Japan’s New Rocket Engine Uses Shock Waves As Propellant

This is a significant advance in the implementation of alternative propulsion systems, aiming at reducing costs and increasing the effectiveness of rocket engines. It is hoped the new engine design will be to be up to the challenge of the new space age, which could see deep space exploration. 

What are rotating detonation engines?

Traditionally, rockets use chemical liquid propellants to lift off, such as hydrazine, high-test peroxide, nitric acid, liquid hydrogen, and others in different combinations. 

Conventional rocket engines have a combustion chamber where stored propellants, fuel, and oxidants are burned to produce hot exhaust gases and, eventually, thrust. This uses Newton’s third law of motion, which states that for every action there is an equal and opposite reaction.

Combustion is a relatively slow and controlled process, which is also very well understood and mature as a technology.

On the other hand, rotating detonation engines use detonation waves to combust the fuel and oxidizer mixture. The explosions move around an annular chamber in a loop, creating gases that are ejected from one end of the ring-shaped channel to produce thrust in the opposite direction. The shockwave from the detonation then propagates – swirling and expanding at around five times the speed of sound. This in turn generates high-frequency shock and compression waves that can be used to generate more detonations in a self-sustaining pattern, aided by the addition of small amounts of fuel. As a result, this type of engine releases significantly more energy from significantly less fuel mass than combustion.

In a similar design, called a pulse detonation engine, the engine is pulsed in order to renew the mixture in the combustion chamber between each detonation wave and the next.

According to NASA, “Pulse detonation rocket engines operate by injecting propellants into long cylinders that are open on one end and closed on the other. When gas fills a cylinder, an igniter—such as a spark plug—is activated. Fuel begins to burn and rapidly transitions to a detonation or powered shock. The shock wave travels through the cylinder at 10 times the speed of sound, so combustion is completed before the gas has time to expand. The explosive pressure of the detonation pushes the exhaust out the open end of the cylinder, providing thrust to the vehicle.”

JAXA’s rocket test also included a pulse detonation engine as a second engine. It was operative for two seconds on three occasions, while the rotating detonation engine worked for six seconds in the liftoff. However, the test still served to demonstrate that both PDEs and RDEs are viable rocket technology. 

Until now, PDEs have been considered inferior to RDEs because, in RDEs, the waves move cyclically around the chamber, while in PDEs, the chambers need to be purged between pulses. Although NASA, and others, continue to research the use of PDEs as rocket engines, so far their utility has been focused on use for military purposes, such as in high-speed reconnaissance aircraft. In fact, before JAXA’s test, PDEs had previously only been tested in 2008, in a modified Rutan Long-EZ aircraft built by the US Air Force Research Laboratory and Innovative Scientific Solutions Incorporated.

But now that PDEs performed so well in space along with RDEs, their applications might be revised and, perhaps,  amplified.

On top of this, a team of researchers from the University of Central Florida (UCF) recently conducted the first demonstration of a third type of detonation engine, the oblique wave detonation engine (OWDE). This produces a stable continuous detonation that’s fixed in space.

It is composed of a hollow tube, divided into three sections. The first section is a mixing chamber, where a jet of hydrogen fuel, pre-mixed with air, is ignited and accelerated. In the second chamber, ultra-high-purity hydrogen fuel is added to the high-pressure air coming down the tube. The tube then tapers, accelerating the mix to Mach 5.0 before heading into the final “test section,” where the detonation takes place. In the last section, the air and fuel mixture is directed up an angled ramp. The pressure wave interactions in the chamber produced a stable, continuous explosion that stayed almost still. Theoretically, an OWDE engine could allow aircraft to travel at 17 times the speed of sound.

How can PDEs and RDEs transform space exploration?

The importance of PDEs and RDEs for future deep space exploration comes from their advantages over conventional rocket engines.

For example, RDEs are estimated to achieve a specific impulse that is 10-15% greater than conventional engines. Specific impulse is the thrust produced per unit rate of consumption of the propellant; it is usually expressed in pounds of thrust per pound of propellant used per second and is a measure of the efficiency of a rocket engine. Overall, RDEs are praised for their potential to give higher performance and greater thermal efficiency. 

Because they need less fuel to function, RDEs could also be more cost-effective and potentially allow rockets to be lighter. By reducing their weight, rockets could reach higher altitudes more quickly and efficiently. 

The RDE tested by JAXA produced around 500 Newtons of thrust. This is tiny compared to SpaceX’s Falcon Heavy rocket, for example, whose 27-Merlin engines together generate more than 5 million pounds of thrust at liftoff – equivalent to around eighteen 747s. However, although the RDE is still in the early stages, JAXA engineers believe that it will eventually allow rockets to use less fuel and weight. This could be of vital importance on interplanetary missions.  

RDEs are also being investigated by U.S. Navy for their ability to reduce fuel consumption. The U.S. Air Force has also built an experimental RDE that uses hydrogen and oxygen fuel to produce about 890 N of thrust. 

Meanwhile, JAXA calculates that RDE-based rockets could be in practical use by around 2026.