 The thrust-to-weight ratio would have been double the F-108 and over double the SR-71.At the request of USAF Chief Scientist, a less radical modification of the Arrow than the PS-2 was pursued which became the Mark 4.The revised intakes of the Mark 3 were retained, but with smaller Curtiss-Wright ramjets, without the canards and nose extension, and with a titanium skin instead of a heat shield.