Rival powers jockey for the lead in hypersonic aircraft
“I’ve spent my career doing things that flew fast,” says Adam Dissel, who heads Reaction Engines’ US operations.
This British company is building engines that can run at breakneck speeds, under conditions that would melt existing jet engines.
The company wants to achieve hypersonic speed, over five times the speed of sound, around 4,000 mph (6,400 km / h) or Mach 5.
The idea is to build high-speed passenger transport by 2030. “It doesn’t have to go to Mach 5. It can be Mach 4.5, which is simpler physics,” says Mr. Dissel.
At that kind of speed you could fly from London to Sydney in four hours or from Los Angeles to Tokyo in two hours.
However, most hypersonic flight research isn’t for civil aviation. It comes from the military, where there has been an explosion of activity in recent years.
James Acton is a British physicist who works for the Carnegie Endowment for International Peace in Washington. Examining the efforts of the United States, China and Russia in hypersonic weapons, he concludes that “there is a whole zoo of hypersonic systems on the drawing board.”
Special materials that can withstand the extreme heat created around Mach 5 and a host of other technologies are making hypersonic flight in Earth’s atmosphere possible.
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The piloted hypersonic flight experiments date back to the American X-15 rocket plane of the 1960s. And intercontinental ballistic missiles (ICBMs) also re-enter the atmosphere at very high hypersonic speeds.
Now rival powers are trying to create weapons that can stay in the atmosphere, without using the cooling properties of outer space, and that can be maneuvered – unlike a static ICBM aimed at a city – at a target. that could move on its own.
Military spending is driving the hypersonic thrust of the three major national players.
At a recent Pentagon press conference Mike White, assistant director for hypersonics in the US military, spoke of the development driven by “our great power competitors and their attempts to challenge our dominance.”
Accuracy is a major challenge for these hypersonic missiles.
The mere possession of hypersonic missiles, dubbed “carrier-killers”, could force US aircraft carriers to stay off the Chinese coast in the mid-Pacific.
But hitting a nuclear-powered aircraft carrier traveling at 30 knots or more (35 mph or 56 km / h) requires fine adjustments of a missile’s course that are difficult to achieve at Mach 5.
The heat generated around a missile’s skin creates a sheath of plasma, or gaseous matter, at hypersonic speeds.
This can block signals received from external sources, such as communications satellites, and can also blind internal tracking systems that try to see outward to locate a moving object.
Plasma only accumulates where the highest temperature is.
A cone-shaped missile will have a uniform coating of plasma, but missiles that resemble shiny-winged darts can push the plasma screen away from surfaces containing the more sensitive antennas.
As if hypersonic flight weren’t difficult enough, chemical dissociation adds to the problems.
At extreme speeds and temperatures this phenomenon causes the atoms that hold the oxygen molecules together to break apart.
This in turn complicates the chemical model on which any air-breathing engine is based.
Progress in the hypersonic arms race has been dramatic. In 2010, the United States flew for five minutes in an unmanned plane and shark jaws across a stretch of the Pacific Ocean at hypersonic speeds.
The goal was more than just speed. It was time.
Five minutes may not seem like a long flight time, but in terms of breaking down the hypersonic barriers it was a triumph.
This fast machine, the X-51A, was launched from a high-flying B-52 bomber and used a rocket to reach Mach 4.5 before its main engine went into action.
Known as the scramjet, this engine combined airflow in a jagged intake with jet fuel, to accelerate to hypersonic speeds.
This meant coping for several minutes with air temperatures entering the intake at 1,000 ° C. Four X-51As eventually made a one-way trip across the Pacific between 2010 and 2013.
Aerojet Rocketdyne is a California-based space and missile engine specialist who worked on the X-51A. It is a measure of the secrecy surrounding this technology that its staff only speak on condition of anonymity, even seven years after the project ends.
A hypersonics expert at the company says of the X-51A: “The really hot part of the machine is in the front where the shock waves form, so that’s where the investment in materials goes.”
He says much has been learned from the 1960s X-15 rocket plane and the subsequent Space Shuttle program.
Reaction Engines has now demonstrated a process that should allow its aircraft engine to ingest superheated hypersonic air smoothly.
Its Saber engine incorporates what it calls a “pre-cooler”. This is the first part of the engine that meets the furious hot hypersonic air.
The challenge then is to mix it with fuel to create thrust.
Hot as lava
The Saber engine underwent an intensive test regime at a Colorado site in October 2019, during which Reaction Engines had to find a way to replicate hypersonic air speeds.
The company took a supersonic engine, nailed it, and channeled the air coming out of its rear into the Saber engine intake.
The Saber pre-cooler did its job, piping the coolant into the high-pressure system and allowing Saber to mix that air with the fuel.
The materials required here are not simple. The Space Shuttle relied on ceramic tiles made from composite materials known as ablatives to protect it during its incandescent reentry into Earth’s atmosphere.
An alternative approach to ablatives is to use a nickel alloy called Inconel which can cope with heated airflow to the same intensity as a lava flow.
Mr. Dissel states that Reaction Engines is on this Inconel league route. “This is kind of where we are now, and the cooling channels are also running to absorb the heat,” he says.
So a sophisticated thermal management system paired with Inconel points the way forward.
If this combination works, the vision of paying passengers on a hypersonic flight could become a reality within 15 years.
The potential of hypersonic travel to allow VIPs to arrive with maximum impact was identified by the US Air Force unit that deals with presidential jets.
He commissioned Atlanta-based hypersonic start-up Hermeus to evaluate a Mach 5 transportation project that carries up to 20 passengers.
It means that in the future the president of the United States may one day join a very select gang of Mach 5 travelers.