Can someone explain this?

[Moonbogg]

I found a video showing someones "hybrid rocket" failing violently. It made me nervous because my cannons are aluminum, like this guy's rocket. Why did this rocket blow up like that? Anyone know whats going on here?

[youtube][/youtube]

[jackssmirkingrevenge]

A hybrid rocket and hybrid spudgun are not the same thing.

[Moonbogg]

Ah, I kind of see that they are loaded with tons of potential energy, unlike a spud gun.

[inonickname]

Yeah, you're talking about liquid nitrous oxide and a solid fuel. Not only that, but they keep up the pressure and temperature for a much longer time than a spudgun.

[jackssmirkingrevenge]

A spudgun is designed to consume all of the potential energy in one go, while a rocket carries all the fuel intended to be used up over a time interval.

It's when something goes wrong and all that fuel is consumed in a signficantly shorter time interval than intended that catastrophic failure tends to occur.

[Moonbogg]

That makes me feel a little better, thanks. I don't know whats been going on with me lately. I have been stricken with a major case of paranoia. I no longer trust my cannon, and when I think about a really heavy duty design, I don't think I would even trust that. I feel like I did when I first made my first advanced combustion. I was full of fear and uncertainty. Thats how I feel all over again, but nothing happened to really cause this fear all of a sudden.
My cannon has held up after many shots. I think the fear comes from not really knowing when it will fail and under what pressures would make it fail. I am also caused anxiety by things like fatigue and stress corrosion cracking that may occure over time without my realizing it.
I have never tested a cannon to failure, so I don't know how dangerous a 2x hybrid failure would be, especially hand held. I don't know how the aluminum would behave if it failed.
200PSI is relatively low pressure and the pressure doesn't last long, but that doesn't make me feel any better. Has anyone else been stricken by this kind of anxiety before?
I swear, even if I made a steel cannon with 1/2" wall thickness all around and used it at 2x, I would still be wondering "what are its true limits. Am I safe after many many shots. What about fatigue" etc.

Sorry for the rant.

[Insomniac]

Well, in that case, build something else. You're using it for enjoyment and recreation, not for a specific purpose. If you can't enjoy shooting it anymore, perhaps its served its purpose?

Either that, or eat some concrete and harden the feck up.

[jackssmirkingrevenge]

As discussed elsewhere I'm happy with my epoxy endcapped hybrids at 17x once I found out what works and what doesn't

Nothing wrong with being a little bit paranoid but safety is relative, I'm sure that your hybrid can take a lot more than you're subjecting it to.

[jrrdw]

IMO the rocket fail had mostly to do with dead space between the fuel and nozzle/exhaust port. Even though there is a port roughly or same size as the fuel tank, the dead space allows pressure to build up causing the explosion.

In my experiences of model rocketry I've seen many fails the most common being to much dead space between the fuel and exhaust port, resulting in my best guess 80% fail rate.

Cannons have very little dead space between the ammo and port etc...

[Insomniac]

That may be true jrrdw, but as JSR has pointed out the big difference is in the way energy is released in rockets vs cannons.

A cannon is designed to contain and then release all of its potential energy in an instant. It has to be designed to be strong enough to handle its entire supply of fuel burning at once.

A rocket on the other hand, is built much more lightly and also has a helluva lot more potential chemical energy. It relies completely on a very careful, controlled burn to keep the pressures and temperetures low enough for the combustion chamber to handle it. Unlike a cannon, if a rocket burns its fuel too fast, the pressure will rise high enough to cause failure.

[ilovefire]

As discussed elsewhere I'm happy with my epoxy endcapped hybrids at 17x once I found out what works and what doesn't

i just have to add iv even taken one of those to 27x and it hasnt failed yet, i mean sure its a little smaller but still 27x for epoxy, thats still pretty good

[Selador]

I found a video showing someones "hybrid rocket" failing violently. It made me nervous because my cannons are aluminum, like this guy's rocket. Why did this rocket blow up like that? Anyone know whats going on here?

