Thursday, May 7, 2020

The Rotating Detonation Rocket Engine

Ordinarily, detonation isn't a really good thing to be have going on in a rocket engine or much of anything in life.  Obvious exceptions for blasting sides off of mountains, mining and military.  There's a difference between explosions and the controlled blast of a rocket engine but the detonation holds out a tantalizing prospect. It appears that the explosion could lower the cost of the engines, make engines simpler, so more reliable, and lower their weights by perhaps 30%.  That in turn would make a launch vehicle's payload that much bigger.

This week's Wired reports on research into Rotating Detonation Engines being carried out at the University of Central Florida.  Coincidentally, UCF is located pretty much due west of the Kennedy Space Center, although closer to Orlando than the KSC.  

It turns out this isn't really a new idea.
Rotating detonation engines, or RDEs, sound like something out of science fiction, but the concept is about as old as the space age itself. In the late 1950s and early '60s, aerospace engineers working on rocket engines envisioned RDEs as a way to turn a problem into a solution. “Sometimes the rocket motors would get a real bad instability and you’d get an explosion,” pioneer Arthur Nicholls recalled in a University of Michigan interview shortly before his death. “Then it led to the idea—well, what if we use that?”
Professor Nicholls noted the problem was an instability in the combustion chamber and saw the potential in creating an annular channel, a ring, in which a continuous shockwave propagated around the ring in a continuous explosion.  He was able to produce working models, but the understanding of the physics behind the behavior was lacking and the experiments were dropped as conventional engines became more successful.

RDEs use fuel and oxidizer, like other liquid fuel engines.  The savings come from the details.  In conventional liquid rocket engines you're familiar with, the fuel and oxidizer are pressurized and fed into the ignition chamber using bulky turbopumps and other complicated machinery. An RDE doesn’t need these pressurization systems, because the shock wave from the detonation provides the pressure.

Fast forward to this century and the idea resurfaced for hypersonic missiles and aircraft in the atmosphere.  Professor Kareem Ahmed, director of the Propulsion and Energy Research Laboratory at UCF, has spent the past few years developing a next-generation rocket engine that uses controlled explosions to boost stuff into space.
In the RDE developed by Ahmed and his colleagues, hydrogen and oxygen are fed into a combustion chamber. A small tube is used to send a shock wave into the chamber, which triggers the detonation. As the pressure wave moves through the chamber, it encounters more hydrogen and oxygen being fed into the front of the engine by dozens of tiny injectors. When the detonation wave hits the fresh fuel and oxidizer, it rapidly raises the temperature and pressure of the gases. This causes them to combust and send a flame shooting out of the rocket engine.

Earlier this month, Ahmed and a team of researchers from the University of Central Florida and the US Air Force published the test results from the first rotating detonation engine to use hydrogen and oxygen for propellant. This chemical cocktail is regularly used to propel the upper stage of a rocket on the final leg of its journey to orbit. But Ahmed says that many engineers believed this chemical mixture was too volatile to be used in a rotating detonation engine. “Hydrogen is a crazy fuel,” he says. “Most believed it wasn’t possible to detonate hydrogen and oxygen, because it would tend to deflagrate like a typical rocket engine, rather than a detonation motor.”
The effort to better understand and model the processes in the engine has been aided by modern technologies being better able to study the process as it's happening.
The number of waves produced by the engine is determined by how much propellant is being pumped into the system. The engine built by Ahmed and his colleagues had five waves, but other RDEs have had up to eight. It’s still unclear how the number of waves affects the performance of the engine, which will require a better understanding of the waves themselves. To study the waves produced by their engine, Ahmed and his colleagues added a chemical tracer into the propellant and filmed the engine using a high-speed camera rolling at over 200,000 frames per second.
The speed of the explosion is "wicked fast"; the wavefront is going around the ring at 4000 miles/hour, and they're concerned about movement of a centimeter.  That takes about 1.4 nanoseconds (billionths of a second). Ultra high-speed cameras are required. 

In 2018, the Air Force Research Laboratory announced a program to develop and fly an RDE that used conventional rocket propellants. The engine developed by Ahmed’s team is a result of that program, and he says the Air Force is aiming for the first flight of an RDE engine by 2025.  As part of that effort, James Koch, an interim postdoctoral researcher in applied mathematics at the University of Washington, began working on simulating the physics in the engines.
To overcome the challenges of working on RDEs, engineers rely on computational fluid dynamics to create detailed simulations of the detonation process. This is the same computing technique also used to design new planes, submarines, and rockets, but modeling a rotating detonation engine pushes a supercomputer to its limits. “It’s a very nitty-gritty, brute force approach,” says Koch. “The Department of Defense has been running simulations on rotating detonation engines that take on the order of three weeks to a month to run on their leading supercomputer.”

