domingo, febrero 5, 2023

Hypersonic scramjets still key to future of aerospace

An 18-hour commercial flight is enough to make anyone squirm. But don’t expect near-term aerospace tech to save you from long hours in that dreaded middle seat.

An 18-hour commercial flight is enough to make anyone squirm. But don’t expect near-term aerospace tech to save you from long hours in that dreaded middle seat.

That’s because the scramjet engine technology needed to cut such flight times to a third or less is still a generation away, say two prominent scramjet researchers. Scramjets, (Supersonic Combustion Ramjet Engines) that allow for supersonic airflow through the engine’s combustion chamber, in fact, may first see use on launchers to low-Earth orbit.

Scramjets have been a concept since the 1950s, says Chris Goyne, an aerospace engineer at the University of Virginia in Charlottesville. But it wasn’t until the 1960s that scramjets were actually tested in the lab. To date, NASA still holds the scramjet speed record for its X-43a experimental aircraft which achieved Mach 9.68 on November 16, 2004.

“Compared to a Boeing BA -1.04% 747, if you look at designs for hypersonic aircraft they really don’t have very much wing surface area at all,” said Goyne. “That’s because when you fly at hypersonic velocities you really don’t need wings; you can get enough lift just from the body of the [fuselage].”

Cruising altitude for a hypersonic aircraft from LAX to Sydney or Los Angeles to Singapore would be about 100,000 ft., about twice that of the Concorde. At Mach 10, ten times the 680 mph speed of sound, Sydney to Los Angeles could be done in about an hour and a half;  making a quick run for sunset drinks at Circular Quay a theoretical possibility.

The absolute upper limit for hypersonic flight is about Mach 15, says Goyne, but scramjet tech will be developed around a paradigm of Mach 5 or 6.

“If you fly too low, there’s a lot of air density friction and the aircraft gets too hot,” said Goyne. “Too high, you start to run out of oxygen. The sweet spot altitude is 100,000 ft. or so.”

Goyne adds that near-term military applications in the next 5 to 10 years include future scramjet cruise missiles, which he says are very close to being fielded by the U.S. military. Beyond that in another 15 years military aircraft; there will be manned or unmanned scramjet military aircraft likely maximized for intelligence gathering.

“If you fly an aircraft at supersonic speeds,” said Goyne, “the efficiency gets so low at Mach 4 or 5 that you are better off operating with a scramjet that will have much higher efficiency.”

Goyne says scramjet operates predominantly using supersonic combustion. Unlike a normal conventional jet engine, he says, a scramjet scoops in air through an inlet; so there’s no mechanical compressor or rotating machinery.

“The biggest challenge in testing hypersonic air flows is fully simulating the velocity of hypersonic flight,” said Goyne. “We use electricity to heat the air to 1800 F and can test scramjets for hours at a time.”

The best computer simulations in the world, says Goyne, are still not accurate enough to predict the performance of the scramjet combustor.

“That’s one reason we are doing ongoing wind tunnel testing here at the University of Virginia with funding from NASA and the U.S. Air Force,” said Goyne. “We have a high-speed, high temperature wind tunnel to simulate an aircraft operating at five times the speed of sound.”

At full scale, to get to their initial scramjet operating velocity of Mach 4, or some 3000 mph, Goyne says scramjets need some other propulsion system to initially accelerate the vehicle. Options now are rocket engines or turbojet engines, says Goyne who notes that both NASA’s X-43 and Boeing’s X-51 were launched from the bottom of B-52 aircraft and then accelerated using a solid rocket engine.

The most practical concept at the moment is the turbine-based combined cycle, says Goyne. So, in this case, he says, a gas turbine or a turbojet engine used to takeoff on the runway and accelerate up to the scramjet takeover speed.

It wasn’t until flight testing in the early 2000s at the University of Queensland in Australia that it was shown that scramjets could actually operate in flight. Researchers there used an Aerobee sounding rocket to get to altitude and then switched on scramjet engines over Woomera Test Range in the Australian Outback.

“The University of Queensland is continuing research in our ground-based testing facilities to find out just how fast scramjets can be made to operate,” said David Mee, a mechanical engineer at the University of Queensland in Australia. “We’re investigating ways to increase the performance of vehicles through efficient inlet design, [as well as] methods of reducing drag in combustors.”

The first civil scramjet application will be access to space, says Goyne.

“The altitude is different,” said Goyne. “But the physics involved between flying at hypersonic velocities at 100,000 ft and hypersonic speeds into space are very similar.”

Goyne says one of the key advantages of a scramjet over a rocket engine is that the scramjet gets the oxidizer to combust its fuel from the air it is flying through, whereas a rocket engine needs to carry liquid oxygen to burn its liquid hydrogen fuel.

Mee says a major component of their research into scramjets is aimed at a three-stage launch system, of which the second stage would be a scramjet. The idea, he says, is that a reusable scramjet second stage would cut in at Mach 6 or 7 and autonomously glide back and return to the launch site.

A third stage would then be released and launch a small 100 kg payload satellite into Earth orbit. Mee says a niche market for scramjets in such launch systems would be geared to small satellites destined for Sun-synchronous polar orbits.

As for scramjet-powered aircraft here on Earth?

The commercial aircraft industry is interested, says Goyne, but the up-front cost of development of hundreds of millions is prohibitive for private aerospace companies. However, he says companies like Boeing will be involved in building aircraft once they are fielded.

Yet if commercial hypersonic flight is economically feasible, Goyne expects it to happen within the next 25 years.

“The fact that scramjets are air breathers makes them more efficient,” said Goyne. “But whether Mach 12 can be commercially viable is questionable.”

As for eventually cutting those long flights by a third?

“We’re still at the stage of just getting scramjets to fly,” said Mee. “So, there’s a lot of development still to be done to get something that you could ‘certify’ to fly people.” – INNOVAticias



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