Author Topic: Shcramjet / ODWE (Read 2623 times)

sanman · « **on:** 06/02/2023 09:09 am »

The shcramjet (Shock-induced Combustion Ramjet) or ODWE (Oblique Detonation Wave Engine) is supposed to operate more efficiently at higher mach numbers, compared to the scramjet:

https://en.wikipedia.org/wiki/Shcramjet

http://fullafterburner.weebly.com/next-gen-weapons/shcramjet

What is the state of development of this technology?

What exactly is the significance of the oblique shockwave? What advantages does it offer, and how?

InterestedEngineer · « **Reply #1 on:** 06/02/2023 04:56 pm »

from wikipedia

Quote

A critical challenge is to confine the detonation in a small area, without allowing it to direct energy upstream towards the fuel source or downstream where it will lose force

Pretty hilarious, since that's fundamental to all heat engines.

They've gotten that to work for 3 seconds so far.

sanman · « **Reply #2 on:** 06/02/2023 06:15 pm »

Quote from: InterestedEngineer on 06/02/2023 04:56 pm

from wikipedia

Quote
A critical challenge is to confine the detonation in a small area, without allowing it to direct energy upstream towards the fuel source or downstream where it will lose force

Pretty hilarious, since that's fundamental to all heat engines.

They've gotten that to work for 3 seconds so far.

But so it sounds like it's all about the shockwave. The shockwave is the key to shaping the combustion in the right ways.

So after initially using a rocket to accelerate you to that optimal flow regime of Mach 10+, that's when your flowstream is fast enough to keep the combustion from moving upstream to the injector. Having that injector located at the inlet means more time for fuel and air to mix.

But I'm not quite sure what exactly the oblique angle of the shockwave is giving you.

InterestedEngineer · « **Reply #3 on:** 06/02/2023 06:35 pm »

Quote from: sanman on 06/02/2023 06:15 pm

Quote from: InterestedEngineer on 06/02/2023 04:56 pm
from wikipedia

Quote
A critical challenge is to confine the detonation in a small area, without allowing it to direct energy upstream towards the fuel source or downstream where it will lose force

Pretty hilarious, since that's fundamental to all heat engines.

They've gotten that to work for 3 seconds so far.

But so it sounds like it's all about the shockwave. The shockwave is the key to shaping the combustion in the right ways.

So after initially using a rocket to accelerate you to that optimal flow regime of Mach 10+, that's when your flowstream is fast enough to keep the combustion from moving upstream to the injector. Having that injector located at the inlet means more time for fuel and air to mix.

But I'm not quite sure what exactly the oblique angle of the shockwave is giving you.

So still two stage to orbit. Or maybe 3, since you need Mach 25 and this only gets you to about Mach 18.

25^2 / 18^2 ~= 2, so still need enough rocket to double the kinetic energy.

But would make a great air launched hypersonic missile, provided one could figure out the heating problems.

How hot is that inlet getting? Maybe it's hydrogen cooled?

sevenperforce · « **Reply #4 on:** 06/02/2023 06:48 pm »

Quote from: sanman on 06/02/2023 09:09 am

The shcramjet (Shock-induced Combustion Ramjet) or ODWE (Oblique Detonation Wave Engine) is supposed to operate more efficiently at higher mach numbers, compared to the scramjet

What exactly is the significance of the oblique shockwave? What advantages does it offer, and how?

An ordinary pressure wave propagates at the speed of sound, while a shockwave propagates above the speed of sound. Because a shockwave is moving faster than the speed of sound, none of its pressure waves can propagate "upstream" in the airflow. This is the principle behind shock-induced compression: while the air inside a turbine jet engine is "trapped" and thus compressed by the mechanical force of the turbine blades, the air flowing into a ramjet or scramjet engine is compressed by the fundamental fluid physics associated with shockwaves. The difference between a ramjet and a scramjet is that the air entering a ramjet eventually reaches (locally) subsonic speeds inside the engine, behind this shock front, while the shockwaves in a scramjet combustor continue to reflect through the vehicle and so there is never a choke point where the local airflow drops below the speed of sound.

While this allows scramjets to operate far above the speed envelope of a ramjet, it also makes them poor combustors. Combustion relies on chemical reactions between molecules, which ordinarily take place in 3-dimensional space, but in a supersonic flow environment you sort of only have two dimensions for your reaction (since molecules can no longer travel upstream). This means you need a very long, heavy section to act as your combustor, and even then you don't get complete combustion.

A shcramjet or oblique detonation wave scramjet would solve this problem by exploiting something ELSE that travels at above the speed of sound: detonation waves. While deflagration (ordinary combustion) happens below the speed of sound in 3-space, detonation occurs when a chemical combustion reaction moves through a well-mixed fuel-oxidizer medium at above the speed of sound. Detonation waves achieve complete combustion in 2-space at the shockwave front.

Ordinarily, detonation waves occur far too fast to be controlled. But in the hypersonic regime of a scramjet intake, it's theoretically possible to have a detonation wavefront which propagates at the same velocity as the shockwave at the front of the combustor, creating a standing wave detonation. This would almost entirely eliminate the need for a long, heavy combustor section, leaving you with an intake that leads directly into the exhaust nozzle.

Quote

What is the state of development of this technology?

Development is largely stalled, as is generally the case with most scramjet dev, because there is very limited use for the tech. It can't be effectively used in orbital spaceflight, because orbital vehicles need high acceleration to achieve orbital velocity. It's far too fast to have a use case for commercial flight. The only real application is for hypersonic cruise missiles, where it would need a rocket booster to get up to the appropriate velocities for scramjet ignition. And even there, you only get the really high efficiencies with liquid hydrogen propellant, which obviates much of the advantages of a cruise missile over a more conventional air-launched short-range ballistic missile.

Quote from: sanman on 06/02/2023 06:15 pm

So after initially using a rocket to accelerate you to that optimal flow regime of Mach 10+, that's when your flowstream is fast enough to keep the combustion from moving upstream to the injector. Having that injector located at the inlet means more time for fuel and air to mix.

But I'm not quite sure what exactly the oblique angle of the shockwave is giving you.

All scramjets (and even ramjets) have oblique shockwaves in the inlet. The point of a shcramjet or ODWE is to have a standing detonation wave induced by the oblique shockwave which acts as a virtual combustor.

Quote from: InterestedEngineer on 06/02/2023 06:35 pm

So still two stage to orbit. Or maybe 3, since you need Mach 25 and this only gets you to about Mach 18.

Scramjets are generally NOT good at accelerating; they are good at cruising. Their advantage is being able to maintain airspeed for a long period of time because they use much less propellant than a rocket operating in comparable regimes.

The cost, weight, and complexity of an intermediate airbreathing stage is almost always going to be prohibitive. There's just not enough acceleration when you've gotta suck up stationary air and then eject it out the back with your exhaust faster than you sucked it up.