Koch was determined to find a better way to model the detonation waves, so he turned to a branch of mathematics called nonlinear waves and pattern dynamics, which uses math to create models that describe how patterns form. When Koch and his colleagues fired up a small rotating detonation engine they built in their lab, Koch found that the fundamental physical processes occurring in the engine could be described with his mathematical models—no supercomputer necessary. “This approach worked really well,” Koch says. “I can run a simulation on my laptop that takes maybe 30 seconds to produce results that are similar to what took the DOD three weeks.”
This combination of bits of progress around the country is re-igniting a lot of interest in RDEs.
RDEs may have a lot more to offer than improved rockets. The US Department of Energy is investing in research on using RDEs for stationary power generation, and companies like General Electric are exploring their application to jet engines. But Ahmed says these systems will first be put to practical use in rockets because there’s so much to gain in reduced weight and fuel efficiency. After 60 years of effort, exploding rocket engines might be just about ready to blast off.

An engine from James Koch/University of Washington.  It's interesting that in the midst of this amazing apparatus, anyone who has changed a spark plug will recognize the spark plug boots on the left.  


  1. I always liked the concept of the original Project Orion .....x

    1. I think it has its place.

      After all, it's just like Star Trek impulse engines.

  2. I assume then that this is better than the variable nozzle?
    I assume that this is still a booster rocket technology that involves lifting the weight of second and third stages.

    1. 1st - I've never seen a comparison. This is the first I've heard of RDEs. I have heard good things about the Aerospike concept, though. Just no numbers.

      2nd - I assume so as well, but I guess that's a system design question.

    2. I'm no rocket scientist. I just realized, a friend of mine is.

  3. I remember reading about another type of Pulse Detonation Engine that was reportedly flying around. It left a very distinct "Donuts-on-a-Rope" contrail, and had a distinct thrumming sound.

    Supposedly it was during flight testing of the "Aurora" aircraft, which doesn't officially exist.

    1. I saw a photo of that contrail in the old Monitoring Times ages ago. Think they were supposed to be from an experienced monitor watching for the Aurora.

    2. Last I heard, speculation was that 'Aurora' was powered by an external combustion jet engine at high speeds. Just like it sounds, apparently at high Mach speeds the airflow over the engine is powerful enough to contain the combustion.

      Kind of an aerospike engine, except in a jet format, kinda, sorta.

    3. Yeah, it doesn't exist.

      Which is why Caltech seismologist Dr. Jim Mori cracked the case on sonic booms moving on a beeline to Tonopah at 6AM at Mach 6, three years after the Air farce retired the SR-71s.

      Because as we all know, DoD just unilaterally gives up strategic reconnaissance defense tech that was untouchable by anyone for 25 years.

      And I have a bridge for sale, cheap.

    4. I forgot the /sarc tags.....

  4. Hmmm. A pulse-rocket (like a pulse-jet.) Still think baffled engine nozzles are a more secure, better way of doing it (like the baffling done in the F1 engines to keep them from going unstable.)

    Now, what about aerospike engines, which have been around, in the J2 aerospike design (an aerospike version of the J2/SSME/RS25 engine) since the mid 1960s?

    Still waiting for that.

    1. I heard Firefly was doing those but now I'm not so sure. Wikipedia says, "Alpha was initially designed with a first stage powered by an FRE-2 engine, which consisted of twelve nozzles arranged in an aerospike configuration." At their web site they have pictures of their Alpha and Beta versions with engines that look pretty conventional.

    2. Yes. But... They've promised us linear and toroidal aerospike engines since the late 1950s, and had working versions that showed promise of better thrust per weight and reduced cost in comparison to normal nozzled engines.

      It's like we advanced and theorized and created, and then decided to go back 10 years or more, and then stayed there.

      Space, where dreams have gone to die.

      Wonder if SpaceX is messing around with anything aerospiky. If they start working on one, then there's a chance we'll actually see one fly.

  5. "This combination of bits of progress around the country is re-igniting a lot of interest in RDEs."

    I saw what you did there.

  6. The first time I read about "detonation" being used for space lift operations was in Footfall by Niven...Those space lift engines were measured in megatons. And we have them laying about today...just waiting to be used.

    1. Before Footfall, I think there was King David's Spaceship.

  7. In the 80s there was a natural gas residential pulse gas furnace. It sounded like a dirt bike in your basement but was rated 95% efficient.

  8. Purdue worked on these things back in the day. My understanding is they don't acheive quite full combustion, and the induced vibrations cause a lot of wear on the engine nozzles/tubes. IIRC, the V-1 buzz-bomb as a PDE - they didn't have compressors for it, so they used pulsed detonation.


  9. Anonymous, the Lenox pulse furnace was and still is basically bombproof. I worked on them as a HVAC Tech back in the 90's.
    Air and Nat gas mix sucked into a heavy stainless combustion chamber by a high temp woven fabric flapper valve actuated by the expansion and contraction of the detonations was the only moving part besides the gas valve and fan motor.
    Only two parts ever went bad in my experience, that flapper wore out from beating on the housing about once a decade, and the electronic control box that ran the spark ignitor, gas valve and sensors occasionally let out sone of it's magic smoke.
    Given the dirty and irregular power we tended to have in coastal N. CA, I don't blame the electronics...control module replacement in all heaters was so frequent I did at least two a week.
    My in-laws have one in their basement that is still going strong, thirty years later.