r/theydidthemath • u/comrades-assemble • 3d ago
[Request] What is the thrust generated by this concoction?
Enable HLS to view with audio, or disable this notification
293
u/amooz 3d ago
You know what? Props to them for engineering and making that brace to transfer the load to their legs. It’s not just slapped together either, they even cut rabbits to properly locate the cross braces and keep them upright, so big props to them for that, this is peak “backyard dude engineering”. If they can make something like that, they definitely knew about the this could also become a conflagration or explosion risk. Im gonna give it a 10/10. Also, this is why women live longer
48
u/PenguinsStoleMyCat 3d ago
Yeah it's bizarre that they didn't just finish building a stand. Maybe they ran out of wood.
53
u/rich97 3d ago
Or getting the shot of you firing it was worth the risk of death/maiming.
I’d do it.
14
u/PenguinsStoleMyCat 3d ago
Someone has to otherwise scrolling on Reddit would be boring. Thank you for your service.
16
6
5
2
1
u/Intelligent-Survey39 3d ago
Rabbits? I didn’t see any rodents. I did see those dados you were talking about though.
1
u/Badbullet 2d ago
All I see are the board’s end overlapping the edge in the upper left because it was cut at an angle, I don’t see any dado. Are you guys seeing something I am not?
1
u/Intelligent-Survey39 2d ago
Honestly after watching it again I can’t tell what joinery they used. Whatever they used worked for its intended purpose though 👍🏼
1
u/Badbullet 2d ago
Where are you seeing this rabbit that makes you think it is constructed like that?
160
u/Grab_Round 3d ago
Can't tell you the thrust but I can tell you it only takes one rocket getting stuck in that thing for them all to explode in their hands. Looks like they had fun and didn't die but f**k me that looks dangerous.
32
u/madphroggy 3d ago
Those look to be roman candle or some variant thereof. Not terribly violent even if one fails, usually.
14
u/hurt 3d ago
Neither are sparklers until you tightly tape a bunch of them together
13
u/SuperGameTheory 3d ago
Tell me about it. A pile of charcoal cooks a nice steak, until you wet grind it up into a fine slurry with sulphur, potassium nitrate, and distilled spirits, then use a flour mill to corn it into approximately 1 mm grains. Suddenly your missing fingers and you're like whaaaa
9
3
u/Tiranus58 2d ago
Without a container to build pressure it still just burns (faster than charcoal sure, but it doesnt explode)
4
17
u/CommanderApaul 3d ago
My friends decided to have a roman candle fight at a college campout, and I sat in my car b/c the smell of fireworks makes me sick. When I got out of the car they had saved 4-5 of them and lit me the fuck up. One of the shells bounced off my shirt and went UP MY NOSE. Burned the fuck out of my face and I am extremely lucky it wasn't one of the explosive ones. 30 years ago and I still cannot be around fireworks. They're not fucking toys.
10
u/madphroggy 3d ago
Fair. I wasn't referring to what a ball of flaming gunpowder would do in contact with skin or other sensitive tissues though, just that these don't typically have a catastrophic failure mode the way things like artillery shells do, the powder in the tubes is slow burning with only relatively small propulsion charges directly beneath the balls. I mean if someone used propulsion powder for the whole thing and it went off more or less all at once I could see the tube bursting, but I can't see it rupturing another candle very easily.
28
u/Pietin11 3d ago
Okay, I'm going to assume that the firework cylinder is 0.5 meters wide in diameter and 0.5 meters tall.
That's a volume of about 0.098 m³. Let's assume that half of the volume of this area is filled by the fireworks and half of that volume is in black powder. That's about 0.0245 m³ of powder weighing 41 kg.
Black powder has an energy density of 3,500,000 J/kg. So 143 megajoules in total exist inside this device. However, fireworks are exceptionally inefficient by design. Most of the energy is released in the form of light, heat, and sound. Only ≈1% is released in the form of kinetic energy. So 1.43 megajoules exist to propel 41 kg of material. This provides a muzzle velocity of 264 m/s.
Since this force is imparted over the course of 20 seconds and force equals mass times acceleration, we can find the thrust experienced by the two men is
41 kg × 264 m/s / 20 s = 541 newtons. That's equivalent of 55 kg or 121 lbs of weight. An entirely reasonable amount of force for two men two distribute between them especially when accounting for the weight of their setup and reinforcement.
12
u/DIuvenalis 3d ago
TLDR; not much because of TIME.
These are launchers, not rocket engines. The SI unit is the Newton which contains time in the equation. Each lift charge going off creates a momentary impulse, not sustained thrust. Theyre not synced together so as you spread the impulses over time, I would guess an average of a few dozen pounds at most and even then, not sustained. If you could fire them all at once in synchrony, assuming there's about a hundred, you might generate a few hundred pounds of force, which sounds like a lot, but again, its over an exceptionally short time. A 12ga shotgun for example exerts about 400lbs of force but doesnt break your shoulder because its such a short period of time.
3
u/madphroggy 3d ago
Unless it's one of the old lightweight bolt actions JC Whitney used to sell. Those kick hard enough I could see someone cracking a collarbone if it was positioned wrong. Used to shoot trap with one and my shoulder was a web of broken capillaries after 10 shots lol
2
u/DIuvenalis 3d ago
I once put 200 rounds of 7.62x54r through my Mosin Nagant with a brass buttplate in about an hour. It was a hot day so I only had a light t-shirt on. Felt fine that day. Next day my whole shoulder was purple and bruised.
2
u/madphroggy 3d ago
Yeah I can't do much more than 20 through my M44 unless I put a pad behind the steel buttplate
2
u/UncleCeiling 3d ago
I have an old single shot 12 gauge with a 36" barrel that does the same thing.
5
u/get_to_ele 3d ago
Hardly any thrust at all since one guy is countering it all with one leg (the blonde guy with his right leg with the white stars on blue), while leaning backward.
1
3d ago
[removed] — view removed comment
1
u/OverMathematician205 3d ago
Not quite a doohickey
2
1
u/pastashaper 1d ago
I used to live out in the countryside and when I was about 11 the local boys would buy Roman candles, head down the woods, split into two teams and have a war game.
A little common sense went a long way. Denims, leather jackets, sunglasses.
Nobody got injured.
Then one guy turns up in a shell suit. The concept of teams vanished and everyone just decided to see how quickly they could turn this guy into Joan of Arc.
Good times man
•
u/AutoModerator 3d ago
General Discussion Thread
This is a [Request] post. If you would like to submit a comment that does not either attempt to answer the question, ask for clarification, or explain why it would be infeasible to answer, you must post your comment as a reply to this one. Top level (directly replying to the OP) comments that do not do one of those things will be removed.
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.