Hybrid velocity

[Pennywise]

I have measured upwards of 3355 feet per second with a hybrid I've been working on. Mind you, I'm using a .510" diameter 3d printed 100% infill aerodynamic projectile weighing very little. I highly doubt this is the projectile velocity, and instead is catching debris or even the gasses themselves

...But still
With the temperature propane burns at, the local speed of sound should be more like 3100fps.

Do the expanding burning gasses in a hybrid burn at a higher temperature than a standard stoichiometric ratio of propane? Could there be further shock-heating as the mixture is forced from the 1.5"ID chamber into the .510" barrel? Or maybe something I'm not considering?

*Edit-i know it's been talked about a bit, but could the expanding zone of flammable gasses being ignited cause the wave to move faster than what should be the local SOS?
**double edit-I'm beginning to wonder if the local SOS of the byproducts of combustion may be significantly higher than that of regular air. There have been rifles capable of 5200fps+. Gunpowder burns at a similar temperature to propane (slightly lower, actually). So the only explanation I can think of is that the chemical components left over after combustion have a higher SOS, allowing their pressure wave to travel faster.

[quinn whitsitt]

dude great job!!!!

[Pennywise]

Are the expanding gasses in a hybrid not limited to local SOS like the released pressure in a pneumatic is?

[mrfoo]

I'm not sure that the speed of sound is an overly useful measure. As I understand it, and I've probably totally misunderstood, even in a "conventional" firearm, the combustion is subsonic even though the actual projectile velocity can be at least supersonic; and that it's pressure and the time the pressure has to act on the projectile (i.e. barrel length) that affects muzzle speed rather than (or, at least, incombination with) the speed of sound in the combustion zone.

The wiki article on light gas guns suggests the following:

Up to the speed of sound, thermodynamics provides a simple, approximate calculation approach: the projectile is accelerated by the pressure difference between its ends, and since such a pressure wave cannot propagate any faster than the speed of sound in the medium, thermodynamic analysis suggests that the muzzle velocity is limited to the speed of sound. However, beyond the speed of sound, the kinetic theory of gases, which determines the speed of sound, provides a more detailed analysis in terms of the gas particles that comprise the working fluid. Kinetic theory indicates that the velocity of the gas particles is Maxwell-Boltzmann distributed, with the velocity of a large fraction of the particles exceeding the speed of sound in the gas. That fraction of the gas can continue to apply pressure to and therefore accelerate the projectile beyond the speed of sound in diminishing amounts as the projectile's speed increases.

That's reaching beyond my pay grade, however. Other regulars, including the designer of VERA, certainly understand this far better than I do. Orders of magnitude better, in fact.

[D_Hall]

A few thoughts in no particular order....

1 - Post-combustion chemistry absolutely matters. It can drastically alter the local sonic velocity.
2 - Traditional powder-based guns can go much faster than OP's listed 5200 ft/s. I've personal experience with a gun that runs at 8,900 ft/s.
3 - I'm a bit bored, so I'll run some numbers regarding nominal SOS (I'll edit this post in a few minutes).
4 - While the quoted bit discussing Maxwell-Boltzmann distributions affecting the performance of guns are logical, I've never attempted to characterize them. Pushing past the local sonic velocity can be done, but the return on the investment has never seemed worth it to me.

edit: For what it's worth, I get that the local speed of sound in post-combustion propane/air is on the order of 3400 ft/s.

[Pennywise]

What are methods of passing beyond the local SOS?
I imagine things like raising temperature beyond that of standard combustion or using a fuel whose combustion produces elements with higher SOS, but these are both modifying the SOS limit rather than surpassing that limit.

The ~3400fps lines up with what I've seen. For the record, the highest velocity I've seen from this gun that actually looks plausible was just over mach 2. Something around 2500fps at a 28ishX mix.

How is it that gunpowder can push 8900fps? I thought the mass of the gunpowder itself made those kinds of velocities impossible? I've read of large caliber shells being necked down to fit a .220 bullet, and those can't touch that velocity.
...Unless there were other charges down the length of the barrel that would ignite immediately as the projectile passed them, perhaps?

[quinn whitsitt]

liquid fertilizer mixed with starter fluid and gas
in 75 to 20/15 mix works well

[D_Hall]

What are methods of passing beyond the local SOS?

The previously mentioned exploitation of Maxwell-Boltzmann distributions. Highly inefficient and has it's own limitations. Not useful for much more than braggin' rights.

I imagine things like raising temperature beyond that of standard combustion or using a fuel whose combustion produces elements with higher SOS, but these are both modifying the SOS limit rather than surpassing that limit.

