View Single Post
Old 08-02-2011, 04:00 AM
Mark II's Avatar
Mark II Mark II is offline
Forest Sprite
Join Date: Dec 2006
Location: Back Up in the Woods
Posts: 3,657

I propose that we retire the phrase "Estes dent" and substitute something more generic, like "balsa smile" since the phenomenon is hardly unique to Estes rockets, or even, for that matter, to LPR.

Bernard, I have seen this happen on rockets with long elastic shock cords. It occurs when the cord gets tangled up, even temporarily, and thus made effectively too short. The nose cone can snap back and strike the top of the tube if a short section of the cord just below the cone is loose while the remainder is coiled up in the tube; the nose cone and the free section of cord are ejected first and the cord stretches and snaps back before the coiled length is even pushed out of the tube.

The solution is to substitute regular poly cord or even Kevlar for most of the shock cord length except for a short section where it emerges from the tube. The short section of flat elastic spreads the load out against the lip of the tube to help prevent zippers, and it provides enough shock absorption to soften the sharp ejection event without storing so much energy that it pulls the nose cone right back to the lip of the tube. By the time that the elastic shortens again, so much non-stretching cord above it has been paid out that it cannot bring tube and nose cone back together again when it recoils. The short section of elastic can only stretch so far and therefore it can only store so much energy; that energy is much less than that of a full length of elastic. You get the advantages of a shock-absorbing tether while avoiding the disadvantages of a cord that can forcefully recoil along its entire length.
Mark S. Kulka NAR #86134 L1,_ASTRE #471_Adirondack Mountains, NY
Opinions Unfettered by Logic Advice Unsullied by Erudition Rocketry Without Pity
SAM # 0011
Reply With Quote