Warning, very OT but since you asked...
Machingbird
Design is a 2 stage, but somewhat different. It isn't very big, but the flight profile is not quite what others have been trying. The overall design and flight profile has been through countless iterations. I started working on this years ago. It took me a long time to figure out a way to make it so it might work.
Full composite 98mm booster with 4 small fins, staging an 88mm sustainer with three small fins and a slight boattail. No asymmetric cores, to eliminate mass asymmetry to help minimize odds of coning. The sustainer is also a bit stubbier than the usual trend for a high altitude 2 stage attempt.
Booster burnout @ 6100ft, M1.9. Sustainer hot fired off the booster a few seconds later @ 15Kft, M1.4 (target for separation to keep it above the transonic range so it can stay as straight as possible).
This is a safer design since there is no igniter built into the sustainer; the booster lights it. If something goes wrong, the sustainer can't light because the booster won't be there. Before it is stacked, it can't light. This sort of design can only really work due to the booster not being much larger OD than the sustainer. Essentially the sustainer plugs into the booster. There is a little penalty for doing it this way but I think the tradeoffs are worth it. Heck, the interface is a 3D printed sleeve; I don't even need to machine a coupler.
Max Q @ about M2.7, near 27Kft. Sustainer burnout @ a bit over 52Kft, M4.81 in the sim, around 23 seconds into the flight. Sim is to 346Kft. N to M motors and the M is quite unusual for thrust profile and duration, a dual propellant double-taper. The slower burning of the sustainer propellants tests at under 1/8"/s and has some other things unusual about it, though it is still APCP. The N is a slightly modified Bates, not all that special really. Everything is case bonded. No casting tubes; no conventional liners.
For the sim, derated sustainer propellant a little, derated nozzle efficiency a fair bit, and I think I could possibly build it for the mass budget. Going by memory it loses about 10Kft per pound over design mass. It's a modest sized rocket with a modest total propellant mass.
I did a lot of work on the flight profile to try to make it as survivable as possible, trying to keep the speed low until it gets high. If it were to survive a first attempt, the motors can be stretched for higher potential altitude.
I no longer have the equipment to do machining like I used to, so it may not get built. Work doesn't help either. I do have much of what is needed for it though; Kate, NC, tubes, chemicals...
I'm planning on bringing at least something to fly at Black Rock this year though probably a much more modest single stage. And the EX hybrid THRP-1 to LDRS.
Gerald