a reply to: Soylent Green Is People
I'm a bit jetlagged, so excuse the coming rambling...
That's been my hunch as well. I have no doubt that there have been aircraft, possibly even airbreathing ones, that flew at Mach 6 (other than the
X-15). That said, I'm fairly certain that what ones flew at those speeds flew in the 70's or early 80's, and was most likely a development of the
X-24C, the FDL-7, or the Isinglass/Rheinberry developments.
What it seems like they bumped up into was all of the issues that you run into when you make a craft fly at those speeds. You essentially need to
air-launch it, which robs it of that precious flexibility compared to the more or less self-sufficient SR-71/A-12. Furthermore, you end up with
either a huge plasma plume if you're an airbreather (since the entire flight envelope could be summed up as "continuous reentry"), which robs you
of any stealth that you might have had because you have an RCS the size of a stadium and the IR signature of a tire fire. Your other option is to go
the Isinglass route and do the boost-glide thing, and then you have a spy platform that has the flight profile of a shallow-trajectory ICBM and could
provoke WWIII simply by overflying Soviet territory.
Speak nothing of the issues of trying to look through that plasma sheath and try to take pictures of any targets.
So my hunch is that they flew a B-52 launched demonstrator or two during the Ford or Carter years to prove that the flight envelope was indeed
achievable, which it indeed was, but that they found that it didn't make much sense for an ISR platform. And so while the technology developed to
allow an X-15 sized demonstrator to fly at Mach 6+ for 30+ minutes ended up being very helpful at allowing the 767-sized Space Shuttle to fly a
crossrange reentry profile at Mach 18 (Does the fact that we went from phenolic ablative shields to reinforced carbon-carbon and a silica tile-based
TPS in less than a decade strike anyone else here as a little, um, suspicious?)
But the SR-71 remained the best ISR platform that we had until the 90's, which is why it remained operational through the 70's and so on.
All the while we developed a pair of competing technology sets to replace it. One built on what was developed for the Shuttle and the
X-24C/FDL-7/Isinglass and led to the X-30, which combined a STS-style thermal management system and a 777-sized airframe with a scramjet for
single-stage-to-mach-6+ capability. But it proved far too expensive/technologically complex to implement, and the best solutions were stuff like the
HOTOL/SABRE technologies, which would take years to make functional.
The other technology set was what we now refer to as stealth, which more or less negated the need for speed to stay hidden from radar, or for 60+ mile
altitudes to get past treaties. The "hail mary" product of these technologies was the AARS/Quartz, a loitering ISR platform so hard to see that the
Soviets would have no clue it was over them as it mapped their mobile ICBM sites for hours at a time.
So where does my hunch take me? The SR-71 WAS deactivated and so it stands to reason that SOMETHING was built to replace it. But all signs point to
the SR-72-type hybrid turbine/scramjet craft still being a bit of a ways off, as as the holy grail of something that can do what AARS/Quartz was
supposed to do.
So what could we have in the interim? Combine the J-58 and that ~Mach 3/90,000ft flight envelope with some improved materials and low-observable
technology to mitigate the IR signature, etc. Add in an advanced TEB-based booster fuel for some extra "oomph" to get you past air defences, etc.