Diminishing Return

[velocity3x]

I did some testing yesterday but. it wasn't on the gun I intended. A great deal of radar and high speed camera equipment with operators was made available to me and after everything was set up and ready....my gun malfunctioned. So the day wouldn't be a total loss, we decided to shoot and track my golf ball cannon....44-mm HUNGUS. The shot results are below and they really illustrate the DIMINISHING RETURN!

All measurements were taken with doppler radar.
Temp. 66F
Barometer 29.91
Humidity 25%
Barrel Bore 1.71 x 72"

Shot 1. Golf Ball / nitrogen / 100psi / 635 fps.
Shot 2. Golf Ball / nitrogen / 100psi / 651.5 fps.
Shot 3. Golf Ball / nitrogen / 500psi / 1,036 fps.
Shot 4. Golf Ball / nitrogen / 700psi / 1,194.22 fps.

7x the gas consumption for about 83% additional velocity....UGH!

[Gippeto]

I know what you mean.

Ggdt predicts only an extra 190fps or so when increasing pressure from 1500psi to 3000psi in the co-ax. But that's also an extra 250+ fpe.

Might not seem to be worth it as I can get a heck of a lot more use out of the N2 if I shoot the bottle down to 1500psi...but then....

[Ragnarok]

7x the gas consumption for about 83% additional velocity....UGH!

However, it's the increase in energy you should be looking at. An increase from 640 fps up to 1190 fps is 345% the energy.
In other words, 7x the gas for 3.45x the energy, so less than half the efficiency.

The problem with those velocities is that you're REALLY pushing the speed of sound in Nitrogen, so you're getting major transonic losses.

If you used about 400 psi, my best guess is that you'd be getting about 950 fps, which seems a fairer trade-off, and about as fast as you can really push a gas of that density without transonic problems.

[MrCrowley]

I'm liking what I see when I change your piston to 2.5", 2" porting and your barrel to 2" diameter at 96" long with a sabot golfball round...

I'm getting over 5500ft/lb at 1900fps
But if you drop the pressure down to 400PSI, you get the same results as you do currently at 700PSI...you only have to build a new cannon

[velocity3x]

Rag,
I've been thinking about switching to Helium. What are your thoughts on He at high pressure? GGDT predicts a huge jump in performance from N2 to He. Can it really make that much difference?

[ramses]

I know you addressed that to rag, but the main limiting factor in these things (besides pressure and flow)is the speed of sound of the gasses

Sos He: 972 m/s
SOS N2: 353 m/s

That is why low mix hybrids perform so much better than high pressure pneumatics; speed of sound is also related to temperature, and hybrids heat the gas.

If you can afford it, hydrogen has a speed of sound around 1286 m/s , over mach 3!

go for it, but leaks will be much more of a problem, since the molecules are so much smaller. And For H2, you face hydrogen embrittlement.

[POLAND_SPUD]

heh that's pretty much the same conclusion I got when I modelled my semiauto MKIII in ggdt at 500 psi and 1000psi

to get twice the muzzle energy it gets at 500 psi I would have to use 1800 psi

I might as well build another version of it firing 12.7mm BBs

[Ragnarok]

As Ramses says, the big difference here is speed of sound. Helium has a speed of sound around 2.9 times that of air (and 2.8 times that of pure Nitrogen), so immediately, all problems with transonic flow happen at around three times the velocity.

Rather than speeds above about 200 m/s starting to risk transonic flows, you're looking at speeds above 600 m/s.
My own modelling shows that Helium is an improvement over air at above 150 m/s (below 100 m/s, air actually has the edge, but we're not talking about those velocities here).

But yes, Helium will be result in much higher velocities. Helium does however leak faster, having the smallest particle size of any gas. (Before anyone comes along and says "but hydrogen is smaller", hydrogen isn't a monatomic gas. Helium is.)

Hydrogen is the "ideal gas" as far as pneumatic performance. With a speed of sound of 3.8 times that of air, as well as a decent ratio of specific heats, hydrogen is capable of insane velocities. Of course, the downside is the potential for explosion and embrittlement in steel.

That said, you can theoretically make hydrogen yourself with a well made electrolysis cell - you'd need to collect the hydrogen and oxygen separately, but that's easy enough.

Either way, if you want to go really fast, you do need a low density gas.

[D_Hall]

And For H2, you face hydrogen embrittlement.

Not really a problem.

Hydrogen embrittlement is caused by the hydrogen gas quite literally disolving into the steel and collecting in inter-crystaline defects. The thing is, to have this happen the hydrogen needs to be in contact with the steel for an extended period. The brief moments one has a spud gun pressurized aren't gonna do it. Further, just as exposure to hydrogen embrittles over time due to the gas disolving into the steel, the embrittlement can be "removed" over time too. All it takes is removing the steel from the hydrogen environment and giving the hydrogen time to dissolve out of the steel.

[ramses]

Hydrogen embrittlement is caused by the hydrogen gas quite literally disolving into the steel and collecting in inter-crystaline defects. The thing is, to have this happen the hydrogen needs to be in contact with the steel for an extended period. The brief moments one has a spud gun pressurized aren't gonna do it. Further, just as exposure to hydrogen embrittles over time due to the gas disolving into the steel, the embrittlement can be "removed" over time too. All it takes is removing the steel from the hydrogen environment and giving the hydrogen time to dissolve out of the steel.

Does this take place faster at elevated temperatures and/or pressures? I would think so, but I'm not an engineer. What kind of time scale are you talking about for an "extended period"? I have googled and such, but no specifics are given.

[jimmy101]

I think hydrogen embrittlement is an over rated potential problem. H2 is stored in steel tanks. It is purified in steel equipment. Yes it happens and under some circumstances it can be a problem but in general it isn't all that big a deal. There are steel tanks that have been used to store hydrogen that are pushing 100 years old.

(Spring steels seem to be particularly supseptable to embrittlement.)

Like D_hall said, it is generally reversible and the steel is gong to spend a heck of a lot more time in air than it does in H2.

[D_Hall]

What kind of time scale are you talking about for an "extended period"?

Months/years. Certainly nothing anybody on this forum needs to concern themselves with for cannon design.

[jeepkahn]

Something else that I've touched on before, In CERTAIN circumstances(projectile weight and amount of pressure mainly) Co2 will outperform air or nitrogen... Call it the exception to the rule, but with heavy projectiles and highpressures sometimes co2 is better...

On the diminishing returns aspect, with heavy projectiles, once I get past 900fps, I'm happy, because it takes SO much more pressure to get so little FPS....

[ramses]

On the diminishing returns aspect, with heavy projectiles, once I get past 900fps, I'm happy, because it takes SO much more pressure to get so little FPS....

The problem is moving close to the speed of the propellent gas. Hydrogen will be below mach .333, whereas the co2 would be near mach 1.

[jeepkahn]

On the diminishing returns aspect, with heavy projectiles, once I get past 900fps, I'm happy, because it takes SO much more pressure to get so little FPS....

The problem is moving close to the speed of the propellent gas. Hydrogen will be below mach .333, whereas the co2 would be near mach 1.

That's why I specified versus air or nitrogen, because without combustion you can't compete with helium or hydrogen...