Rocket Fuel Injectors - Things Kerbal Space Program Doesn't Teach

oh it's Scott Manley here today we're gonna talk about what Kerbal space program doesn't teach you about rocket fuel injectors so rocket fuel injectors aren't a critical part of rocket engines for propellants are fed into the combustion chamber these are hidden away in the heart of the engine and in many cases when I'm looking at real engines there are security people there that are telling me please don't photograph these because they are actually protected under ITAR and various other export regulations now of course we know a lot about older engines and in many cases we know what newer engines are using but we don't know the exact measurements that for the injector so we can't you'll figure out the exact performance or derive our own engine problem good propellant injectors can make all the difference between your engine performing excellently and it exploding due to combustion instability their task is to take the propellants and ensure that they are distributed and mixed well enough the combustion is as complete as possible before the exhaust escapes out the throat and the nozzle of the engine this is measured in something called combustion efficiency you want this to be as high as possible otherwise you are literally losing performance at the back of the nozzle on most rocket engines the propellants are supplied as liquids the injectors spray these into the combustion chamber making tiny drops and you might imagine that this is like a shower head but a shower head is a pure design even ignoring the fact that your average shower head wouldn't survive very long in the combustion chamber of rocket but shower heads are a good starting point for understanding how injectors work on one side you have a high-pressure liquid going into a manifold at a lot of small holes where the fluid flows out the narrow holes result in an increase in fluid velocity any drop in its pressure the Jets of liquid then break up into small drops and these mix in the combustion chamber and burn quickly so the primary goal of thing Jetta is to maximize the surface area to mass ratio off your propellant the process of breaking down to propellant into small droplets is generally referred to as atomization the other primary goal of a fuel injector is to ensure that the fuel and the oxidizer get well mixed otherwise you can create a fine spray of fuel drops but they won't burn unless they mix with a fine spray of oxidizer drops so fuel and oxidizer jets also need to get placed so they mix well with each other if an injector design doesn't produce small enough drops or the mixing isn't particularly good then the combustion efficiency won't be as high one way the compensate is to make your combustion chamber bigger but then of course you add more mass to your engine and you lose performance also because you have to then flow your cooling fluid through a larger volume so shower head style injectors were used in early rockets but they don't generate great atomization or mixing however they are relatively easy to make and loss of amateur Rockets actually we'll start out with those in their engines in fact I've heard legends of amateur rockets using actual shower heads they bought from the store the problem with the simple jets from a showerhead is that they go in a straight line without much spread until they hit something so they rely on turbulence in the combustion chamber to drive the mixing process so many rocket designs improve on the showerhead by using impinging injectors that is the propellant Jets are angled so that they collide with each other the colliding Jets scatter the liquid over a wider angle and create much smaller droplets so that they offer a significant improvement in both atomization and mixing the primary disadvantage is that you need to drill all your injector holes with much higher accuracy because messing up one will mean the Jets do an imp engine you lose a lot of performance and that means pretty much throwing the way your entire injector assembly there are a whole host of impinging jet injectors designers can choose to vary the number of Jets involved and they can also choose to match Jets to a single propellant or they can mix and match them if you've got to and if you could Jets that are all the same type those are called lye if you're firing fuel into oxidizer that's called an unlike impinging injector so the most simple is the like doublet where you have two Jets of the same propellant they meet and they produce a spray that fans out across a wide angle so it delivers good atomization and spreads out the propellant but the mixing still happens downstream from the injection so this design was used in a lot of classic rockets that we used an Atlas Thor Titan and the Saturn 5 because the Jets are all the same it was very easy to design the manifold to deliver the propellant to both of the Jets the f1 propellant injectors on the Saturn 5 are probably the most famous propellant injectors in rocket history partly on account of the problems they had where the engines would explore due to combustion instability but this problem was really more to do with the size of the combustion chamber rather than the fact that they were choosing these impinging injectors we have some great photos off the injector plates showing all the pairs of the orifices in the injector system along with the baffles that they had to add to suppress the combustion instability on the final design there were 1428 oxidizer orifices and a 