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

In total there are 49 users online :: 2 registered, 0 hidden and 47 guests

Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

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

The Team
Global Moderators
global_moderators.png CS

Llamas with Hats

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

Llamas with Hats

Unread postAuthor: DR » Mon Apr 12, 2010 4:17 pm

Sitting in the garage, wondering why Llamas with Hats is so damn funny, I had an epiphany:
If a pulsejet's cycling draws in fresh air to ingnite the new fuel air charge, then why couldn't we do the same to vent a basic metered-propane combustion launcher?

We all pretty much understand; That when the fuel/air mixture is ignited... as the gas expands and the combustion chamber empties, the pressure inside the Combustion Launcher drops. Due to inertia of the moving gas, this drop continues for some time even after the pressure falls back to atmospheric. The expansion stops only when the momentum of the gas pulse is completely spent. At that point, there is a partial vacuum inside the Combustion Launcher.

The process can now reverse itself. The outside (atmospheric) pressure is now higher than the pressure inside the engine and fresh air can rush into the end with the PVC Check Valve.

The words from the last 2 paragraphs, were copied and pasted out of a pdf file about Valveless Pulsejets, which was created by Bruno Ogorelec. The words were edited to fit the current application of a new Combustion Launcher design concept.

It may take quite a bit of experimenting to find an ideal C:B ratio, such as the "general rule-of-thumb 0.7:1" for most basic metered-propane combustion launchers. But, how cool would it be to have a launcher that now requires no (additional) lapse in the reloading sequence, to vent the chamber?

My idea is:
If the check valve were mounted so that the flow was towards the inside of the combustion chamber, then perhaps the 14.7 pounds per square inch of force would be more than sufficient to overcome the spring pressure in the check valve and allow fresh air to be drawn in?

Starting with a 0.7:1 C:B ratio, the barrel can be progressively shortened, until the tester experiences a noticeable reduction in performance... Much like Latke' did in his testing.
  • 0

CVAV.JPG (22.69 KiB) Viewed 870 times
User avatar
Sergeant Major
Sergeant Major
Posts: 179
Joined: Fri Oct 08, 2004 8:16 pm
Reputation: 0

Unread postAuthor: Ragnarok » Mon Apr 12, 2010 6:52 pm

While an interesting idea, I turned up a few problems when I looked at borrowing ideas from pulse jets a while back.

The big part is that pulse jets don't transfer the momentum of their gasses to a projectile. Robbing the gasses of momentum to drive a projectile will lessen venting effectiveness.
And with that considered, I'm not sure the gasses will have the remaining momentum to continue their flow out of the muzzle against the vacuum needed to open the check valve.

Yes, the idea can theoretically work, but it will demand something more sophisticated than a standard check valve. (I'm sure have a design for a valve that might have fulfilled the role, but I can't find the file on my computer at the moment. Meh, it'll turn up eventually.)

... and as you suggest, it'll need a less efficient C:B ratio to leave some reasonable momentum in the gasses.
  • 0

Does that thing kinda look like a big cat to you?
User avatar
Chief of Staff
Chief of Staff
Posts: 5401
Joined: Tue Dec 19, 2006 8:23 am
Location: The UK
Reputation: 13

Unread postAuthor: jimmy101 » Mon Apr 12, 2010 7:05 pm

It would be interesting to know just how well a generic combustion spudgun ventilates all by itself, without any kind of breech valve system.

I would WAG it that 50% or more of the combustion gases are expelled during firing and replaced with fresh air.

I can't think of any easy way to actually measure the CO2 to O2 ratio after firing, if I just had a mass spec...
  • 0


Lieutenant General
Lieutenant General
Posts: 3160
Joined: Wed Mar 28, 2007 9:48 am
Location: Greenwood, Indiana
Country: United States (us)
Reputation: 14

Unread postAuthor: POLAND_SPUD » Tue Apr 13, 2010 3:23 am

someone has pointed out that in a piston valve hybrid there is a slight vacuum inside the chamber after each shot... probably due to the fact that the gases are first exhausted, then the piston closes and the gasses inside cool down
  • 0

Children are the future

unless we stop them now
User avatar
Chief of Staff
Chief of Staff
Posts: 5405
Joined: Sat Oct 13, 2007 4:43 pm
Country: Israel (il)
Reputation: 11

Unread postAuthor: psycix » Tue Apr 13, 2010 8:09 am

Holes in an endcap, rubber flap on the inside that can open, but also seal depending on the pressure differential.

However, I don't think that the gases have enough momentum to suck in all the air you need.
  • 0

Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

Spudfiles steam group, join!
User avatar
Donating Member
Donating Member
Posts: 3684
Joined: Mon Jun 25, 2007 7:12 am
Location: The Netherlands
Reputation: 0

Return to Combustion Cannon Discussion

Who is online

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

Reputation System ©'