Bet me to posting this ! From the Boramae engine post and the link there, to Breakingdefense.com. Studies like that have existed since Project NOTS / CALEB, vintage 1958 in the wake of Sputnik. Or perhaps even before.
A Mach 2 fighter jet can scrap 1600 m/s out of an ascent to orbit 9000 m/s. Not negligible, except what matters is the raw energy rather than the delta-v: and bad luck, there is an exponential nested in the equation. So energy wise it is not much, plus the Gripen has a limited bombload, translating into a very limited rocket weight, plus hydrolox is a giant PITA but anything else has much less specific impulse... end result of the whole train wreck: perhaps 100 kg to orbit. Then again: cubesats. Albeit they existed since 25 years and air-launch is still stuck in limbo. The 800 pound gorilla is SpaceX F9 Rideshare, which is presently making a massacre of all the small rocket startups. Even Rocketlab's Beck is freaking out.
A decade ago or two, Dassault had similar plans for the Rafale. So had the Israelis from their F-15s. Or was it ALASA ? same plane, but DARPA, and it went nowhere. Also the russians from their MiG-31s.
We shall see...