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Comment by pengaru | Hacker News Reader

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.

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>> 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?

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.

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.

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!

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