1,400 and for fuel orifices all paired up into doublets and you can imagine this takes a very long time to make you can also see that some pairs are larger than others because then the engine you have more liquid oxygen than liquid kerosene so you have the oxygen holes are larger you can see from this photo that they also experimented with triplets where there would be a third orifice added in the middle and this changes the spread of the droplets and various other parameters but I don't think it provided any real benefits to justify the extra 1,400 holes that they would have to drill in the injector plate moving on there is a much greater variety of unlike impinging injectors you can have a simple doublet where the fuel and the oxidizer Jets intersect and this gives atomization that starts the thing right at the injector but the fuel and oxidizers are end up somewhat segregated because the fuel will tend to bouncing in one direction and the oxidizer in the other also because the mass of the fuel and the oxidizer are generally different your spray will tend to go off at one angle or another and of course designers have to accommodate for that so there is an improvement which is the unlike Triplett where you have one propellant jet in the center of one type or the other and then on the outside you have the other propellant any pairs in pinching on the center and this is an improvement because your spray pattern is no symmetrical and your mixing is good I believe this was used on the XLR 81 which is the engine on the upper stage the Agena upper stage and I've also seen photos of the lunar module ascent engine injector that looks like it uses triplets - somewhat more common though is the unlike quad lit and this has four Jets meeting at a point so you've got fuel coming in and one side and then at 90 degrees you'll have a pair of oxidizer and they all meet at the same point so this produces another nice symmetric spray pattern it's more like a cone rather than a fan you get from the linear injectors so this design was used on the lr 87 engine which was of course the system for the Titan rockets and you know if you really want you can make a pentad where you add an extra jet in the middle but you know the more holes you drill the harder to build these things it is to build these things and the truth is that these impinging jet injectors have sort of faded out over time know also in most of the examples I've mentioned the I've talked about propellant plates being on the top of the engine blowing down it's also possible to have your injectors along the side and if you remember when I was at ISA they I looked at the Viking engine on the area and one through four and that had the propellant injecting through the side so that's another way to do things also while I remember I've talked a bit main combustion engines but your also gonna need injector systems for your pre-burners and your gas generators and of course those will have much more oxidizer or much more fuel depending upon with how you know how with your fuel rich or oxidizer rich and early on in the 1940s and 50s there was another type of jet injector which was somewhat popular this is called a splash plate injector where you would have your fuel and your oxidizer jets and you would meet a point and you would also have a hard plate so we'd scatter off that and splash off at an angle and I guess those fell out of favor they kind of make remind me of sprinkler systems in buildings to you know extinguish fires so the other major class of injectors which are actually more common today than the jet types are sheet or spray type injectors and these produce conical sheets which then spread out and they separate into the droplets and a very early example of this is the simple swirl type injector and these just the fluid flows into the base at an angle and then the swirl around and then spray out because the fluid is spinning when it emerges from the hole it spreads out and these were actually used in the v2 the v2 was a very interesting rocket because of we're obviously trying to solve the problems instead of having an injector plate at the top of the engine they had these sort of cups or warrants that were stuck onto the top of the combustion chamber and they would have like her shower head at the top and then spray swirl or types injecting in on the sides and of course that would then mix and go into the engine they were obviously trying to figure this all out back then it's not a great design but it's interesting to see how they did it back then now it's much more common these days to see the swirl mechanism used in concentric tube injectors so concentric tube conjecture has one propellant flowing up a tube in the middle and then you have a concentric pipe basically the fluid flows through an annulus and you know they flow into the engine and this doesn't provide on its own doesn't provide a great way of mixing if there's no swirl because the things kind of flaw with nuoc banshan it does kind of work if you've got say hydrogen and oxygen because the speed of sound or the speed of flow is different by a factor of 10 but for you know fluids which are more consistent in their mass you need to make that central one expand outwards by swirling it so that it collides with the other propellant and so you've got caught coaxial swirl injectors and this design is being used by Copenhagen suborbitals and they actually sent me some fine video examples of this so yeah you have the one prepare propellant jacket which is produces a