Supersonic Trebuchet [video] (youtube.com)
118 points by CharlesW 4 days ago
roomey an hour ago
Thinking this approach out, could someone correct me if I have this wrong.
So he gradually increases the energy transfer from the weight to the arm over the course of the drop. But that arm swinging around must have a lot of energy loss due to friction and air resistance.
This loss will rise exponentially with the speed increase right? So in a sense he is holding his energy budget in a very inefficient phase, with high losses.
Two things support this. A) he spent a lot of time trying to make the arm more arodynamic. B) The spool widening at the end, which is essentially changing back down gears, applying more torque, right at the end when you want speed.
An alternative would be to drop the weight, and let it fall unrestricted until it gets to it's highest speed (this would minimise friction and air resistance) and only at the end of its fall, transfer the energy into the arm.
In this case the arm would only need to rotate 180 degrees or whatever, and not waste energy rotating right?
And then, to take this a step further to make this work there would need to be some flexibility or elasticity in the system to take energy from the high speed falling weight to the arm.
And what you have ended up with is something that looks like.... A whip!
pletnes 11 minutes ago
Energy loss from air resistance typically is quadratic, not exponential, with regard to velocity. So it’s worth much less than you’re arguing. And the arm has to rotate from what I can understand, so the end rotational velocity - and hence, rotational energy - would be the same?
fooqux an hour ago
If I'm picturing what you're explaining correctly, I think this would be difficult. Suddenly accelerating the arm from zero to hundreds of mph would put immense stress on the arm, not to mention whatever you're planning on using to transfer that energy (you kinda hand waved that bit).
I think your assessment is correct about the energy losses. I'm just not sure about the fix.
roomey 29 minutes ago
I think the fix is essentially a whip, that's what I meant by elasticity and flexibility.
If you want to accelerate a weight quickly in a single swipe, (simplified) gradually reducing the mass along the length of the whip imparts more kenetic energy at the end (the whip crack).
So the arm would no longer be rigid, although you can see in the OPs video it isn't actually fully rigid.
arlattimore 6 hours ago
Tom Stanton has a raft of cool videos, I like the engineering iterations he often goes through to find a working prototype.
kreelman 5 hours ago
Agree!
He has some great videos. I watched this one and was impressed at the calculations, the transparency and the celebration at the end when he reached supersonic speeds.
Mashimo 6 hours ago
grahar64 4 hours ago
At least this is a direct link to the video and not blog spam
pino83 an hour ago
Or, in other words: There was some text to read and some pictures to look at. For topics in the range of "supersonic trebuchets", I'm absolutely okay with quickly reading some lines of text; dynamically deciding how quickly/thoroughly I read, what paragraphs to skip, etc. What I'll definitely NOT do: Watch a video clip about it.
londons_explore 4 hours ago
I suspect that with a little iterative modelling of the whole system with derivatives, efficiency could be bought from ~40% to ~80%, giving you an extra few hundred mph...
repeekad 4 hours ago
I wonder if Claude has guardrails against hyper optimizing a trebuchet projectile into a proper bullet
ezst 4 hours ago
How would you even get Claude involved in that? This isn't your git repo that a language model can scrape for textual content.
MomsAVoxell 3 hours ago
bonoboTP 3 hours ago
voidUpdate 4 hours ago
I'm pretty sure the only reason the video trebuchet wasn't a proper bullet is because it's made out of plastic instead of a ball bearing. Aren't most handguns subsonic?
foo42 3 hours ago
Great video.
Other trebuchet fans who dream of building one some day may be interested in googling "walking trebuchet" for a surprisingly high complexity/results ratio.
aa-jv 6 minutes ago
Every time I see a strandbeest[1] on the beach somewhere, I wonder to myself why it hasn't got an array of trebuchet included, also recharged by the wind, to pick up sand balls and throw at people.
Just for the fun of it. Pure whimsy.
kitd 38 minutes ago
Love it! But shouldn't that be "high results/complexity ratio"?
fragmede 6 minutes ago
Depends on if the journey and not the destination is what's important to you. ;)
londons_explore 4 hours ago
Interesting that his spool goes bigger at the end...?
I would think you want it decreasing to near zero to extract all the kinetic energy from the mass, leaving the mass stationary as it hits the ground.
vanderZwan 4 hours ago
I think the idea is that at the end the sling starts to unwind too, increasing the effective arm-length (that non-linearity is one of the reasons trebuchets are so efficient to begin with). As a result it's possible that decreasing the spool to near zero is like putting the gears of your car too high. Reducing the spool size ensures there's enough torque left to put more power into the final swing.
drtoboggan 4 hours ago
Really wondering up to what point I can build a trebuchet in my backyard before the authorities start to complain
xdertz 2 hours ago
There are three issues that arise
- structural: There are height and size limits on things you can build without a permit
- trespassing: flinging objects onto others people property is illegal
- endangerment: any activity that can endanger other people is illegal
All of these three depend on how rural you live I guess. So in a suburban neighborhood I would assume they get involved very quickly.
rwmj 2 hours ago
Surely that depends on the side of your back yard?
aa-jv 5 minutes ago
Maybe it depends more on the vicinity to others' back yards... ?
arielpetit an hour ago
Interesting
manlymuppet 3 hours ago
Fuckin love this guy
mywacaday 2 hours ago
If you like Tom you will probably also enjoy https://youtube.com/@stuffmadehere
manlymuppet an hour ago
I am familiar, and I love that guy too!
manlymuppet an hour ago
MomsAVoxell 3 hours ago
I would love for this to be optimized to the point that its portable and can be carried around, assembled on the side of the mountain, and used as a low-cost, efficient way to throw things from one mountain to another, maybe for avalanche control, or cloud seeding, or indeed for seed bombs intended for re-wilding hard to access places, or something.
It just seems like such an obvious tool, rather than a weapon.
xdertz 2 hours ago
if you want low cost and portable, chemistry (i.e. gunpowder) always wins over gravity powered.
MomsAVoxell 39 minutes ago
This is so obviously wrong. Show me a field-rechargeable gunpowder-based device for launching things, and I will show you someone who is about to run out of ammo on the side of a mountain they’ve just climbed after a single day of work.
Honestly, what the hell. Where do you think gunpowder comes from, thin air? Oh, is gravity a subscription service in your country?
Come on, try to at least be sensible about your response.
anon_cow1111 3 minutes ago
alehlopeh an hour ago
You’ve got the solution. Now you just need the problem.
MomsAVoxell 39 minutes ago
Seed bombing is pretty fun. Re-wilding is a huge industry. Field-reloadable ultra-trebuchets that can be carried and deployed across the landscape, instead of stored in a weapons depot, also a big win for energy harvesters, everywhere.
roer 3 hours ago
You'd probably be better off using a rail gun if you want a carriable launcher
MomsAVoxell 38 minutes ago
Really? With the required infrastructure to charge it, also in the backpack, for a few weeks of work too?
Is gravity a subscription service in your world?
sawjet 3 hours ago
So, a firearm?
__alexs 2 hours ago
Why move gun, when gun can move bullet?
MomsAVoxell 38 minutes ago
You do know that this thing doesn’t require consumables to operate, right?