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Pulsed Detonation Wave engine

blackkite

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Pulse Detonation Engine Overview. The University of British Columbia November 26-2004.
1.The main objective of PDF research is to develop an efficient engine that is primarily used for high
speeds.(potentially Mach 5)
2.The PDE technology is more efficient than current engine type by virtue of it's mechanical simplicity
and thermodynamic efficiency.
3. The PDE can provide static thrust for a ramjet engine or scramjet engine or operate in combination
with turbofan systems.
4.P&W and GE have developed different process to accomplish the refinement current method for
initiating detonation process.
5.Current materials used jet engines, such as Nickel- based super-alloys are inadequate to withstand
the extreme heat and pressure generated by detonation cycle.

http://www.phas.ubc.ca/~lamm/docs/201report.pdf
http://www.flightglobal.com/articles/2008/03/05/222008/us-afrl-proves-pulse-detonation-engine-can-power-aircraft.html
 

KJ_Lesnick

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Has it reached the point where a PDE can be essentially incorporated into a retractable "strut-jet" like system? Could such a device be set up to run like a rocket too (i.e. LOX pumped in and it works in space)?

KJ Lesnick
 

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If I remember correctly, during the late 90s there was a proposal to equip F-15 owned by NASA with PDE instead of its right engine. It went nowhere.
 

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quellish

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KJ_Lesnick said:
Has it reached the point where a PDE can be essentially incorporated into a retractable "strut-jet" like system? Could such a device be set up to run like a rocket too (i.e. LOX pumped in and it works in space)?

KJ Lesnick

That is correct, yes. A PDE can be cycled to perform as a rocket.
 

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blackkite said:
Hi! Pulsed Detonation Wave engine displayed in U.S Air Force Museum Dayton.

Yes, it is:


More at my blog:
http://up-ship.com/blog/?p=2088
 

KJ_Lesnick

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That sounds totally bad-ass. You could essentially have a PDE that could seemingly seemlessly transition into a pure ramjet, and with variable geometry changes transition into a scramjet, using active cooling you can squeeze some extra mach numbers out of it, then cycle the PDE back into the equation as a rocket.

You'd have an integrated propulsion system that could go from zero to Mach 25, and would be even more elegant (smaller, simpler, lighter, and more powerful in both air-breathing and rocket-modes) than the RBCC/Turbocompressor-based Strutjet design. And if if you could get the PDE to operate at high frequencies (thousands of cycles a second) as it would ride relatively smoothly.


KJ Lesnick
 

quellish

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KJ_Lesnick said:
That sounds totally bad-ass. You could essentially have a PDE that could seemingly seemlessly transition into a pure ramjet, and with variable geometry changes transition into a scramjet, using active cooling you can squeeze some extra mach numbers out of it, then cycle the PDE back into the equation as a rocket.

You'd have an integrated propulsion system that could go from zero to Mach 25, and would be even more elegant (smaller, simpler, lighter, and more powerful in both air-breathing and rocket-modes) than the RBCC/Turbocompressor-based Strutjet design. And if if you could get the PDE to operate at high frequencies (thousands of cycles a second) as it would ride relatively smoothly.


KJ Lesnick

Yup, looks great on paper, doesn't it?
 

lantinian

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Here is the diagram that give me everything I need to know about the potential of these engines. If they can be adopted to work in space (Pulse Detonation Rocket Engine) by using onboard oxigen then, you have a single engine that no only operates in the whole mach regime, but has the highest specific impulse.
 

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blackkite

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Hi! You mean PDE has big potential for SSTO space plane engine?
 

lantinian

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PDE (in theory) only works from 0 to Mach 6, as it uses atmospheric oxygen which needs to be at a certain temperature, pressure and proportion with the fuel for the PDE to work. In the absence of that and above Mach 6 speeds, you could in theory supply onboard oxygen and still operate it in the same way. That is the Pulse Detonation Rocket Engine in concept.

