In 2007 NASA began to look at reusing AIM-54 Phoenix missiles, retired in 2004 along with the F-14, as hypersonic boosters. The idea was that the NASA F-15B would carry accelerate to Mach 2 then launch the Phoenix, which would keep going to Mach 5 and carry out whatever research was needed (thermal protection materials, scramjet propulsion, guidance & control, boundary layer transition, aerodynamics at hypersonic speeds, etc.)
Captive carry flights were carried out in late 2007, with a demiled AIM-54 mounted to a pylon, mounted to another pylon under the centerline of the Eagle. Funding was not provided to continue the experiments however, and the remaining Phoenixes were scrapped or sent off to museums.