Bipropellant Fuel Injector Question

[the_Demongod]

Can somebody help me understand why liquid fuel rocket engine injectors work in their typical configuration?

Assuming a self-impinging doublet, how does such a method of mixing the propellants sustain itself in the conditions of the combustion chamber? My intuition tells me that the pressure, acoustic resonance, and turbulence from the combustion would interfere with the clean mixing of the propellants either by igniting them prematurely before they have mixed properly, or by disrupting the jets of liquid and preventing them from mixing properly before combustion. How far down the chamber does the combustion take place, is it closer to the throat of the nozzle, or to the injector plate itself? Is the propellant injected with sufficient pressure to keep the actual combustion away from the injection mechanism?

I guess my confusion stems from the fact that it would seem that the combustion would put sufficient backpressure on the injection process to interfere with an even and stable mixing of the fuels, or if the combustion is happening right up against the plate, to cause them to begin to ignite before they've mixed properly, but obviously they work or real life rocket engines wouldn't function as they do so obviously I just don't understand it fully.

Thanks

[Streetwind]

While I'm in no way qualified to talk about this topic, I would personally make it a default assumption that the combustion chamber pressure never gets anywhere near the injection pressure.

Turbopumps are extremely hardcore.

[p1t1o]

As far as I know, turbulence increases mixing, rather than preventing "clean" mixing, I would imagine that the environment within the combustion chamber results in almost instant and complete. Pressure-wise, the pressure that your engine is able to achieve at the injector is one of the things that directly limits thrust, ie: you must design your combustion chamber not to produce pressures in excess of this, otherwise you get catostrophic deconstruction. The higher pressure you can generate, the higher pressure you can allow to develop in the combustion. Turbopumps are essentially rocket-powered anyway, so no problems there.

Is "premature" ignition a thing? I mean once the fuel is in the combustion chamber I'd imagine that anything that isn't burning is only reducing performance.

 

[kerbiloid]

Closed cycle engine: turbopump pressure <=600 atm, combustion chamber just <=200 atm,

Open cycle engine: turbopump pressure <=60 atm, combustion chamber just <=20 atm,

Just a brutal force.

Edited June 16, 2016 by kerbiloid

[the_Demongod]

2 hours ago, p1t1o said:

Is "premature" ignition a thing? I mean once the fuel is in the combustion chamber I'd imagine that anything that isn't burning is only reducing performance.

My thought was that if combustion was taking place up against the injector plate (to give an extreme example), the propellants would begin to combust before they'd sufficiently mixed and causing pressure that might push propellants apart (imagine our rocket's injector is like a gas stove burner or something, where the fuel is ignited as soon as it leaves the injector), as opposed to impinging cleanly and then combusting together a short distance later but it sounds like the sheer force and output volume of the turbopump is enough to keep the combustion from "climbing" up too close to the injector, like in this video I just found.

[p1t1o]

1 minute ago, the_Demongod said:

My thought was that if combustion was taking place up against the injector plate (to give an extreme example), the propellants would begin to combust before they'd sufficiently mixed and causing pressure that might push propellants apart (imagine our rocket's injector is like a gas stove burner or something, where the fuel is ignited as soon as it leaves the injector), as opposed to impinging cleanly and then combusting together a short distance later but it sounds like the sheer force and output volume of the turbopump is enough to keep the combustion from "climbing" up too close to the injector, like in this video I just found.

I think you pretty much nailed it on the head about the turbopumps just there

[magnemoe]

42 minutes ago, the_Demongod said:

My thought was that if combustion was taking place up against the injector plate (to give an extreme example), the propellants would begin to combust before they'd sufficiently mixed and causing pressure that might push propellants apart (imagine our rocket's injector is like a gas stove burner or something, where the fuel is ignited as soon as it leaves the injector), as opposed to impinging cleanly and then combusting together a short distance later but it sounds like the sheer force and output volume of the turbopump is enough to keep the combustion from "climbing" up too close to the injector, like in this video I just found.

If it climb to close you will not get fuel or oxidizer anymore and the burning will stop. Hard too get burn back with him pressure and no air

[cubinator]

From my very limited knowledge of rocket engines, I would say that if the fuel combusts too far up the throat of the engine, you will have a very big problem and you will not go to space today.

[shynung]

4 hours ago, p1t1o said:

I'd imagine that anything that isn't burning is only reducing performance.

