Everything is coated with highly flammable powder, so they have no trouble igniting from the burst.

It's a challenge actually to prevent everything from igniting prematurely. Especially with mortars, since they're propelled out of a tube with an explosion that can compromise the shell, igniting its contents immediately rather than up in the air as intended.

If you've ever seen a public fireworks display where the stars exploded awkwardly close to the ground instead of way up with everything else, it's likely because the explosion for thrusting it into the air managed to ignite the contents instead of the delayed fuse.

>> they're propelled out of a tube with an explosion that can compromise the shell

I've always wondered why large fireworks use explosive 'lifting charges' rather than a proper rocket design (slower burning fuel). Is it a legal limitation against creating missiles?

I haven’t done the math, but I would assume it’s more efficient. Ignoring drag, if you launch out of a tube, you don’t need to carry any of your fuel with you, and, at least for very subsonic muzzle velocities, you might be able to arrange for the pressure in the barrel behind the shell to be near atmospheric when the shell exits. In other words, most of the launch charge energy could actually go to kinetic energy of the shell.

In contrast, rockets are very inefficient. Except in the special case where the exhaust velocity equals the rocket’s velocity, a large amount of the engine’s energy ends up as kinetic energy in the exhaust.

This is partially addressed by the article:

>Simple stick stabilized rockets work out well for small pyrotechnics, but don’t scale well. If you want bigger bangs and larger bursts in the air you need to get more payload up there. A larger rocket, with a few pounds of excitement at the front end, would require a stick 20-30 feet in length; not very practical.

En essence, rocket like designs are less stable, less predictable, and more complicated.

It is hard to have something simpler and cheaper than a ball.

Except even the article acknowledges that the stick's just doing the job that fins normally would do on other rockets. I know from first-hand (well, 1.5 hands; my stepdad was the one building and launching, and I was just there to watch and occasionally help out) that model rockets are capable of putting some decently-heavy payloads quite a ways into the air without a 20-30 foot long stick, and while model rockets are typically designed to be a lot sturdier than bottle rockets, they're also typically designed to be reused whereas bottle rockets are not (if such a requirement didn't exist, then they could be built to disintegrate and explode in the air, and be dirt cheap).

While I can't seem to find any specific rules from the FAA, Tripoli, or NAR specifically prohibiting the launch of a rocket that deliberately explodes in mid-air, my impression from my recollections of talking to expert rocketeers at launches (and asking them the sorts of questions a 10-year-old might reasonably ask, like "can I launch a rocket from an RC plane?" or "Can I launch a rocket from another rocket?") is that this would be thoroughly frowned upon my launch officials and/or regulatory bodies, and if the rocket exceeds the specifications for "Class 1" it'd require prior FAA authorization (and I can't imagine "Hey I'm gonna strap an H motor to this explosive ball and hope for the best" to go well on that front).

Author of article here. Thanks for kinds comments.

Yes, it's possible to stabilize bottle rockets with fins (to adjust the CP), but as the article hints, the stick is multipurpose in helping with launch, as well as stability (plus being easy to manufacture with low precision).

I just published a follow-up article talking about bigger rockets. http://datagenetics.com/blog/july22020/index.html

Aeronautics is a complex science and my articles are intended to pique interest, not oversimplify or belittle. It's a hard balance to explain things in a few hundred words. I hope you enjoy them and, if there is interest, they become the catalyst for you to learn more. It's a fascininating subject.

That is a fantastic follow-up article. Bookmarked :)

Re: the stick...

> the stick is multipurpose in helping with launch, as well as stability (plus being easy to manufacture with low precision)

True. Model rockets achieve mostly the same, except instead of keeping the stick attached to the rocket, the stick is instead attached to the launchpad, and is stuck through two (or more) holes on the side of the rocket (fancier/bigger rockets typically opt for rails, but a simple metal rod is typically good enough for smaller rockets).

Like I mentioned, if such a rocket were to be built to be disposable (i.e. with a "recovery system" that just disintegrates the rocket instead of something like a parachute), they could probably be cost-competitive and performance-competitive with mortars (probably a slightly-higher upfront cost due to the launchpad and electric ignition system instead of a traditional fuse, but not by a whole lot); that they ain't typically used for pyrotechnics seems to imply a regulatory issue with doing so.

With a mortar shell most of the lofted mass is burnt or converted into tiny shreds. With a rocket the casing plus stick/other-aerodynamic-guide falls in a few coherent pieces, sometimes burning, which means for large (public display) fireworks would restrict the launch site and direction, ideally out over a body of water. Somewhere like Cape Canaveral in fact...

Thanks for that info!

Read the article, explains it and shows cross sections of fireworks to aid understanding.

Yeah, I saw that photo, and thanks. But it didn't really explain to me how pieces of disconnected carbon ignite each other without some physical link. Unless somehow the inner particles create a cloud of fire that ignites the larger balls? That's the part I'm not getting.

I agree the article doesn’t really explain it. The short answer is the burst from the shell generates enough heat to ignite the payload (the stars) before they’re blown out into the sky. No fuse is needed because the payload is flammable enough to ignite but not so flammable that nothing interesting is seen from the ground. That sounds like a cop-out but it’s why designing fireworks is a science and art. :)

The purpose of gunpowder is to burn rapidly. Burning generates a lot of hot gas that tries to expand. The bomb's physical shell, along with the inertia of the stars, resists the expansion for a brief moment, creating pressure that's used to propel the stars away at high speeds, but also ensures that the burning gunpowder charge has time to ignite the stars before they fly away.

Gunpowder is meant to burn at controlled rates, but traditional gunpowder, black powder, used in fireworks and older firearms typically burns very quickly. Smokeless gunpowder for modern firearms have controlled burn rates. Depending on barrel length, etc. you may want the gunpowder to burn faster or slower to affect the ballistics. I imagine they use certain ratios of differing ingredients to control the rate of burn in fireworks as well.

Burn rate for black powder depends on particle size. Fine powder burns faster than coarse powder. So one characteristic of high-quality powder is uniform particle size.

But yes, changing the formulation also affects burn rate. For example, replacing potassium nitrate with potassium perchlorate yields a much faster burn.[0] Also replacing the carbon with aluminum dust gives you flash powder.[1]

0) https://pyrodata.com/compositions/Black-Powder-Perchlorate

1) https://pyrodata.com/composition/flash-powder

The photo seems to show a layer of the same material as the exploding core between the layers of spheres. I presume it's gun powder.