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.
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.
There has been some speculation about this outside this thread though:
https://theaviationist.com/2026/09/23/nasa-chief-teases-myst...
I don't know how credible those reports are, but Jared Isaacman's comments do suggest more than just a testbed. Do you have any comment on that?
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.
Thanks for the insights!