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Supersonic crack or what?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Supersonic crack or what?

Unread postAuthor: elitesniper » Fri Apr 24, 2009 11:50 pm

Hey guys, I was just wondering why this happens..on my micro advanced combustion seen here http://www.spudfiles.com/forums/advanced-extreme-small-combustion-v2-must-see-t17950.html
when I put nothing in the barrel and just inject propane as i normally would with ammo in the barrel..when I press the trigger an extremely loud ear piercing sound that leaves everyones ear ringing it echos for a while, this is on my 40'' sch 40 3/4 inch pvc barrel..what is this that is happening? Is the gasses expanding extremely fast and accelerating down the barrel making this sound?
When ammo is in the barrel it does not make this ear piercing sound anymore
Does this only happens on smaller bore combustions?
Thanks guys! :)
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Unread postAuthor: inonickname » Sat Apr 25, 2009 12:42 am

Of course it's a supersonic crack..most soundwaves are supersonic.

What was the ammo? May be able to shed some more light. I doubt a 1x will push supersonic.
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Unread postAuthor: jon_89 » Sat Apr 25, 2009 12:50 am

My small combustion with a .5 inch barrel sounds like a .22 with no ammo. I dont think it is only small bores because a hooked my .40 cal blowgun barrel up to it and it was near silent. :?
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Unread postAuthor: Ragnarok » Sat Apr 25, 2009 6:40 am

inonickname wrote:Of course it's a supersonic crack..most soundwaves are supersonic.

Um... Sound waves, by definition, travel at the speed of sound.

However, in this case, with no projectile to slow the expansion of gases, it's possible that a supersonic shockwave is formed as a process. The sound of that shockwave still propagates at the speed of sound though.
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Unread postAuthor: inonickname » Sat Apr 25, 2009 6:52 am

That's sort of what I meant Rag :lol:

Possibly correct Ragnarok. I believe someone theorized in the spud wiki that a pneumatic cannon with a large volume of dead space could allow the well..gas front to accelerate to such speeds. Though this of course has not been tested and would no doubt be impractical.

Could be similar
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Unread postAuthor: Ragnarok » Sat Apr 25, 2009 8:10 am

inonickname wrote:I believe someone theorized in the spud wiki that a pneumatic cannon with a large volume of dead space could allow the well..gas front to accelerate to such speeds. Though this of course has not been tested and would no doubt be impractical.

It may not have been tested, but I have done some modelling on the matter.

The volume of dead space is fairly irrelevant to allowing the gas front to accelerate to such speeds - the volume does however become important for sustaining such speeds.

However, we're not talking about a pneumatic here, and a combustion's gases are undoubtedly in excess of Mach 1 in RTP air, hence, it's possible for a supersonic shockwave to form when the expanding gases meet still air.
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Unread postAuthor: covey12 » Mon Apr 27, 2009 4:35 pm

inonickname wrote:Of course it's a supersonic crack..most soundwaves are supersonic.

What was the ammo? May be able to shed some more light. I doubt a 1x will push supersonic.


sound is not supersonic, for a supersonic crack you have to go more than 1250fps which is the speed of sound, or sound barrier, when the sound barrier is broken, it makes a loud crack, since you have no ammo, there is no visible shockwave like you see when f-16s go supersonic, i hope that helped
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Unread postAuthor: Technician1002 » Mon Apr 27, 2009 5:20 pm

covey12 wrote:
inonickname wrote:Of course it's a supersonic crack..most soundwaves are supersonic.

What was the ammo? May be able to shed some more light. I doubt a 1x will push supersonic.


sound is not supersonic, for a supersonic crack you have to go more than 1250fps which is the speed of sound, or sound barrier, when the sound barrier is broken, it makes a loud crack, since you have no ammo, there is no visible shockwave like you see when f-16s go supersonic, i hope that helped


Good explanation. I think I have some photos to support that taken with a high speed video camera at 1,000 frames/second. Sorry the color is off. The borrowed camera was set for indoor and we shot outside. I brightened it up some with The Gimp.

