I finished the booster section of the Phobos this morning, over a pot of coffee and a big batch of epoxy. Fin fillets went on smoothly, using tape "dams" to contain the epoxy to the fillet area. Some sanding will be needed after curing, but it should turn out fairly neat. The next step was to do internal fillets, anchoring the fins to the motor mount and inside of the airframe. This proved fairly tricky, since the 2.1 inch airframe and 38mm ID motor mount leaves only enough room for a smallish dowel. I solved this by mixing West Systems epoxy with a generous amount of colloidal silica, to the "peanut butter" stage of consistency, and then loading this into a syringe. The epoxy could then be injected (with considerable effort given the thickness) into the narrow areas of the fin mounts, and then filleted with a dowel. This worked relatively well, although the fillets deep into the fin root are nowhere near as thick and nice as the exterior fin joints. The rearmost 1" of the fin root received a fairly heavy fillet to make up for this, and I suspect the fins are fairly well supported. Finally, I placed the rear centering ring and applied more fillets of the thickened epoxy.
The aft centering ring is plain — all of my attempts to construct a 4-40 hex bolt retaining ring failed, given the narrowness of the aft ring. I split two of them, in fact, trying to carefully drill the pilot holes. Instead, for next week’s launch I’m going to use friction fits for the F and G motors in the adapter, and worry about serious motor retention when I try to use reloadable 38mm motors for L1 certification and launches. At that point, I may try to install an Aeropack Slimline retention system.
So what’s left? The recovery system and payload bay, but these are fairly quick to construct compared to the booster section. Once the aft fillets are set and curing, I can start on the piston, and if I have time later today I can begin on the payload bay bulkhead and eyebolts.
Both of the BT-55 scratch builds are completed; the shock cords went in yesterday morning, with a loop for a parachute to be clipped onto each. I did not permanently mount parachutes because I believe they should always be modular, to allow different choices for different conditions. Even in small rockets such as this. Both rockets turned out well, I think, and despite minor imperfections in fin mounting should fly just fine. But we’ll see.
Sadly, Bill Barnes isn’t going to make it to next week’s launch due to a family emergency, so I’m going to collect his van and gear mid-week and drive it to the launch. So we’ll have all the pads, launch controllers, and his collection of rockets for the kids and visitors to launch. It should still be a good time, even though Bill can’t make it.