SpudFiles

Ok, this topic isn't about the the absolutely most effiecient combuston. I'm talking about making the most efficient combustion without any rediculous upgrades or pricey mods. How efficient can you make the average potato gun. There are many questions to be answered before a design can be made. There will be many obvious answers, but if this becomes detailed enough I'm going to try to make this a tutorial for those who want to have a weekend of fun with there first spudgun. If there are any disputes or input please post. I want this to be universally accepted.

Assume
metering is too expensive
PVC is sch 40
ignition is a piezoelectric grill ignitor
It has to fit a typical potato.
The soon-to-be spudders don't have a machine shop.
everything used has to be legal and accessable to any kid
$50 is relatively expensive

Questions to be answered and applied to the gun

Q: What is the perfect C:B volume ratio?
A: 1:1.5

Q: What is the ideal reduction ratio from chamber to barrel?
A: 2:1

Q: What is the ideal diameter to length ratio of the chamber?
A: 1:1

Q: Where is the best placement of the spark gap?
A: Centered in the chamber

Q: What is the best type of cheap fuel
A: propane

Q: What type of combustion is most efficient?
A: Inline cannon

Q: To carry the gun, what body weight to cannon weight ratio is ideal?
A: 10:1

Q: What is the best ratio of propane to air
A: 1:10

Key:
New Question
Debatable Answer
Agreeable Answer

New questions or answers are apprecitated.

Q: What is the ideal diameter to length ratio of the chamber?
A: as close to 'spherical' as possible if you know what i mean, like if its 4" pipe make it 4" long and so on, but in most cases that is impracticable.

Q: What is the best type of cheap fuel
A: get a propane tank used for hiking stoves and the cap off a WD40 can and give her a spray

Q: At what size and weight would a cannon be considered unweildly?
A: depends on how built you are...


im normally on the pneumatic side of things, so i'm just reiterating what i have heard around the place...

"....What is the ideal C:B Ratio..."

I'd say for non metered propane make it 1.5 to 1

I think the best type of cheap fuel is aerosol starter fluid....

I wouldn't build anything smaller then a 18in long 4inch diameter chamber with a five foot barrel. Great power, and very flexible in what you can shoot from it. And the chamber is big enough so its not finicky about correct fuel ratios. (I.e. spray times)

Lastly I would keep the spark gap in the back...just because it would be drilled through two layers of pipe (fitting and pipe) and if they use a paper clip like I did for my first cannon its easer to adjust the spark gap.

Jared Haehnel wrote:"Lastly I would keep the spark gap in the back...just because it would be drilled through two layers of pipe (fitting and pipe) and if they use a paper clip like I did for my first cannon its easer to adjust the spark gap."

the spark is best in the middle of the chamber, a coupler in the middle of the chamber provides you with double thickness

That was my bad I quote the wrong thing. I meant C:B ratio... I'll see if I can fix it...

The spark is the best in the middle but given his criteria, I figured it would save a few bucks and a few minutes of labor if he just left it in the back. While giving a good performance.

Q: What is the perfect C:B volume ratio?
A: If not using metered propane, it will be around 1:1-1.5:1, but not really enough test information.

Q: What is the ideal reduction ratio from chamber to barrel?
A: Not enough test information.

Q: What is the ideal diameter to length ratio of the chamber?
A: 1:1. Allows flame front to progress further in spherical mode.

Q: Where is the best placement of the spark gap?
A: Centered in the chamber

Q: What is the best type of cheap fuel
A: Butane

Q: What type of combustion is most efficient?
A: Inline cannon

Q: At what size and weight would a cannon be considered unweildly?
A: I'd say only limited to the shooter's body weight. A proper balance of the cannon and a good harness can allow such a thing.

1. Metering is never too expensive. A 60cc plastic syringe is less than $2.

2. Like markfh11q said, butane is, by far, the cheapest fuel for a combustion gun. It's pretty hard to beat a 99 cent disposable butane lighter as a fuel source.
And, it's easy to meter.
And, you can use it to light "other" things.

