What you see in that video is a test firing of the latest breakthrough in aerospace propulsion. Courtesy of the Air Force Research Lab (AFRL), the video shows a reverse detonation rocket engine (RDRE) in operation. This engine could revolutionize aerospace technology.
Since the dawn of the space age, back in the 1950s when Robert Goddard launched his first liquid-fueled rocket burning gasoline and liquid oxygen, rocket engines have always used the same process. You mix a combustible fuel with oxygen, light it up, and let it burn. As it burns it heats and expands. It’s a subsonic process called deflagration — the flame front is advancing through the fuel at less than the speed of sound. Same thing happens with candles, barbecues, wildfires, internal combustion engines, and turbines.
By contrast, in an RDRE, the flame front advances at supersonic speed. The burns come in bursts, with the flame front circulating around the annular combustion chamber at a rate of 30,000 cycles per second. That pulsating effect can be seen in this high-speed image of the exhaust:
The concept of RDRE has been around for decades, but only recently has technology enabled a serious investigation. Computer modeling, 3D printing, materials science, and AI have all played a part in these advances. Here’s an animation of the basic operation of an RDRE. Note that, unlike a conventional combustion chamber with a flaring exhaust nozzle, in an RDRE the combustion is confined to an annular chamber:
This combustion chamber geometry has several effects:
- Instead of deflagration you get detonation. The flame front advances so fast it creates its own shock wave, compressing the fuel and burning it with up to 25% greater efficiency. You get more power with less fuel.
- Because it “eats fuel” so fast, the power density (energy release per unit volume) is 10X that of conventional engines. The fuel is burned at 1200 °C — hotter than other engine designs driving more exhaust expansion.
- It allows for the use of gaseous fuels instead of cryogenic fuels, which require special handling to reduce boil-off and lost fuel. The AFRL rocket in the video was burning gaseous oxygen and methane.
According to NASA:
The RDRE achieved its primary test objective by demonstrating that its hardware – made from novel additive manufacturing (aka 3D printing) designs and processes – could operate for long durations while withstanding the extreme heat and pressure environments generated by detonations. While operating at full throttle, the RDRE produced over 4,000 pounds of thrust for nearly a minute at an average chamber pressure of 622 pounds per square inch, the highest pressure rating for this design on record.
A company called Venus Aerospace recently launched the first US RDRE. It performed to expectations. Watch it here:
https://www.youtube.com/watch?v=Fecy8W7XZ0c
I fully expect we’ll be seeing RDREs in a host of application based on research already done. Beyond rockets, RDREs could power supersonic commercial travel in the atmosphere, engines for heavy vehicles that currently use conventional turbines (like the M1 Abrams tank), and electricity-generating turbines that currently burn methane (aka natural gas).
They also fit nicely with goals to combat global warming by getting the same job done with less fuel burned.
Next week in Sky Lights ⇒ How 3D Printers Work