conical shape sorry a cylindrical shape and the core you produce the conical shape they collide and they mix and they atomized simultaneously and these are very popular these days because they're kind of quite easy to manufacture compared to say drilling all the holes in an injector plate and they provide really good mixing and performance however the coaxial design you've probably heard mentioned by rocket nerds is the pintle injector instead of swirling what you have is a structure at the top such as an inverted cone where the propellant flows up into it and then pushed out sideways so this is very this is actually very popular because you can adjust this pintle up and down and you can do this on two levels so you can adjust the geometry of your injector while you're changing your throttle on your engine so this inherently lends itself to throttle engines and the first really good example of a pintle injector is the Apollo design engine which of course needed to throttle all the way down to ten percent the other really good example of a pintle injector is the Merlin engine on the Falcon name both of these engines by the way they're fed by a single injector assembly whereas when I've talked about something it's also possible to have large numbers of pinholes or large numbers of swirl injectors it's really down to the designer so the get the great thing is you can adjust the geometry this by pulling her and with the correct design you can actually use this as a valve so you could Clause up your injectors and close the valve simultaneously and this means that you don't have to worry about fluid or fuel or propellant in the pipes flowing into your engine after you've shot them down which is of course really good if you're trying to land a rocket it means you can start up and shut down very quickly so this is basically closing off the fuel flow at the injector face or face shut off which is a phrase you might have heard one disadvantage of using the single large pintle injector that a SpaceX encountered was that they wanted to use an ablative combustion chamber but because the propellant comes out any cord it was hitting the walls and producing a hotspot all the way around that would inevitably burn through which was what gave them the impetus to switch their Merlin's over to regenerative lis cooled combustion chambers and to be clear whatever kind of injector element you're using you need to ensure that the propellant is distributed in such a manner that the engine doesn't have hot spots or the sections with the fuel isn't mixed in all these cases about they can have cases where this doesn't work it is common though to take advantage of this of the geometry and have sea fuel injectors along the edges that you create if fuel rich your layer around the outside to protect the walls that's of course film cooling okay we're almost there so far I've only discussed injectors for both propellants are in liquid form but sometimes one or more of your propellants has as a gas and gases are actually easier in many ways I mean if you think about it the reason why we are wanting the fuel in tiny drops is so that they evaporate into a gas faster so for the RL 10 the expander cycle means that the hydrogen arrives and ninja' at the injector as a gas so a coaxial injector is used with the oxygen in the middle and the hydrogen flowing rapidly around the outside and it's the speed difference between the oxygen and hydrogen which really drives the mixing more recent RL tens the a dirty swirl to the oxygen floor the Space Shuttle main engine they use a coaxial swirl system with the oxygen flowing down the middle but the central pipes use of the oxygen are also heated by the hot hydrogen rich exhaust from the pre-burners which means your oxygen is coming into the chamber warmed up and you're more readily turning itself into a gas and if you want to know a bit more about this injector you should to watch my video on STS 93 explaining how a gold bullet almost destroyed a space shuttle finally we don't know very much about the Raptor engine but we do know that it's a full floor staged combustion cycle engine which means the propellants that have come out of both the pre-burners are all gas and according to Elon Musk they've they're gonna use a large number of coaxial swirl injectors to make sure that the fuel is mixed correctly which is pretty cool because you've got Copenhagen suborbital and SpaceX both using this concept which just shows this is the injector that everyone's loving these days so at this point I hope you've learned that there is a ton of engineering and science that's been applied to the problem of injecting those propellants into the engines of course there are probably a bunch of you out there who already writing comments explaining what they got wrong or what I missed and honestly I look forward to learning more about the subject from you all it's not a simple problem either indeed computational tools haven't come close to solving all the problems and sometimes when you think you have the best solution it can be off limits because it's just too complicated to actually build it reliably that's where rocket science realizes that it has to work within rocket engineering I'm Scott Manley fly safe [Music]

Channel: Scott Manley

Views: 774,499

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Id: aa4ATJGRqA0

Length: 17min 51sec (1071 seconds)

Published: Mon Mar 09 2020

Reddit Comments

[u/Boncester2018]

Love his videos. He does a great job if really breaking down the tech!