Springs vs. Rubber bands is not a single answer question.
I tried putting together a comparison chart, but they are just too different in their nature, here goes my try.Spring
- Heavy (slower, but better for accelerating heavier projectiles/assemblies)
- Durable (takes shock well, occasional dings and minor scratches have no effect on performance)
- Better weather resistance (withstands elements like UV light, moisture, natural temperature changes and with various protective coatings and treatments can be even more weather resistant)
- Longer Life (takes repeated use well, but again it's not indestructible and can crack or take set/stretch over a period of time especially if left stressed for an extended period of time....we are talking days)
- Consistency of energy storage (not many elements will have a significant enough impact on the springs performance and thus it will maintain consistent energy storage making use of sights possible)
- Light (fast, better at accelerating relatively light weight projectiles/assemblies)
- Brittle (shock is not a problem since it's absorbed by rubber, but even the slightest scratch/tear render the bend useless as it deteriorates more and more with every single use)
- Bad weather resistance (UV light makes rubber more brittle, moisture content is important - rubber dries up, temperature changes effect the elasticity of rubber making it stiffer if temperature is lower or more elastic if temperature is greater)
- Short life (with all the factors combined...it is evident that rubber degrades with weather and repeated use)
- Consistency (temperature and moisture content play a great role in rubber's performance, energy storage for a given stress will vary with temperature/moisture, rubber also loses some of it's power if left stressed for even a short period of time - we are talking minutes....thus resulting in inconsistent energy storage in different environments and sights will have to be adjusted frequently and perhaps not always necessarily)
There is also a Composite
(fiberglass for example) which has some advantages over both as well as some weaknesses.
- Relatively Light (Lighter than steel, heavier than rubber...a balanced material good for accelerating mid weight to heavy projectiles/assemblies)
- Relatively Durable (Not very shock resistant, dings and scratches can be a problem in the long run...rubberized coatings can help protect the surface from damage)
- Weather Resistant (UV light can have a shorten the life of a composite and protective coatings have to be used to prevent that, moisture has no effect on composites, temperature slightly effects the performance of a composite)
- Long life (Modern composites take no noticeable set if any at all, can be left stressed for long periods of time)
- Consistency (Since it takes no set, can withstand repeated use and long stress periods better than steel and rubber, keeps it's energy storage properties in a variety of environments - consistency is at it's best and use of signs is at it's most accurate)
Just a few things off the top of the head.
To return to the original question...Rubber is inexpensive, widely available, and easily adjustable to a variety of applications, a bundle of various thickness can be collected to adjust power.
An inexpensive material, good for quick projects I guess.
As for compactness and improved efficiency...
There have been a few things mentioned about lever bows.
When taking The Flipper as a platform to improve on...the first thing jumping to mind is allowing the levers to be bent past the point where they are perpendicular to the stock. This will allow for longer draw and thus a more compact version will be able to draw as far as the bigger one. Also the shorter levers will be lighter and will swing faster.
Another thought that came across in the few discussions I had was to recurve the short lever arm which would achieve a similar effect to the one explained above - winding the string/bend in a smoother fashion and thus theoretically resulting in a smoother shot. An even more compact and even more efficient version would have cams instead of the shorter lever arms, which if calculated correctly will give a force draw curve similar to compounds where energy transfer will be close to maximum every inch the projectile accelerates >>> similar to what Igora did in his ballista design.
Now for compactness during storage/transportation...there has been a few things said and they have all landed in the same bucket. The bar on which the levers are mounted can be split in two and made to fold forward and locked. Kinda what our buddy Clyde did with his little bow.
Though again, any kind of mount for folding limbs will make the assembly heavier and more demanding as it's made more powerful...winning in compactness, but adding weight.Ivo