Hypersonic passenger travel still seems more like dreamy work of science fiction than a feasible commercial project, but this is now a fact: Boeing is working on it, and it’s a serious project.
“It may not be as hard as people think it is,” says Boeing chief technology officer Greg Hyslop, who quickly adds a caveat: “It’s still going to be hard.”
Boeing first unveiled its hypersonic airliner concept on 26 June at an aviation technology conference and highlighted it again at the Farnborough air show in July.
In an industry that has lacked a supersonic transport option since 2003, suddenly proposing a hypersonic airliner as a viable option within 20-30 years seems to register somewhere between the ambitious and the absurd.
But Boeing insists that it has found a combination of speed, materials and propulsion that can make a Mach 5-capable aircraft not only technologically achievable, but financially profitable at some point after around 2040.
They even think they have worked out a solution for one of the trickiest problems associated with hypersonic flight. To cruise at speeds around M5.0, the aircraft has to fly at an altitude between 90,000 and 95,000 feet. The passengers won’t be wearing space suits, so they’ll be seated in a pressurised cabin. So any event that causes a depressurisation at that altitude would be catastrophic.
“We’re aware of it, and we’d thought about it,” says Kevin Bowcutt, a Boeing senior technical fellow and chief scientist for hypersonics.
“And we have some very innovative approaches for dealing with it,” he adds. “Basically, we’ve devised a concept that would keep the cabin pressurised even in a depressurisation event.”
Bowcutt declines to elaborate further on the technical solution to the depressurisation problem.
Boeing’s concept proposes a top velocity of five times the speed of sound. That happens to equal the commonly accepted threshold of hypersonic speed, but that wasn’t the reason. According to Boeing’s analysis, M5.0 is the limit achievable with the available structural, propulsive and fuel technology.
“We have the technology today and the design tools to do this,” Bowcutt says. “We don’t have to invent some new thing.”
The engine for the hypersonic airliner is a good example. A M6.0 aircraft requires a supersonic combustion ramjet (or “scramjet”) engine, a technology that still isn’t mature after decades of research and demonstration. A M5.0 aircraft, however, has access to other propulsion options, Bowcutt says.
The fastest passenger-carrying aircraft with an air-breathing engine in history is the Lockheed SR-71A. It flew at speeds up to Mach 3.2 using two Pratt & Whitney J58 engines. The J58 featured a unique configuration called a turboramjet. The engine functioned like a turbojet up to about M2, then diverted air from the compressor into ducts that emptied in the afterburner. The effect was similar to a ramjet.
Boeing’s hypersonic airliner also would use a turboramjet configuration, with some variations compared to the J58, Bowcutt says. Instead of ducting only a portion of the airflow around combustor over M2, Boeing’s concept might bypass all of the airflow around the engine core at higher speeds, he says.
Bowcutt is familiar with hypersonic vehicles. He is credited as the “father of the X-51”. He designed the vehicle in 1995 to become a missile, but it turned into a hypersonic demonstrator funded by the Air Force Research Laboratory. The X-51 used JP-7 fuel — the same kerosene formula that powered the SR-71A — as both a source of combustion and as a coolant system. Since the hypersonic airliner concept won’t travel than M5.0, Bowcutt says, it doesn’t require JP-7. Standard Jet-A fuel. Liquid methane or some combination of those fuels are options for the commercial aircraft.
Finally, Boeing selected M5.0 as the top speed because that greatly simplifies the structural materials. Instead of exotic, heat-absorbing materials like ceramic matrix composites, standard titanium alloys used in aircraft and jet engines today are strong enough to survive the surface temperatures ranging up to 600°C (1,100°F).
M5.0 is already 625% faster than a typical subsonic airliner cruising at M0.8, Bowcutt says. Higher speeds produce significantly greater challenges, with sharply dwindling returns. A M6.0 aircraft, for example, can fly only 20% faster than Boeing’s concept, he explains, but requires the designer to use nickel instead of titanium and unproven scramjets instead of turboramjets.
That more than six-fold increase in speed also drives the business case for the hypersonic airliner.
Hyslop compares hypersonic technology to a supersonic airliner, like the M2.0 British Aerospace/Aerospatiale Concorde. That speed allowed the Concorde, in theory, to cross the Atlantic twice a day using the same crew (although British Airways and Air France chose not to exploit that advantage). By contrast, the hypersonic airliner, in theory, might be able to cross the Atlantic four or five times a day with the same crew, he says. The difference in utilisation rate potentially makes the hypersonic airliner more attractive than the costs of operating a supersonic jet, he says.
“Would that be the tipping out where it economically makes sense?” Hyslop asks, rhetorically. “This whole business about how frequently can you turn an airplane — the economic engineering — looks more intriguing to us the faster you can go. That might be a sweet spot.”