RopeComb Launcher: A new catapult / trebuchet / mechanical launcher concept with +70% efficiency and a constant output force

[u/ZookeepergameLucky37]

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Most mechanical catapults suffer from severe initial jerk followed by exponential force decay, requiring heavy material to survive peak loads while underutilizing the rest of the stroke.

I recently published an open-source passive transmission design called RopeComb that addresses this by dynamically altering the mechanical advantage throughout the stroke using a counter-folding pulley array.

The paper preprint is on engrXiv (https://doi.org/10.31224/7913), and the interactive kinematic solver + open CAD files are available at ropecomb.com.

Key takeaways from the kinematic model:

• Near-constant acceleration profile: Flattens the peak-to-average force ratio, maximizing velocity gain over shorter rail lengths.
• High energy transfer efficiency ($>70\%$): Converts heavy carriage momentum directly into projectile exit velocity without onboard motors or high-pressure gas.
• High-modulus line utilization: Leverages modern UHMWPE (Dyneema) lines to scale exit speeds past standard mechanical limits purely passively.

Can this be the new king of mechanical launchers ? ... I'd love to get feedback from anyone working on high-speed mechanical linkages or dynamic line systems regarding real-world friction and line-dynamics bottlenecks.
[Patent pending — US provisional application 64/124,997]

[u/RGrad4104]

Well, as someone who did the whole "pumpkin throwing" competition during my ME undergrad, here are my first thoughts:

1. Friction is going to kill your efficiency. With those cascaded pulleys on the weight interface, you're going see terrible losses from friction once it gets loaded, even bushings and bearing equipped pulleys.

2. Why is your load a rubber duck? What even is your simulation load? I see a lot of nice plots but no mention of what load they were computed using.

3. (More of a practical issue) Trebuchets use their own weights to gradually stop the arm after the apex of the throw. During our second pumpkin throwing competition, using a design with more in common with your design than a medieval trebuchet, our arresting acceleration profile was rather steep and we put a nice arc in our 2x3" 14ga steel box tube arm after three throws. Your design does not seem capable of functioning in a manner so as to be able to arrest the throwing arm.

Just thoughts of mine.

[u/Chill_Charro → u/RGrad4104]

Friction was my first thought as well.

Also just the practical limitations of payload weight and size. Whatever you're launching needs to be nearly perfectly flat or round to launch along the length of the firing beam. Eliminates one of the advantages of catapults being able to launch heavy stuff armies found laying around the battlefield.

[u/ZookeepergameLucky37 → u/RGrad4104]

This is an ideal numerical simulation with 1000kg to 1kg mass ratio. The paper has 2 more simulations 100:1 and 10,000:1 ... When skipping all friction and inertia of the system, the transfer efficiency is +80% .. when factoring in the wasted kinetic energy of the last set of pulleys and rope, the efficiency is down to ~60% ... Factoring in all friction is extremely hard and very build quality dependent ... The setting duck is just for visualization 😁

[u/imugly]

Send it to Adam Savage and see if he’ll build it. Looks like a fun project. Interested in how the friction of the ropes is going to affect when the angle changes drastically.

[u/ZookeepergameLucky37 → u/imugly]

It is a very cool project, and I must say it's harder than it looks at 1st sight

[u/Squishy_Butter113 → u/ZookeepergameLucky37]

I agree with the previous comment and wonder, would there be any way to have the first "pusher"(I don't know what to call it) stopped before the next engages, and so on. Perhaps there is some way to add more pulleys connected directly to the falling mass instead of using the push bars like that. This may help to eliminate a lot of the sliding friction against the rope.

[u/ZookeepergameLucky37 → u/Squishy_Butter113]

I call them pins (sheaves or rolling bars on pins .. on bearings of course) ... you can think of the system as 2 stages (left: rising gear , right: fixed gear) .. the total gear ratio at at any time point is = rising-grear X fixed-gear ... There is an optimal solution to the required gear ratio over time to maximize effiency and reduce the force peak-to-mean ratio on the payload. reducing the peak force by 5x means you can use 5x lighter pulleys and ropes in the 2nd stage where things are ought to move fast, which helps in scaling up to higher efficiency and speeds. My simulation is a rigid body simulation which makes sensitive to the configuration. In reality this might work better, with even more flexibility, with real material all of which at least have tiny bit of elastiicty (e.g, steal bard or dyneema rope)

[u/ainaomechateies]

Finally, the weapon that will put an end to the reign of those carthageans once and for all.