Main obstacles for this engines are vibration, noise and mostly primary ignition under various conditions and with various types of fuels. Once you start the first explosion, it kind of sustains itself.
 

blackkite

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I want to ask you some questions.
1.According to explanation board for long-EZ, " first manned flight of an aircraft powered by PDE ".
This means unmanned vehicle already tested?
2.Why is this engine so long? Pulse cycle is controlled by the length of tube?
3.What is the material of long-EZ's engine tube?
 

lantinian

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1.According to explanation board for long-EZ, " first manned flight of an aircraft powered by PDE ".
This means unmanned vehicle already tested?
Well, I personally doubt that. It's was strictly a low cost proof of concept demonstrator. The unmanned option is generally used to test a new flight configuration or engines that require extreme testing conditions. Since PDE are supposed to work from 0 kph, then attaching them to an existing manned aircraft seams to be a lower cost approach. Besides, I think one of the things they tested were noise levels on the crew, and you need crew for that ;)

2.Why is this engine so long? Pulse cycle is controlled by the length of tube
?
PDR works on the principle of supersonic combustion, unlike turbojets which work on subsonic combustion. I do not pretend to understand the principle fully but the length and diameter of the exhaust аre used to accelerate the the cyclic combustion pulse to supersonic speeds.

3.What is the material of long-EZ's engine tube?
From what I read, Scaled Composites used auto parts extensively, Remember, this is not a high performance PDE, just a proof of concept. I will be more interested in the overall trust to weight ration of such a perspective engine based on of the shelf components.
 

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lantinian said:
3.What is the material of long-EZ's engine tube?
From what I read, Scaled Composites used auto parts extensively,

Supposedly: General Motors Quad 4 car engine.

The tubes looked like steel to me. As a low-cost demonstrator, advanced materials were not likely used.
 

KJ_Lesnick

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Blackkite,

KJ! Fightingirish already post following site for Blackswift topic in June 20 2008.
Please confirm the site.

http://www.darpa.mil/TTO/solicit/BAA08-53/VULCAN_Industry_Day_Presentations.pdf

I checked the site. From what it seems the US Government, and DARPA does intend to ultimately combine the PDE along with other engines so as to ultimately produce a design for space-access. Unfortunately it does bother me that all of this technology will probably only be used for military applications (putting up satellites -- most of them surveillance oriented, into orbit along with maintaining said satellites, and for potentially direct surveillance in the form of the Blackswift) with any potentially interesting civilian ideas such as hypersonic transports, and affordable space-access for civilians (While Virgin Galactic is quite impressive, it's is not a single-stage to orbit, it's relatively small, and extremely expensive -- who could afford $200,000 bucks to go into space a few minutes? Actually I can think of quite a couple, such as celebrities, heads of big business, including ultra rich individuals such as Bill Gates, and Warren Buffet and such, but I myself certainly cannot).

One thing that's interesting is the mention of a Continuous Detonation Engine (CDE), which I have wondered about it's practicality as it wouldn't have the jackhammer vibration problems that the PDE would have (I've been told that it wouldn't be as efficient though however I have no idea what ideas are being cooked up now)

Hi! You mean PDE has big potential for SSTO space plane engine?

That is correct. It would be useful to get a ramjet up to ignition speed, could work along with the ramjet to improve thrust at certain speeds. It is possible to design a ramjet which features variable geometry which can transition from a ramjet to a scramjet. Active cooling can increase the maximum speed at which a ramjet and/or scramjet can operate at. Since the PDE can be cycled into a rocket, it can provide the final push into space.


Quellish,

Yup, looks great on paper, doesn't it?

Well, I have a pretty good feeling it wouldn't work as well under real world conditions as it would on paper, but I think such a design could actually work. The exception is the pulse-frequency rate of the PDE's currently being developed which are around 40-80 cycles per second, I am not certain that could be increased, although there are things I didn't think were possible in the past (though not all of these things were good). Having a multitude of PDE tubes each firing at slightly different times could produce the effect of a higher frequency-rate. Some of the research in the link depicted did mention a Continuous-Detonation Engine which would get around the jack-hammer vibration issue, but I've been told it wouldn't be as efficient, but for all I know it would still have modern engines beat by miles.


Matej,

Yes, like the warp drive :)

I think a PDE that could be used to boost a ramjet up to speed, be cycled to behave as a rocket, and developing a ramjet that can alter it's geometry to transition into a ramjet combined with active cooling to increase efficiency and maximum top speed is far more plausible than developing a warp-drive.

I agree that the manufacture it will be a little more difficult task.