IMO that "rocket" failed because it moved. And because the parts were not solidly connected, to each other, and to the same surface.

If the rocket had been bolted down to something that wouldn't move... And the "valve" and nitro tank were bolted down, likewise, to the same surface. So that every piece was individually solid, and solid to each other. I think they would have had a pretty good test run on that rocket.

As it is, to set something like that up, watch it all move and parts collide with/destroy, each other... then to say that "We need a bigger cylinder", indicates to me that they shouldn't be playing with this stuff at all.

I mean, what kind of idiot understands how to build a rocket, but doesn't understand the need to buckle said rocket down to something more solid than some little concret block... Because that rocket is going to want to move !!! And it's going to want to move whatever it is attached to. I mean, what is the purpose of a rocket anyway ???

~~~

IOW: It's just like setting up a pressure tank, of any sort, filled with a combustible gas or mixture... And releasing the pressure. As long as the tank is set solidly somewhere, the pressure will simply release in the easiest direction... Whatever valve you use to release the pressure. Even if the gas being released, is ignited.

People do it every day. It's called propane torches, welding torches, etc.

But you start releasing the gas, ignite it... Then shoot the tank and/or it's connection to the rocket/torch... It's liable to explode. Likewise, smack the hose, (whether rubber or metal), that feeds the "rocket" (Torch), with something that will break the hose, and release the gas in an uncontrolled manner, anywhere near the already ignited torch.. And there you have another probable kaboom.

The above being more likely if your "tank" and/or "rocket" are only just barely able to handle the pressure. And can be easily pierced.

These guys used their brains to design and build the rocket. But then turned their brains off when it came to testing/utilizing it.

~~~

Imagine what would happen if DHall had built Vera... Just sat it on the ground with the propane tank sitting behind it, and all the connections being loosely laid about between them... Then shot it.

Vera is likely to fly backward into the propane tank and connections, and boom !

~~~

Your cannon is not going to come anywhere NEAR being subjected to the same sorts of forces and dynamics. Unless you plan on smacking the chamber with a BFH, at the moment that combustion is taking place...

If you keep your cannon in the house, when not in use. On a shelf, or even in a case. And if it doesn't get kicked around, dropped, or banged up... Then you have little to nothing to worry about. You have more to worry about when you get in your car and go someplace.

You might want to disassemble it and have a look at all the parts after the first thousand rounds. But I don't think you'll find anything to worry about, even then.

[Insomniac]

Selador, the type of explosion you are describing is a BLEVE, which is commonly seen with liquified propane canisters and the like...

This was not a BLEVE, and I doubt the rocket moving had anything to do with it. This rocket simply failed as the materials could not handle the rate of combustion.

[Selador]

Selador, the type of explosion you are describing is a BLEVE, which is commonly seen with liquified propane canisters and the like...

This was not a BLEVE, and I doubt the rocket moving had anything to do with it. This rocket simply failed as the materials could not handle the rate of combustion.

It was a BLEVE.

Watch the vid again.

The "rocket" not only goes backward, it flips. It makes contact with the other elements.

The movement led to a probable puncture, or other damage of the sort, and the BLEVE.

Without the movement, there would have been no puncture, and possibly no explosion.

I suppose this is one of those things where we just can't know for sure.

If all the elements had been secured, as I suggest, and the explosion had happened anyway, then I would have to agree with you.

But it DID move. It did make contact with the other elements. It was easily puncturable by several of the other elements. And it does look like this caused a BLEVE.

It didn't even have to actually be punctured. It could have been that one of the through bolts got bent. Or the endcap got cracked. Or the fitting, in the endcap. Or a failure in the junction. Any of these caused by the flipping and crashing of the rocket. And any, leading to the catastrauphic failure.

The postulation in it's simplest form... The "rocket" may as well have been hit with a hammer at the critical moment.

[Moonbogg]

Hmm, looks like I need a bigger cannon! That will cure the fear. Thanks for the feedback. I think I may be suffering from some age related pussification.