Remember, each frame is only 0.001 seconds apart. How fast is the flow? This is an air cannon. Yes, it was loud.

Specs,
700 cubic inch chamber
~80 PSI
2 inch quick dump valve
2.5 inch barrel, 7 feet long.
Broken apple in barrel provides the applesauce mist.
Video speed 1,000 frames/sec.
metal measure in photo is 5 feet long with each foot marked in masking tape and magic marker.

Just how fast is that applesause traveling? :?:
The apple shows up Time +3 and initial impact is in frame +4.

This video was shot to confirm the valve speed. Full flow in under 2 ms.

The Freon tank the water bottle is sitting on was made into the winning t shirt launcher. :)
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Attachments
Applebottle1a.jpg
Last frame of stillness.
Applebottle2a.jpg
Time + 1 ms. Label is loose.
Applebottle3a.jpg
Time + 2 ms. Apple sauce has traveled almost 1.5 feet in 1 ms in the center of the flow.

If I get a chance to re shoot this, I would like to dump a handful of packing peanuts in the barrel.
Last edited by Technician1002 on Mon Apr 27, 2009 6:05 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Mon Apr 27, 2009 5:40 pm

covey12 wrote:sound is not supersonic, for a supersonic crack you have to go more than 1250fps which is the speed of sound

You're about 40 metres per second too high - it's more like 1120 fps, and of course it's not a constant, as it varies with temperature, pressure and humidity.
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Unread postAuthor: jimmy101 » Tue Apr 28, 2009 12:00 pm

Ragnarok wrote:
covey12 wrote:sound is not supersonic, for a supersonic crack you have to go more than 1250fps which is the speed of sound

You're about 40 metres per second too high - it's more like 1120 fps, and of course it's not a constant, as it varies with temperature, pressure and humidity.

Temperature --> big affect on SOS
(10X in absolute temperature is about 3X is SOS,
for a 10C temperature change the SOS changes about 2%)

Humidy --> small affect on SOS
(difference between 0% humidity and 100% humidity is less than a 0.5% change is SOS)

Pressure --> unmeasurably small affect on SOS in most situations.
(difference between 1 ATM and 10 ATM is less than 0.1% change in SOS).
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Unread postAuthor: Ragnarok » Tue Apr 28, 2009 12:53 pm

Which I am quite aware of.
However, what am I meant to say? That it has no effect?

Honestly. If I'd omitted it, someone would have had a go at me for that. If I leave it in, and someone has a go at me. You can't win.
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Unread postAuthor: Biopyro » Tue Apr 28, 2009 1:08 pm

You never can rag, haven't you learnt that by now?

Think of it as, the ammo usually absorbs the energy, but without it, all that goes into sound.
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Unread postAuthor: Ragnarok » Tue Apr 28, 2009 1:15 pm

Biopyro wrote:Think of it as, the ammo usually absorbs the energy, but without it, all that goes into sound.

Not really "all of it" - sound is mercifully low amounts of energy.

Bear in mind, although a speaker might be labelled as 10 watts, only around 1% of that actually becomes sound energy. Most is lost to magnetic fields and heating. The sound output of most things is usually similarly low percentages.

However, more energy is available to become sound.
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Unread postAuthor: Biopyro » Tue Apr 28, 2009 1:18 pm

I see what you mean, you just can't win!
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Unread postAuthor: jimmy101 » Tue Apr 28, 2009 2:41 pm

Ragnarok wrote:Which I am quite aware of.
However, what am I meant to say? That it has no effect?

Honestly. If I'd omitted it, someone would have had a go at me for that. If I leave it in, and someone has a go at me. You can't win.

Well, since you are aware of it I posted to make others aware of the facts. You aren't the only person that reads the forum.

Besides, for a typical spudder, humidity and pressure do indeed have no effect. If you can't measure it then the effect, for all practical purposes, doesn't exist.
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