The speed of modern jet fighters is generally limited to around Mach 2 to Mach 2.5, whereas the SR-71 Blackbird could reach speeds of Mach 3.5, even in the 1960s. Several factors explain why modern fighters are not designed to achieve such extreme speeds.
Firstly, the **design requirements** of modern jet fighters prioritize a balance between speed, maneuverability, range, and stealth. Modern fighters, such as the F-22 Raptor or the F-35 Lightning II, are optimized for versatility, including air superiority, ground attack, and stealth, while also considering factors like fuel efficiency and operational range. Achieving extreme speeds like Mach 3.5 would demand compromises in other areas, such as maneuverability or weapon payload capacity.
Secondly, **engine limitations** play a significant role in speed restrictions. The SR-71 used specialized turbojet engines, known as Pratt & Whitney J58s, which were capable of sustaining high speeds at high altitudes. These engines were designed for high-speed flight at the edge of the atmosphere, where air is less dense, allowing the SR-71 to maintain Mach 3.5. Modern jet engines, particularly those in fighters, are optimized for different operational parameters. To reach speeds above Mach 2.5, engines must cope with extreme temperatures and airflow conditions, requiring advanced cooling systems and materials.
Furthermore, **aerodynamic and material challenges** at high speeds become significant. The heat generated by air friction at speeds above Mach 2 is immense and can lead to structural stress on the aircraft. The SR-71 was constructed from titanium and other materials to withstand these conditions, but such materials are expensive and difficult to work with. Modern fighters, while capable of high speeds, are built with a focus on more sustainable and cost-effective materials that are suited for a broader range of missions, rather than achieving extreme velocities.
Finally, **practical considerations** like the high fuel consumption required to sustain Mach 3 speeds, the risks to pilot safety, and the need for efficient operational readiness all contribute to the decision to limit speed in modern fighters. While the SR-71's speed was impressive and suited to reconnaissance missions, modern combat needs and technology have shifted focus toward multirole capabilities, stealth, and survivability rather than raw speed.