SpudFiles

A Crony can definitely give false readings. I got the result shown below from a pneumatic shot. Needless to say it wasn't actually going that fast.

Nice shootin' there Tex... I don't think you have the energy in 3X to be anywhere close to mach 4, at least according to HGDT. 1400 fps is much more in the ball park. That was a golfball shot wasn't it?

Also I don't believe you acheived DDT there...conditions just not there. I think you can chalk up the loud shot to ideal fuel/air/burst disk/projectile conditions.

picture

no really it is holes NOT THE DISK!

SpudFarm wrote:yes, i have used $1400 on it now and are not going to retrie it i am just not going higher than between 1x and 5x

UPGBH10x may have to change to UPGBH5x

yeah i thought of it but if i don't get the strange effect anymore i just take a chance.

That sounds optimistic to me, I think I'll have to call a false chrony reading caused by unseen factors.

The speed of sound in an adiabatic propane-air combustion is only about 3200 fps. You're doing about Mach 1.35 relative to the combustion gases, and although that's not completely impossible, it's infinitesimally unlikely.

Personally, I agree with Starman - most likely you hit some near perfect conditions, a nearly stoichiometric and homogeneous mix in the chamber that ignited particularly well, and a good burst disk thickness.

That will have affected the muzzle blast, and that will have interfered with the chrony reading.

And using a Beta version of a chrony? If you use a Beta version, little wonder you'll get odd results. Use the proper release man!
...Sorry, bad joke.

but if you say the speed of sound in it is 3200fps the muzzle blast could not have gone that fast either.

SpudFarm wrote:but if you say the speed of sound in it is 3200fps the muzzle blast could not have gone that fast either.

It doesn't need to - all it needs to do is confuse the chronograph.

The chronograph measures the time it takes for whatever to travel between the two points, by detecting when something is in the way of the sensor - and that could be muzzle blast or golfball.

If the muzzle blast blocks the sensors in the right/wrong way, you could get some very weird results.

Let's say the golfball takes 10ms to travel between two points (it's actually a lot less, that's just a nominal figure).
However, if the muzzle blast blocks the sensor closest to the muzzle for an extra 2 ms after the golfball has passed, the chronograph will assume the ball left the first sensor 2 ms later, so it'll assume it only took 8ms to make the trip, and will predict velocity 25% higher than it actually is.

That's the problem with putting your chrono too close to the muzzle.

and for once i am happy to be wrong because it was some sort of aluminium

Either that what Ragnarok, or when the chrony's sensors are optical, the light from the muzzle flash may also confuse it.
Anyways, there are alot of possibilities which mess up a chrony and you should never look at one shot, always at the average of a row of shots, and scratch out the unlikely ones.

How long is the chamber and how many spark gaps have you in there? Where is the spark gap placed?
We might find out if DDT is likely to have happened.

we can't find out anything because we have no clue about DDT.

look at the pic and the chamber is 53cm on the inside. if we count the end plates it is 60cm

Spudfarm sent me the details of the gun and I've calculated the efficiency of the "Mach 4" shot;

Projectile KE:
45g Golfball at 1350 m/s(~Mach 4, 4420 FPS)
KE = 0.5(0.045kg)(1350m/s)^2
KE = 41.0 KJ

Chamber Potential Energy (PE):
60cm L by 12.5cm ID (23.6" L by 4.92" ID)
Chamber volume = 7.36L = 449ci
Assume temperature at launch of 75F (27C)
For 1X propane+air PE=2220 KJ/mol (530 KCal/mol, 50500 KJ/Kg)
Assuming a 3X mixture is three times the energy, 6660 KJ/mol
the math is in the attached image
PE = 83.8 KJ

Efficiency:
Efficiency = Projectile KE / Chamber PE = 41.0 KJ / 83.8 KJ *100
Efficiency = 48.9%

(assuming no errors in my calculation )

Typical 1x combustion guns are ~10% efficient. A hybrid really isn't meant to be more efficient, just more powerful. ~50% efficiency doesn't sound very likely to me. Gasoline ICE's running at their most efficient speeds and using ~10:1 compression ratios are usually less than 30% efficient.

So it looks to me like some type of timing error; perhaps the shock wave, perhaps the flame, perhaps vibration through the metal case of the Chrony (speed of sound through steel is ~5,100 m/s or more).

Your calculation also match my numbers, so i think your calculation are correct.

Wasnt there some sort of "rule of thumb" which told the approximate DDT runup distance on a propane/air mix, where the distance gets shorter as the mix gets higher.

no.. there is no one that knows