We initially started with (I think) 6 x 17m bungees but they didn't have quite the 'Oompf' to get the Swift airborne from a flat field, so we doubled the length of the bungees (linking the extensions to the original ropes using light weight aluminium carabiners)... and the results were most pleasing!
Initial tests of the bungee launch system:
Skip to 7.00minutes to see the Swift finally get off the ground!
Hill launching Ken's Swift using the bungee system:
I can post more photos and detail of the bungee system components if you like. The bungee rope we used is not the normal (agricultural variety) but has about double the energy density - pricey, but worth it.
We used 10mm diameter bungee rope from Sandowtechnic.com, which yields around 22daN / 22Kg of force when stretched 75% beyond its unstretched length. I would be reluctant to use anything thicker than 10mm as this is on the limit of what a single person can safely stretch on their own - and even then, one slip and you can launch yourself 'over the back!'
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