Well, it would not be easy to build, but I think it could be done.

A great deal of the technology actually exists already to do the job

- We already have metallic high temperature TPS which in addition to eliminating much of the need for the highly elaborate thermal management systems used on a "hot" design which requires the fuel be used as a heat-sink and elaborate cooling schemes. The only part of the plane that might require a cooling system would be the nose (If the nose was a sharp-design) which could use a liquid piped coolant. These metallic TPS systems, which have been around since the mid-1960's, according to former McDonnell/McDonnell-Douglas employee (and, to the best of my knowledge, current Aerospace Engineering Professor) Paul Csysz, are actually lighter, sturdier and more rugged than the thermal tiles used on the Space Shuttle.

- We have the means to achieve hypersonic-flight (in some cases up to orbital velocity) without the need for blunt-noses. To an extent this capability has existed since the late 1950's (The AeroSpacePlane program which was from 1957 to 1963 did feature at least some sharp nosed designs, as did some designs out of Wright Patterson's Flight-Dynamics Laboratory or FDL.), and definitely by the 1980's (While the X-30/NASP design was often depicted as having a blunt-nose, there are some diagrams, pictures of at least one wind-tunnel model, and statements from Paul Csysz that the actual design was to have had a sharp nose). A sharp nose while producing a much higher nose-temp actually produces a great deal less drag, and for a hypersonic waverider design which uses it's forward underbelly as both an inlet and lifting device, it yields a higher pressure recovery and a higher L/D ratio which are all desirable qualities.

- We have scramjet technology, which McDonnell, according to Paul Csysz, had begun research (IIRC in the late 1950's) and had been tested in wind-tunnels at least by the early 1960's (if not in the late 1950's) which could generate positive thrust, with some designs able to yield thrust up to Mach 22. Regardless, we certainly do have technology now to produce scramjets with the X-43 being testament to it (which to the best of my knowledge has achieved Mach 10). A great deal of research has been done to the best of my knowledge which shows that a scramjet duct could be designed to undergo a variable geometry change to "transform" it into a ramjet duct for low speeds. Above a certain speed, the engine would undergo a VG change back into a scramjet duct.


KJ Lesnick
 

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Yup. Looks great on paper.
Thermal protection systems of the kind you're talking about don't help much with a PDE. The temperatures and pressures of an operating PDE are challenging, but solvable. Regenerative cooling like with a rocket is an obvious solution. The vibration problem, again, is very solvable. For example, multiple tubes can be operated out of phase of each other.
The real problem is initiating detonation and keeping it going in a flight vehicle. THAT is a difficult problem to solve. Both Pratt and GE have solutions, which are very different approaches to the same problem, that have not flown yet.

One of the major advantages of a PDE is that you do not *need* additional cycles. Why convert to a ramjet if the PDE cycle is viable at higher mach numbers? You would only be adding more weight and cutting into your fuel fraction. A PDE->rocket cycle could very well take a vehicle all the way to orbit. Combined cycle systems carry an awful lot of risk and weight.
Sort of like a Toyota Prius. A cute idea, but at the end of the day your mileage still isn't that great.
 

blackkite

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I used to see strange contrail picture (continuous donut like smoke) taken in the United States.
I can't find it now. Any one has the picture of this strange contrail?
 

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Quellish,

Thermal protection systems of the kind you're talking about don't help much with a PDE.

I'm confused. Why wouldn't such a TPS work on an aircraft mounting a PDE, when it would work on an aircraft mounting a rocket, a ramjet, or a scramjet?

Unless you're talking about cooling the PDE itself, in that case I think the best solution would simply be to cycle the fuel around the engine then run it through the engine and burn it all up. Historically it's worked with rockets.

The temperatures and pressures of an operating PDE are challenging, but solvable.

While it's kind of good to hear that the problems are solvable, what kind of temperatures do a PDE produce?

Regenerative cooling like with a rocket is an obvious solution.

I guess I wasn't all that far off after all...

The vibration problem, again, is very solvable. For example, multiple tubes can be operated out of phase of each other.

LOL -- Isn't that basically what I said? (No offense or anything) Regardless to have each tube operating slightly out of phase wouldn't that require the engine to be a bit oversized so that the desired thrust can be produced even with out all the tubes firing at once?

