Exhaust Gas...11 pics @ .04 seconds apart

[velocity3x]

This past weekend, a friend came over to help test my cannon (I was the sorry a** camera operator). The shot was done with a golf ball, 300 psi carbon dioxide and the 3/4 plywood box was 9'-10" from the muzzle tip.
Barrel = 72" x 1.71"


The wheel brakes were locked and yet the cannon recoiled back 12 inches.

I posted the pics to show how the exhaust gas flows from a large, barrel sealing piston valve.


I think I need a much longer barrel to take advantage to the gas volume. Any thoughts?

[Hotwired]

I think the pilot gas blast looks more impressive than the muzzle blast, it's like a little rocket motor

Aside from that.... I could have a 12 foot barrel on my signature cannon and I could still get more power by adding more barrel.

Keep dimensions to what you're comfortable with, you do not get twice the energy if you add twice the barrel, it's a diminishing returns game. Only if you start off with a small volume of gas can you really be efficient with it in a practical way.

If you have a large chamber you will get massive muzzle blast. Just how it goes if you want high energy in a short package.

[Technician1002]

Model it in GGDT. Play with various barrel lengths. In regards to the tank volume.. I am presuming this is the quanity of gas that the volume would take at 1 atm pressure.

Tank vol @ 300psi = 8ft^3

What is the physical tank volume. It's obvious it is smaller than my refrigerator. My small chest freezer is only 5 cubic feet.

Same question applies for the actual pilot volume. My marshmallow cannon has an actual chamber volume of 54 cubic inches. You will need the physical size for GGDT modeling.

[velocity3x]

Model it in GGDT. Play with various barrel lengths. In regards to the tank volume.. I am presuming this is the quanity of gas that the volume would take at 1 atm pressure.

Tank vol @ 300psi = 8ft^3

quote]

Tank vol @ 300psi = 8ft^3

300psi /14.7 (1 atmosphere)=20.48 atmospheres
8ft^3 / 20.48 atmospheres = 0.39ft^3 @ 1 atmosphere

Pilot volume is 6in^3 at 1 atmosphere

Looking at the pics makes me think that the Full retract of the piston is of no value. The projectal is already through the target before 0.08 seconds and the piston seems to fully retract just before 0.12 seconds.

[Technician1002]

Model it in GGDT. Play with various barrel lengths. In regards to the tank volume.. I am presuming this is the quanity of gas that the volume would take at 1 atm pressure.

Tank vol @ 300psi = 8ft^3

quote]

Tank vol @ 300psi = 8ft^3

300psi /14.7 (1 atmosphere)=20.48 atmospheres
8ft^3 / 20.48 atmospheres = 0.39ft^3 @ 1 atmosphere

Pilot volume is 6in^3 at 1 atmosphere

Looking at the pics makes me think that the Full retract of the piston is of no value. The projectal is already through the target before 0.08 seconds and the piston seems to fully retract just before 0.12 seconds.

120 ms is a very slow piston.. I suspect the piston is open faster and the gas from the high pressure is still venting from the pilot area in the video. If it is opening that slowly, it could use a closer ratio OD to valve seat diameter so the pilot area pressure is much lower before the valve starts to open so when it does open, it opens faster.

The chamber volume is about 4X too big.. The incremental increase on the golf ball is minimal due to the flow you can get out of the chamber. Your chamber is about the same size as my 2 inch QDV cannon which is 700 cubic inches. For that size of chamber, great improvements can be realized with a larger valve seat.

Plug a 2inch valve into GGDT to see what I mean. Then try a 3 inch. I'm building a 3 inch valve seat cannon into a tank slightly smaller than your chamber.

Looking at your pilot vent, I suspect the valve opening pressure is varying due to the gas spring in the pilot area. The vent shows a hard blast followed by a minimum flow followed again by a hard blast. This indicates the piston bounced in the pilot on the bumper and or gas pressure. From the flow, I suspect the piston opened with quite a bit of pressure remaining in the pilot area. This may indicate the piston is opening slower than possible due to high pilot area pressure. A larger valve seat or larger pilot valve may improve piston speed.

[Hotwired]

I'd never considered someone would write their chamber volume as the volume of atmospheric pressure gas it would hold at another specific pressure.

There must be a reason for that... it'll come to me...

I'm personally very much in favour of writing the volume as the volume of water you could stuff into the chamber which would be an 11 litre chamber or 674 cubic inches.

Your barrel on the other hand has a volume of... 2.66 litres or 162 cubic inches.

Essentially your chamber is oversize for the barrel you are using, roughly a 4:1 ratio, compare to the cannon in my signature which uses something like 1.6:1 and is still fairly wasteful.


By the way, while it's still in my mind, what were you using to take such a decent set of frames at 1/25 of a second?

[velocity3x]

120 ms is a very slow piston.. I suspect the piston is open faster and the gas from the high pressure is still venting from the pilot area in the video. If it is opening that slowly, it could use a closer ratio OD to valve seat diameter so the pilot area pressure is much lower before the valve starts to open so when it does open, it opens faster.

Tech,
I can't make the ratio any closer than it already is. It's a 2.0" piston with a 1.875" o-ring seal on the face.

