Not sure what he means. According to 'Doc' Horowitz, when he gave his talk at the Mars Society Conference last summer (and I had a chance to ask the specific questions), there is no need to beef up the SRB for the "Stick" configuration as there is less bending moment than is applied during ignition of the SSME's before liftoff. That could, of course, be subject to change, based on a number of things that hadn't been defined yet at that time.<br /><br />At the time I talked with him, I asked about the possibility of thrust-terminating to back the SRM away from the rest of the vehicle. Doc said that would open up a whole can of worms (or words to that effect), since you add the potential for the TT port Safe-And Arms to malfunction, either premature firing or failure to fire. Although using TT on ballistic missile top stages works, don't have the statistical data to prove exactly how reliable it is, and we don't need any more problems. So I'd bet THAT technique is out, as is "fire-in-the-hole" ignition of the 2nd stage engine to push the 1st stage away. (Too much chance of overpressures in the interstage messing up something. I seem to recall that Titan II/Gemini did use FITH, igniting the 2nd stage at the same time that 1st Sep was initiated. NOT sure, as I didn't get involved with Titan on that level.)<br /><br />Besides, the SRM's will still be producing some thrust at tailoff and separation. So the best technique would seem to be using retro-thrust motors.<br /><br />Things that will have to be changed will be the attach points for the interstage on the front end of the SRM (which is the new designation for the SRB); figuring out some means of roll control, probably on the interstage; re-orienting the separation motors pointing forward and outboard (to prevent exhaust impingement on the 2nd stage)...or possibly designing new ones; and determining if fins are needed on the bottom of the SRM to increase attitude command authority. I would also suspect there might b