The real problem is initiating detonation and keeping it going in a flight vehicle. THAT is a difficult problem to solve.

I was actually under the impression that they have solved those problems...

Both Pratt and GE have solutions, which are very different approaches to the same problem, that have not flown yet.

I guess the fact that it hasn't flown yet is a good point... By the way, if it's not classified, may I ask what P&W and GE's approaches are to the problem?

One of the major advantages of a PDE is that you do not *need* additional cycles.

I was under the impression that the design worked from sitting still on the ramp up to the ramjet speeds. I was not under the impression that the design could operate at scramjet-speeds...

Why convert to a ramjet if the PDE cycle is viable at higher mach numbers?

Well you wouldn't -- PDE straight to rocket would be preferred. But as I said before I wasn't under the impression that the PDE could work at the speeds where a scramjet would work ideally at. Also I remember hearing at least once that the PDE might have a slightly lower mach-limit over the absolute maximum-limit of a ramjet though I could be wrong.

You would only be adding more weight and cutting into your fuel fraction. A PDE->rocket cycle could very well take a vehicle all the way to orbit.

As I said, if it could do it, it would be a better choice to just go PDE to Rocket.

Combined cycle systems carry an awful lot of risk and weight.

Some combined-cycle systems add a lot of weight -- I don't think all of them do. From what I remember the strut-jet/RBCC design actually had a very good power to weight ratio.


KJ Lesnick
 

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KJ_Lesnick said:
Quellish,

I'm confused. Why wouldn't such a TPS work on an aircraft mounting a PDE, when it would work on an aircraft mounting a rocket, a ramjet, or a scramjet?
Based on your original posts, I was under the impression you were talking about TPS in the engine. Nonetheless, the structural, thermal, and aerodynamic requirements for a PDE-powered aircraft vs. one powered by a ramjet, etc. are indeed very different. Different engines have very different integration requirements. A scramjet requires a very different inlet than a rocket ;)
This, again, is why combined cycle powerplants that move from ramjet to scramjet to rocket or whatever are heavy. It's not always the engine(s), it's usually what it does to the rest of the vehicle.

KJ_Lesnick said:
The real problem is initiating detonation and keeping it going in a flight vehicle. THAT is a difficult problem to solve.

I was actually under the impression that they have solved those problems...

Both Pratt and GE have solutions, which are very different approaches to the same problem, that have not flown yet.

I guess the fact that it hasn't flown yet is a good point... By the way, if it's not classified, may I ask what P&W and GE's approaches are to the problem?

Popular Science did a good article on this several years ago:
http://www.popsci.com/military-aviation-space/article/2003-08/after-combustion-detonation?page=1
But this quote from elsewhere sums it up well:
"According to GE officials, their approach to pulse detonation technology is significantly different from that of their competitors. "Other companies are using mechanical valving systems which limit the frequency of their detonations. We're relying on an aerodynamic valving system," said Harvey Maclin, GE's manager, advanced technology marketing and government programs."

That leads to very, very different solutions. Detonation to deflagration transition is still the critical problem to solve. Pratt, GE, and Rolls have all taken different approaches to that set of problems, and each thinks their solution gives them an advantage.
It is a very, very hard problem. If you want more details on how these work, you would have to look up the relevant patents.

KJ_Lesnick said:
One of the major advantages of a PDE is that you do not *need* additional cycles.

I was under the impression that the design worked from sitting still on the ramp up to the ramjet speeds. I was not under the impression that the design could operate at scramjet-speeds...

Why convert to a ramjet if the PDE cycle is viable at higher mach numbers?

Well you wouldn't -- PDE straight to rocket would be preferred. But as I said before I wasn't under the impression that the PDE could work at the speeds where a scramjet would work ideally at. Also I remember hearing at least once that the PDE might have a slightly lower mach-limit over the absolute maximum-limit of a ramjet though I could be wrong.

You would only be adding more weight and cutting into your fuel fraction. A PDE->rocket cycle could very well take a vehicle all the way to orbit.

As I said, if it could do it, it would be a better choice to just go PDE to Rocket.

There is no reason a PDE would not work even at very high mach numbers.

KJ_Lesnick said:
Combined cycle systems carry an awful lot of risk and weight.

