[HN Gopher] Supersonic Trebuchet
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Supersonic Trebuchet
Author : wyrm
Score : 228 points
Date : 2021-12-01 19:35 UTC (3 hours ago)
(HTM) web link (hackaday.com)
(TXT) w3m dump (hackaday.com)
| thomasfromcdnjs wrote:
| How far does the projectile go?
| nuccy wrote:
| Fun fact, you can accelerate an almost-everyday object to
| supersonic speeds: whip tip [1,2].
|
| 1. https://en.wikipedia.org/wiki/Whipcracking
|
| 2. https://www.youtube.com/watch?v=AnaASTBn_K4 (very much
| recommended)
| wscott wrote:
| Yes he ends up with a cute toy that flings a marble but sounds
| like a rifle. But in the video he explains the engineering
| process to start with the desired goal and then optimized and
| derive the parameters he needed to achieve that result. It was a
| good example of engineering.
| jacquesm wrote:
| That cute toy would probably shoot the ball straight through
| you at appreciable distances.
| jaggederest wrote:
| It's about the same muzzle energy as a .38 special -
| definitely not a toy!
| ISL wrote:
| I found his lack of safety-glasses and ballistic shields
| disturbing, especially given that one of the rotors had
| already disintegrated.
|
| A piece of 3/4" plywood to stand behind would slow down a lot
| of lightweight shrapnel.
|
| Alas, one of the few useful things that comes with age and
| experience is an awareness of consequence...
| jacquesm wrote:
| My biggest worry would be the joint at the bottom (the four
| bolts visible in the video) giving way because the tension
| is across the top of the wooden beams. That won't end well
| because suddenly the winch becomes the projectile.
| exabrial wrote:
| I would definitely not watch Colin Furze
| bee_rider wrote:
| Interesting how different his device is, from historical
| trebuchets. In particular, they seem to maximize arm length,
| while he seemed to go for the shortest arm he could. I wonder
| if it is entirely the materials difference, or if his
| optimization was also much better than theirs. It would be cool
| to see his design built in period materials.
| Aunche wrote:
| His trebuchet is quite high off the ground, so the arm and
| sling has room to accelerate. This wouldn't really be
| practical in a medieval trebuchet, especially if you wanted
| to move it to a siege.
| dragontamer wrote:
| Medieval Trebuchets didn't move. You hired engineers to cut
| down trees and build Trebuchets for a particular siege.
|
| Honestly, "Castle Drops" in AOE2 are more akin to how
| Trebuchets were used. You build an army, the army gains
| control of an area, and then you use your engineers to
| build a long-range siege device at that location.
|
| ------
|
| https://en.wikipedia.org/wiki/Trebuchet#/media/File:Ms.Thot
| t...
|
| See? No wheels. Those things couldn't move at all.
| Aunche wrote:
| My impression was that they were assembled on site, but
| they were pre-made and could be dissembled. From this
| thread, it looks like both methods could have been used:
|
| https://www.reddit.com/r/AskHistorians/comments/3z597l/in
| _me...
| masklinn wrote:
| > Interesting how different his device is, from historical
| trebuchets.
|
| It's more of an onager really: it's a torsion engine, whereas
| the trebuchet you'd think of is a gravity (counterweight)
| engine.
|
| > It would be cool to see his design built in period
| materials.
|
| "Period material" would be limited to sinew and / or hair,
| wood, and relatively small pieces of metal (mostly
| fasteners).
| Robotbeat wrote:
| He built his arm first out of plywood and I'm pretty sure you
| could easily build the same design out of a high quality
| single piece of wood. It really comes down to an extremely
| well-optimized design.
| ggcdn wrote:
| Right, but I think its the high quality elastic bands that
| would be difficult to replace.
| Robotbeat wrote:
| They could be replaced with either a weight, steel
| springs, or perhaps rope or sinew cords like the Romans
| used. Rubber is more effective and compact, but watch the
| video and you see there's nothing particularly special
| about using rubber & he could've picked something else.
| The rubber puller itself does not exceed 10 m/s.
| creato wrote:
| I agree he could have used some other spring, but I don't
| think he could have used a weight. The acceleration of
| springs/rubber is potentially much greater than a weight
| (at least one starting at rest).
| salmonellaeater wrote:
| It might have to do with the fact that he's using rubber
| bands to turn the arm, while historical trebuchets used
| weights. A weight is limited in how fast it can accelerate so
| you need to use a lever (the arm) to increase the speed.
