Optimising a Trebuchet [video] | Hacker News Reader

Optimising a Trebuchet [video]

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128 pointsmontrose22 comments

If you enjoyed this one, the video where he adds the wheels is equally fascinating: https://www.youtube.com/watch?v=KpFTyE-wiNo

Wow that was awesome.

To me it's way more interesting than TFA, as it's really not obvious why the wheels would work, and the wheels don't work "out of the box" either, you can't just slap wheels on a tuned "static" trebuchet and need to do some re-tuning to get the correct synchronisation between the arm and counterweight.

Really neat.

Most of the means of optimizing trebuchets make sense after the fact, but would not be intuitive to me at first glance.

For example, making the frame more rigid improved the outcome here, but if you put the whole thing on wheels you would likely still get more increase because the weight could take a more direct path downward as the frame moves.

https://classes.engineering.wustl.edu/2009/fall/ese251/prese...

>the weight could take a more direct path downward as the frame moves.

That should change how long it takes the arm to drop, but it shouldn't change the amount of potential energy released (and thus, projectile speed).

It does, another commenter posted the same person's explanation. But, the idea is that it doesn't matter how much GPE is released, it matters what the velocity of the projectile at release is. If the trebuchet can't move then the weight starts to slow down before all it's GPE is used up. When it can move, it coincides with the time that the projectile is at maximum velocity.

Projectile velocity is a function of power rather than energy alone because you only have so much time to accelerate it.

Very interesting. Is it possible to make a trebuchet that can throw stuffs beyond escape velocity?

Probably not exactly what you had in mind, but there's something called a skyhook which might act kind of like an orbital trebuchet.

https://en.wikipedia.org/wiki/Skyhook_(structure)

Video going into detail on Skyhooks: https://youtu.be/TlpFzn_Y-F0

The closest thing I can think of is the 'slingatron'.

https://en.m.wikipedia.org/wiki/Non-rocket_spacelaunch

Naively, I'd think a simple lever could do that, but it becomes a materials science problem: with no constraints on the length of the lever lenght, and no constraints in the density of the counterweight, you should be able to achieve any velocity, right? Well, up to where relativistic effects become an impediment, of course. The question is, is it possible to create such materials as this would require?

As you can probably guess, I'm not a physicist. Maybe someone with expertise can chime in here.

> Well, up to where relativistic effects become an impediment, of course.

I don't think relativistic effect would enter the equation for escape velocity (there's a 5 orders of magnitude difference), your issue will be basic material science, namely:

* That you'll need to add enough headroom that the payload is still at escape velocity after going through the atmosphere, which means the projectile needs to be launched way beyond escape velocity in a very dense atmosphere. Which means it will instantly burn up / blow up as it leaves the launch system, probably alongside the sling and part of the arm…

* except for the counterweight and material rigidity you need for the arm to not be feasible either, you'd need a long arm multiple miles long and a counterweight in the megaton range… before even accounting for the length of the long arm itself, or how you'll actually bring the bloody arm down

Orbital Velocity at grazing altitude is 8 km/s, and escape velocity is sqrt(2) times that, or about 11.3 km/s. Thing is, anything travelling that fast in the atmosphere tries to turn to plasma, so your launch arm would need to be a bit special.

Or at very high altitude.

Or have ablative heat shielding.

But atmospheric drag would probably prevent you from getting anything like those speeds with this kind of mechanism.

So on the moon it might work ;)

Vacuum helps, certainly. Also the escape velocity on the Moon is about 2.4 km/s.

That's still rather fast ...

This happened before: https://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_...

Wow, so they basically made a cannon powered by 300t nuclear explosion.

It has been researched:

>"In Project HARP, a 1960s joint United States and Canada defence project, a U.S. Navy 16 in (410 mm) 100 caliber gun was used to fire a 180 kg (400 lb) projectile at 3600 m/s or 12,960 km/h (8,050 mph), reaching an apogee of 180 km (110 mi), hence performing a suborbital spaceflight. However, a space gun has never been successfully used to launch an object into orbit or out of Earth's gravitational pull."

maybe these guys are planning that https://techcrunch.com/2018/02/22/spinlaunch-2/

The superior siege weapon.

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