I have been flying a lot of 38mm rockets, and challenging myself with staging, with partial success. My single-stage 29mm and 38mm flights have been going well, though, with clean recoveries on Mach 2+ and/or record breaking flights on 6 of my last 6 attempts.
I'm not sure why I caught the level-3 bug, but recently I've been spending a lot of time thinking about high-performance 3" rockets. I always try to fly my rockets as high as I can on my chosen impulse class, so I've been doing some simulation on what's the highest that I could go with an M motor.
Two motors that I've been looking into are:
CTI M2020: 2nd-highest impulse 3" commercial motor. The highest-impulse motor, the Loki M3000 has a little more oomph but is 30% longer, and has a shorter burn time. RASAero sims this to about 50kft from Black Rock, but has a peak speed of Mach 3-3.5, depending on how light I build it. That's well into the range where airframe heating is a major design problem.
CTI M840: This motor has significantly less impulse, but a 9 second long burn duration. Better yet, it only gets up to about Mach 2, well within what I have successfully flown before (Mach 2.5). It goes almost as high, simming to about 46kft, so it would still contend for the M record.
My current plan is to certify L3 at Balls next year with the CTI M840, and if everything goes o.k., fly it again on the M2020. So I'll do what I can to design and build for a Mach 3 flight, but I wouldn't need to go over about Mach 2-2.5 in order cert successfully.
Preliminary rocket description:
6:1 Von Karman nosecone. I'll either use a Shockwave Rocketry cone directly, or use it as a mandrel to fab my own cone. The nosecone will slip over the motor for a single separation at apogee, and the main chute will be contained in a separate chute holder that will be bolted to the top of my motor along with the av-bay. I have used this configuration for all of my recent flights. The advantages of this av-bay and nosecone configuration for high-performance flights are:
- It allows av-bay and chute to be packaged within the curved part of the nosecone, minimizing the rocket length and drag
- Only 1 airframe break reduces aerodynamic drag
- An internal structure is provided to restrain the tracking antenna in a nice, straight line
- The motor keeps the av-bay warm at very high altitudes
- The apogee deployment doesn't pull on anything restraining the main chute, so accidental main deployments are very unlikely.
The av-bay will have GPS tracking and high-altitude barometric altimetry. Hopefully it will be the new Featherweight Raven+GPS+radio unit if I can get it done in time. Otherwise, it will probably use a Telemetrum and Raven like I've been flying in my 38mm birds.
The aft tube will be shorter than the length of the motor, allowing the motor to act as the coupler. That also gives me the option of using the motor case as a mandrel to make my own carbon tube. I priced out an aluminum mandrel, and it's about the same price as a 3" carbon tube from Performance Rocketry, so I don't think I'll make a long 3" tube on my own.
The fins will be carbon fiber, with almost all of the fibers running in the direction away from the body tube. I have some carbon sheet stock that is already mostly uni-directional that way. I would start with the sheet stock and apply tip-to-tip unidirectional carbon fiber, up to about 0.060: to 0.080 inches. My 38mm carbon fins, made the same way, are very stiff at about 0.045" thick.
I'm shooting for stability margin of a bare minimum of 10% of the rocket length, which works out to about 1.5 calibers of stability. But I'll probably go with bigger fins than that, to get a straighter boost, and provide more options for future staging using the motor as a coupler. I have found in my record flights that it doesn't pay to make the fins too small.
I'll use Cotronics Durapot 862 resin for the tip-to-tip layup, which has a 500F allowable temperature. I have used the 861 resin for a fin layup before, and it worked great, other than not having a room-temperature cure. I'll use that resin for the nosecone and booster tube too, if I decide to make my own.
I even haven't asked the person I want to be my TAP yet, so this is very preliminary. But I'm in the mood for kicking off this design/build thread anyway, so here we are.