Some combined-cycle systems add a lot of weight -- I don't think all of them do. From what I remember the strut-jet/RBCC design actually had a very good power to weight ratio.
[/quote]

For each cycle you different things, and carrying all of that is heavy (and draggy). Some engines look good (on paper) until you see the bigger picture of how they integrate into the vehicle. A strutjet looks good until you weigh the oxidizer, for example.
 

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blackkite said:
I used to see strange contrail picture (continuous donut like smoke) taken in the United States.
I can't find it now. Any one has the picture of this strange contrail?

It's not that strange. The "donuts on a rope" contrails are formed by regular jetliner turbofan engines. I've watched 'em form with my own two eyes.
 

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Lots of confusion in this thread.

Pulse detonation engine is a bit like an ordinary gasoline or diesel piston engine.
Except there is no detonation in a (well working) ordinary piston engine. (Detonation has supersonic combustion front, compared to deflagaration, but the propellants are practically stationary themselves, so it's not a ramjet vs scramjet type supersonic flow thing, ie the supersonic "keyword" is a red herring here.)

In a PDE, the fuel and air are put in a chamber, the valves are closed, the propellants are compressed and they detonate / are detonated (I don't know if spark ignition is used).

Also, naturally the pulse detonation engine doesn't take the drive power through the crank shaft and propeller like an ordinary piston engine, but from the hot exhaust gases.

The quickest description of a PDE would be a piston engine with intended detonation.

I think it is mainly intended to be used in the burner cans of turbine engines to increase efficiency. Ie a compressor wheel before the pulse detonator and a turbine wheel after it.

In theory one could run a pulse detonation engine with liquid oxygen and fuel too. Then it could work like a rocket engine. I don't know if that has been tested. Again you perhaps could get to higher pressures and better efficiencies than with standard turbopumps. (Though when looking at the complete cycle, I'm not certain if that makes sense.)

I think GE (or was it P&W?) was working on a valveless pulse detonation engine on a test stand. If you tune the dynamics very carefully, you can get very high compression without having to close off the space. I think I read it on AW&ST years ago. There are valveless amateur built pulse jets (not PDE's), which show how you can achieve compression in a valveless design, though of course they have much much much lower pressures.

I don't really know if taking a canard plane, and putting in a modified car engine would take that long for anyone to demonstrate a PDE airplane. I'm no car techie and don't know how you could best achieve detonation without overly taxing the engine, I'd guess increase compression and decrease fuel flow.
 

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Orionblamblam said:
blackkite said:
I used to see strange contrail picture (continuous donut like smoke) taken in the United States.
I can't find it now. Any one has the picture of this strange contrail?

It's not that strange. The "donuts on a rope" contrails are formed by regular jetliner turbofan engines. I've watched 'em form with my own two eyes.

I have too.
But they are different from the reported "doughnuts on a rope" contrail.

A commercial turbofan can indeed form something that looks like a doughnuts on a rope
contrail over time, but the reported "doughnuts on a rope" comes out of the back of the
airplane in that form.

Or at least according to the witnesses, if we believe them.

Larry
 

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>(Detonation has supersonic combustion front, compared to deflagaration, but the propellants are practically stationary themselves, so >it's not a ramjet vs scramjet type supersonic flow thing, ... .)

Well said!!
In fact, there is a nice explanation of the effect in John D. Anderson's "Modern Compressible Flow" in the chapter
on shock tunnels. The chapter doesn't explain PDE's but it does explain the propogation velocities of shock waves
as well as the interactions between such travelling shock waves and the gasses they pass through. Shock tunnels
create their shock waves in a different manner, but the physics is similar.

>In a PDE, the fuel and air are put in a chamber, the valves are closed, the propellants are compressed and they detonate / are detonated (I don't know if spark ignition is used).
In a PDE, the detonation is created by a seperate mechanism. I suspect that the USAF engine did indeed use an automobile spark plug
as well as an automobile ignition circuit to create these shock waves. I'd be curious as to what fuel they used for the shock generator.

I learned a lot about the SAIC (Science Applications International Corp.) PDE's back in the early to mid 90's.
If I recall, they also used a spark plug but they used an O2 riich gas as the fuel to achieve reliable detonations
for their engine. Their engine was not valved. Indeed some combustion products flowed out the side inlets creating the doughnuts on a rope
contrail. They generated Mach 2+ (if I recall correctly) CFD particle traces for me that showed the doughnuts on a rope contrail being formed.

