How may an aircraft that is promised to travel at Mach 6, in 2026, still be more myth than menu? Lockheed Martin In an engineering lane too skinny to be called a runway, the SR-72, dubbed by some as the “Son of Blackbird,” is an idea of a hypersonic aircraft which is to provide something the retirement of the SR-71 removed, namely a reusable, runway-launched platform capable of appearing quickly, collecting valuable information, and leaving before defenses can respond in any meaningful way to it. The concept gained hype when a movie-ready “Darkstar” lookalike was introduced to the popular culture, but the actual SR-72 has continued to remain unnecessarily secretive, with no publicly confirmed flights taking place, and the speculation moving around as to when this would actually take flight.
It is unsurprising that the fascination is easy to explain. The SR-72 is generally characterized as an air-breather which was capable of operating during the same conversation type as experimental rockets without having to use rocket logistics. In open-source accounts, it is positioned as a second-generation complement to satellites and traditional aircraft: not as predictable as orbital resources, faster than aircraft designed to run on stealth, and capable of traversing continents with gobsmacking speed-usually; “New York to London” in half an hour. The performance of that headline however, is the culmination of a very long chain of technical dependencies which have their own failure modes.
The propulsion system is central, and its focus is a combined-cycle architecture based on turbines that can be used to lift off under turbine power, accelerate to the challenging middle of the flight envelope and then switch to a ramjet/scramjet regime when conditions permit sustained hypersonic cruise. The transition has traditionally been defined as approaching Mach 3, at which point inlet conditions can provide the transition between the airflow control characteristics of turbines to the crude simplicity of the ramjet physics. A NASA-funded propulsion feasibility study held that the scramjet requirement to generate the so-called “bridge” between the turbine limits and the scramjet has been a fundamental barrier not only to this airframe, but to the broader hypersonics community. That is a problem which can be solved with paper, be tried in the earth and be chastised in the air. And though the engines may be good, the airframe must be able to endure the ride.
At hypersonic speed, there is no longer a design constraint of heat but it is the design. The heat flux and temperature environment that Mach 5+ flight represents places a strain on the repeated use of conventional metallic structures that exceeds what conventional materials can withstand, making high-temperature mixes, advanced composites, and active cooling of structures a topic of ongoing discussion in relation to the SR-72. Lockheed leadership too have publicly associated feasibility with contemporary manufacturing; in 2017, the executive vice president Rob Weiss said, “We’ve been saying hypersonics [are] two years away for the last 20 years,” but the technology behind it has actually evolved. Similarly in the wider context, company briefings identified additive manufacturing as a route to engrave cooling capabilities within hot structures-beneficial in engines and potentially absolutely vital on leading edges and other hot places.
Another layer is the debate between mission and mission. The SR-72 is most frequently described as an ISR aircraft, although the concept of the “dual-role” strike continues to reappear: a hypersonic airplane that is able to arrive quickly, fire its weapons and vanish. This is not a stuff of theory engineering. Internal as a packaging weapon battles directly with thermal management volume at very high velocities, and the dynamics of releasing weapons are no longer a trivial problem when the launch aircraft is traveling at a speed high enough to run the risk of catching up its own stores. The concepts of cleanest strike that best fit the physics is not the normal sense of “bombing;” instead it is more like an airstrike that uses speedy release and standoff enabled launch profile, where the plane is required to place a weapon into the appropriate energy state and geometry and continue on its way.
Why, then, does the SR-72 remain like a question mark? One of them is that highly classified programs can fly without the official recognition. The other one is the type of work that results in long spells of silence interspersed with well thought-out signals. Some of the more vivid indications include money: Lockheed filings have mentioned almost 1.7 billion in classified program losses, associated with “very complicated design and systems integration.” That does not verify an SR-72 in the air, but it does concur with the type of expensive, iterative work that is found in the area of advanced propulsion, thermal structures and flight-test risk. As far as no one is ready to accept the demonstration- an Engineering problem is an audacious set of engineering problems, contained in one airframe-shaped promise.
