The only realistic Mach 4 engine that was mass-produced (38 units) is the Marquardt RJ43-MA-20 Ramjet engine. Due to the high operating temperatures at 2500+ mph and low pressures at 80,000 - 95,000 ft altitude much of the center body construction was a Magnesium Thorium alloy, not exactly a conventional material.

Other drawbacks of the ramjet are that it would self-destruct at lower altitudes, had no re-start capability, and had to be launched at Mach 3 from atop an M21 Blackbird (the precursor to the SR-71). The D-21 did not carry a pilot, it was essentially a disposable cruise missile with an airbreathing engine.

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The MIPCC technology would use a conventional aircraft, taking off from a runway, flying 250 miles out to sea, then pulling into a climb above 40 degrees. Activating its afterburners and the MIPCC unit it would then accelerate vertically to a high Mach number with a rocket launch around 100,000 ft. The aircraft would then be able to re-enter the troposphere, using only a spray-on ceramic coating for heat shielding, shut down MIPCC and afterburners, and fly back to base. It could also re-start its engines if they stalled. The MIPCC aircraft carries a pilot, and is re-usable, flying unlimited number of missions. In fact RASCAL will be designed with a 24-hr turnaround, and a 1-hr scramble capability. With that capability built into an ARMXER Orbital Skyhook rendezvous aircraft, a fleet of just 48 aircraft, stationed around the world and MIPCC-equipped, could keep up with and achieve the 30-minute launch frequency.