From the buzz bomb to hypersonic transport: first US flight of a novel type of engine

No don't worry, this is not too techie of a post... it's reasonably simple and it's very cool.

This post is coming out later than usual because I was waiting for some questions to be answered by Venus Aerospace, which has just announced the first US flight of a novel type of engine which, stacked in front of their ramjet into a single engine, will hopefully make global hypersonic travel a reality with their Stargazer M4:

Rocket engines are great because they provide tremendous thrust and can operate in space where there’s no air.

Jet engines are great because they use incoming air as part of the fuel, so can run for far longer and more efficiently than rockets.

What if we could combine the two? We’d have the best of both worlds, the unequaled power of a rocket combined with a jet’s long range and efficiency. This is the idea behind various types of pulse detonation engines and ramjets.

Perhaps the most familiar example of this was the Nazi WWII V-1 cruise missile, nicknamed the “buzz bomb” because of the distinctive (very loud) sound made by its pulsejet engine. This is essentially a hollow tube with fuel injected into the center, which then explodes. The V-1 had to be launched from a steam catapult to reach 250 mph, a speed at which it had sufficient incoming airflow to work. Each detonation was contained to direct the thrust out the back by shutter doors opening and closing at the intake — yes, actual doors flapping open and closed.

A V-1 on its launch catapult

The “buzz bomb” sound came from the limitation that such an engine cannot run continuously. Each detonation had to be cleared out before the next could be made, thus the name pulsejet.

The next evolution of this idea was the PDE, or pulse detonation engine; a very similar concept but making use of shock waves inside the tube to shape and control each detonation. But this is still limited by only being able to run in pulses rather than continuously.

So what people have been working on for a long time — and I do mean a lot of people — is the RDRE, or rotating detonation rocket engine (also called an RDE). Nothing is actually physically rotating; here’s how it works. Imagine a roll of Life Savers. The foil on the outside of the roll is the tube. The hole through the center is a supersonic flow of atmospheric air. The area occupied by the Life Savers themselves is where detonation happens, fueled at the front end by rocket fuel and an oxidizer injected through tiny ports all the way around the ring, in a rotating sequence going around and around. Each detonation is controlled by the central flow of air and has no need to wait for the previous detonation to get cleaned out, because unlike in a PDE, they are not happening in the same place; they happen in continuous sequence around and around the circle.

Many RDEs have been successfully ground tested, and a few even flown. JAXA, the Japanese Space Agency, even flew one in 2021 as the second stage of a sounding rocket which used a conventional rocket engine in its first stage to get it up to speed fast enough for the RDE to take over. Also in 2021, the Warsaw Institute of Aviation flew one to an altitude of 450m. China flew a drone powered by an RDE in 2023. So far it’s the US that’s been playing catchup.

Until now.