| krasin wrote:
| Springs could be used instead of rubber bands. While "coil
| spring" is a relatively modern invention, other kinds of
| springs are known from the Bronze Age, accoding to https://
| www.designingbuildings.co.uk/wiki/History_of_the_Spr...
| fabianhjr wrote:
| Well, this might be supersonic but doubt it would throw heavy
| boulders at fortresses anytime soon to be useful as a siege
| weapon.
| 0_____0 wrote:
| wouldn't need to be a boulder at those speeds. energy goes
| up with the square of velocity
|
| 50g @ 460m/s = 5290J
|
| 25kg @ 50m/s = 31250J
|
| that is to say, the small but fast projectile is 1/6th the
| energy but only 1/500th the mass
|
| edit: formatting
|
| edit2: a 300g projectile at 460m/s would have approx the
| same energy as a the 25kg boulder at 50 m/s. at point blank
| range anyway.
| technobabbler wrote:
| At a certain point, wouldn't the tiny fast pebble just
| drill through the fort and leave a small, hot hole in its
| wake? Like a big neutrino?
| blacksmith_tb wrote:
| A small projectile like a pebble would be vaporized
| pretty quickly by friction with the air at those speeds
| (and of course a bigger projectile would take more energy
| to launch).
| 0_____0 wrote:
| You fire the weapon. The tiny pebble breaks up on impact,
| creating a little crater in the rock. To you, the
| medieval siege engineer, this is annoying. The frenchmen
| taunt you from their parapet, yelling maternal insults
| and farting in your direction. Your supersonic trebuchet
| is a failure, tactically speaking.
|
| You can't take their mockery anymore, so you set about
| fixing your projectile.
|
| The pebble lost much of its energy in flight - drag is a
| huge issue with any distance to target. You make a
| projectile in the shape of a rod. This has the effect of
| decreasing the cross-sectional area of the projectile
| relative to its mass, minimizing drag losses. There's a
| limit to the length:width ratio, however- go too skinny
| and it'll deform under firing forces.
|
| The projectile's length also allows it to burrow deeper
| into the rock -- when material at the tip breaks off on
| impact, it's got the rest of the shaft behind it, just as
| mad.
|
| To pack the most punch into your projectile, you craft it
| from tungsten (then known as Wolfram). In order to keep
| it from yawing in flight, you add some rigid vanes at the
| back. It looks like a small, superheavy arrow.
|
| Now you have the issue wherein it doesn't fit well into
| your cannon anymore. No mind -- you set it in a cup of
| sorts that seals the barrel. Wait, where did you get a
| cannon? This is the year 1217 AD...???
|
| Congratulations: you have constructed an armor-piercing
| fin-stabilized discarding sabot munition (APFSDS). You
| load it into your M256A1 120mm smoothbore cannon. You
| peer through the sights.... now where were those _sac a
| merde?_
| rich_sasha wrote:
| Smaller projectiles lose more energy to air drag.
| ambicapter wrote:
| You mean objects with lower surface area to mass ratio,
| AKA objects with lower density. If he wants some high-
| density objects to throw I think there are stores in the
| US that sell small, dense metal objects he could use.
| dragontamer wrote:
| Mass is not a factor in drag equations.
|
| https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocke
| t/a...
|
| Velocity-squared is the factor in drag.
| bernulli wrote:
| But very much in inertia, which is acting against the
| drag, trying to sustain its initial velocity.
|
| https://en.wikipedia.org/wiki/Ballistic_coefficient
| jpollock wrote:
| Density is important when comparing two spheres of the
| same mass.
|
| https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocke
| t/a...
|
| Where frontal volume is determined by mass and density
| for a sphere.
|
| So, two speheres with the same mass, but different
| densities will have different drag forces.
| dragontamer wrote:
| Doubly so when you consider the supersonic barrier.
|
| Air drag is proportional to the drag-coefficient *
| velocity^2. So doubling your speed quadruples the drag.
|
| Except... the drag-coefficient itself is a complex curve
| over velocity. It barely changes from 0mph through
| 300mph, but once you reach transonic and supersonic
| speeds, the drag-coefficient skyrockets.
|
| As a result: 250mph is more than 4x more efficient than
| 500mph from a drag perspective (4x predicted from Drag
| equation, but in practice might be 8x or more). This is
| mostly a problem for airplane efficiency and fuel
| consumption, but I'm sure it applies to war-machines and
| projectiles too.
|
| --------
|
| But modern airplanes fly at very high heights, so the air
| is thinner, reducing drag, but requiring a pressurized
| cabin lest everyone gets hypoxia.
