Ballista and torsion engines

Hi,

Im starting a thread regarding to all ballistas (arrow or stone throwers) as a possible start for ANY disscussions of theories/construtions/experience and brain 'dump' for beginners to veterans. Im a novice but I've been following many projects on the interwebs.

My first progress would be contructing and studying the Mark IVa (arrow thrower) ballista. This inswinger seems easier to start off.

I've read and noticed a few blogs, "Nick's" project and warhammer1. There is also a great wiki page, etc. Which are all interesting/overwhelming, but they dont detail the phases of design.

A gold standard for reading is probably Marsden's Technical Treatises.

For a start, Im curious of how would one determine optimal rope diameter, total spring diameter and length, besides using optimisation formulaes and tables from the ancients and trial 'n' error?

Im a novice at mechanics. The traditional method for starting off would be choosing projectile (arrow) length, then base it on ancient "formulaes". Im more interested in modern ways of starting off.

"Im a novice at mechanics. The traditional method for starting off would
be choosing projectile (arrow) length, then base it on ancient
"formulaes". Im more interested in modern ways of starting off."
So as I understand it, you don't want to use the proven traditional formulae. You prefer to start from scratch with engineering principles you don't fully understand? Hmmm, I detect a possible problem there.
Ahh well, there are plenty of catapult guys hereabouts who can give you a hand. I'm back to medieval crossbows. Geezer.

I've built a couple ballista and run into tons of dead ends. The Ancients knew what they were doing.
Build a spread sheet with the formulas they used and plug into it your desired projectile. You will save yourself the humiliation of projects reduced to firewood in an instant and get results before you run out of money.
I have 4 more ballista in the basement under construction. I'm hoping half of them work.

I will post a link to a ballista site. The guy may be active on here I thought this is where I got the link to begin with but maybe not. It is an awesome machine and he was posting often about upgrades and TONS of information... He did post he was taking a break for the honey do list season but I will still post the link...

http://wattsunique.com/blog/

Welcome to the forum Scorpio, great place we got here and indeed we do have some really cool folks dealing with ballista like crossbows.

If you read through this forum (gosh - the whole 2 topics! )Jake had an introductory topic rolling ...but yeh, this forum needs some more love and a practical application topic is just the thing.

Funny you mention Ballistas and modern approach. I was actually sitting at my computer over a cup of coffee and playing with my favorite program (that I normally didn't have time for) and did exactly ballista mechanics.





Enjoy your stay and build something cool, we are bound to chip in with some ideas and feedback - all cool peeps here.

Yes, I don't understand torsion springs of rope bundles. A start for that would be great.

Marsden details/uses ancient spreadsheet and formulaes. I don't think that he knows the "engineering princples" neither. For example, he has a typo of KE=mv^2

Kind of trivial, but you know what I mean.

I ll take small steps, but I think Im determined reach my goal.

Haha, yep! That's the man!!!

Noticed that I was messing with your system in that video, huh?

I really grasped what you meant by *shorter arms*...the way you used idler wheels in the tips combined with string anchored to the riser...maaan I still can't get enough of it - it's brilliant!

Warhammer, you ever use Working Model 2D? ...I got it from the trebuchet guys a while back...really cool app.

Ivo

PS: You really need to post pictures and not links man.

I just posted a vid of my torsion assist slingshot destroying two inch thick concrete block on my facebook wall. This is the link facebook gave when I clicked on link: http://app.talkfusion.com/fusion2/viewTemplate.asp?mailId=MTc5OTkxMA%3D%3D&glb=0#.T1kjtrcCYls.facebook

The block just doesnt break but kinda explodes. The ball bearing doing the destruction is 110 grams. The draw weight is up there, but Im not that strong so probly around 50lbs. Its speargun tubing 1/4 inch inside diameter, 1/8 walls. I need arms like JoergS to use these tubes...

