An explosion! Send us into space? Exploring the energy secrets of spacecraft

Endless explosions could send hypersonic vehicles into space

Illustration: This is a conceptual illustration of a hypersonic aircraft powered by an oblique detonation wave engine. Image source: Daniel A. Rosato, NASA

Endless explosions could be the key to hypersonic flight and space planes that could rocket from Earth into orbit, and researchers have simulated in the lab the phenomena that could make such planes explode.

These explosions are not simple explosions as we understand them, but rather a particularly powerful explosion that moves outward faster than the speed of sound. The explosion that rocked the port of Beirut in Lebanon last year was a similar detonation, and the huge energy they can produce causes radiation damage over a wide area.

A long-standing dream of scientists is to build an airplane engine that can use this energy to fly; such a craft could theoretically fly from New York to London in an hour. But the detonation is extremely difficult to control, usually lasting less than a microsecond, so no scientist has yet been able to make it a reality.

Now, a team at the University of Central Florida has created an experimental device that allows them to continue detonating in a fixed position for several seconds, which the researchers believe is an important step toward future hypersonic propulsion systems.

"What we're trying to do here is control this detonation," said Kareem Ahmed, an associate professor of mechanical and aerospace engineering at the University of Central Florida and lead author of a paper on the research published in the Proceedings of the National Academy of Sciences.

"We want to freeze it in space and harness that energy. Instead of having it destroy buildings like you saw in Lebanon, now I want to harness it and use it to produce thrust," Ahmed told Live Science. "If we can do that, we can travel at superspeed."

The breakthrough builds on decades of research into a theoretical propulsion system known as the Oblique Detonation Wave Engine (ODWE). The concept works by sending a mixture of air and fuel at hypersonic speeds (more than five times the speed of sound) down an inclined plane, which creates a shock wave. This shock wave will rapidly heat the fuel-air mixture and cause it to explode, ejecting exhaust gases out the back of the engine at high speeds. The result is a massive amount of thrust.

When the air and fuel mixture is detonated in this way, the resulting combustion is extremely efficient because close to 100 percent of the fuel is burned. The explosion also creates a large amount of pressure, which means the engine can generate more thrust than it would otherwise. Theoretically, the energy from such a detonation should be able to propel an aircraft at up to 17 times the speed of sound, which could be fast enough for a spacecraft to simply fly out of the atmosphere, rather than needing to hitch a ride on a rocket, the researchers said.

The technical challenge is to maintain the detonation long enough to power such a flight, but previous experiments have shown only milliseconds at best. The main difficulty, Ahmed said, is to prevent the explosion from moving upstream of the fuel source, where it could cause serious damage, or further downstream, where it would extinguish.

"There's always this question of: If you hold it for a millisecond or so, are you just holding it momentarily?" Ahmed said. "You don't know if you've stabilized."

To test whether they could improve on the previous record, Ahmed and his colleagues built a series of chambers about 2.5 feet long (0.76 meters) to mix and heat air and hydrogen before accelerating them to hypersonic speeds and firing them up a ramp.

By carefully balancing the ratio of the air-to-fuel mixture, the speed of the gas flow, and the angle of the ramp, the team was able to produce a fixed-position explosion for about three seconds. That was enough to confirm that the explosion was stable and occurred in a fixed position, rather than moving upward or downstream, Ahmed said, which is the first important step toward realizing a real-life ODWE.

Demonstrating stable detonation is a major achievement, said Frank Lu, a professor of mechanical and aerospace engineering at the University of Texas at Arlington. To develop a practical engine, researchers must now figure out how to operate over a range of speeds and altitudes and deal with combustion instabilities caused by factors such as uneven mixing of fuel and air.

"I think the investigators have done a great job and look forward to further results," Lu told Live Science.

Ahmed explained that the researchers ran the experiment for only a few seconds, mainly because the intensity of the blast quickly eroded the glass sides of the test chamber. He said they had to use glass in their initial tests so that they could make optical measurements of the blast, but that they should be able to run the blast for longer if they used metal sides instead.

And hopefully, Ahmed said, the test rig's structure is not that different from the design of the full-scale ODWE. The main challenge now for the researchers is to work out how they can vary the three key ingredients—fuel mix, air velocity, and bevel angle—and still keep the explosion stable.

“Now that we’ve proven it’s feasible, it’s more of an engineering problem — figuring out how to sustain it over a larger operational domain,” Ahmed said.

BY:Edd Gent

FY: Qiu Bai

If there is any infringement of related content, please contact the author to delete it after the work is published.

Please obtain authorization for reprinting, and pay attention to maintaining integrity and indicating the source