Some observers believe the Future Strike System will be similar to the airplane-like two-stage space launch systems. Some argue that the technology for a two-stage system is now at hand, and these technologies are the same ones the United States Air Force could use for a future strike system, the next generation bombers. Airplane-like access to space refers to a first stage of a two-stage system that is highly reliable, has very short turn-around times, is launched on demand versus on schedule, is easily maintained, and can use multiple existing runways. The staging speed would be around Mach 4, in order to allow the airframe to be constructed with conventional materials. A combined cycle engine that starts out as a conventional turbine engine and converts to a ram-scram jet would provide for a gradual speed increase from 0 to Mach 4.