time I have spent
blasting old motorcycle parts, small animals, fruit, and whatever into
the stratosphere in the name of science and beer for anything. If
someone dies in the process, so what. Science and beer are worth much
more to me than safety. Safety is a four letter word as far as I am
concerned. Fun and insanity are the best entertainment. Get the most
volitale fuel you can find and blast the biggest tomatoe as far as you
can. I wouldn't quit until I fired one at least a quarter of a mile.

I made this spreadsheet a little while back so we could play with the
barrel length and other parameters of our water-balloon launchers.  It
assumes adiabatic expansion of the air (no temperature passes through
the PVC while the air expands) and uses the base equation
P1*V1^(gamma)=P2*V2^(gamma) where P = Pressure [Pascals] (remember to
add atmospheric in with the gauge pressure) V = volume [m^3] and gamma
is approximately 1.4 for air.  What is not accounted for is air
passing around the projectile as it goes down the barrel and air
resistance once it exits the barrel - as those are hard to model and
depend heavily on the specific parameters of the projectile.  The
numbers should give a good starting point though.

Here is the spreadsheet in excel format:  http://www.bavetta.com/aircannon.xls