Yup! If you want to go fast with a (non-electromagnetic) gun, you have three options for your working fluid....
1 - A hot gas.
2 - A gas with a low molecular weight.
3 - A hot gas with a low molecular weight.

How is it that gunpowder can push 8900fps? I thought the mass of the gunpowder itself made those kinds of velocities impossible? I've read of large caliber shells being necked down to fit a .220 bullet, and those can't touch that velocity.

First, it's worth noting that (roughly) half of the gases associated with the combustion of nitrocellulose is H2O. Hey, that's got a molecular weight of 18. That's a lot less than air (28ish). Oh, and it's hot. In other words, firearms use option 3 from above. That said...

Admittedly, 8,900 is right at the ragged edge of what's possible with a powder gun. As for a .22 bullet not being able to pull it off.... I've never tried that particular trick, but a couple issues come to mind. The first is bullet density. When we're shooting 8,900 we aren't shooting lead (sabots are involved). Secondly, a .22 is small bore. The smaller the bore, the more heat the gun's gases will lose to the walls of the barrel (square/cube law in action!). This will make it more difficult to push things to the limit.

...Unless there were other charges down the length of the barrel that would ignite immediately as the projectile passed them, perhaps?

Nope.

[quinn whitsitt]

not true as I am a friend with a gunsmith about that powder gun you just have to have a massive case with much higher powder concentration and faster than 8900 is possible with enough fuel and air for spudgun if you use the right feul

[Pennywise]

Quinn, please stop.

D_Hall,
I'll have to learn more about Maxwell-Boltzmann and related topics, thanks.
I may be at the upper limits of what can be accomplished with propane, in that case. Given your reasoning for the velocity potential with powder, it would stand to reason that hydrogen/oxygen would be the best possible fuel to use in a hybrid. The reaction should produce exclusively H2O, though I've read that DDT is an immediate danger regardless of chamber shape with that mix (LARDA comes to mind). If correctly harnessed, would H/O be able to keep up with a powdered gun? I feel like, if anything, the light molecular mass of the gasses relative to gunpowder should give them a chance, even though the change in volume between a solid->gas vs a gas->gas is hard to compensate for.

As for the 8900fps gun, do you recall what the actual projectile was? You mentioned saboted, so it could really be any size under the given diameter. I can't imagine this was a rifled barrel (the rotation would likely cause failures), so I assume the testing was purely for high velocity as opposed to any kind of accuracy.

Edit-another thought I've been having is whether preheating the mixture prior to ignition would aid in increasing the SOS. The easier method would likely be to shroud the interior of the chamber with a surface that doesn't absorb as much heat as the current stainless steel does. With as quickly as the projectile leaves the barrel, I can't imagine this would make too big of a difference, though.

[D_Hall]

Yes, H and O are great. That's why you see 'em used in rockets. For bonus points, run it just a bit fuel rich and you get even lower molecular weight without much loss in temperature. And yes, that's why rockets run such. The same physics apply! That said, running H/O in a gun at anything that resembled high pressures would be a scary proposition (it LOVES to detonate)...but in theory it could keep up with a powder gun.

But if you really want to go down the pure velocity rabbit hole, you could build a two stage gun. Run the first stage as a standard propane/air hybrid with the second stage being hydrogen (or helium for slightly reduced performance, but increased safety). I don't imagine 10,000 ft/s would be particularly difficult (but VERY expensive!!!).

As for the projectile of the 8900 ft/s gun... you can read all about it. https://imemg.org/wp-content/uploads/20 ... ts2015.pdf

[quinn whitsitt]

it did not use a sabot and had to use the deflagation dang it that word is very hard to spell anyway back to the topic it did not use a sabot just a solid steel projectile besides I have used liquid guns a lot more and they seam to be more reliable though if you really wanted some velocity I whould use hephintonitrocubane mixed with elemental fluorine to give it some extra punch

[D_Hall]

Hint: Side 7 has the title "Typical FI Laboratory Gun Sabot" and photographs of 2 different sabot designs.

[quinn whitsitt]

I was saying my gun did and sorry I should have clarified

[Pennywise]

Very interesting. At those velocities, does the sabot shatter on exit?
I would be lying if I said I haven't been thinking of ways to pack a 2 stage gun into a rifle-esque design.

Quinn, as I've said before, PLEASE post any picture/video/drawing of any of the guns you've built. I would LOVE to see the design of any of your claimed accomplishments. If you can't figure it out, you can email the pictures to me, and I can post them for you.