Depends on the propellant mix. In an LH2/LOX rocket, sometimes making the O/F ratio a little too rich, the net result is greater specific impulse. This is because there are unburned hydrogen gas in the exhaust, which are lighter than water. Given the same temperature and pressure, a lighter gas gives greater specific impulse. This is also the reason why some kerosene/LOX rockets run a little oxigen-rich; oxygen gas is lighter than CO2 or water.

9 minutes ago, cubinator said:

From my very limited knowledge of rocket engines, I would say that if the fuel combusts too far up the throat of the engine, you will have a very big problem and you will not go to space today.

Actually, just before the nozzle throat is a component called the combustion chamber, where, as you guessed, the (chemical) propellant is injected and reacted. The dangerous part is when the fuel injected into the chamber doesn't ignite immediately, but collects in a puddle inside the chamber. When this puddle inevitably reacts, it doesn't make a clean burn, but rather an explosion that, if not disassemble the engine outright, would damage the engine. This is called a 'hard start'.

Edited June 16, 2016 by shynung

[VaPaL]

Read the RD-180 Engine section in here: http://spaceflight101.com/spacerockets/atlas-v-551/

It's not a specific answer to your question, but there's a lot of info on there.

[p1t1o]

@the_Demongod

 A lot of what is being talked about is described quite well, and quite entertainingly, in this book:

http://library.sciencemadness.org/library/books/ignition.pdf

Its a useful reference if you're perusing these fora, this is at least the third time I have posted that link! Well worth a read

[the_Demongod]

@p1t1o  maybe it's on my end, but that PDF is completely scrambled and unintelligible, maybe I'll just have to buy the book, it definitely sounds interesting.

[p1t1o]

@the_Demongod Oh yeah, it only loaded the first couple pages...shucks, thats a shame, it was a legit release as well.

Err...also, having just found it on amazon... o_0 ...kinda really makes me wanna dig out that pdf...I hope I still have it...

https://www.amazon.com/Ignition-informal-history-liquid-propellants/dp/0813507251/ref=sr_1_1?ie=UTF8&qid=1466115152&sr=8-1&keywords=ignition+john+clark

[the_Demongod]

@p1t1o Ah good news, downloading the .pdf and opening it in Adobe Acrobat makes it display correctly, I guess it was just Chrome's built in pdf reader that was scrambling it. Will definitely give it a read, thanks!

[p1t1o]

@the_Demongod

Found the following page to have a huge amount of info on the Saturn V F1 fuel injection regime:

http://heroicrelics.org/info/f-1/f-1-injector.html

Looks like it contains almost everything one would want to know about injectors! Includes excellent diagrams, info on instabilities and acoustics.

 

Sorry mods, for the minor necro!

[A Fuzzy Velociraptor]

On 6/16/2016 at 11:46 AM, the_Demongod said:

My thought was that if combustion was taking place up against the injector plate (to give an extreme example), the propellants would begin to combust before they'd sufficiently mixed and causing pressure that might push propellants apart (imagine our rocket's injector is like a gas stove burner or something, where the fuel is ignited as soon as it leaves the injector), as opposed to impinging cleanly and then combusting together a short distance later but it sounds like the sheer force and output volume of the turbopump is enough to keep the combustion from "climbing" up too close to the injector, like in this video I just found.

The phenomenon you are referring to is called "blowapart" and can be seen with hypergolic mixtures. I think that the greater number of impinging streams (doublet, triplet, pentad etc) the greater the likelihood of blowapart. Also the greater the number of impinging streams the more heat will be transferred into the injector plate. With non-hypergolic mixtures I believe the flame front is designed to be maintained further down the combustion chamber to allow for more complete mixing and avoid blowapart though I have only ever worked with hypergolic mixtures.

As for the topic, yes that can happen. The combustion can cause an acoustic vibration which can affect the injector system slowing down the injection and causing a larger vibration. To prevent this the injector flow is "decoupled" from the combustion chamber so that vibrations in the combustion chamber will, ideally, not affect the combustion chamber. To do this the pressure drop across the injectors is normally made to be around 20% of the total pressure such that fluctuations in the chamber pressure will not affect the flow rate across the injectors. Huzel and Huang do a good job explaining this in DESIGN OF LIQUID ROCKET PROPELLANT ENGINES (1992) pgs 104-116,

[the_Demongod]

@A Fuzzy Velociraptor Great answer, this was what I was looking for. Basically, having looked at how injectors work and seeing some videos and simulations of impinging streams mixing two fluids and such, I sort of found it hard to believe that such a process could work just as well under the conditions of combustion, so it's good to know that my suspicions were not entirely unfounded.

Thanks again