Supercruise is the capacity of a jet fighter to sustain supersonic flight. The current generation in fighters, which includes the
MiG-29 and
F/A-18 Hornet can attain supersonic speeds. However, such speeds are only attained using the afterburners.
Afterburners operate more or less by spraying fuel in the hot exhaust of a jet engine. High thrust can be attained, but at the cost of high
fuel consumption. An hour's worth of fuel can be burned in a minute. No current jet fighter can sustain
supersonic flight for this reason. All are
subsonic aircraft with a limited supersonic capability.
Supercruise capability represents a leap as substantial as the move from propellers to jets. Tactically, this offers significant advantages, as the supercruise capable aircraft enters combat at a high energy state. US Fighter doctrine teaches that speed is life! Speed greatly complicates any interception problem. Weapons launch windows are greatly shrunk. Subsonic fighters may be able to attain equal, or greater speeds than a supercruise aircraft, but the acceleration times required to attain such will make that point moot in most tactical situations. Supercruise capable aircraft will enjoy the choice to engage or not engage in most situations. The sum of these capabilities grant supercruise capable aircraft distinct tactical advantages over all current fighters.
An aside, the SR-71 is in effect a ramjet, because the operation of its nose creates a ramjet effect, reducing its fuel consumption to roughly equal the 737. Jet engine inlet design is quite a challenge, and particularly so for a supercruise aircraft. The reason is that air entering a jet engine, must always travel at subsonic speeds, regardless of aircraft speed. Otherwise compressability waves--- or shock waves-- will create uncontrollable vibrations among the compressor vanes. Engine inlet design thefore can effectively limit the speed of the aircraft, regardless of thrust. The SR-71's distinctive, but not unique- nosecone is designed to funnel air around the J-58 engine for that very reason.