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Pulse Detonation Engines: The Future of Aviation
noah_fense writes "Popular Science is running an interesting article about the race to replace the jet turbine with a more efficient source of Mach-breaking airpower: the pulse-detonation engine. It works by detonating (instead of slow burning) fuel hundreds to thousands of times a second. PDE technology is poised to make supersonic passenger flights and space travel affordable. 'Pulse detonation is a hot topic in combustion research,' says Gabriel Roy of the Office of Naval Research. 'Compared with gas turbines, the PDE has a much simpler configuration. It has the capability of going from subsonic to supersonic using less fuel, and it's thermodynamically more efficient. But there are big engineering issues--thermal fatigue, noise. It's very challenging research.'"
If any research is beeing done into the bladeless (Tesla) turbine?
The bladeless turbine pump is hailed as the best thing to hit industrial pumps ever.
All you need to reverse the intake and exaust and it is an engine (was orignally designed as an engine)
Pulse detonation seems to be the best way to power these turbines. Tesla claims over 10 horsepower to the pound of engine weight.
With this horsepower to weight ratio, I wonder what could be acomplished using this instead of a conventional turbine.
More info on the tesla turbine here.
It's easier to fight for one's principles than to live up to them.
Yep, you should go RTFA. PDE is very explosive. The idea was first thought of in the 1930's. The article says that the Germans tried it on the V-1 rocket, but didn't succede. The article states that detonation is different from deflagration. I don't know what the internal combustion engine uses, but PDE is very complicated and has only recently been showing signs of success.
...interesting if true.
from the article:
" Imagine a tube, closed at one end and filled with a mixture of fuel and air. A spark ignites the fuel at the closed end, and a combustion reaction propagates down the tube. In deflagration?even in "fast flame" situations ordinarily called explosions?that reaction moves at tens of meters per second at most. But in detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone. "
Bruce's pulse jets are very low-tech, deflagration engines. The PopSci article is about detonation pulse engines.
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I actually did my Master's Research on a Pulsed Detonation Engine (Rocket actually, since we were providing the oxygen). It is a more efficient form of propulsion (for thermo geeks, detonation can be modeled by a constant volume Humphrey thermo cycle, rather than the constant pressure Brayton cycle and a comparison of efficiencies results in a vast improvement for the pulsed detonation engine). It certainly isn't too new as far as the idea being thrown around, but it certainly is gaining momentum as being more and more plausible. Aside from the efficiency benefits, the engine itself results in a much simpler design and weight savings rather than relying on today's complex turbomachinery. Furthermore, pulsed detonation engines offer the potential for substantiative performance increases; finally bringing hypersonic flight to within a practical reach. A detonation is different than a deflagration in basically the speed at which combustion occurs. Deflagration occurs at relatively low flame speeds on the order of 1 or 2 m/sec.; whereas, detonation is a supersonic mode of combustion. Most forms of combustion that we are familiar with today utilize the deflagrative mode. The article was accurate in stating that this technology still has a few hurdles to overcome. Primarily, the pulsed detonation engine is an unsteady flow phenomenon that requires a periodic input to control fuel injection into the detonation chamber coupled with a very large energy input to ignite the fuel and reach a critical Chapman-Jouguet velocity. Such energy input has been accomplished so far using an arc igniter, but doing so on a reliable basis at frequencies of at least 100Hz, necessary for practical use have been somewhat of a challenge thus far.
The AirForce research guys demoed a pulsejet made out of automotive parts at Oshkosh this year. There's a link and a picture at Avweb
The United States Air Force Research Lab Propulsion Directorate has a pulse detonation engine program as well. Pics and story here. Apparently their engine is made mostly of off-the-shelf automotive parts. It's powered by any type of general aviation fuel (Jet-A, JP-8), and even gasoline.
A pulse jet and a pulse detonation engine is not the same thing.
Pulse jet's was what the germans used in their "buzz-bombs" during WWII.
As far as I've been able to conclude the greatest difference is in the burnrate of the fuel.
In a pulse jet you have a series of "slow" burns or explotions at a fairly low rate.
In a pulse detonation engine you've got insanely fast burns (hence "detonation") at serveral hundred detonations per second.
One of the greatest enginering tasks was apparently to be able to not only achieve a detonation instead of a burn or explosion, but to also do this continously at a high rate.
