
An aerospace engineer explains fireworks - raybb
https://datagenetics.com/blog/july12020/index.html
======
WalterBright
When I was a kid, every 4th I'd buy a bunch of bottle rockets and attempt to
make a multi-stage one by taping them together and varying the length of the
fuses. What would happen is the one with the shortest fuse would go bang first
and blow off the fuse of the other.

I finally resolved this problem by locating stage 2 further down the stick
(instead of adjacent), with its fuse up into the exhaust of stage one. Then it
would light as it was ascending, and the stage 1 bang was far enough away not
to disturb stage 2.

3 stages did not work because stage 1 could not lift the weight. A dual
booster bottle rocket did not work because the fuse timings were too erratic.
Always one would go first and blow the fuse off the other. (Wasn't that the
problem with the Soviet moon rocket?)

Naturally, in college I gravitated towards aerospace and got a minor in
Aeronautical Engineering.

Finally, when I had a real job, I could buy all the fireworks I wanted. I
bought a huge pile, and set about lighting and throwing them. After an hour or
so, I got bored and lit off the rest as strings. It was like someone threw a
switch in my brain, I completely lost interest in fireworks and it never
returned.

But my interest in jet engines, rocket engines and V8s has never waned.

~~~
tomcam
If compilers paid the way games do you’d be laughing at John Carmack way back
there in your rearview mirror!

~~~
WalterBright
Carmack is a far better programmer than I am.

I was originally a game programmer (Empire, Mattel Intellivision) but
wrestling with the compiler convinced me I could write a better one, and so it
went.

~~~
tomcam
I’d almost believe that if I hadn’t been following your work for about 27
years

------
pengaru
Decades ago I purchased a printed DIY fireworks guide from skylighter.com, it
looks like they've since made a lot of that information freely available on
their web site:

[https://www.skylighter.com/pages/free-homemade-fireworks-
pro...](https://www.skylighter.com/pages/free-homemade-fireworks-projects-and-
pyrotechnic-formulas)

~~~
magnetowasright
Thanks for sharing, this is awesome. When I inevitably lose my eyebrows, I'm
blaming you!

------
supernova87a
When you've got a firework with hundreds of little flammable bits inside
making the star pattern, how do they all get ignited? Does the center fuse
ignite some main charge, which through heat and expansion ignites the rest of
the bits? I mean, the puzzle to me is that the parts are not all connected by
fuses, so how do they go off?

~~~
pengaru
_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.

~~~
sandworm101
>> 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?

~~~
s1artibartfast
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.

~~~
yellowapple
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).

~~~
squeakynick
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](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.

~~~
yellowapple
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.

------
supernova87a
That video of the world record-setting firework is impressive. I started to
think, the amount of lifting charge needed to get a car's worth of mass up
that high must be big enough to start worrying about blowing the launch tube
(and firework itself) into pieces?

~~~
dmurray
Yes for amateurs. But we have plenty of experience from military applications
about how to launch a massive explosive really far. WWI artillery shells could
weigh that much and travel much further, though they weren't launched straight
up.

~~~
supernova87a
The caption says it was a 26 ft launch tube. I wonder, would you put the
explosive at the very bottom, or in the middle? Packed right next to the
firework, or some distance from it, for the gases to be able to act most
effectively?

------
aloer
Years ago I was watching a public firework + music and was a bit disappointed
in how out of sync everything looked.

I figured there must be an economically feasible way to trigger the explosions
remotely and in sync for such public displays. So essentially shooting the
firework the traditional way but igniting the payload on demand. Is someone
working on this?

~~~
milkytron
I'd imagine fireworks + music would be difficult because of the difference in
the speed of sound vs the speed of light, which would explain the out of sync
experience you had.

~~~
aloer
Apparently there are software simulations for this.

Something like
[https://www.youtube.com/watch?v=CNjggrxUQ78](https://www.youtube.com/watch?v=CNjggrxUQ78)

Not sure at what distance the speed of sound vs speed of light becomes
disrupting

~~~
TheSpiceIsLife
At 343 meters / 1125 feet the sound is going to be about 1 second out of sync
with the light, and that's going to sound terrible.

Wind direction and atmospheric pressure is going to make a small difference.

------
gorgoiler
Very interesting — I can’t wait for the opportunity to see this
counterintuitive property whereby bottle rockets launched into a wind will
steer themselves _into the wind_.

See also: what happens when you have a helium balloon in your car and you hit
the brakes.

------
platz
does anyone know where you can find bottle rockets without a report or a
whistle? i do not want to cut open existing rockets.