Hotwired....I'd never considered someone would write their chamber volume as the volume of atmospheric pressure gas it would hold at another specific pressure.
There must be a reason for that... it'll come to me...

Considering that the total volume of gas is part of the equation, why would that seem so peculiar to you ?

[Ragnarok]

120 ms is a very slow piston.. I suspect the piston is open faster and the gas from the high pressure is still venting from the pilot area in the video.

On this note, it should be borne in mind that it takes some (well, relatively) time for the majority of the vapour to condense.

I've got videos where the projectile is long since through the target before the muzzle vapour cloud starts to form.

The vapour is no useful marker for anything much at all.

[spudtyrrant]

I can't make the ratio any closer than it already is. It's a 2.0" piston with a 1.875" o-ring seal on the face.

he means because 1.5" valve seat in a 2" housing you could have made the housing maybe 1.75" considering i'm sure you had the tools and it would have performed better because the diameter of the piston housing would have been less and therefore there would have been less to pilot.

On another note i just plugged you cannons specs into the ggdt and i got results as followed @ 300psi co2
72" barrel: 767 fps
84" barrel: 791fps
96" barrel: 812fps
108" barrel: 830fps
120" barrel: 848 fps

so based on those results yes you would get better performance if you increased the length of the barrel but the question is what is practical for you i mean 80fps increase is good but are you willing to drag around a barrel 4' longer to achieve that added boost if so then be my guest but its usually a compromise between power and practicability

p.s. its 1178fps @ 300psi helium btw

[Hotwired]

Hotwired....I'd never considered someone would write their chamber volume as the volume of atmospheric pressure gas it would hold at another specific pressure.
There must be a reason for that... it'll come to me...

Considering that the total volume of gas is part of the equation, why would that seem so peculiar to you ?

If you did the calculations by hand you possibly wouldn't have used a chamber 4x the volume of the barrel or you'd have noticed that even changing the chamber by quite a lot was having very little effect on muzzle energy.

If you used GGDT as everyone who's hung around here does in this place you'd have listed internal chamber dimensions + pressure used as that's what it takes as input. It doesn't show the actual equations or require any knowledge of them and I'd bet the vast majority of it's users are much happier that way too.

Reservoirs, cylinders or chambers to hold pressure are primarily listed by physical internal volume and things running on pressurised gas tend to have gas consumption based on the volume of pressurised gas used as opposed to the volume of gas it expands to.

That's all. Not getting at you, it's just different from the expected


In case you missed it: what did you film the shot with?

[Technician1002]

120 ms is a very slow piston.. I suspect the piston is open faster and the gas from the high pressure is still venting from the pilot area in the video.

On this note, it should be borne in mind that it takes some (well, relatively) time for the majority of the vapour to condense.

I've got videos where the projectile is long since through the target before the muzzle vapour cloud starts to form.

The vapor is no useful marker for anything much at all.

From the muzzle this is to be expected. The initial blast is a wave of high pressure which has only a little cooling. As the gas continues to expand it cools further. With CO2 there is no moisture in the gas stream. Only when it mixes with humid air do you get condensation. The dew point is lowered by the combining of the humid air with the dry CO2.

From the pilot, the expansion is immediate and there is no barrel full of warm air atmospheric pressure to lead the way, so the condensation near the vent would appear first. In addition the pilot vents a large volume before the piston opens.

As far as condensation speed.. It's pretty much instantaneous when temp drops below dew point. High speed video of the vortex cannon clearly shows very rapid condensation and return to vapor in the high speed ring.

From the data you provided on the piston OD, your ratio looks fantastic. The double spurt of pilot vent velocity may be simply due to piston bounce off the bumper. A more energy absorbing bumper may improve this. It is also possible this is happening after the projectile has left. If so, I wouldn't worry about it except wonder why the piston takes longer to move an inch than the golfball takes to move 6 feet.

A faster video camera to capture the vent pressure stream in detail could be helpful.

Nice job on the cannon.. May your next one be even better.
:wav:

[velocity3x]

In case you missed it: what did you film the shot with?

It's just a Sony digital still cam. It also shoots 1.5 minutes of video on the internal memory card. Nothing expensive...just middle of the road. If you're interested I'll get the model#.

spudtyrrant

p.s. its 1178fps @ 300psi helium btw

I had 2 tanks of helium delivered this just this morning...one for the tig
welder and one for the cannon.

Hotwired
If you did the calculations by hand you possibly wouldn't have used a chamber 4x the volume of the barrel or you'd have noticed that even changing the chamber by quite a lot was having very little effect on muzzle energy.

I was quite familiar with the calcs before I built it. Like most everyone here, when building.... "you use what you've got". The chamber is 4 times bigger because my original plan is for a cannon that fires combustible fuel OR pneumatically with only a slight change-over.

[Hotwired]

*facepalm*

I managed to read it as 1/50 of a second not 1/25 of a second which is the difference between 60fps and 30fps ^_^

[jackssmirkingrevenge]

I think I need a much longer barrel to take advantage to the gas volume. Any thoughts?

I thought we already mentioned that