Pulse Detonation Wave engines (PDE)

F119Doctor said:
sferrin said:
F119Doctor said:
In any heat machine, the efficiency is determined by delta temperature and delta pressure. The higher temperature and pressure that can be achieved in the working fluid when heat is added, the higher the efficiency. In air breathing engines, the pressure is limited by the capability of the fan and compressor. With a rotating detonation combustor, the supersonic detonation wave raises the pressure and temperature significantly higher than than the inlet pressure while shielding the compressor from that elevated pressure, greatly increasing the efficiency of the thermal cycle.

For a liquid fueled rocket engine, the fuel and oxidizer pumps have to deliver their liquids to the combustion chamber at higher pressures than the chamber pressure. Pressurizing liquid flow is much easier than than compressing air, but it is still a challenge. Rotating detonation combustion can once again raise the combustion pressure higher than the pump pressure, but it would not seem to be as much of an ISP advantage as for an air breathing engine.
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That's the explanation I was looking for. :D

One question though. "significantly higher than than the inlet pressure while shielding the compressor from that elevated pressure" How does it do that? Shield the compressor that is?
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The detonation shock wave compresses the gas in front of it, so the gas being fed in by the pump (liquid) or compressor (air) behind the supersonic shock wave doesn’t feel that pressure increase.
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But what's keeping that shock from traveling forward?
 
It's an Aerospike. Linear aerospike came after axial ones that are axial symetric but function on the same principle. Rocketdyne was the inventor when they were looking for an alternative to the massive F-1 engine nozzle.
 
sferrin said:
F119Doctor said:
sferrin said:
F119Doctor said:
In any heat machine, the efficiency is determined by delta temperature and delta pressure. The higher temperature and pressure that can be achieved in the working fluid when heat is added, the higher the efficiency. In air breathing engines, the pressure is limited by the capability of the fan and compressor. With a rotating detonation combustor, the supersonic detonation wave raises the pressure and temperature significantly higher than than the inlet pressure while shielding the compressor from that elevated pressure, greatly increasing the efficiency of the thermal cycle.

For a liquid fueled rocket engine, the fuel and oxidizer pumps have to deliver their liquids to the combustion chamber at higher pressures than the chamber pressure. Pressurizing liquid flow is much easier than than compressing air, but it is still a challenge. Rotating detonation combustion can once again raise the combustion pressure higher than the pump pressure, but it would not seem to be as much of an ISP advantage as for an air breathing engine.
Click to expand...
That's the explanation I was looking for. :D

One question though. "significantly higher than than the inlet pressure while shielding the compressor from that elevated pressure" How does it do that? Shield the compressor that is?
Click to expand...
The detonation shock wave compresses the gas in front of it, so the gas being fed in by the pump (liquid) or compressor (air) behind the supersonic shock wave doesn’t feel that pressure increase.
Click to expand...
But what's keeping that shock from traveling forward?
Click to expand...
That’s part of the secret sauce. First, you have to set up the conditions and mixture to initiate the detonation. 2nd, you need to get the detonation to travel in the right direction.

In a pulse detonation engine, the detonation travels from front to rear in an intermittent manner.

In the RDE, multiple detonation waves travel around the combustion section, not 90 degrees to the flow, but on a diagonal with the detonation compressing and pushing the gas flow to the rear exit.
 
Archibald said:
Ok so I downloaded one tech report about it (will attach it later).

That thing has potential to improve "classic" specific impulses by 10% average.

And thus, behold... (page 7 of the pdf, now attached)

-H202/ kerosene: 330 seconds plus 10% = 360 seconds
-Kerolox: 360 seconds - up to 400
-Methalox: from 375 to way above 400, 420 something.
-And hydrolox is even more startling: from the usual 460 to way, way above 500. Exactly what is needed to make SSTO happens with slightly less insane propellant mass fraction than 0.88 (with zero payload) or 0.90 and above (small payload).

And since that engine is high thrust, hence not an electric thruster with minuscule thrust...
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The article is just a study by persons who don't seem to know or understand the First Law of Thermodynamics and consequently they don't see that there must be something wrong in their simulation model.

The high Isp number of 554 s in table on page 6 is for gOX/gH2 which means gaseous H2 and O2, not really a suitable rocket fuel, but even then their predicted Isp number is way too high.

The maximum possible Isp of a rocket engine is limited by the First Law: the sum of Enthalpy (function of temperature) and Kinetic Energy (function of velocity) can never exceed the energy content (Enthalpy plus Lower Heating Value) of the propellants, but the authors of the article don't seem to realize that. Thermodynamic laws don't play any role in their article. Often enough scientist forget the First Law when they are working on a new invention. Fixated on the interesting physics of their theory they don't think about doing a simple check of all energy in and out.

The maximum possible Isp for a rocket engine using LOX/LH2 in a ratio of 6 is below 500 s.
With an LOX/LH2 ratio of only 2.6 (see table in article) the max possible Isp would be below 450 s.

SSME RS-25 gives about 450 s, RL-10 nowadays about 465 s so there is very little to gain by using a new type engine like RDRE. Only using nuclear would result in a big increase in Isp.

It might however (many years from now) result in a rocket engine that has a lower weight and/or lower cost than other rocket engines.

RDE may in future result in turbofans with better fuel economy and therefor it would be better that all researchers now wasting their time with RDRE would switch to RDE research for turbofans.
 
Could the plug nozzle be sacrificial? Throat opens up as nozzle itself adds energy sloughing off and taking heat with it. New cone each flight?
 
The first time I heard about pulse detonation wave engines was in stories regarding the alleged hypersonic Aurora aircraft. Then an article appeared. An interesting aspect of the story was regarding the leading edges of the aircraft removing air pressure built up by micro explosions. I'm assuming similar explosives to rocket stage separations.

Anyone hear something similar?
 
Wave propogation is the easy part. Coordination of the wave generation is critical because with each pulse you are creating a low pressure zone that collapses. Your next pulse needs to take place precisely in a space that has regained static pressure, post-wave. That static pressure must be utilized in a sliver of time or it becomes wasted potential. The secret sauce is in how many waves can be simultaneous formed in a continous coordinated effort. Density is everything. Your fuel choice affects that density achievable in both storage and wave size, your limitations for the desired thermal gradient, and the maximum velocity of exit gases. Hydrogen is neither the ideal fuel for large scale nozzles nor fuel storage. Wouldn't it be oxymoronic if fuels with the consistency of grease or sludge be ideal clean fuel?
 
MadRat said:
Wave propogation is the easy part. Coordination of the wave generation is critical because with each pulse you are creating a low pressure zone that collapses. Your next pulse needs to take place precisely in a space that has regained static pressure, post-wave. That static pressure must be utilized in a sliver of time or it becomes wasted potential. The secret sauce is in how many waves can be simultaneous formed in a continous coordinated effort. Density is everything. Your fuel choice affects that density achievable in both storage and wave size, your limitations for the desired thermal gradient, and the maximum velocity of exit gases. Hydrogen is neither the ideal fuel for large scale nozzles nor fuel storage. Wouldn't it be oxymoronic if fuels with the consistency of grease or sludge be ideal clean fuel?
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Maybe they'll finally find a use for the mythical "gel-fuel".
 

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