..but has anyone thought about making a scorpio or a full sized ballista? http://www.legionxxiv.org/catapulta/
I think I am going to start making a mini model of one for fun, hopefully leading to a larger one. It would be cool to design one based on modern materials just to see how much it could be improved upon.

Yeah, I have. I was looking to make it like quarter scale. Make it small enough that I could carry it by myself and that they wouldn't have huge issues with it at the range. I was also trying to convince some friends to build other small scale siege weapons and create a target siege weapon competition. But of course first I have to get off my lazy but and build one.

I really like the scorpio, I am thinking of starting with two large copper pipe sections and making twisting ratchet caps to tension the string.

Scorpions, catapultae, ballistas, Oh-my! Having had some long conversations with Darius the catapult guy and another old friend who regularly participates in the national-championship pumpkin-chucking competition, I will offer this warning. A catapult built correctly is an awesome machine, with LOTS of stored energy. As one experienced builder said: "It's easy enough to make a machine that will shoot a two-pound stone a hundred yards. The trick is to make one that will do it repeatedly without exploding.'
So if you're gonna make a catapult, do a LOT of research first and spend the money to buy the best materials. Junkyard catapults thrown together with nails and glue are liable to fly apart!
Otherwise: Go for it! Geezer

My drawings so far for the first model are for an all metal, mini scorpio made of commercially available hardware and nylon rope. It will shoot slingshot balls so you have an idea of the size.

I s doing some research on Roman crossbows today and changed my mind on making a mini ballista. Going to try and make a manuballista instead.
http://www.romanhideout.com/news/2005/20050224.asp
They are basically a shoulder fired ballista, or a torsion bow.

That link wasn't working but I googled it. Looks similar in size to the scorpio which is probably heavier due to brass construction. Looks like fun, keep us posted!

Ok corrected the link. Little copy and paste error which added an extra letter to the end.
Jake- You mentioned using nylon rope but it is my understanding that natural material rope is a better choice. Nylon rope stretches under tension and will permently deform all of which results in lost energy. From my limit experiance making rope and comments from others who have experiemented with this (and rope making) natural fiber rope can take and hold a considerable amount of torsion energy with out deforming. In fact during the rope making process the rope builds up a lot of stored energy.

basilisk120 wrote:Ok corrected the link. Little copy and paste error which added an extra letter to the end.
Jake- You mentioned using nylon rope but it is my understanding that natural material rope is a better choice. Nylon rope stretches under tension and will permently deform all of which results in lost energy. From my limit experiance making rope and comments from others who have experiemented with this (and rope making) natural fiber rope can take and hold a considerable amount of torsion energy with out deforming. In fact during the rope making process the rope builds up a lot of stored energy.

Most of the modern ballista remakes I have seen were with nylon. I am sure you are right but where am I going to get a bunch of sinew? I have hemp and I could use that but I was hoping to add modern material strength to the design....maybe steel cable?

For modern materials 550 paracord might be a good choice. I believe that is what is used by the SCA combat seige weapon ballistas. Its strong but has a bit of stretch that is going to have to be dealt with but I don't think it will take much of a set. Just make sure to get the real 550 paracord, there are several different types and imitations but mil-spec 550 paracord is the strongest. I know of a few people that play around with seige weapons so I'll ask them, If i can find there contact information again.

As for natural rope I was thinking of a hemp or jute rope. the type that you can pick up at a hardware store. That should be more than strong enought and have less stretch that the 550 paracord. I'm wasn't thinking of anything crazy like sinew. I have heard of sinew being used for this sort of thing and for ropes but not sure how how well it would really work. (may work great just need to research it, I heard about it from a History channel show so I take that with a grain of salt without further research)
Depending on what I need, making a simple rope is easy so I may go that route.

I am pretty sure a simpler version of a skein could be done in steel but then other problems arise, limb material, steel, rubber bumpers for stops....I really think it can be done with junkyard parts and exceed the original. It may take a few rebuilds but when I will have the means to get it all together...who knows?

I better set my sights a little lower for the first one, say, wood, glue, screws , some metal straps and round stock and twine?

1/2 scale of this.

Check this guy out, read the part about the direction of the arms.
http://stormthewalls.kicks-ass.net/Siege/Aitor/Aitor%20Iriarte.htm

Storm the Walls is our friend Darius' web-site. He checks in here occasionally. And now for tonight's caution-message from the old Geezer.
I will repeat myself just this once. You can store an amazing amount of energery in the rope-skeins of a catapulta. For this reason, recreational builders should be careful to do careful research about catapult construction. The originals were made of the strongest woods... ash, oak, rock maple and the like, and all joints were very strong mortise/tenon joints, reinforced with powerful pegs and iron strapping. Why do you suppose they went to all that trouble? Because they were gluttons for punishment? No, i'ts because otherwise, their machines would fly apart and hurt people. A catapult is a wonderful project, by all means go to it, but don't try to go cheap and slap something together unless you've got really good health insurance.
Years ago, One of my buddies broke one of the arms on his catapult... a dinky little thing that drew about 1200 lb at 30 inches. The broken bit flew through a 3/4 inch thick 90 year old yellow-pine wall. If it had hit someone, they would have been badly injured or dead. So take heed, and do it right the first time. Okay? Nuff sed? So get to it. Geezer

Yeah, thanks for that. I am not one to take safety lightly and I never underbuild. I HAVE been researching carefully and plan to have a bulletproof design and assembly method before the build even starts. This part of the reasoning behind the model.

Thanks for the heads up Geezer. Its easy to forget how much energy these devices can store and how little warning it can give before things can turn bad.
I was thinking of building something in the 150 to 200 lb range, depending on how effecent the thing is. While not 1200lb it will still have plenty of poptential for damage.

Random thought on these different types of ballistas. I was wondering if making the ballista stronger doesn't add speed but makes it possible to shoot a heavier object at the same speed.
Which of course brings up the question of what is the max speed of a projectile from a ballista and if a lighter draw weigh ballista could give a higher flight speed because the limbs could be made lighter?
So based on the projectile weight is there an ideal draw weight? of course that would depend on several factors so not sure it would be possible to create an "Ideal Weight" Table or something.