How a startup plans to chase SR-71 speeds with F-16-sized jet

For nearly three decades, sustained high-Mach flight has been more memory than reality. When the Lockheed SR-71 Blackbird stopped flying in the 1990s, the United States lost more than just a fast plane. It lost the regular process of building and testing reusable aircraft capable of flying faster than Mach 3.

Work on hypersonic technology moved toward missiles and research projects. High-speed planes with pilots or that could be used again were no longer part of regular plans. Now, a new aerospace company with private funding says it wants to bring that ability back, not with one big risky project, but by quickly building and improving real hardware step by step.

Hermeus’ Quarterhorse Mk 2.1 is about the same size as an F-16 fighter jet. It is built to fly faster than the speed of sound. Later versions will try to go even faster, first reaching Mach 2.5, then aiming for steady hypersonic flight by switching from one engine type to another at high speeds.

The speed numbers are compelling. But they are not the real story. The real story is how the company is trying to rebuild everything needed for high-speed flight, like engines, testing facilities, materials know-how, and the speed of development, in a way that is very different from how things were done during the Cold War.

Speed as architecture

Breaking the sound barrier is no longer a big deal. Fighter jets have done that for years. What matters now is flying at high speeds for a long time, where factors like air temperature, how air enters the engine, and switching engine modes determine whether the plane can survive and how far it can go.

The SR-71 specially handled this by combining its air intake and engines. Its J58 engines worked partly like regular jet engines and partly like ramjets at high speed. Movable spikes controlled shockwaves very precisely. This was a clever engineering fix made possible by urgent national needs and almost unlimited defense money.

Hermeus is attempting to reach into that same regime, but through a different path.

Quarterhorse Mk 2 uses a Pratt & Whitney F100 engine, a well-known jet engine with extra power from an afterburner and a special cooling system. This cooler is not just for show. At high speeds, the air entering the engine becomes very hot. Controlling that heat before it reaches the engine is necessary to keep the engine running well and prevent damage.

But precooling alone does not make an aircraft hypersonic. The company’s long-term goal is to build an engine called Chimera that can work in two ways. Simply put, it uses a regular jet engine for takeoff and speeding up, then switches to a ramjet at higher speeds. Changing from one type of airflow to another at high speed is the hardest part of making engines for very fast planes.

It is not about top speed. It is about mode transition without flameout, structural shock, or catastrophic instability. That engineering problem has not been operationally solved in a reusable aircraft since the Blackbird era.

Iteration as industrial strategy

Modern fighter jet programs use advanced computer models and take a long time to put together. But they are also slow to finish.

Hermeus believes that building and testing real planes is more important than spending a long time perfecting designs on paper. The company’s process is simple: build something, fly it, learn from it, and do it again. Quarterhorse Mk 1 proved it could take off and land. Mk 2.1 will try to fly faster than the speed of sound. Mk 2.2 and later versions will try to go even faster.

Each new plane gives information that helps improve the next one. This way of working is more like SpaceX’s than how old, big aerospace companies do.

That tempo does more than accelerate development. It rebuilds muscle memory. For years, the US aerospace industry has built fewer brand-new planes from scratch. Skilled workers gathered in fewer places. Test planes became rare. Engineering teams grew accustomed to working on updates rather than building new, fast planes.

Flying one new jet each year makes problems show up quickly. Things like flight controls, how much the plane can handle, how the engine connects, data systems, and heat control are all tested in real life, not just on computers. The result may not be elegant at first. But it builds competence quickly.

And high-Mach aviation requires competence more than perfection.

Infrastructure is destiny

A lack of suitable facilities has limited the number of test engines for high-speed flight in the United States. Special wind tunnels and test rooms for these engines are costly and often already in use.

Hermeus responded by building its own special test center for high-speed engines, called HEAT. This center is in Florida and uses former Navy buildings that have been converted into modern engine-testing facilities. That decision may prove as important as the aircraft itself.

Having their own test center means they do not have to wait for government schedules. It makes each test cheaper. It lets them make small improvements and check them quickly. It turns engine development from something that happens once in a while into something that happens all the time.

In high-Mach engineering, speed of testing equals speed of learning.

The SR-71 benefited from a whole national industry that worked together quickly during the Cold War. Today, things are more scattered. A private test center for high-speed engine testing is an attempt to rebuild that teamwork on a smaller scale.

Thermal reality

At speeds of Mach 2.5 and higher, the way air moves around the plane becomes more about heat than just movement.

Friction with the air heats the plane’s surface. The plane’s structure actually gets bigger from the heat. The materials have to handle repeated stress as the plane speeds up and slows down through different temperatures.

The Blackbird famously leaked fuel on the ground because its titanium skin expanded at cruise temperatures. That was not a flaw. It was physics.

Quarterhorse will face the same kind of physics. Even at Mach 2.5, the outside of the plane gets much hotter. The moving parts that steer the plane work in areas with strong shockwaves. The air intake must handle these waves effectively to maintain smooth airflow.

The cooler that Hermeus adds to its engine system is meant to enable the plane to fly in more conditions and protect engine components. But it also brings new challenges. The heat exchanger must cool the air sufficiently without losing too much pressure. If too much pressure is lost, the engine loses power. If the system is too heavy, the plane performs less well.

High-Mach flight punishes inefficiency. That is why propulsion architecture, not airframe styling, determines success.

Strategic relevance

Why does reusable high-Mach capability matter now? Missiles already travel at hypersonic speeds. But missiles are expendable. Aircraft are reusable.

A high-speed plane could be used to test hypersonic equipment quickly, gather intelligence much more quickly, or demonstrate military strength without making things permanently more tense.

Speed shortens geography. It complicates adversary planning. It creates options. The US military once relied on sustained high-speed aircraft as part of its strategic toolkit. After the SR-71, that tool disappeared. Hypersonic efforts became largely missile-centric.

Hermeus is trying to restore a reusable, air-breathing component to that ecosystem. Whether it succeeds remains uncertain. But the attempt itself signals a shift.

Invoking the SR-71 invites romanticism. But nostalgia does not solve the problem of propulsion physics. The important question is not if Quarterhorse will be faster than the Blackbird. The question is whether the United States can once again design, test, and deploy high-speed planes without needing a massive 10-year program.

If a startup can demonstrate sustained turbine-to-ramjet transition in flight, it will not simply match an old speed record.

It will show that high-speed flight can come from private companies building and improving step by step, instead of only from giant government projects.

That shift would redefine how advanced military aviation evolves. Speed may be returning as a strategic currency. But the deeper return is industrial.

High-speed flight requires advanced engines, robust heat resistance, and ongoing testing. It needs a team that is willing to build real planes, fly them, learn from them, and do it again.

Hermeus is betting that this culture can be rebuilt. If Hermeus is right, the next generation of American high-speed planes will not just be made in secret. They will be built step by step, in a steady rhythm. And that may matter more than Mach numbers.