| deegles wrote:
| A scaled up version of this would be terrifying... I'm sure
| with the right materials (and budget) it could launch a
| cannonball at those speeds.
| kingcharles wrote:
| I was thinking it would be more fun with a rotten cow.
| Someone wrote:
| A rotten cow might disintegrate before it leaves the
| launch site. I wouldn't classify that as fun.
| twic wrote:
| At 5:30 in the video he claims that the problem is infinite-
| dimensional. How so?
| bunje wrote:
| Solutions to differential equations belong to infinite
| dimensional function spaces unless discretized.
| jacquesm wrote:
| At 13:12 in that video notice the tension in the frame and how
| the main spars are getting out of alignment with the part the
| winch is mounted on. I wonder how much spare strength there still
| is but it can't be much. Pretty scary machine!
| pjkundert wrote:
| No kidding; when he was cranking the final few inches of
| tension, I wonder if he realized what would happen if anything
| snapped: instant death was definitely one of the potential
| outcomes.
| jacquesm wrote:
| Exactly. It would be interesting to hook the whole thing up
| to an electric motor and to winch it (from a safe distance)
| to the point where it breaks just to determine how much
| margin there was. Probably not a whole lot past the point
| that he hand cranked it.
| tempestn wrote:
| Yep, that was pretty terrifying to watch.
| jacquesm wrote:
| Indeed, I wasn't comfortable watching it until I realized
| that he was the one that posted the video. But instant
| desintegration was definitely on the menu there and once
| stuff comes apart and starts flying it's impossible to
| predict trajectories. That's a very large amount of energy on
| what looks to be two run-of-the-mill boards, they're
| fortunately under compression but the joint at the bottom
| looks (far) less than ideal for this, especially with the
| holes drilled in line with each other (that's a definite 'no'
| if you want to avoid splitting the beams).
|
| A++ for the theory, but the implementation is not something
| I'd want to be near when it is in operation, it is just too
| unsafe judging by my somewhat experienced DIY eye.
| pjkundert wrote:
| This is a perfect illustration of why I chuckle when people claim
| to be able to "ban guns".
|
| He is projecting a 3/8" (.375 caliber) round steel projectile at
| ~3x the maximum velocity limit for an unrestricted air rifle in
| Canada.
|
| Using a crank and some hefty rubber bands.
| bfung wrote:
| Sure, but this project isn't really hand held and mobile and
| reloadable in a small amount of time. It took some smart enough
| person to operate as well. Some nuance and differentiation
| still appropriate; this is as close to an air rifle as vacuum
| tubes are to SoC.
| muglug wrote:
| I'm not sure what point you're trying to make... that machine
| is not particularly mobile. It takes him 5 minutes to load a
| projectile.
|
| The UK has banned guns. People there generally don't build
| trebuchets to compensate.
|
| Murder rate in the US is 18x higher than in the UK, and 23x
| higher than in Canada.
|
| Edit: there are obviously other ways to murder someone, knives
| being the most common in the UK. If you're a good aim, guns are
| a lot less messy, and I think that makes a big difference to
| would-be murderers.
| _Microft wrote:
| ... using a device that is meters long and that is reloaded and
| fired every minute or so.
|
| BTW: I think open carry of trebuchets should be forbidden.
| Concealed carry, maybe. Open, no way
| [deleted]
| smudgy wrote:
| I know a large group of weirdos that are probably flipping their
| shit with this.
|
| Not only the superior siege engine but the supersonic one too.
| beckerdo wrote:
| It is interesting to me how many engineers hop from English ("3
| inches to go on the rubber band") to the metric system ("400
| meters per second"). Andy Weir touches on this in his book "Hail
| Mary".
| pharke wrote:
| Reminds me of the unlikely idea of using a whip crack to send
| something into orbit[0] maybe SpinLaunch needs to consider adding
| another joint to their arm. I'd love to see a massive trebuchet
| launch something into orbit.
|
| [0] https://futurism.com/the-byte/amazon-patented-system-
| whips-s...
| grayrest wrote:
| The idea of using whip cracks for acceleration features fairly
| prominently in Neal Stephenson's _Seveneves_. None of the
| instances in the books are sized for orbital launch but that
| was the first time I encountered the idea.
| jamestimmins wrote:
| In general, it was fascinating to see how much of Seveneves
| was devoted to acceleration and orbital mechanics.
