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 64 users online :: 4 registered, 0 hidden and 60 guests


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

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

The Team
Administrators
Global Moderators
global_moderators.png CS

combustion coxial

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 » Fri Apr 13, 2007 11:26 am

Willard:

I agree completely, different builders have different goals. Clever design is always of interest.

I just try to correct misconceptions that many people post on this site.

For example, you said "Any additional heat loss is to the interior fo the gun, thus minimizing the effect." Sorry, but that is not true. Where the heat is lost to is irrelevant. Lost is lost. The high heat capacity and high mass of the pvc will suck a tremendous amount of energy out of the low heat capacity, low mass, but high temperature, combustion gases.

Or, "Use a spark strip," spark strips are OK but not very effective. It takes two sparks on a strip to equal one spark on the central axis of the chamber. And, the sparks must be spaced roughly the chamber diameter apart for the multiple sparks to make much difference.

So, back to my original post, a coaxial design is interesting but it basicly sucks.
  • 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: lukemc » Fri Apr 13, 2007 11:54 am

willarddaniels wrote:
could you make a coaxial hybrid? a regular piston valve but instead of droping the pressure on the pilot side increase the pressure on the chamber side (combustion) or have a simultaneous ignition pilot valve release

You can, you just need to use a compression spring instead of trying to use a pneumatically pressurized piston. The problem comes in trying to release the pilot at the same moment moment of combustion... get it wrong and you just made a bomb.


hence the burst disk on the diagram i showed
  • 0

"Those who are different change the world. Those who are the same keep it that way"
User avatar
lukemc
Colonel
Colonel
 
Posts: 565
Joined: Sun Feb 25, 2007 8:48 am
Location: NY
Reputation: 0

Unread postAuthor: spudthug » Fri Apr 13, 2007 1:55 pm

ok so back to the being ok for combustion part...so u guys dont think this will blow up and kill me??


edit: i plan on using propane...

edit 2...ok since im bored ill go try it..if i die someone who lives in pa search for me and take my cannons...take care of them for me mmkay
  • 0

4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
User avatar
spudthug
Brigadier General
Brigadier General
 
Posts: 936
Joined: Mon Feb 26, 2007 9:02 pm
Location: johnsonburg, Pa USA
Reputation: 0

Unread postAuthor: SpudBlaster15 » Fri Apr 13, 2007 1:59 pm

Why would you build a launcher, then wonder if it will blow up and kill you?

Sounds like you are lacking in the common sense department.

BTW, no, it will not blow up and kill you.
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: FeLeX » Fri Apr 13, 2007 2:03 pm

Are you dead? You posted an hour ago.
  • 0

User avatar
FeLeX
Captain
Captain
 
Posts: 310
Joined: Sat Mar 10, 2007 10:45 pm
Reputation: 0

Unread postAuthor: spudthug » Fri Apr 13, 2007 2:08 pm

actually i posted like 15 mins ago...and it works but since i am not using metere propane how many seconds of propane should i use??? ill go try again with more propane and post back



omg this thing sucks...i used right guard i used propane and the potato comes out like "phwoop" and goes like 20 feet....what is wrong??
  • 0

4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
User avatar
spudthug
Brigadier General
Brigadier General
 
Posts: 936
Joined: Mon Feb 26, 2007 9:02 pm
Location: johnsonburg, Pa USA
Reputation: 0

Unread postAuthor: FeLeX » Fri Apr 13, 2007 2:21 pm

Wrong mix. Happend with my cannon once too, my friends almost peeeeed themselves. On my cannon I use 30-40 in warm weather and 50-60 when its cold. But it really depends on the size of your gun. Id say start at 15-20 seconds and then go up by 5 seconds. Find the sweet spot and rember it, at least tahts how I did it. Good luck!
  • 0

User avatar
FeLeX
Captain
Captain
 
Posts: 310
Joined: Sat Mar 10, 2007 10:45 pm
Reputation: 0

Sponsored

Sponsor
 


Unread postAuthor: boilingleadbath » Fri Apr 13, 2007 3:33 pm

...I wonder if putting lengthwise beads on the outside of the barrel would increase performance enough to be noticed. (think schelkin spiral)

Regardless, I think we have two contradicting factors with one's standard coaxial launcher:
1) Increased chamber volume
2) Increased heat loss and increased time of combustion - discounting going overboard on the ignition points.