Here's a good bow statics and dynamics: ( which is similar to a catapult/ballista )

http://markdenny.shawwebspace.ca/asset/view/2104/bows.pdf


Here's a good journal on the "hatra (Mark V) and mark IV":

The Hatra ballista a secret weapon of the past? by Hart, V. G and Lewis, M. J. T


Im still stuck on how to determine theoretically a good spring constant for the torsion bundles. I have little experience with draw weights physically possible to wind up.

Couldn't you use some archery spring scale to calculate draw weight?

" Hatra Ballista - Secret Weapon of the Past" is an academic paper not available to general public. It is an experiment using two identically built and powered machines at 20kg's draw weight. One was an inswinger (Hatra) and the other a regular outswinger. Naturally the inswinger outperformed the outswinger by 50% aprox. on range and speeds.

Go to www.romanarmytalk.com and search Hatra Ballista. There are references to the paper there.

The best available journal of inswinger Orsova ballista is Nicks blog bar none. www.wattsunique.com

Me, I got my own ideas. There are other ballista builders who use rope bundle power here if you look around at some diffent threads, and that gives you access to expertise in such matters. A good and cheap learning curve would be to buy some ballista and catapult kits, and study the construction if historically accurate.

In any case a kit would at least give you something to compare performance with, and provide a wealth of experience and learning.

Good luck.

L.

Geezer here on balistas: I don't claim to be an expert in this field, however I will note: 1. In-swinger catapults appear to allow a much longer power-stroke than out-swingers, so they may provide greater power/accelerations.
2. The biggest problem with in-swingers from an ancient perspective would probably lie in necessity to make the frame holding the arms and torsion-skeins much wider, which would require either stronger materials, or much heavier frame.
3. Marsden and Payne-Gallwey seem to agree that torsion-springs from twisted rope, or some such, store lots more power with less weight than any sort of bow-stave will provide.
4. A torsion-spring made of rope will work better, if all the ropes used in the spring are on the outside of the skein (hollow in the middle) as any ropes inside the bundle get stressed less, but still add weight and build up heat in operation.
Looks like fun. I look forward to seeing what comes next. Geezer.

Geezer, the premise of my works involves argument by doctrines and treatises of construction for maximum effeciences allowed by available technology. You will note I argue with both VG Hart and Nick on testing and design.

Despite argument from the math dept. my experiments as verified by Nick provided an immediate 22% increase in velocity. This effort supported my argument against Mr. Hart, while I admit I may have interpreted his opposition in argument incorrectly.

I am convinced the shorter arm theory I support is the happier of the two opposing views, and modern crossbow manufacturers seem to agree. Its seems improbable to me that if the use of the pulley was utilized in those days, it would not be utilized elsewhere aside from cranking back the slider/trigger assembly.

these folks had hundreds of years to perfect the design, and using compression is a huge part of structural integrety.

I have access to the Hatra journal, Im just pointing out that it's a good read.

Model kits won't help since I have far better resources, for example, Technical Treatises by Marsden. Nick's blog is just a blog of his train of thought, it's hard for noobs (like me) to follow. Yes, inswinger is better. My goal is outswinger.

Im rather confused and can't follow your progress, since I wasn't there at the beggining. Could you explain a bit more? Are you in contact with crossbow manfacturer and "Nick"? What projects are you doing?

I'd like to follow but I feel that my questions and this thread are being burried by further private discussions which don't benefit outsiders (like me).

It would help if there was recognition of questions, like if anyone could help me with finding the torsion coefficient of a torsion bundle, and a relationship of arm length and total bowstring length.

Thanks.

everyone who wants to build ballistas or torsion machines have to start somewhere, and that is usually at the beginning or more specfiically by effort and experimentation. You also started out perhaps on the wrong foot, dissing Marsden and efforts (as I have in the past) while at the same time admitting you are a newb and not much into mechanics.

If you have followed any of my work you know I dont work with rope spring bundles, but VG Hart does say the rope bundles enjoy ineffeciences as high as 58% if I remember correctly. Furthermore, the outswinger and inswinger would have varying ceffecients as one is subject to more twist than the other.

My argument(s) with HArts work is outlined on RAT as I attempted to point out to you, but passed by unnoticed. Dane who posts here, can help you out better than I as he and several other members are heavily into rope spingbundle powered ballistas (historically correct as interpreted by Marsden et al).

