>we would probably go in another design direction now

Which begs the question, "what direction woult that be?"

It looks like we are going to build a small roton-like vehicle at Armadillo (in internal competition with our pure-rocket designs), and I would certainly be interested in hearing any retrospective thoughts from you. Because we don't have aspirations of being an SSTO, we can make some notable simplifications:

Peroxide monoprop only. With rotor air augmentation, that is sufficient for a 100km vehicle. Because we use catalyst packs, the plumbing is then just a single fluid going through the hub.

Fixed pitch blades. Instead of having a single engine on each blade that pitches with collective change, we will have a small side firing engine and a separate, larger, down firing engine. This avoids deep throttling during atmospheric ascent, and may allow us to pulse the side firing engines in phase with the rotor for attitude control. If the pulsed attitude control doesn't work out, we will be gimballing the entire rotor instead of using a cyclic. Without collective, it won't be able to auto-rotate, so on descent it will be more like two deployed drag brakes instead of a rotor, but that will still be sufficient to slow it significantly before we spin the rotor back up.

We are still debating the stability question. While the control system can probably fly it unstable, we would rather not. An early design with a pusher rotor pitched over and landed on a ground penetrator like a lawn dart, but that required the blades to reverse their pitch on descent. Worst case, if we really want stability and can't get it any other way, we could have an ejectable fin can that we pop off at apogee. My current idea is that there should be some arrangement of angled winglets that can be placed on the ends of the rotors to make a larger corrective force in ascent than descent, which can be used to make it stable both ways.

The question of the transonic flight regime is still completely unknown, but the goal for this year is just to take the world unlimited time to climb record to 3000m, so we can deal with that next year...

We have already fired a rotating peroxide tip engine:

http://media.armadilloaerospace.com/FirstSpin.mpg

John Carmack
www.armadilloaerospace.com