OVERVIEW AND BACKGROUND
The HC Mojave Sphinx works great exactly as designed in HCR-5100. It is battle tested with over 50 firings and flights.
After building, firing, and flying our Sinnoh Sphinx+ we noticed a small amount of IPA leaking from our fuel bulkhead when we went horizontal after filling and we noticed some small fuel leaks while pressurizing on the pad from camera footage post-flight. These leaks were minor and did not prevent the rocket from properly pressurizing, firing, or launching. The stock 4” aluminum tank tube and the +/- .013” ID tolerance is a wide variable, so we wanted to design a one-piece bulkhead with two o-rings to minimize the risk of a leak.
We also were not fond of the two recovery bulkhead posts and their gasket washers, so we redesigned the aft recovery anchor plate to not use the bolts. That freed up room in the fuel bulkhead, so we added a 1/8” NPT tube and small valve we could use to fill fuel after the bulkhead had been secured to the tank. This allows us to assemble the piston and fuel bulkhead at home and then “load and go” on the range.
SPECIAL CONSIDERATIONS
This mod, as designed, will only work if you are using a standard 4” fiberglass airframe for your recovery section. That was an upgrade in our original build. The stock Mojave Sphinx uses a cardboard recovery airframe that cannot support the snap forces of the drogue with the relocated aft recovery bulkhead (anchor).
Read the attached PDF for more details.
Mike & Preston