Since you have a preference for rope bundle outswingers (in opposition to your first post), I cannot help you further as I have no interest whatsoever in a technologically inferior design where performance and range is primary. Therefore, any optimization of the outswinger in my mind is a moot point given the inswingers accepted superiority.

The relationship between arm length and total bowstring length at its most basic controls the length of draw, and the power required to draw the arms back fully into firing position.

In ballista building there are no real shortcuts and knowledge comes at a price. I occasionally go back and read Nicks blog front to back. Research, research and more research is the price you must pay for any real knowledge that means anything. You seem unwilling to do that so far, and unwilling to take advantage of the knowledge already offered.

For instance, on NIcks blog there is a good read from a link - Duncan Campbell. By reading the entire paper one or twice I was able to glean some information by Philon I had previoused missed or dismissed, backing up a rope bundle theory I have. It may take days or weeks of intense research (or experimentation) to answer a single question, and what you dont know about mechanics or physics you must be willing to learn.

In the end, it is only through results driven research will you be able to answer and UNDERSTAND the principles involved. It is not enough to to simply know it works, but the understanding of the bow and why.

Also sometimes answers to questions here or elsewhere may take weeks or even months.

Many ballistas built by members heavily involve the work of folks like Marsden, which you have already dismissed. Perhaps that is why no immediate help can be forthcoming. My work involves steel springs and inswingers, while you prefer the outswinger rope bundle tech, thus eliminating me from further interest in the matter.

Geezer here: Regarding rope-bundle balistas, Outswingers vs. inswingers. Years ago I had a pal who made a lovely 'cheirobalist' small balista. It was an outswinger, based on Marsden's experiments. Ol' Dave fitted his balista with deeply recurved laminated maple arms, rather than the common hickroy axe-handle solution many builders prefer. The laminated arms flexed, storing energy, and their deep recurve shape gave him at least 5 degrees more throw, compared to straight arms. Indeed the machine shot beautifully, getting something like 350 yards with a 3 foot long one-inch diameter bolt. So there may indeed be substantial improvements available to out-swingers, without going the extra steps required to build effective in-swingers.
Just thought you guys might like to know. Geezer

I believe Vetruvious does discuss recurved arms. Been a while since I worked on any torsion projects. I am building a hand held, and have factored that into my built. The arms will be steamed, so there is no cutting into the grain. That would invite disaster. This is going to be for a machined based on Vet. tables. And Scorpio, probably sticking to the formulas is not a bad idea at first. These machines can be really hazardous to your health. Go slow in any case.

I've stated earlier that Im interested in the mechanics of ballistas.
Marsdens works are a gold standard, I've never dissed the ancient formulaes.

I wish to determine optimal torsion springs (a good torsion coefficient).

The problem is that I can't exactly match diameters of rope (for example) to the ancients, nor the material.
I don't plan on buying 3-4 kinds of rope dimensions, just to eventually test which one's "better".

I don't know the equations or fundamentals of that, since "ropes" aren't a popular topic.

I dont feel that discussing inswinger vs outswinger is relevant at the current pace of this thread.
I think that we need to start somewhere, and the focus of this thread's purpose is going all over the place.

Thanks.

If it is rope you are wanting to focus on, you have only a few viable choices to match what Greek and Roman engineers were working with. Sinew and hair. Hair was a popular alternative to sinew, which was considered much better. First, making the volume of rope you will need out of sinew is going to be a hurculearn task. And what kind of sinew? The Roman army consumed a vast amount of meat, probably goat being one of the staples, based on the use of goat leather for tents, so they were slaugtering a lot of animals. And according to one of the writers, Heron maybe, pretensioning the rope as you created each spring was critical. You simply cant do that with hair rope. And sinew is not all made the same. Cattle sinew is very greasy and not all that suitable for backing wooden bows, for instance, whereas deer sinew is considered much better, so there has to be differences in the properties of vaious animal sinew. Cattle sinew would be a good choice though for making, say, 500 yards of rope, but then I can imagine the only way to collect that much sinew is to spend a lot of time in slaughterhouses. That is going to be really ugly work. They destory the legs of cattle during the slaughter process (grind them up, if I am recallling right), so if you build a relationship, they would probably be happy to give you all the dead cow legs you could want, and they you have to extract them. Backstrap sinew is much longer and much better, but it may not be possible to get that at all, depending on how the animals are slaughtered.

