DARPA has a concept program called ALASA(Airborne Launch Assist Space Access) that uses small jets to assist in launching small payloads into orbit.
“DARPA’s Airborne Launch Assist Space Access program (ALASA) seeks to propel 100-pound satellites into low Earth orbit (LEO) within 24 hours of call-up, all for less than $1 million per launch. The current ALASA design envisions launching a low-cost, expendable launch vehicle from conventional aircraft. Serving as a reusable first stage, the plane would fly to high altitude and release the launch vehicle, which would carry the payload to the desired location. The program is moving ahead with rigorous testing of new technologies that one day could enable revolutionary satellite launch systems that provide more affordable, routine and reliable access to space.” – DARPA