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 69 users online :: 5 registered, 0 hidden and 64 guests


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

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

The Team
Administrators
Global Moderators
global_moderators.png CS

N33b who needs some pro 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: Technician1002 » Thu Feb 10, 2011 1:11 am

An overly long barrel will slow the projectile. I have seen and measured this effect. Longer barrel = more drag and more air mass in front of the projectile resulting in back pressure in front of the projectile slowing it.

Depending on your valve, chamber and projectile factors the ideal barrel length can be found. Do not be fooled in GGDT looking at the projectile still accelerating at the muzzle. I fell for this too as it implies longer will permit a longer acceleration with higher muzzle velocity. The reality of a longer barrel resulted in LOWER acceleration with it still accelerating at the muzzle for a net LOWER muzzle velocity.

To utilize a long barrel, a large diameter is required along with a large chamber and high pressure. A small diameter has too much flow restrictions and a long barrel hinders performance.

In the forum, I have covered this topic before with in barrel acceleration measurements to support the theory. A 10 foot barrel for a 1.5 inch combustion spudgun is too long. A 5 foot barrel will outperform it. A 96 foot long barrel regardless of chamber size on an advanced combustion will underperform. The flow restriction and projectile friction will severely cut performance. The projectile will cease accelerating long before it reaches the muzzle.

Here is a graph with a scope picking up a magnet in a projectile as it traveled a 10 foot barrel. By measuring the diminishing time between pulses you can see the delta velocity or acceleration. Note the last 4 feet of this shot shows little to no acceleration. After trimming to about 7 feet the acceleration was much greater as well as a higher muzzle velocity.

This graph does not show the entire 10 feet. It shows the last few feet. the muzzle coil pick up has reversed polarity to show it is the last pick up.
Image
  • 0

Last edited by Technician1002 on Thu Feb 10, 2011 1:20 am, edited 1 time in total.
User avatar
Technician1002
Chief of Staff
Chief of Staff
 
Posts: 5190
Joined: Sat Apr 04, 2009 11:10 am
Reputation: 14

Unread postAuthor: jackssmirkingrevenge » Thu Feb 10, 2011 1:18 am

Technician1002 wrote:In the forum, I have covered this topic before with in barrel acceleration measurements to support the theory. A 10 foot barrel for a 1.5 inch combustion spudgun is too long. A 5 foot barrel will outperform it. A 96 foot long barrel regardless of chamber size on an advanced combustion will underperform. The flow restriction and projectile friction will severely cut performance. The projectile will cease accelerating long before it reaches the muzzle.


There is plenty of good data supporting this on Burnt Latke.

Image

The above example clearly shows that there is an ideal barrel length for a given chamber size (and to some extent, projectile type), and having the barrel too long or too short will result in less than optimal performance.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Unread postAuthor: shmoesus » Thu Feb 10, 2011 2:05 am

I think if my question were a target and your answer was the bullet, you just hit a bulls eye! Than 110% answered my question.
  • 0


shmoesus
Private
Private
 
Posts: 14
Joined: Tue Feb 08, 2011 6:58 pm
Location: NY
Reputation: 0

Unread postAuthor: shmoesus » Thu Feb 10, 2011 2:09 am

Oh i just thought of a question for you potato gun guru's,
ok since I have been shooting, dissembling and cleaning my own guns since I was 5 I know the in's and outs and lots of info about fire arms

My question is, would putting gun oil (lubricant) down the barrel of my potato gun help in adding less friction to the projectile in the barrel so that the end result will help the potato fly possibly farther, longer etc.... ?
  • 0


shmoesus
Private
Private
 
Posts: 14
Joined: Tue Feb 08, 2011 6:58 pm
Location: NY
Reputation: 0

Unread postAuthor: jackssmirkingrevenge » Thu Feb 10, 2011 2:30 am

shmoesus wrote:My question is, would putting gun oil (lubricant) down the barrel of my potato gun help in adding less friction to the projectile in the barrel so that the end result will help the potato fly possibly farther, longer etc.... ?


Less friction is not necessarily a good thing in this case.

If a projectile is tight in the barrel, it means that it will take a larger pressure in the chamber to get it to start moving. This will allow a greater pressure to build up inside the chamber, technically resulting in more power.

It's the same logic behind adding a burst disk between the chamber and barrel.

Whether this extra impulse to the projectile is enough to result in a net gain in power after overcoming the increased friction in the barrel depends on the individual case.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Unread postAuthor: shmoesus » Thu Feb 10, 2011 3:53 am

Ah I see, I had thought of that but I was 50/50 in my head weather it would be a good idea or not but I dont think I will be adding any oil this time around.
  • 0


shmoesus
Private
Private
 
Posts: 14
Joined: Tue Feb 08, 2011 6:58 pm
Location: NY
Reputation: 0

Unread postAuthor: Technician1002 » Thu Feb 10, 2011 12:28 pm

On the combustion side, the time to build peak pressure is needed as JSR noted. In an air cannon with a fast valve, the outside of the spud tends to liquefy due to the sudden and rapid movement providing lubrication so no additional lube is needed. I have not noticed any measurable difference with a dry barrel or one with a shot of cooking spray.

On tennis balls, lube of either oil or simply a wet ball helps on smaller tight fit barrels, but has no noticeable improvement on the thin wall tennis ball or soda can barrel. If your launch is burning the fuzz off the ball from friction, you could use some lube.
  • 0

User avatar
Technician1002
Chief of Staff
Chief of Staff
 
Posts: 5190
Joined: Sat Apr 04, 2009 11:10 am
Reputation: 14

Sponsored

Sponsor
 


Unread postAuthor: shmoesus » Thu Feb 10, 2011 1:04 pm

Ah indeed well I have finished my plans for my new potato gun and I would be more than happy to share them with everybody but I guess I can't put pictures on here of them.
The specs:
Barrel - 1 1/2" PVC
Converter - 1 1/2" PVC to 4" PVC
Combustion Chamber - 4" PVC

Hardware:
Case fan - Do not have a specified Fan at moment
Butt Stock will be added (personal design)
Ignition - Grill Ignition

?:
Case fan
- Also can some one provide me a easy to read design for the energy source for the case fan, also If anyone could I would like to have a On/Off switch.

And if there anything that anyone could suggest to improve or just make the potato gun look more interesting I am open to change.

THANKS
  • 0


shmoesus
Private
Private
 
Posts: 14
Joined: Tue Feb 08, 2011 6:58 pm
Location: NY
Reputation: 0

Previous

Return to Combustion Cannon Discussion

Who is online

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

Reputation System ©'