Horse hair is a good alternative, and not to expensive. You can find 1" diameter bundles of horse hair, about 16"+ long, for around 20 dollars each from places like Crazy Crow. There are also instances in classical literature of the use of women's hair for use in catapults. Women's hair, not just human hair.

Maybe you are focused too much on the diameter of the individual ropes used. The idea was to cram as much rope into a hole carrier as possible. Whether that is 1/4" or 3/8" or 5 mm doesn't seem as importanat as getting as much rope as you can to fill out a skein of rope that is 2" in outside diameter, for instance. I was mocking up a rope bundle for a 3/4" OD handheld, and it took something like 50 meters of small diamter rope for each hold carrier. So you are going to need a LOT of rope no matter what size you decide your projectile length / rope spring is going to be.

Then you have to make all that rope. You can construct a rope walk with associated machines, or you can think about braided rope. Braided rope will not I think perform as well as twisted rope. When I was focused on making rope, I seem to recall that they didnt have rope walks in the classical world, so I am not sure how the Romans and Greeks did it. They were a seafaring peoples, so they must have had some way of producing a large volume of rope, though. Slave labor and hand twisting, maybe?

A buddy of mine who is heavily into catapult research feels that paracord is a good alternative to sinew rope. He may be right. Use that and you dont have to worry about the diameter of the cord, as it only comes in one size. Other reconstructors use nylon rope, or sisell rope, or hemp rope. Experimenting and spending a great deal of time and money on different ropes is about the only way you can go about this, I am guessing.

Dane

PS there simply is no easy way to go about building a scorpion or ballista, and if you are unwilling to do a lot of experimentation to find the "best" type of rope, I dont see how you can come up with the best results possible, which equates to me in ballistic performance.

The guys at www.torsionsfire.de built a nice repro of a inswinger ballista.
Also nice: a download exel spreadsheet with simulation for inswinger ballista. They switched from outswinger to inswinger design and got a decent upgrade in performance.
Site is in german of cause but even if you only can look at the pics its well worth it. Lots of details on the build.
What also intrigued me was that when you play around with the simulator the mass of the swinging arms seems to have very little effect on the end velocity. Not nearly as much as I thought it would have!

scorpio wrote:Hi,

Im starting a thread regarding to all ballistas (arrow or stone throwers) as a possible start for ANY disscussions of theories/construtions/experience and brain 'dump' for beginners to veterans. Im a novice but I've been following many projects on the interwebs.

My first progress would be contructing and studying the Mark IVa (arrow thrower) ballista. This inswinger seems easier to start off.

I've read and noticed a few blogs, "Nick's" project and warhammer1. There is also a great wiki page, etc. Which are all interesting/overwhelming, but they dont detail the phases of design.

A gold standard for reading is probably Marsden's Technical Treatises.

For a start, Im curious of how would one determine optimal rope diameter, total spring diameter and length, besides using optimisation formulaes and tables from the ancients and trial 'n' error?

Im a novice at mechanics. The traditional method for starting off would be choosing projectile (arrow) length, then base it on ancient "formulaes". Im more interested in modern ways of starting off.

I have both of Marsdens books and a few others. You need Marsdens technical treatises and historical development books because neither is complete without the other. All measurements are based on the diameter of the spring hole. For example, the most common size had a 3 inch spring hole and the arrow was 9 times that or 27 inches. There is a table of measurements in the technical book, but you really need both books. Roman Artillery, by Alan Wilkins helps too. As for the rope, I have read that twisted nylon is good. It has to have a little stretch and springiness. Nylon is used for acoustic musical instrument strings and holds its tune while still being springy. Stiff fibers will lose their strength the closer the arrow gets to being released.