F-15s are not stealth. You don't want to risk escalation by sending fighter jets towards someone else's airspace. To me, they don't make sense for this kind of mission.
It does sound like a lot of work, but that's the part of the puzzle that is not a mystery. Someone is in fact putting one of those back to work, we just don't know why.
I'm missing something - why would you be wanting to intercept balloons over someone else's airspace? The 2023 incident was in US (and Canadian) airspace.
Sorry, I should have explained it: you want to intercept a spy balloon before it enters your airspace, not after it has already entered and possibly gathered any information.
Well there's a whole lot of ocean one doesn't need to be stealth to fly over but really, your comments read like endless goal-post shifting from someone who has a preferred solution in mind and is working backwards to justify it. Also the SR-71 was never stealth, only reduced observability. The USSR was very aware of the overflights taking place.
It seems no one else offered any other concrete uses that would justify the efforts that went public regarding the frame reuse.
That likely concentrated the discussions towards this single idea I presented. However, I'm not actually that committed to it. If you have any other, I would very much would like to hear about it.
This is a NASA/JPL project; there are a limited number of airworthy SR-71s, they are not modernized for combat, and they cannot scramble to intercept. The M-21 variant has a well-documented conversion to a mothership[0] that can ferry payloads externally to-altitude and at high speed. This is incredibly ideal for testing captive ramjet/scramjet technology, since they require Mach 3+ speeds to attain ram pressure through the engine.
Sending the SR-71 into denied or uncontrolled airspace seems like a terrible idea, in 2026. Chinese and Russian naval interceptor missiles could shoot down the plane at it's maximum speed and altitude; it's not going up against 1980s missiles anymore. The risk is absurd, and there's not enough SR-71s to treat them like they're expendable.
I don't have access to any privileged information, here. My best bet is that DARPA is lighting a fire under JPL's ass to explore variable-cycle engine technology: https://en.wikipedia.org/wiki/Variable_cycle_engine
The US has tested ramjets before, but mostly in the context of missiles and boost-glide vehicles. The "holy grail" of efficient propulsion would be to taxi/accelerate with a turbofan engine, bypass into a ramjet for Mach 3-to-6ish, and then transition to a liquid-fuel rocket motor for the last Mach 6-10 climb. An engine like that could escape the Earth's atmosphere without sacrificing efficiency at lower altitudes. I realize that "space fighter engine" is also a longshot, but China has ramped up investment in variable-cycle technology quite a bit in the past 2 years. Simply solving the turbofan-to-ramjet transition would be a huge boon for high and fast flight.
Issacman's quotes in that article suggest, to me, that he wants to reboot the SR-72 program (or something similar) with cleansheet propulsion as a feature. The SR-71 could be the engine testbed, and a future airframe could be designed around the requirements of the engine. Just my $0.02 though.
It does sound like a lot of work, but that's the part of the puzzle that is not a mystery. Someone is in fact putting one of those back to work, we just don't know why.