/.Mattsson - My native language is not English, so please don't whine over linguistic errors. (That's lame anyway...)
ok, once again.... the V1 was a missile propelled by a jet engine, not a rocket. A rocket carries it's own oxidizer with it. The V2 was a missile propelled by a rocket.
Neither was the first successfull guided missile, and the V2 was not the first successfull Liquid-Fueled Rocket. The germans had wire-guided air-launched anti-ship missiles before either.
The PDWE has been rumored for years to be the propulsion for the fabled Aurora...this type of engine leaves "donuts on a rope" contrails behind the aircraft. The PDWE is so much different from any other engine that it's silly...First, there are few, if any, moving parts. Fuel is injected, and causes a traveling wave of combustion to move down a tube...which is reflected inside the engine, and comes back up the tube. This wave compresses fuel and air still being injected and inhaled, enough so that it detonates, instead of combusting...think of it as the "pinging" in your car engine when you have crappy fuel. But harnesses correctly (as in a diesel engine,) it's actually more efficient. So this fuel detonates, which creates a pulse which partially blows out the back, but also partially reflects back up the tube to compress more fuel. Since there are no moving parts, this can take place at a very high rate of speed...The biggest problems I've read of are starting the thing...which was supposed to be the source of low-frequency rumbles at the Groom Lake site. The tube is "tuned" to a certain speed of waves inside it, and it doesn't want to run at other speeds. And...of course...noise. The thing is capable of producing lots of power...but its operation is much like that of the German pulse-jets, which sounded like flying jackhammers. But it definately could be propulsion for the future...but not to the extent that people would dream of...
If your car is detonating its not gonna last long. When a car engine detonates the shock wave produced often destroys the engines internals, or atleast causes some damage.
A cars combustion engine when working normally uses deflagation to produce power. Its easy smooth and works well. The octain or lead (1980) in petrol helps prevent detonation.
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Bruce's pulse jets are very low-tech, deflagration engines
Not strictly true. Although simple, the X-Jet design is not really "very low-tech" -- a lot of time and money has been invested in analysing a phenomenon called "high magnitude combustion" which, while not "detonation" still provides combustion efficiencies almost three times higher than the deflagration that occurs in a conventional pulsejet.
Whereas the flame-front in a normal pulsejet travels at just a few tens of meters per second, HMC occurs with a flame-front that travels at the speed of sound in the air/fuel mixture.
While this is still well short of the Mach 5-6 flamefront that is produced in a PDE, the X-Jet using HMC is an engine that can be produced now in commercial quantities and with power to weight ratios that make it an extremely viable source of propulsion for a wide range of flying craft.
The other advantage is that it can be manufactured at a much lower cost than a PDE and without many of the other problems.
Nope, I wasn't smoking anything at the time.
I originally wrote that paper for an Honors Seminar at the University of Maryland. It was called Science and Pseudoscience: An Investigative Approach. Pretty nifty class that helped you to look at things differently. I'm not sure what the conspiracy angle is that you're talking about aside from it discussing aircraft technologies that are still under wraps. As you can see from the bibliography section of the report I wrote, Popular Science and other news organizations have known about the existence of this technology for a while. More than a decade in fact.
Space craft take off using a continuous propulsion system in the form of gasses leaving the rocket. The force exerted by a pulse-detonation engine is more powerful than a continuous propulsion system when it comes to force exerted over a smaller amount of time. Also by having a series of detonations instead of a continuous burn, the craft doesn't have as many problem when it comes to ignitions back-tracking up the fuel supply lines to the main fuel storage area.
are old hat. There are even toys that work like that (Dynajet). Dynajets are so noisy that most towns and cities banned these toys, decades ago. The German buzz bombs used it in WW2.
Oh well, what the hell...
Definitionally, what differentiates a pulse-jet engine from a rocket?
As the grandparent post says (and you even quote it), a rocket carries its own oxidizer. A jet uses atmospheric air for its oxygen supply. Illustration: Rockets can potentially work in space or underwater whereas jets can't.
Perhaps you were thinking about turbine engine fan propulsion versus exhaust gas only propulsion? In that case pulse-jet and rockets are similar.
