Figure 4.1 shows the history of progress in speed and altitude capabilities. The impressive growth in one century is astounding – leaving the Earth’s surface in a heavier-than-air vehicle and returning from the Moon in fewer than 66 years!
It is interesting that for air-breathing engine powered aircraft, the speed – altitude record is still held by the more than 40-year-old design, the SR71 (Blackbird; see Figure 1.11), capable of operating at around Mach 3.0 and a 100,000-ft altitude. Aluminum-alloy properties would allow a flight speed up to Mach 2.5. Above Mach 2.5, a change in material and/or cooling would be required because the stagnation temperature would approach 600° K, exceeding the strength limit of aluminum alloys. Aircraft speed-altitude capabilities have remained stagnant since the 1960s. A recent breakthrough was the success of “Spaceship One” which took aircraft to
the atmosphere edge to 100 km altitude. In civil aviation, the SST aircraft “Concorde” was designed nearly four decades ago and has not yet been supplanted. The Concorde’s speed-altitude capability is Mach 2.2 at around 60,000 ft.
In military aircraft scenarios, gone (almost) are the days of “dogfights” that demanded a high-speed chase to bring an adversary within machine-gun firing range (i. e., low projectile speed, low impact energy, and no homing); if the target was missed, the hunter became the hunted. In the post-World War II period, around the late 1960s, air-superiority combat required fast acceleration and speed (e. g., the Lockheed F104 Starfighter) to engage with infrared homing missiles firing at a relatively short distance from the target. As missile capabilities advanced, the current combat aircraft design trend showed a decrease in speed capabilities. Instead, high turning rates and acceleration, integrated with superior missile capabilities (i. e., guided, high speed, and high impact even when detonated in proximity of the target), comprise the current trend. Target acquisition beyond visual range (BVR) – using an advance warning system from a separate platform – and rapid aiming comprise the combat rules for mission accomplishment and survivability. Current military aircraft operate below Mach 2.5; hypersonic aircraft are in the offing.