A type of rocket engine once thought impossible has just been fired up in the lab. Engineers have built and successfully tested what is known as a rotating detonation engine, which generates thrust via a self-sustaining wave of detonations that travel around a circular channel.
The idea of the rotating detonation engine goes back to the 1950s. It consists of a ring-shaped – annular – thrust chamber created by two cylinders of different diameters stacked inside one another, creating a gap in between.
Gas fuel and oxidiser are then injected into this chamber through small holes and ignited. This creates the first detonation, which produces a supersonic shockwave that bounces around the chamber. That shockwave ignites the next detonation, which ignites the next, and so forth, producing an ongoing supersonic shockwave to generate thrust.
In practice, however, there’s a reason rockets are generally powered by internal combustion instead, in which the fuel and oxidiser are mixed to produce a slower, controlled reaction to generate thrust.
Mach 5 explosions create bursts of energy that travel 4,500 to 5,600 miles per hour, which is more than five times the speed of sound. They are contained within a durable engine body constructed of copper and brass.
MACH NUMBER is defined as a speed ratio, referenced to the speed of sound, i.e. Since the temperature and density of air decreases with altitude, so does the speed of sound, hence a given true velocity results in a higher MACH number at higher altitudes
The fuel used, the oxidiser ratio, the size of the holes, the size of the annular chamber, the size and shape of the reactor, when and where the fuel is injected – these all need to be considered, and change in relation to each other. For example, different types of fuel could require injection holes of different size or shape.
This fine-tuning is what Ahmed and his team have been working on. They created a careful balance of the hydrogen and oxygen, and tried it out on their test engine, a small, 7.6-centimetre (3-inch) rotating detonation rocket engine modelled after one designed by the US Air Force Research Laboratory.
“We have to tune the sizes of the jets releasing the propellants to enhance the mixing for a local hydrogen-oxygen mixture,” Ahmed explained.
“So, when the rotating explosion comes by for this fresh mixture, it’s still sustained. Because if you have your composition mixture slightly off, it will tend to deflagrate, or burn slowly instead of detonating.”
To show that the rig worked, the team also injected a methane tracer into the hydrogen, and used a high-speed camera to capture the detonation waves. Their images, they say, show continuous five-wave co-rotating detonations moving in a counter-clockwise direction.
If it can be scaled up, this technology could significantly lighten rocket payloads, and reduce the costs of rocket launches, but it has other potential applications, too., the US Navy predicted that rotating detonation engines could result in a 25 percent reduction in fuel use in its ships, and shave 300-400 million of its annual US$2 billion fuel bill.
“These research results already are having repercussions across the international research community,” said William Hargus of the Air Force Research Laboratory’s Rotating Detonation Rocket Engine Program.
“Several projects are now re-examining hydrogen detonation combustion within rotating detonation rocket engines because of these results. I am very proud to be associated with this high-quality research.”
The research has been published in Combustion and Flame.