SpudFiles

jackssmirkingrevenge wrote:Given the uneven nature of pumpkins and the unpredictable nature of their flight, I think this is next to impossible.

There are quite a few ways of finding Cd, so I'm sure one of them could work.

The thing is, if you could find an approximation of the Cd, even if that proves to be quite a wide range, it could still give you a good idea as to things such as ideal launch angle and mass.

And knowing these things could prove an edge.

jeepkahn wrote:I guess you've got a year to finish yours

I hope it won't be that long. I know it's already seen lots of delays, but I would like to have it available without too many more added to the list.

better yet if you know what you are using for a barrel you can grow the pumpkin in a piece of that pipe and it should grow so that it is a nice fit in the barrel without cutting or anything. they do this with watermelons except in a box to make shipping easier.

That is definatley possible. I grow pumpkins every year and I know for a fact that you can grow pumpkins in molds like this.

Punkins are supplied, BUT you can bring your own...

I would try for a massively long barrel and even pressure around the pumpkin. Maybe some small scale tests to see

1. What frags a veggy?

If it's acceleration that can be solved with a long barrel

2. How does rifling affect soft ammo?

How does rifling affect round pumpkins?

How does rifling affect elongated pumpkins?

3. What part of the pumpkin is the weakest?

4. How do things with similar cd's react with rifling and angle of launch?

Are you allowed to grow your own pumpkins?
If you were, I'd decide on a barrel diameter and get a bucket or something slightly smaller in diameter. Cut the bottom out of the bucket and place that over your pumpkin while it grows. That ways, it'll be shaped more like a cylinder as opposed to a well, pumpkin. You have plenty of time to set this up seeing as its a 2010 contest.

Edit- God dammit, someone beat me to it. I guess thats what I get for not reading the previous posts. Well, the idea is still viable.

pumpkin chunkin!
ya I love that contest! They also allow onagers and trebuchets(love trebuchets ) and stuff like that, not in the spud gun section though

Next question....

Aside from open flame or combustion, what would be the most effective way to heat up the chamber volume??? you have 3 minutes to pressurize and fire, and I'd like to get the chamber air temp as high as is safely possible(i don't think i need to explain why) with the given time constraints... my thought is an inline heater between the tanks/compressor and the chamber...

How much current do you have, and how much money? Nichrome wire comes to mind for this application..

It's commonly used for electronic ignition for pyrotechnics, 'muscle' wire and in applications like hair dryers. If you can get a spool of it fairly cheap, just coil it through your chamber on plastic supports (insulated..ceramic probably) and run a DC current through it.

Nichrome wire heats up and reacts almost instantly, and makes a lot of heat (enough to ignite a BP priming charge, and that is plenty for this)..it also does not require much current. If you could get enough of it and coil it through the chamber, it would be up to temperature in under 30 seconds most likely. Think of how long a hair dryer takes to get consistently hot.

If you have the money, the air could be made extremely hot through shock compression, like a springer but with air moving the piston. Simple adiabatic calculations show that almost twice ambient temperature could easily be reached by compressing a sample of air to one quarter of its original volume. This would result in a 15psig volume of air reaching 190psig and 520K. The actual execution of this idea would likely be overwhelming though (stopping a 24" or so diameter piston without destroying the entire launcher would be a true feat of engineering).

If you use electric heating, you'll need a LOT of power. Heating 1 cubic metre of air at 100psi by 100K in 2 minutes would require 5.5kW or power being transferred into the gas as heat. You need to account for inefficiency and heat loss to the chamber walls as well. That's going to be a hell of heater

inonickname wrote:How much current do you have, and how much money? Nichrome wire comes to mind for this application..

It's commonly used for electronic ignition for pyrotechnics, 'muscle' wire and in applications like hair dryers. If you can get a spool of it fairly cheap, just coil it through your chamber on plastic supports (insulated..ceramic probably) and run a DC current through it.

Nichrome wire heats up and reacts almost instantly, and makes a lot of heat (enough to ignite a BP priming charge, and that is plenty for this)..it also does not require much current. If you could get enough of it and coil it through the chamber, it would be up to temperature in under 30 seconds most likely. Think of how long a hair dryer takes to get consistently hot.

Why not just make a giant blow dryer? So your setup is a giant compressor then a small length of hose then a big chamber that contains the wire then hose out to the chamber so you don't screw with the chamber volume.

Kinetic ambitions is right, you probably don't have enough current to pull this off.

I'm going to have to say burning something is the way to go..only way you could even get close to a suitable pressure.*

Edit: * temperature

Kinetic, what if the volume was compressed 4:1 in about 30sec to a minute... With the compressed volume being 35,000ci???

I am considering an integrated intensifier(hydraulic ram used to reduce the already pressurized volume to an even greater pressure)...

I'm not trying to cook the pumpkin, just trying to raise internal sos...


And on the vacuum side of things, if the barrel is 20ft long, that equates to approximately 1lb of air in the barrel without vacuum, I'm sure that would creat a big differance in performance, not too mention during acceleration that 1lb of air creates a fair amount pressure on the punkin...

inonickname wrote:Kinetic ambitions is right, you probably don't have enough current to pull this off.

I'm going to have to say burning something is the way to go..only way you could even get close to a suitable pressure.*

Edit: * temperature

A variation of a fire tube boiler would work. A coaxial pipe in a pipe in a cross flow heat exchanger configuration. Burner heats the inner cylinder from the air outlet side and exhausts near the compressor side. It would heat the gas to oven temperatures on the way through and can be built for high pressure air heating. Consideration will need to be given for metal expansion and contraction. Built as a heat exchanger in a 1/2 turn or more coil will take care of expansion issues.




The images are for water and freon heat exchange. Dimensions will need to be modified to be suitable for fire tube and air.

jeepkahn wrote:Kinetic, what if the volume was compressed 4:1 in about 30sec to a minute... With the compressed volume being 35,000ci???

Next to useless. In that time, any heat gained would bleed away.

Here's a question. Is there any rule that specifically prevents the chamber being preheated before your filling time starts?

Ragnarok wrote:

jeepkahn wrote:Kinetic, what if the volume was compressed 4:1 in about 30sec to a minute... With the compressed volume being 35,000ci???

Next to useless. In that time, any heat gained would bleed away.

Here's a question. Is there any rule that specifically prevents the chamber being preheated before your filling time starts?

None that I've seen... I did, BTW, link to the rules on an earlier post...