Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 62 users online :: 4 registered, 0 hidden and 58 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Third cannon help.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
Sponsored 
  • Author
    Message

Unread postAuthor: jimmy101 » Thu Aug 18, 2011 6:33 pm

Labtecpower wrote:
jackssmirkingrevenge wrote:Anything above 1:1 is going to be bad for performance, for a combustion you want the chamber to be between 0.9 and 0.5 times the barrel volume.

...

It could be that a larger chamber would have performed the same or better with the same barrel as the small chamber. Or am I wrong?
Ahh, the yearly "performance" vs. "efficiency" thread.

Performance and efficiency are not the same thing. And there are many ways to quantify both.

Efficiency is, in the combustion spud gun world, totally irrelevant. Latke's CB of 0.8 is the most efficient one. That means the largest percentage of the energy available in the fuel is converted into kinetic energy of the spud. He started with a fixed chamber size then changed the barrel length until he had maximized the muzzle velocity. If he would have then take the most efficient barrel length and put it on a larger chamber the performance would have gone up, but the efficiency would have gone down. But efficiency is irrelevant, so by any logical view of the matter a chamber bigger than CB 0.8 is better.

There is a a point where a large enough chamber will actually start to decrease muzzle velocity for a given barrel and ammo. I don't think anyone really knows what that ratio is but it is certainly well above a CB of 5:1.

Bottom line #1: Make the CB bigger than 1:1 but anything above 1.5:1 is getting into greatly diminishings returns.

Bottom line #2: There are a number of much better ways to significantly improve the muzzle energy (i.e., the only "performance" you really care about), and Bottom Line #1 isn't even on the list.

In rough order of biggest improvement to least improvement:

1. Meter your fuel: Figure that will double your muzzle kinetic energy (on average). And, your shots, and ignition, will be much more consistent.

2. Include a chamber fan: Figure that will also double you muzzle kinetic energy, particularly if you are impatient. (Or, omit the fan and just let the gun sit for a few tens of minutes before firing.)

3. Increase both the sealing capability and static friction of the ammo to the barrel. This is easily done for spud ammo by beveling both the inside and the outside of the barrel when making the spud cutter. You want a tight seal and a fair amount of static friction.

...



87. Optimize the CB ratio
  • 0

Image

jimmy101
Lieutenant General
Lieutenant General
 
Posts: 3127
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 7

Unread postAuthor: Bias_P11 » Thu Aug 18, 2011 10:58 pm

The reason that a large chamber can damage performance is if the fuel does not burn fully before the projectile leaves the barrel, so not all the available energy is converted to pressure quickly, lowering psi. However, small chambers do not have enough fuel to burn. If you can get multiple ignition points, a chamber fan, a fast burning fuel, or if the cannon is small, a larger c:b ratio wont hurt much but greatly increases reliability and shot replication.

As for having more psi than needed (fuel), no one really cares. We like big boom.

The 2" wide chamber this guy wants is small enough that c:b ratio is okay to go higher.
  • 0

User avatar
Bias_P11
Private
Private
 
Posts: 24
Joined: Mon Aug 15, 2011 4:09 pm
Reputation: 0

Unread postAuthor: Bias_P11 » Thu Aug 18, 2011 11:01 pm

Oh and spuds are low density projectiles. A high density projectile such as a magnetite slug needs a slightly higher C:B ratio. Use a sabot for stuff like that!
  • 0

User avatar
Bias_P11
Private
Private
 
Posts: 24
Joined: Mon Aug 15, 2011 4:09 pm
Reputation: 0

Previous

Return to Combustion Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot]

Reputation System ©'