Here is an essay I wrote for my LA class on the subject of Hurling (January 2007). Many things are oversimplified or left out for the purpose of it being a essay intended for non hurlers.
Modern Model Siege Engine Superiority
It has been discovered that, no siege engine design is completely superior to any other. Most of these designs fall into three categories, onagers (Powered by a single torsion bundle of rope swinging an arm and sling), ballistae (Crossbow like, powered by two torsion bundles) and trebuchets (Powered by a falling counterweight swinging an arm and sling). Superiority in this case means several things. Effectiveness which is determined by range, projectile and accuracy and is very important. Effectiveness is combined with production, which consists of labor and the materials to build the device. Also related to production, is the labor for repairs and operation. Last, but not least is safety. Even a puny siege engine can kill someone. The optimal device would have the best range with biggest projectile while being easy to make and use and function completely safely with no repairs.
A trebuchet has many advantages. Its simple counterweight powered design makes the production easy. Also, there are few unknown factors about the stresses, and the stresses are generally low or spread out. This reduces the supplies needed and the labor a lot, which lets someone build a much larger trebuchet to throw larger projectiles. These larger trebuchets are hard to move to use because they are simply large and heavy with a large counterweight. Unfortunately, the low stresses reduce the power and range. Trebuchets can be built safely, but there are always dangers with large counterweights up in the air of falling projectiles. One of the most dangerous things with trebuchets is that they can throw backwards almost as far as they can throw forwards is something goes wrong. One other thing about trebuchets is that they are simple mechanisms, and to some people operate with a great mathematically beauty. Trebuchets are the simplest cheapest easiest, and the best for large projectiles, but lack the massive range of onagers and ballista. Trebuchets are also not as compact as the torsion devices, but tend to be safer when used correctly.
The two torsion devices (Onager and Ballista) operate on a difference principal, brute force. Tension is often added to the bundles (By twisting the ends, called the modiolus) until strange cracking sounds are produced or steel bends and wood shatters. They rely of massive unknown loads based on the rope tension which is often impossible for even an expert to guess at with any accuracy. This requires very strong frames using lots of supplies and careful work. Also the arms need to be stoped, usually by a hard stop, which causes huge impact forces which tend to break arms and stops. This means they can be expensive and use hard to make or hard to get parts leading to lots of labor for both production and repairs but, all the forces do produce amazingly powerful results.
Onagers and ballistae use the power in different ways with different advantages and disadvantages. Onagers have a single arm with one end in the torsion bundle and a sling to carry the projectile on the other end. They, like trebuchets, can fire backwards, or straight up, which is very dangerous. As a general rule, all parts or projectiles, that go flying off at high speeds, will be in the plain of rotation, which is vertical for an onager. This means the the operator is relatively safe on the sides, but loading the projectile and pulling the arm down is still dangerous. The transportation of onagers is one of the beast things about them. They are a great shape, usually rectangular, and quite small for their range. A one foot long onager can throw golf ball 100 yards. A trebuchet built with comparable effort might throw a much larger projectile, like a baseball, or fist sized rock, 100 feet. The onager clearly would be easier to transport, and would throw much further, but as said before, the trebuchet could throw something much more massive. Onagers are probably the least work for the range, but are beat by trebuchets for the least for for total power output (Range times projectile mass). Onagers certainly have the most range and power for their size.
Ballistae are similar to onagers in many ways. They both have the advantages and disadvantages of their torsion power source, but the ballista has two torsion bundles. Two bundles are harder to make than one and clearly take more supplies. The frame for a ballista, resembling a cross bow, is an inconvenient shape to transport and store, as well as taking more supplies to build than comparable trebuchet or onager. The two bundles and arms are also a pain to maintain because they have to kept identical and functioning. Also, the ballista's arms swing horizontally which means if they break peaces can end up almost anywhere. One major advantage unique to the ballista is its accuracy. The projectile is accelerated along a straight path which allows for superior aim and prevents dangerous backwards shots. Also, ballistae can fire arrows, or stones, which offers great variety of ammo choices. Arrows work excellent for maximum range and aim. For ammo weights, ballista are very similar to onagers. Overall, ballistae have the best accuracy, great power and superior range, but are the hardest to build. They also are the only siege one that can throw arrows, which further increases range and accuracy.
It is clear that each device has its own advantages and disadvantages. One may be superior for a particular task, but none is best for all. A trebuchet is great for hurling massive stones large distances, but an onager would be a better choice to throw a smaller stone further. A ballista is needed if accuracy is required. Trebuchets are very consistent, but a ballista can be aimed far easier. Ballista are also needed if one needs to fire arrows. Overall, safety is relativity balanced, but trebuchets can be made safer than onagers and ballistae. Ballistae have the clear advantage that they can not backfire. have the Therefor, it is clear that, no siege engine design is completely superior to any other.