As such, I'm not certain it's as strait forwards as "coaxials won't preform as well".
  • 0

User avatar
boilingleadbath
Moderator
Moderator
 
Posts: 1642
Joined: Sat Mar 12, 2005 10:35 pm
Location: Pennsylvania, USA
Reputation: 0

Unread postAuthor: FeLeX » Fri Apr 13, 2007 3:51 pm

You mean decreased volume since the barrel is in the chamber.
  • 0

User avatar
FeLeX
Captain
Captain
 
Posts: 310
Joined: Sat Mar 10, 2007 10:45 pm
Reputation: 0

Unread postAuthor: SpudBlaster15 » Fri Apr 13, 2007 5:26 pm

The heat loss issue is, as with most aspects of spud gunning, dependent upon more than just a simple factor. One cannot state that "because the barrel extends into the chamber, the heat loss will double, and the performance will decrease drastically". Obviously, the relative heat losses will be much different for say, a 4" diameter chamber with a 3" diameter barrel than it is for say, a 4" diameter chamber with a 1.5" diameter barrel.

...That said, assuming identical heat capacities and wall thicknesses between the barrel (1.9" OD) and chamber (4" ID, 12" length), and a barrel that extends into the chamber as far as (chamber length - 2"), the heat loss will be approximately 25% greater than that of a standard launcher with similar dimensions, which is quite considerable.
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: BC Pneumatics » Sat Apr 14, 2007 6:04 pm

A step back? That is like saying a bolt action is a 'step back' form a semi auto. Both have there places. Performance isn't always everything.

The best place of ignition is as far from the barrel as possible, so the flamefront can accelerate, as far as anyone has ever been able to show. the whole 'put it in the middle some more is burning' has never been proven as effective. (Most people seem to think the opposite, it hurts performance.
  • 0

<a href="http://www.bcarms.com/"><img src="http://www.bcarms.com/images/store_logo.png" border="0"> </a>
User avatar
BC Pneumatics
Donating Member
Donating Member
 
Posts: 1089
Joined: Tue Jan 18, 2005 6:55 pm
Location: Fresno, CA
Reputation: 0

Unread postAuthor: boilingleadbath » Sat Apr 14, 2007 8:27 pm

I was assuming that the standard coaxial has the chamber extend for most of the length of the barrel, in which case you'd normally have a pretty high ratio.

Hmmm....
Perhaps one could use the small barrel-chamber gap to our advantage in the coaxial design, by ribbing the barrel? (again, think schelkin spiral)

This idea should be really f'ing scary to anyone who knew what a schelkin spiral is designed to do, and what 4' of schelkin spiral is capable of, but perhaps one could make a only 'slightly coaxial' chamber with, say, 6" of 'submerged' ribbed barrel?

With the obvious caveat that it is never, <i>ever</i>, to be used with enriched or sensitive fuel-air mixtures.
  • 0

User avatar
boilingleadbath
Moderator
Moderator
 
Posts: 1642
Joined: Sat Mar 12, 2005 10:35 pm
Location: Pennsylvania, USA
Reputation: 0

Unread postAuthor: frankrede » Sat Apr 14, 2007 10:11 pm

The barrel, if used in used in a hybrid mixture would collapse on itself from the pressure. SCH-80 pvc actually is weaker when pressure is coming from the exterior.
  • 0

Current project: Afghanistan deployment
User avatar
frankrede
Lieutenant General
Lieutenant General
 
Posts: 3220
Joined: Thu Jul 20, 2006 9:47 pm
Reputation: 0

Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 11:00 pm

BC Pneumatics wrote:The best place of ignition is as far from the barrel as possible, so the flamefront can accelerate, as far as anyone has ever been able to show. the whole 'put it in the middle some more is burning' has never been proven as effective. (Most people seem to think the opposite, it hurts performance.


I believe jimmy modeled an adiabatic combustion launcher scenario, and determined that the theoretical optimal spark positioning for a single gap is slightly rearward from the center. I will try to dig up the spudtech archive to verify this.

I remember reading something about schelkin spirals a while back, and I believe they are designed to rapidly accelerate the flame front to DDT. I doubt this would be a modification one would want to perform inside the chamber of a PVC coaxial, but a steel gun could most likely handle it.
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: LucyInTheSky » Sun Apr 15, 2007 6:20 am

For the hybrid idea, you could just make the piolet abit larger and only fill it to about 5psi higher than the pressure of the mix. This way the piston will still be able to move back as it will just compress the air behind it until it is equal with the combustion at the front so it wont be a bomb, plus it is like a self setting bust disc.
  • 0

If your getting chased off a police dog, try not to run through a tunnel then onto a small see-saw then through a ring of fire. They are trained for that!
User avatar
LucyInTheSky
1st Lieutenant
1st Lieutenant
 
Posts: 293
Joined: Fri Apr 21, 2006 11:46 am
Reputation: 0

PreviousNext

Return to Combustion Cannon Discussion

Who is online

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

Reputation System ©'