It takes a lot of fuel to launch something into space. Sending NASA’s Space Shuttle into orbit required more than 3.5 million pounds of fuel, which is about 15 times heavier than a blue whale. But a new type of engine — called a rotating detonation engine — promises to make rockets not only more fuel-efficient but also more lightweight and less complicated to construct.
A rocket engine is a reaction engine, an engine that expels mass to generate thrust. Thrust is the force that propels a rocket or spacecraft and is measured in pounds, kilograms or Newtons. Physically speaking, it is the result of pressure which is exerted on the wall of the combustion chamber. Rockets carry all of their own working fluid, called propellant, in contrast with air-breathing engines. Furthermore reaction engines are only considered rockets when they use gasdynamic expansion of the working propellant to accelerate it. This contrasts with, for instance, many types of electric propulsion thrusters which use body forces such as the Coulomb electrostatic force to accelerate gasses.
In a normal rocket engine we use fuel and oxidizer in a chemical reaction to create hot combustion products. The combustion process involves the oxidation of constituents in the fuel that are capable of being oxidized, and can therefore be represented by a chemical equation. During a combustion process the mass of each element remains the same. t should be pointed out that in the combustion process there will be a dissociation of molecules among the products. That is, the high heat of combustion causes the separation of molecules into simpler constituents that are then capable of recombining.
The common feature of all of these engines is a converging / diverging nozzle De Laval nozzle. Such nozzles are the essential feature of rocket engines as this is what enables the engine to gasdynamically accelerate propellant gasses from a stagnant state to high velocity imparting momentum on the engine. To create high speed exhaust gases, the necessary high temperatures and pressures of combustion are obtained by using a very energetic fuel and by having the molecular weight of the exhaust gases as low as possible. It is also necessary to reduce the pressure of the gas as much as possible inside the nozzle by creating a large section ratio. The section ratio, or expansion ratio, is defined as the area of the exit Ae divided by the area of the throat At.
A good parameter for the effectiveness of a rocket is called effective exhaust velocity which is the quotient of thrust (what we want) with propellant mass flowrate (what we have to pay). Specific Impulse, is popularly spoken of as the “gas mileage” for a rocket cycle and it fundamentally indicates how much bang for the buck you get .For a rocket with Isp = 100s a unit mass, m of propellant can generate enough thrust to support its weight in Earth’s gravity for 100 seconds or 100 times its weight for one second.
Researchers at the University of Washington have developed a mathematical model that describes howrotating detonation engine work.
A conventional rocket engine works by burning propellant and then pushing it out of the back of the engine to create thrust. “A rotating detonation engine takes a different approach to how it combusts propellant,” Koch said. “It’s made of concentric cylinders. Propellant flows in the gap between the cylinders, and, after ignition, the rapid heat release forms a shock wave, a strong pulse of gas with significantly higher pressure and temperature that is moving faster than the speed of sound.
Researchers at the University of Washington have developed a mathematical model that describes how these engines work. With this information, engineers can, for the first time, develop tests to improve these engines and make them more stable. The team published these findings Jan. 10 in Physical Review E. “This combustion process is literally a detonation — an explosion — but behind this initial start-up phase, we see a number of stable combustion pulses form that continue to consume available propellant. This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust.”
“The combustion-driven shocks naturally compress the flow as they travel around the combustion chamber,” Koch said. “The downside of that is that these detonations have a mind of their own. Once you detonate something, it just goes. It’s so violent.” To try to be able to describe how these engines work, the researchers first developed an experimental rotating detonation engine where they could control different parameters, such as the size of the gap between the cylinders. Then they recorded the combustion processes with a high-speed camera. Each experiment took only 0.5 seconds to complete, but the researchers recorded these experiments at 240,000 frames per second so they could see what was happening in slow motion.
“This is the only model in the literature currently capable of describing the diverse and complex dynamics of these rotating detonation engines that we observe in experiments,” said co-author J. Nathan Kutz, a UW professor of applied mathematics. The model allowed the researchers to determine for the first time whether an engine of this type would be stable or unstable. It also allowed them to assess how well a specific engine was performing.
Researchers Develop Groundbreaking New Rocket-Propulsion System
A University of Central Florida researcher and his team have developed an advanced new rocket-propulsion system once thought to be impossible. The system, known as a rotating detonation rocket engine, will allow upper stage rockets for space missions to become lighter, travel farther, and burn more cleanly. The results were published this month in the journal Combustion and Flame. “The study presents, for the first time, experimental evidence of a safe and functioning hydrogen and oxygen propellant detonation in a rotating detonation rocket engine,” says Kareem Ahmed, an assistant professor in UCF’s Department of Mechanical and Aerospace Engineering who led the research.
The rotating detonations are continuous, Mach 5 explosions that rotate around the inside of a rocket engine, and the explosions are sustained by feeding hydrogen and oxygen propellant into the system at just the right amounts. This system improves rocket-engine efficiency so that more power is generated while using less fuel than traditional rocket energies, thus lightening the rocket’s load and reducing its costs and emissions. 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.The technology has been studied since the 1960s but had not been successful due to the chemical propellants used or the ways they were mixed.
Ahmed’s group made it work by carefully balancing the rate of the propellants, hydrogen and oxygen, released into the engine. “We have to tune the sizes of the jets releasing the propellants to enhance the mixing for a local hydrogen-oxygen mixture,” Ahmed says. “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.” Ahmed’s team also had to capture evidence of their finding. They did this by injecting a tracer in the hydrogen fuel flow and quantifying the detonation waves using a high-speed camera.
“You need the tracer to actually see that explosion that is happening inside and track its motion,” he says. “Developing this method to characterize the detonation wave dynamics is another contribution of this article.” William Hargus, lead of the Air Force Research Laboratory’s Rotating Detonation Rocket Engine Program, is a co-author of the study and began working with Ahmed on the project last summer. “As an advanced propulsion spectroscopist, I recognized some of the unique challenges in the observation of hydrogen-detonation structures,” Hargus says. “After consulting with Professor Ahmed, we were able to formulate a slightly modified experimental apparatus that significantly increased the relevant signal strength.”
“These research results already are having repercussions across the international research community,” Hargus says. “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.” “These research results already are having repercussions across the international research community.” — William Hargus, study co-author The study was supported with funding from the U.S. Air Force Office of Scientific Research and an Air Force Research Laboratory Contract.