Once the shock is created, as you explained above, it is caused to travel through the fuel/air mixture.
As the shock propogates through this gas, it compresses it (similar to gas flowing through the shock wave(s)
of a supersonic inlet) and raises its pressure (also similar to a supersonic inlet), and it also raises its static temperature
(also similar to a supersonic inlet) except that here, static temp is increased to the point where it causes combustion
which releases the energy of the fuel.

The propogating shock also does something else really cool, it imparts momentum to the gas it propogates through,
or in laymens terms, it drags the combustion gas along with it.

The SAIC engine shock generator actually was downstream of the combustor, so once the shock was generated,
it flowed upstream towards the front of the engine, towards the combustor, went through the fuel/air gas in the
combustor, doing all the above mentioned stuff, kept going upstream, encountered a thrust wall, bounced off it and
came back again for another pass through the fuel air mixture, dragging it with it.

In the above you can see how the shock actually seems to be like a piston in an auto engine where the
thrust wall is like the combustion chamber head in an auto engine, and the shock wave is like a piston.

If you read the Anderson reference above, he calculates the theoretical maximum mach no. that a propogating shock
wave of infinite velocity, could accelerate an at rest gas to, as the shock passes through the gas. I don't have my copy of
Anderson with me at the moment, but from memory I believe it is on the order of around Mach 4 !!

So the propogating shock also takes the gas and propogates it down the nozzle as well.

I recall asking the SAIC researchers if their nozzle was like a normal gas dynamic nozzle. I recall them telling
me that it seemed to work better without using a gas dynamic nozzle. I always wanted to learn more about
that. The USAF engine has those long exhaust tubes. I'd be curious if they have any normal nozzle properties.

My collaboration with SAIC ended right after that, unfortunately.

PDE's are cool engines. There are just one class of gas-dynamic wave engines.

This is causing me to get curious about them again!

Larry
 

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Oh! PDE is very difficult to understand. I can understand gas turbine engine, but PDE ???
 

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Quellish,

Could you get a PDE to Mach 20 or 22 while in air breathing mode?

If you could do that, why did that DARPA link, which discussed PDE's and all sorts of other engines and it often involved a PDE and another engine?


KJ Lesnick
 

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KJ_Lesnick said:
Could you get a PDE to Mach 20 or 22 while in air breathing mode?
I think but I'm not sure that the Chapman Jouguet detonation speed is (one of the things that is) putting an upper limit on the speed of an airbreathing PDE. I've rarely seen it recommended to operate beyond Mach 4.
 

quellish

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KJ_Lesnick said:
Quellish,

Could you get a PDE to Mach 20 or 22 while in air breathing mode?

If you could do that, why did that DARPA link, which discussed PDE's and all sorts of other engines and it often involved a PDE and another engine?


KJ Lesnick

The limiters there are not the type of engine, so yes. "Mach 20 in air breathing mode" would produce skin friction, etc. etc. that would make the rest of the vehicle... challenging.

PDEs are immature and have (obvious) limitations right now. DARPA is very interested in using PDEs as augmentors, etc. with conventional engines, as conventional engines are nearing their limitations - the Popular Science article mentioned before touches on this. One of VULCAN's primary goals is to use unsteady combustion (PDE, or other constant volume system) to enable the use of a off-the-shelf turbine engine at much higher speeds. Integrating a PDE and a F101 (for example) together to be more than the sum of it's parts and enable operational Mach 4+ vehicles.
This article talks about VULCAN at a high level:
http://www.aviationweek.com/aw/blogs/defense/index.jsp?plckController=Blog&plckScript=blogScript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%3A27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3A6adbffb4-6533-460f-b531-62f7dd64ea61

This should also be of interest:
http://www.scribd.com/doc/3615888/VULCAN-Industry-Day-Presentations
 

KJ_Lesnick

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Quellish,

How would a PDE operating at Mach 20 be more challenging than say a scramjet with active cooling operating at Mach 20? Or are you simply talking about the general challenge of operating any air-breathing vehicle at that kind of speed.