Actually the V-1 wasn't a rocket. It was a pulse jet powered cruise missile. A pulsejet is sort of like a ramjet with venetian blinds on the air intake, It fires in pulses rather than a continuous stream. Unlike a ramjets, pulsejets can be started while standing still.
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scramjets are completely different. They work on a principle of compressing the incoming air and then using a combustion chamber to blow it out the back at higher speeds. The big difference is that the air intake is compressed down slightly and that the combustion chamber has a constant combustion going on.
A pulsejet/detonation engine uses the previous detonation to compress the air/oxidizer for the next one. I've seen some designs with two outputs, it actually just oscillates between them. It's in a U shape and the detonation on one side send the shockwave to the otherside to compress that detonation.
Here's a good site with pics and even audio of pulsejet engines. http://home3.inet.tele.dk/kennethm/ There's also a section on ramjets which are just variants of scramjets for slower speeds.
This PDF has the osciliatory pulsejet design with pics starting on about page 5 or 6. The link is from the Valveless Pulsejet Engine article linked under pulsejets in the above site.
In the UK this is done by catalytic reforming to produce benzene and other ring-shaped molecules. This certainly gets the octane rating up to 95 or 97 ("premium" and "super" unleaded respectively) but, from a health point of view, may actually be more harmful than a bit of lead bromide in the exhaust. The alleged link between lead in exhaust fumes and childhood development was always hard to prove, although lead itself is undeniably toxic in the wrong forms. Benzene is carcinogenic in any quantity. We may have swapped one problem for another.
As an aside, a friend of mine worked at a factory where they had a lot of bulk benzene available - all the managers were running their cars on a 50/50 mix of benzene and unleaded petrol. Naughty.
When I am king, you will be first against the wall.
Google for German Pulse Jet Fighters...
...and he grinned, like a fox eating shit out of a wire brush.
To demonstrate the difference between a deflagration (the slow combustion you get in your auto engine or a pulsejet) and detonation (the rapid combusiton that occurs in a PDE) I like to draw the following comparison.
:-).
1. Take a can of gasoline and pour a trail on the ground as you walk along. That trail might end up being 20-30 yards long.
Above that trail there is a stoichiometric mixture (ie: a mixture capable of burning) of gasoline vapor and air -- just as you'd find inside an engine.
Now light one end of the trail and watch how long it takes for the flame to travel along to the far end.
It actually takes several seconds. That's the speed of a flame-front during deflagration.
2. Now take a very long piece of cordite or some other "high explosive" and lay it along the ground for some distance.
Then place a detonator at one end, stand well back and energize it.
The entire length of the explosive will appear to explode at once. The shockwave that propogates the explosion down the length of explosive material will travel far to quickly for you to see. Instead of taking several seconds to travel just 20-30 yards, the detonation will travel over a mile per SECOND or faster.
That's the difference in speed between deflagration and detonation.
But there's one other very important difference:
If you pour a gallon of gasoline out onto the ground and light it it will go "woof" (just like a dog
You can safely stand within just a few yards of such a deflagration without fear of being harmed.
However, if you were to *detonate* (rather than deflagrate) that same amount of gasoline it would blow you right into the middle of next week and further.
With a detonation, all the available energy is released in a very tiny fraction of a second and this generates huge pressures (thus huge thrust).
With deflagration, the energy is released far more gradually so the pressures are lower.
What's more, because deflagration is such a slow process, when the fuel is burnt inside an engine, there's far more time for the heat of combustion to be transfered to the engine itself. That means the engine will require more cooling and a greater percentage of the fuel's energy will be wasted as radiated heat rather than in producing work.
I hope that clarifies the key differences between deflagration and detonation -- and goes some way to explaining why a PDE could provide greater efficiencies than an engine that simply "burns" its fuel through deflagration.
The problem is not a theoretical limit for thrust development, but a physical one. As the gas goes through a compressor and combustor, it gets very hot. So hot, in fact, that without the special current ceramic coatings the turbine would melt. If one could develop a way to make the turbine survive even hotter temperatures then turbojet/turbofan engines can produce even more power, at least subsonically.
Let me repeat: pulse jets are not pulse detonation jets. Unlike pulse detonation jets, pure pulse jet research efforts have been all but abandoned as they were concluded to be too inefficent and overall inferior to any other jet design.