| JoeAltmaier wrote:
| I've always thought the trebuchet cord should be wrapped around a
| wheel. Then the force would always be rotational - no component
| of fource would pull against the hub and be wasted.
| mleonhard wrote:
| The arm is just a large wheel and a small wheel on the same
| axle, with the unnecessary parts cut away. Round (not-cut-away)
| wheels will capture more of the energy as rotational inertia,
| leaving less for the projectile.
| mgsouth wrote:
| The most striking aspect is that he was explicitly optimizing for
| _project difficulty_. I smiled when he said, in effect, 'let's
| mathematically calculate How Hard Can it Be'. Sure kid...
|
| But that's what he did. Fantastic job determining the criticial
| criteria and engineering to it.
|
| 3 weeks total project time.
|
| Kid's got a future...
| chronolitus wrote:
| Amazing project! Although, I would probably err on the side of
| caution and at least wear some safety glasses when hand-loading a
| supersonic trebuchet.
| orangepenguin wrote:
| I had the same thought! Earplugs for the sound might not be a
| bad idea either.
| spir wrote:
| I'm curious, could this be a realistic and cost-effective weapon
| for, for example, militias in third-world countries?
| FetusP wrote:
| I can't imagine so. Even if it was somehow accurate, it
| wouldn't be very mobile or quick..
|
| I'd imagine one person with a pistol could take out a good
| sized group of people shooting marble trebuchets
| Nomentatus wrote:
| Next stop lunar surface! Something like this may be what you need
| to export lunar materials from the surface, say to the LaGrange
| point.
| leephillips wrote:
| I'd like to suggest linking directly to the video
| (https://www.youtube.com/watch?v=gdXOS-B0Bus) instead of the
| hackaday page, which doesn't even pretend to add anything and is
| largely incoherent besides.
| jacquesm wrote:
| Hackaday is just blogspam. They happily rip off your work.
| ok_dad wrote:
| I agree this is a bad textual article, but am I one of the few
| people who prefer text and images over video? I literally never
| watch a video link here but I'll read almost any textual
| content. Not judging others, but I'm sad that technical videos
| are so prevalent and most don't have any textual counterpart.
| rhn_mk1 wrote:
| I hope the paper that was explained in the video is available
| published somewhere.
| jacquesm wrote:
| The video is very good, actually, for once.
| leephillips wrote:
| I absolutely prefer text to video for tutorials or
| documentation. I don't think this preference is unusual. But
| the linked article is not a text version of the video. It
| has, in fact, no content. Everything is in the video. And the
| final bit, where you hear the snap of the sonic boom, is
| worth watching the whole thing.
| ok_dad wrote:
| Agreed, thanks for responding. I guess it was more bitching
| than I should have done and off topic here; I agree with
| your assessment of this "article".
| sbierwagen wrote:
| Direct link to the timestamp of first full power shot:
| https://youtu.be/gdXOS-B0Bus?t=806
|
| Second shot: https://youtu.be/gdXOS-B0Bus?t=871
| EamonnMR wrote:
| Having to stand downrange of it to windlass it is intimidating.
| jacquesm wrote:
| Real Men (tm) stand in the plane of rotation.
| [deleted]
| throwaway81523 wrote:
| Cute. It is around 8 feet long and per the comments, it throws
| tennis balls. Throwing a tennis ball at supersonic speed is
| nothing to sneeze at, but the really impressive thing about
| historical and modern trebuchets is how big some of them got.
| They could throw things like pianos, as in a famous Northern
| Exposure episode. Doing that supersonically would have really
| been something ;).
| tantalor wrote:
| No, it throws ball bearings.
| Robotbeat wrote:
| Literally just watch the video.
| pacificmint wrote:
| > They could throw things like pianos
|
| One example: Trebuchet throws a piano that has been set on fire
| at Burning Man: https://www.youtube.com/watch?v=SgboQNcRN5w
| speedgoose wrote:
| Such a dense crowd doesn't seem to be the best environment to
| throw a burning piano with a trebuchet, but I guess I don't
| get the point of the burning man.
| dougmwne wrote:
| I know not with what weapons World War III will be fought, but
| World War IV will be fought with the supersonic trebuchet.
|
| Excellent engineering video and it hope it inspires more young
| engineers than it lands in the ER. Stay safe kids!
| mootzville wrote:
| That's pretty awesome. May have to give a whirl. Thanks!
|
| I remember years ago seeing some show about weapons seized in
| jail, and one of them was a crossbow made out of underwear that
| could shoot a plastic fork through a steel filing cabinet.
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