Thursday, April 26, 2012
As high-end hobby rockets begin to exceed mach 3, and in a few cases even probe the lower hypersonic regime, good conventional epoxies like aeropoxy may not be enough. For certain rocket surfaces, such as fin leading edges, there are a few possible solutions. The most obvious choice is to simply use metal in the fins. Aluminum should be plenty for flights to mach 5, and most space shots. Steel is even better, but only needed in extreme conditions such as boosted dart space shots.
As a sort of compromise, metal strips could also be used on the fin leading edge to protect the composites below. (Image above is from "tfish" at the rocketry forum. This rocket was not recovered so the efficacy of this aluminum tape is not known.) Composite damage to fins around mach 3 seems to arrive in the form of delamination, starting near the fin leading edge. There is little doubt that very high temperatures cause this problem. Metal is a good heatsink, protecting the composites below to some extent. Metals can spread the heat out, away from the fin edge, and buffer the transfer down into the composite matrix. Since most rockets only spend a few seconds above mach 3 at most, simply delaying the input of heat may be enough. Even at high temperatures, I expect that strips of metal would protect heat damaged composites from delamination. It is not heat alone that destroys fins - it is the combination of heat weakening and extremely high dynamic pressures.
Metals may not be attractive for various reasons, including cost, weight, and machining difficulties. An additional alternative, but one that has not been employed much in the hobby, is ablative coating. Properly designed ablatives are more than sufficient to protect a rocket under just about any flight condition. Just look at the Sprint ABM that could survive dozens of seconds over mach 5. Thick layers of epoxy or high temperature paint can offer ablative protection. But I suspect that these layers should have a suitable opacity to protect the layers below them. For epoxies and ceramic adhesives that can either protect through ablation or actually resist high stagnation temperatures, I think Cotronics is a good choice. A cheap solution would be to build up many layers of JB weld over the edge of the fin. This will resist modest temperatures, and serve the rocket well at mach 2. There are other fairly conventional epoxies that resist higher temperatures still. Cotronics offers some epoxies like this. Those that are most thermally resistant require protracted high temperature cures, something that wont work with composites. (But would be fine for an all-metal rocket with a metal fin can.) Another solution that I propose is the use of ceramic adhesives. Cotronics offers some solutions that may be useful. Durabond 950 cures at room temperature, and is resistant to 1200 degrees. It is thermally conductive, however, because it has metal in it. This adhesive is not terribly strong, only offering hundreds of pounds per square inch at low temperatures. Protecting a fin edge may not require that much strength.
I still think that metal fins and motor-as-airframe is the inevitable configuration for all top performance 75 and 98mm rockets in the hobby. This is already known in P-Q motors and space shots, and it would be a natural extension to bring that technology down into level 3. But composites are cheap(er), lightweight, and easy to mold into almost any shape. They will remain the go-to choice for a while. This does mean that fin protection, perhaps with better epoxies, is increasingly important.