Originally Posted by
tdracer
I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.
Yes. They had an existing engine which fitted in the allowed nacelle diameter. The challenge was modifying the existing engine with least carve-up possible and still give what was required, hence no concentric bypass duct instead of 6 tubes, for example. Never mind a huge duct bypassing the whole turbomachine which would have had to pass more than five times as much air as the tubes. For insight into other paper-only options for "the problem/challenge" to get a J58 to do M3.2 , eg GE-style variable stators, see U S patent 3,344,606.