Carlman wrote:Q: What is the best type of cheap fuel
A: get a propane tank used for hiking stoves and the cap off a WD40 can and give her a spray

That doesn't conform with the assumption that the builders are age restricted kids.

markfh11q wrote:Q: What is the best type of cheap fuel
A: Butane

That also doesn't conform with the assumption that the builders are age-restricted kids.

jimmy101 wrote:Metering is never too expensive. A 60cc plastic syringe is less than $2.

That doesn't conform with assumption that any external piping is too complicated.

These statements are all good though. Keep on thinking, we'll be able to perfect it. I'm hoping to make an ultimate how-to for combustions. I'm thinking about breaking down all the aspects of a combustion into difficulty levels to fit into one post. An example would be the basic inline cannon is level one. Adding a chamber fan would be level two. On the same level would be a basic over-under cannon. Level three would deal with metering and custom ignitons. It would go on from there. Perhaps I'll branch out into pneumatics and hybrids. (levels 3-4 and 5-6 accordingly) This system would be able to identify skill based on what one has and has not done. That's a way down the road though. Let's stick on the questions I have up for now.

rcman50166 wrote:

Carlman wrote:Q: What is the best type of cheap fuel
A: get a propane tank used for hiking stoves and the cap off a WD40 can and give her a spray

That doesn't conform with the assumption that the builders are age restricted kids.

markfh11q wrote:Q: What is the best type of cheap fuel
A: Butane

That also doesn't conform with the assumption that the builders are age-restricted kids.

jimmy101 wrote:Metering is never too expensive. A 60cc plastic syringe is less than $2.

That doesn't conform with assumption that any external piping is too complicated.

This below is not a complicated metering system. I'm using these for some noise making cannons. Just attach the quick connect to the chamber in a double walled area. Calculate the size and PSI of your meter tube and you are done. This one works in a 3660 cc chamber when filled at around 63 PSI propane.

Edit: This will also keep the weight of your cannon down by not including the propane tank and metering system as a built-in part of the gun.

You seemed determined to use any and every kind of fuel other than the one that has been deemed generally by time and consensus (propane) to be the best bang for the buck. Why is that? Anyone that is too young to walk into a Home Depot and buy a tank of propane should probably have their parents overseeing things anyway.

If you don't want any of these things you're down to a basic spray and pray PVC model, easily slapped together in a few minutes time. Do you really want to waste your time documenting such an already plenty documented beast?

Q: To carry the gun, what body weight to cannon weight ratio is ideal?

A: I would have to say the best ratio would be 1:10

I hope you mean 10:1.

And starman, this is for the noobs. When I first saw a propane metering system, I was scarred away by it's seeming complexity. I'm sure other new users would feel the same way.

rcman50166 wrote: I hope you mean 10:1.

And starman, this is for the noobs. When I first saw a propane metering system, I was scarred away by it's seeming complexity. I'm sure other new users would feel the same way.

Take another look at the meter....very simple. Noobs need to be building a spray and pray like I mentioned before....not much need for all this documentation you're attempting. www.burntlatke.com is the defacto standard for combustions and answers many of the issues you put forth.

rcman50166 wrote:I hope you mean 10:1.

oops, yeah. my bad

Q: What is the ideal reduction from chamber to barrel?

A: None. The reducer increases surface area and therefore heat loss, and decreases the amount of force that can act on a projectile.

And just in case you didn't know, there is no age restriction on buying propane.

DYI wrote:Q: What is the ideal reduction from chamber to barrel?

A: None. The reducer increases surface area and therefore heat loss, and decreases the amount of force that can act on a projectile.

And just in case you didn't know, there is no age restriction on buying propane.

Walmart has instituted some assinine age checks for things like gas treatment, paint thinner, etc...potential huffing components...probably some checks for things from the tobacco dept...ie. butane. However, I haven't run into these checks at general hardware stores.