Truthfully speaking with current technology I think using a PDE as a booster and rocket-cycle is more practical at this stage. Ramjet and scramjet technology exists, and active cooling can be used to jack up the maximum allowable mach-numbers.


KJ Lesnick
 

quellish

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KJ_Lesnick said:
Quellish,

How would a PDE operating at Mach 20 be more challenging than say a scramjet with active cooling operating at Mach 20? Or are you simply talking about the general challenge of operating any air-breathing vehicle at that kind of speed.

Truthfully speaking with current technology I think using a PDE as a booster and rocket-cycle is more practical at this stage. Ramjet and scramjet technology exists, and active cooling can be used to jack up the maximum allowable mach-numbers.


KJ Lesnick

Well, any air breathing vehicle travelling at mach 20 is going to be a challenge. That's a lot of air, moving quickly, to resist you. Most things traveling that fast are up where air is very scarce.

Scramjets, until very recently, were confined to the laboratory. Now only a handful have flown, for very brief periods of time. Ramjets have been used on operational aircraft, but nearly all of those have used rockets to get to a speed where a ramjet can be lit. Not very practical.

To make a long story short, if a high-mach air-breathing vehicle was practical, it would be flying right now.
 

lantinian

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How would a PDE operating at Mach 20 be more challenging than say a scramjet with active cooling operating at Mach 20?

From my understanding of the limitation of the technology, incoming air cannot come at speeds more than Mach 6 for PDE operation. So above Mach 6, in theory you have two option:
1. Slow the incoming air somehow
2. Cary your own air and operate it as PDRE.

I also believe, for the PDE to work the fuel mixture must be within much more strict proportions than say a Scramjet, for if you do not get the right mixture in the detonation chamber, you will not achieve detonation, or it will not happen at the correct frequency.

In other words, PDE not only have a speed limit but also an altitude one, since air content changes with altitude.

In short, by the nature of their operation PDEs require much more strict condition to be efficient than other engines.

Still, Mach 20 indeed poses a whole new set of challenges indeed
 

quellish

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lantinian said:
How would a PDE operating at Mach 20 be more challenging than say a scramjet with active cooling operating at Mach 20?

From my understanding of the limitation of the technology, incoming air cannot come at speeds more than Mach 6 for PDE operation. So above Mach 6, in theory you have two option:
1. Slow the incoming air somehow
2. Cary your own air and operate it as PDRE.

Yes, and that is true of pretty much any air breathing engine - the inlet matters as much, if not more than the engine itself. The SR-71 is a good example of this. HyStrike was designed for flying faster than Mach 6, and it was powered by a PDE.

lantinian said:
I also believe, for the PDE to work the fuel mixture must be within much more strict proportions than say a Scramjet, for if you do not get the right mixture in the detonation chamber, you will not achieve detonation, or it will not happen at the correct frequency.

Yes and no, it depends on the PDE more than anything else - the technique used to initiate detonation is what matters most here, rather than the fuel air mixture in the primary detonation tube.
Many PDE designs use what we'll call igniter tubes. Smaller secondary detonation tubes used to initiate detonation and propagate into the primary detonation chamber. Those are very controlled environments, where the mixture is strictly controlled to ensure precise detonation properties. This is not *necessary*, but for these PDE designs it is very desirable. More recent designs use techniques other than these igniter tubes.

More generally, you do not *need* precise control of the fuel and oxidant mixture. Fuel-air explosives are a good example of this, they do not require this precision to operate reliably or with predictable effects.

lantinian said:
In other words, PDE not only have a speed limit but also an altitude one, since air content changes with altitude.

Again, this is true of any air breathing engine. The inlet is more of a limiter than the engine.

lantinian said:
In short, by the nature of their operation PDEs require much more strict condition to be efficient than other engines.

Still, Mach 20 indeed poses a whole new set of challenges indeed

Not necessarily, as long as you can reliably initiate and propogate detonation to deflagration transition, you do not require precise control of the primary detonation tube to ensure efficiency - the nature of the Humphrey cycle itself ensures you will get better efficiency even under less than ideal conditions.
 

