Opinions needed on first spud gun

[herve001]

Hello everyone,

I'm new to the forum and I'm about to create my first combustion spud gun. I'd like it to be max 2 meters long.
I've read many things across many websites on the best ratio and it is very confusing (0.8, 1, 1.2, 1.5, 1.7, 2,...).
Anyways as I understand the best studies on the subject were conducted by Burnt Latke which states that the best for propane and hairspray is 0.8:1.

Here are the tools I bought:
- for the barrel: 2 meters of 4cm diam PVC pipe (3mm thick) - inside diam: 3.4cm = 1.34in
- for the chamber: 1 meter of 10cm diam PVC pipe (3mm thick) - inside diam: 9.4cm = 3.7in
- ignition: DC 3.6-6V to 30kV-40kV Boost Step-up Power Module
- Fuel: Aqua Net hairspray or propane

With the current couplings and stuff the smallest chamber I can do is ~25cm in length.

I realize now that I should have bought a smaller diameter pipe for the chamber because 9.4cm diam pipe makes the barrel really long, really fast.

I'll be going with a 1.2 C:B ratio for convenience which should give me around 93% power of the 0.8 ratio (only 7% loss).
So to recap here's the gun I plan to do:
- barrel: length 170cm (~67in), inside diam 3.4cm (~1.34in)
- chamber: length 26cm (~10.23in), inside diam 9.4cm (~3.7in)

I'm still surprised by how long the barrel needs to be compared to the chamber.
Do these calculations make sense?
what is the tipical total length and chamber diameter of hand held spud guns you guys make?

Any advice is more than welcome.
Thank you very much

[jackssmirkingrevenge]

Welcome to the forum!

For a combustion, a good idea to predict performance is to download DHall's excellent design tool:

http://www.thehalls-in-bfe.com/HGDT/

This will allow you to experiment with different parameters and estimate performance before gluing your first section of PVC to see what is important. I took the liberty of modelling your design with the parameters you posted:

As you can see it will launch a 100 gram projectile at 310 feet per second for 329 ft lbs of muzzle energy.

Increasing barrel length to 100 inches with the same chamber gives us 384 ft lbs. Keeping the same barrel length and increasing the chamber length to 20 inches gives us 433 ft lbs. Playing with these parameters will let you optimize the design of your launcher.

Fuel: Aqua Net hairspray or propane

It's 2017, please don't use hairspray - a large syringe will allow you to carefully meter the right amount of propane and get the best and most consistent performance out of your launcher.

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[herve001]

Thank you very much jackssmirkingrevenge.

I was thinking about a 1.2 ratio because of this article: http://www.inpharmix.com/jps/Optimal%20 ... 0Size.html

They state:

As you can see, the scaling model predicts that, for a fixed size barrel, the maximum muzzle velocity is obtained with a CB ratio of about 1.2. If the CB ratio is increased (the chamber is made bigger) beyond 1.2 then the muzzle velocity drops off dramatically. It looks like the performance would be pretty poor at a CB of 1.5.

Which is not in par with the HGDT predictions. I'm not sure about how they scale the muzzle velocity in that article...
What do you think?

Maybe I'm overthinking this
I'll have a fan and multiple ignition sparks. How many sparks do you recommend? 2, 3 or 4?

Thank you

[jackssmirkingrevenge]

I was thinking about a 1.2 ratio because of this article: http://www.inpharmix.com/jps/Optimal%20 ... 0Size.html

This is jimmy101's website, I'm sure he can chime in but note that his conclusions are based on data extrapolated from Latke's experiment, and at the bottom of the page using a similar program to HGDT he did not get the same result. Unless you go to a hugely disproportionate ratio, in practical terms for a fixed barrel length moar chamber = moar power, especially if you have a fan and multiple ignition points that ensure rapid ignition.

I'll have a fan and multiple ignition sparks. How many sparks do you recommend? 2, 3 or 4?

3 or 4 is usually sufficient, just try and spread them out for maximum effect.

[RMD]

Someone Knows How It Works This weapon needed to be explained to me how it works and how I can elaborate it I want to combine ideas of that weapon with my ideas I do not want to copy myself from the weapon only I would like to know how the combustion chamber works since Ban two parties The Fan watch the video Of the Weapon I told them

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[jackssmirkingrevenge]

Someone Knows How It Works This weapon needed to be explained to me how it works and how I can elaborate it

The maker of that launcher is active on this forum, you can ask him directly