It’s interesting how you can hear a part of an idea, file it away in the back of your brain, and then all of the sudden have something pop into your mind months later.
A couple of months ago, my arch-nemesis from ATK (ok, he’s actually a friend of mine, we just tend to disagree about…work related issues…) posted a rather interesting article about inflatable nozzle extensions that was done by Goodrich back around the time of the Apollo program. Basically, the idea was that they wanted to see if they could put a huge deployable nozzle extension on the bottom of a J-2 engine, so that it could have a much longer nozzle without paying a huge weight penalty for a long interstage section. The nozzle was made of a stainless steel “airmat” mesh, with the outside of the bell sealed off with a high temperature silicone rubber, and the whole thing was inflated with the turbine exhaust gas (which also acted as a warm-gas transpiration coolant for the nozzle extension). They did some testing with the concept including deployment tests and such, though they didn’t get an opportunity to hot-fire altitude test the concept.
The RS-68 has a fairly short nozzle (21.5 expansion ratio) so that it won’t get flow separation at sea level, so it can have enough thrust to lift the vehicle off the pad (since in some Delta configurations all the thrust is provided by RS-68s). However this lower expansion ratio severely hurts the vacuum Isp of the system. The RS-68 has a vacuum Isp of only 420s (similar to the J-2 which had an Isp of 421s), compared to the ~450s of the SSME or RL-10. The SSME has a much higher chamber pressure, so it can have a longer nozzle extension (~75:1 if I’m recalling correctly) without flow separation at sea level. The RL-10 only operates at high altitude, so it doesn’t have to worry about flow separation at all, and can have a really large expansion ratio (84:1).
Interestingly enough, if you could put a large enough inflatable nozzle extension on the RS-68, you could boost its Isp into the 430-445s range. And unlike the sliding nozzle extension like on the RL-10B-2 used on the Delta IV, this doesn’t involve any sliding seals. You’d probably want to go with a regen cooled nozzle upgrade while you’re at it, which would also help with things.
If this technology pans out, it could probably increase the performance of Delta IV vehicles by a substantial margin, while also bringing ideas like DIRECT back into the realm of feasibility.
It’d definitely be fun to see if the idea could be dusted off (now that the patents have expired).
Just a random thought.