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If you use an engine where the air must be slowed down to subsonic speed before combustion, then the faster you go, the more it heats up when you slow it down (in the inlet). There is an upper limit when the chemical products of combustion are not stable anymore and hence no energy from the combustion can be generated. This sets the upper speed limit of ramjets to Mach 6-7 AFAIK IIRC.
I think the PDE has to slow down the air flow to subsonic - at least. I am not sure about this.
If it can do with Mach 4 internal air flow speed, then it could be possible that you could go to Mach 9-10 free air flow with it. (The inlet takes 5-6 Machs away.)
 

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mz said:
KJ_Lesnick said:
scramjet technology exists

Afaik, no scramjet has ever accelerated under its own power.

Evidently the 2nd X-43A flight did some acceleration.

http://www.nasa.gov/centers/dryden/news/FactSheets/FS-040-DFRC.html
NASA "Hyper-X" Program Demonstrates Scramjet Techologies
X-43A Flight Makes Aviation History 03.09.07
...
The engine thrust was very close to its design value in each flight - sufficient to accelerate
the vehicle during the Mach 7 flight and to allow the vehicle to cruise at constant velocity
in the Mach 10 flight.
...

And also from: AIAA 2009-1523
"Learning from Experience: Case Studies of the Hyper-X Project
by: Curtis Peebles
...
"The second Hyper-X flight was made on March 27, 2004. Despite the unknowns of the off-loaded booster,
stability through transonic speeds, the FAS, the separation sequence, and airflow between the adapter and X-43A,
the launch and boost were successful. The scramjet ignited and accelerated the vehicle. The engine’s performance
was close to that predicted. After the ten-second burn, the X-43A performed the aerodynamic test maneuvers. The
top speed was Mach 6.83, making the X-43A the first hypersonic airbreathing vehicle to fly."

...
 

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blackkite said:
Oh! PDE is very difficult to understand. I can understand gas turbine engine, but PDE ???

I hope I didn't confuse you more.

I am looking into these things again, and when I make some more progress maybe I
will be able to do a better job.
 

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quellish said:
lantinian said:
How would a PDE operating at Mach 20 be more challenging than say a scramjet with active cooling operating at Mach 20?

... the inlet matters as much, if not more than the engine itself. ...



... The inlet is more of a limiter than the engine.


Hmmmm.

I thought the inlet was part of the engine.

Anyway, using your words, after the first quote I might say, well then let's get rid of the engine and just keep the inlet !

And after the second quote above I may just say, well lets get rid of the inlet (on this airbreathing engine)
and just keep the engine.

You bring up some interesting points. I can tell you've read quite a bit about PDEs.
I would just check what you're saying. You can confuse an old fart like me.
 

quellish

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shockonlip said:
Hmmmm.

I thought the inlet was part of the engine.

Depends on who you ask! As a propulsion guy, he'll argue it's part of the engine. Ask an airframe guy, he'll say it's part of the drag (or the source of all evil). Ask a structures guy, he'll say he wishes it was someone else's problem.
For a hypersonic aircraft the inlet is where it all comes together, which is why on designs like the X-30 it's so dominant, yet fully integrated with the rest of the aircraft, unlike subsonic or supersonic aircraft.

shockonlip said:
Anyway, using your words, after the first quote I might say, well then let's get rid of the engine and just keep the inlet !

At cruise speeds, the SR-71 almost does that - the inlet itself generates substantial thrust.

shockonlip said:
And after the second quote above I may just say, well lets get rid of the inlet (on this airbreathing engine)
and just keep the engine.

You bring up some interesting points. I can tell you've read quite a bit about PDEs.
I would just check what you're saying. You can confuse an old fart like me.

A PDE is a very different animal than a turbine or ramjet, so they can be pretty confusing.
Besides that, I think we stopped talking strictly about PDEs in this thread a while ago. This has turned into something more focused on hypersonic vehicles I'm afraid. While I'd love to see the day a manned, airbreathing hypersonic vehicle flies (in the unclassified world), I do not think I will live that long.
I do think we will see PDEs on unmanned vehicles within the next few years though.
 

KJ_Lesnick

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Lantanian,

From my understanding of the limitation of the technology, incoming air cannot come at speeds more than Mach 6 for PDE operation.

You mean you don't use the inlet to slow the flow down to subsonic speeds before running it through the PDE?


KJ Lesnick
 

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