Devices and concepts classified as “impossible”, is becoming more possible now. Thanks to technological advancements! In the early 2000s people made fun of multi-planetary life. But now, an ambitious technophile is building a rocket to get there. Speaking of rockets, there was this specific rocket engine that was considered to be impossible. Though the concept was solid, the scientists couldn’t find a proper design and method to make the propulsion system possible.
However, it seems as if the Engineers have finally figured out how to make it work. Recently, a group of researchers from the University of Central Florida developed a design for this propulsion system. This is the first breakthrough in this advanced rocket propulsion system. If this rocket engine is commercially developed, it can revolutionize the way we picture space travel! But, before we get there, it is important to get a picture as to how a rotating detonation engine works.
Rotating Detonation Engine(RDE)
As the name suggests, a rotating detonation engine uses a series of spinning detonations to produce the required thrust. This is different from a conventional rocket engine. While the conventional propulsion system works under the principle of combustion, the RDE follows detonation. Though the combustion-based propulsion system is more mature, they are inefficient. In the sense, the process is slow and requires a lot of fuel. Meanwhile, if the rotating detonation engine is to replace the existing system, the efficiency would increase by 25%. In addition to that, a lot of fuel can be saved.
Arthur Nicholls
The origin of this technology dates back to the early 1960s. Arthur Nicholls of the University of Michigan College of Engineering was the first one to idealise this concept. Being a pioneer in the field of detonation physics, he believed he could devise a technology that could naturally manage the flow of fuel without mechanical controls. His idea, though ambitious, was to confine a spinning detonation wave in a ring over a circle of fuel injectors.
Although he was able to start the process, he couldn’t control it. He concluded that this design would be an efficient replacement for combustors. After years of testing, he still couldn’t stabilise the system. Hence, he closed his research in 1964. He presented detailed documentation citing his work to the military. At that time, many practical designs for combustors were developed. Therefore the U.S lost interest in his design. Hence, he wasn’t able to materialise his design.
The science behind the rotating detonation engine
The working of detonation engines is simple. It makes use of one or more detonations that continuously travel around an annular channel. The design for such engines usually comprises a concentric cylinder with small holes or slits. The fuel and oxidizer are injected through these holes. Generally, both fuel and oxidizer are mixed together before injecting them into the cylindrical chamber. Above all, the injection takes place in a sequential, circular manner. Consequently, using an igniter, this mixture is burnt. This forms the first detonation. Once the engine is started, the detonations are self-sustaining. This is as a result of energy released during the first detonation. The burnt gases are pushed out of the channel by the next cycle of fuel/oxidiser mixture.
These detonations create excessive pressure within the system. Hence, eliminating the need for an auxiliary compressor. In addition to that, it is this pressure from each ignition that keeps the cycle moving. Each ignition is lighted in a sequential manner. To sum up, in a rotating detonation engine, concentric circles host chemical reactions. The product of this reaction pushes pulses of supersonic gas out. Hence generating thrust.
Why was it considered to be “impossible”?
Detonation is a fast and chaotic process. It is hard to control the flow of fuel in such engines. Meanwhile, in combustors, fuel and oxidiser are mixed to produce a slower, controlled reaction. Hence, safety-wise, Internal combustion engines were better. Moreover, in a Rotating detonation engine, for everything to stabilise and not blow up(literally), everything must be carefully calibrated. Precision is quintessential for a Rotating detonation engine. This is a tedious task.
One of the researchers who worked on this technology said: “We can get something stable, but how far are we from the optimal? And what do we need to get to the optimal point? Those are practical questions that are still being answered”. Before the researchers of Central Florida found success, it was Mirko Gamba from Michigan who worked on this system. He was trying to optimise the propagation of the detonation wave. Yet another factor that proved detrimental for the success of this system was the ideal air-fuel mixture. Hence, it was majorly these uncertainties and unpredictability of this system that made it impossible.
The new prototype: How they made it possible
The researchers at the University of Central Florida defied the odds and devised a practical design for this engine. Their design boasts Mach 5 explosion, meaning it could create bursts of energy that travel at 5600 miles per hour. This is equivalent to the speed that is 5 times to that of the sound! For the mixture, they used hydrogen and oxygen. Before injecting them, they calibrated it at the right amounts. This is crucial for the system to stabilise.
In addition to that, the researchers made a small change in the design. “We have to tune the sizes of the jets releasing the propellants to enhance the mixing for a local hydrogen-oxygen mixture,” Ahmed, one of the researchers, said. “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 capture the evidence for this finding, Ahmed injected a tracer into the system. This tracer helps in analysing the detonation. Moreover, it tracks the motion of the burnt gases. Also, to quantify this data, Ahmed and his team used a high-speed camera.
Applications of Rotating detonation engines
Detonative combustion is the future. Flaunting a simplistic design with low mechanical complexity, scientists are planning to implement this engineering marvel in various industries. These include the Aerospace and Marine industries. RDE promises a better, lighter and efficient means of propulsion for either of these sectors.
Aerospace Industry
RDE was initially dismissed as a potential rocket engine by experts. “Just a few months prior, a number of US rocket engine experts had publicly declared that hydrogen-oxygen detonation engines were not possible,” Ahmed said. However, his research shows that RDE is a prudent technology. On his paper, he has demonstrated experimental evidence to prove the feasibility of Rotating detonation rocket engines.
As of now, Ahmed is evaluating RDE for Aerojet Rocketdyne’s RL-10 rocket. In addition to that, he is working on the modern versions for the upper stages of Delta IV and Atlas V rockets. The Air Force is expecting a rocket launch flight test by 2025. This would be a benchmark for Rotating detonation rocket engines.
Terrestrial Applications
In addition to the aerospace industry, retrofitting RDE in place of Gas turbines can significantly help the Navy. Using RDE would decrease at about 15-20% of the annual fuel bill. Currently, the Navy pays around $2 Billion in fuel. RDE can serve as a booster engine, but it requires a different propellant configuration. In short, RDE can potentially replace conventional systems in the near future!