Pulse jets are composed of a combustion section, a set of inlet shutters and an exhaust valve. Air enters the combustion chamber and the inlet shutters closes, forcing the combusting fuel-air mixture out through the exhaust valve, producing thrust. Pulse detonation jets have no such valves.
The USAF kept saying "We have no idea what that is, it's not ours, etc". While Aviation Week kept publishing pictures of the things flying over Nevada and Utah, well away from the airline flight routes.
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I was at the Oshkosh airshow recently and the Air Force had a pulse-detonation powered Long-EZ homebuilt aircraft on display. Doing a quick search on the 'net finds some links to it here: http://www.af.mil/stories/story.asp?storyID=123005 352
I spoke with one of the engineers for awhile. This engine only produces 200 lbs of thrust, which is barely enough to get the Long-EZ airborne (contrast this to the EZ-Rocket project, in which each of two engines produce 400lbs of thrust IIRC). It is built from low-cost autmotive parts -- imagine a 4cyl engine with ~4' exhaust tubes coming straight off the exhaust ports of the head. The exhaust reaches speeds of up to mach 5 IIRC.
They do not forsee commercial applications for their design, but rather for use as an efficient missile powerplant.
For example, Edison(who strongly believed DC was much safer, outweighing transmission problems) was mostly responsible for death by electrocution; he figured the public would be shocked by how easily a man was killed by AC, and would fear it as a result...putting an end to Tesla, who was quickly taking Edison Electric to the cleaners, with more efficient generation and transmission.
Actually Edision had it in for George Westinghouse (not Tesla) when he aided in the creation of the electric chair, leading to the once popular, though short lived term "Westinghoused" for someone who was executed in the chair.
After the first electrocution, which didn't exactly go off without a hitch, Westinghouse was quoted as saying "they could have done it better with an axe".
With the engine in the back and the plane going super-sonic (and with decent vibration issolation between the engine and the plane) the noise will be literally left behind.
How could a helicopter replace the SR-71?
Most likely, as was proven during the secrecy of the Stealth program (Have Blue) in the late 70's - early 80's, this project was the source of rumors for the Aurora Spyplane.
--"The perfect example of the man of action is the suicide." - William Carlos Williams
Your professor is correct. Engineering utilizes numbers. However when expressing these results to an audience without the technical background in a specific area, these numbers aren't going to get your point across. So, it's rarely good form to explain things in such terms. Furthermore, you're rarely going to see a publication, such as Popular Science, providing such information since it's purpose is to describe new and noteworthy developments in technology so the person without any formal technical training can appreciate such things.
That being said, as I mentoned earlier if you take the equation for efficiency of the Brayton cycle, which is the constant pressure thermodynamic cycle that models today's turbofan engines:
Brayton_eff = 1 -T_0/T_1
where T_0 and T_1 are the absolute temperatures at their respective engine stations (or cycle stations if you're a thermo geek) with the constant volume Humphrey cycle efficiency which the PDE cycle closely follows:
Humphrey_eff = 1 - gamma*T_0/T_1* [(T2/T1)^(1/gamma) -1/((T_2/T_1)-1)]
you will see that the difference between these cycle efficiencies is the multiplier:
gamma*[(T2/T1)^(1/gamma) -1/((T_2/T_1)-1)]
For typical detonation combustion, the value of this multiplier is always less than one; therefore, resulting in a higher cycle efficiency than the efficiency of the Brayton cycle.
Furthermore, pulse detonation engines offer the potential to operate at very high densities, allowing the designer to use very compact combustor designs which has been desirable in the aerospace industry to allow for lengthening the nozzle.
As far as typical numbers with regards to specific impulse, you'll find that the time-averaged values are very comparable to those of an arcjet on the order of 10^4/sec. You might not find too many specifics with regards to performance as more than likely this sort of detail is proprietary still at this point.
Solid rockets are basically long tubes, open on one end, filled with solid propellants. Once they're ignited they keep going until the fuel is exhausted - there is no way to stop the combustion easily. Think solid rocket booster on the space shuttle.
Liquid fueled rockets, on the other hand, use liquid propellants and oxidizers, fed into the engine from storage tanks through a system of pumps and valves. They can more or less be started, stopped, and throttled at will by controlling the rate of fuel / oxidizer flow through manipulation of these pumps and valves. The main engines on the space shuttle are liquid fueled rockets.
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