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LS6: Injector design         

Send questions to Alex Koenig (koe@mit.edu)

Relevant links for further reading!

https://en.wikipedia.org/wiki/Liquid-propellant_rocket#Types_of_injectors

Why use injectors at all?

Although fuel and oxidizer could theoretically be directly piped into the combustion chamber with no special mechanisms, doing so would have disastrous consequences for both the efficiency and stability of the combustion. The purpose of the injector is to deliver fuel and oxidizer to the combustion chamber in a manner such that the propellants atomize and mix within the combustion chamber as thoroughly as possible. The more thoroughly the propellants mix, the more the propellants will actually end up combusting, so the injector is key to obtaining high engine efficiency -- any uncombusted fuel that escapes the engine is wasted, which is more likely to happen with imperfect injector designs. The droplet size of individual fuel and oxidizer particles is also ideally reduced by a good injector design, which additionally helps achieve thorough combustion.

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As another interesting note about injectors in liquid engines, in almost all other engine types (internal combustion engines, gas turbine engines, etc.), fuel and oxidizer enter the combustion chamber premixed. For those of you familiar with general aviation aircraft engines and older car engines, you will know the premixer as a carburetor. Bipropellant liquid rocket engines are fairly unique in that fuel and oxidizer are intended to only make contact upon entering the combustion chamber, primarily due to their volatility -- many fuel-oxidizer combinations are prone to detonation when mixed, particularly at the high pressures and velocities experienced within the fluid system upstream of the engine, so we keep them unmixed outside of the combustion chamber. 

Helios injector layout

In a typical injector design, propellants first enter the manifold behind the injector, where they get evenly spread across the injector plate; then propellants are forced through small orifices (holes) in the injector plate, and become mixed inside the combustion chamber. Such is the design for the Helios engine, which is depicted in the diagrams below.

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Fig. 2: a top-down view of the Helios injector, more clearly showing the shape of the two ring-shaped chambers that comprise the manifold, along with the 16 orifices that allow propellant into the combustion chamber. (The large holes furthest away from the center of the injector plate are just for bolts to hold the injector plate onto the combustion chamber.)

Main categories of injection schemes

Non-impinging

In non-impinging injection schemes, propellants are sprayed into the engine but the streams do not directly collide with other propellants. In general, these injectors achieve the most stable combustion out of all categories, but are the most complex and difficult to machine. 

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To point out another famous system, the Space Shuttle main engines (RS-25 engines) -- which are now in use for the Space Launch System -- employ coaxial elements (top right).

Like-impinging

In like-impinging injectors, fuel streams impinge on other fuel streams, and oxidizer streams impinge on other oxidizer streams. These are generally not quite as stable as non-impinging injectors, but are often favorable due to their relative ease to machine -- the only machining operations required are drilling small holes at angles such that the fuel and oxidizer impinges.

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(Notably, used in the Rocketdyne F-1 engine of the Saturn V rockets!)

Unlike-impinging

In unlike-impinging injectors, fuel and oxidizer impinge on each other. Generally these achieve the least stable combustion compared to other injector schemes, but that does not necessarily mean that they are unstable, per se, just that it presents a greater design concern. It is also the case that these are the easiest to machine, which is why they were selected for use in the Helios engine (as well as the previous Viper engine), since Helios was intended to be the simplest possible bipropellant engine that we could make. These happen to be a more common choice for systems which use liquid oxygen as the oxidizer.

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The table above gives a good overview of attributes of like- and unlike-impinging injector elements. Doublets refer to those which have 2 orifices per impingement; triplets have 3. (Like-impinging injectors have double the orifices per element because each ‘element’ consists of one fuel like-impinging injector and one oxidizer like-impinging injector, rather than one injector for both, as in the unlike-impinging system).

Injector element geometries

The following equations are referenced with regard to like and unlike doublets in particular, since those are the ones we most closely work with in liquid prop, but the same equations hold for other injector types as well.

Sizing

Where q = mass flow rate (kg/s), K = head-loss coefficient, ρ = fluid density (kg/m3), and ΔP = pressure drop (Pa). The pressure drop will be discussed more further on in this LSET; it is the primary parameter around which the injector area is changed in order to achieve a certain value (the other parameters like fluid density are typically known by the time the injector area is determined).

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In the diagram above, one fluid stream points inward towards the central axis of the chamber, and the other points in the opposite direction towards the chamber wall. Generally, injection elements are arranged with the LOX orifices angled towards the chamber wall and ethanol orifices angled towards the central axis. This provides minor film cooling with the spray deflection on the outer ring angled back towards the chamber wall (the LOX is at cryogenic temperatures, of course, and so can cool down the chamber wall upon contact, assuming it has not yet been combusted by that point). The opposite arrangement would expose the chamber wall to oxidizer spray and contribute to corrosion and deterioration, although that consideration in particular is not a significant factor for the Helios engine since it is only designed to fire for a few seconds.

Manifold

As briefly mentioned in the Helios injector layout section, the manifold is the component upstream of the injectors which is intended to evenly distribute the flow across all the injector elements/orifices so as to achieve even combustion. In the Helios design, there is one manifold section for LOX, and one for ethanol; both are annulus-shaped cavities in the injector plate. Even distribution of propellants by the manifold generally dictates the use of large passageways, rounded corners, and smooth finishes; friction, cavitation sites, turbulence, and restrictions in the pathway are detrimental to consistent mass flow. Manifolds for different engines can take on a wide variety of forms, but for our purposes the ‘best’ designs are ones which evenly distribute propellants, are easy to manufacture, and don’t require complicated sealing mechanisms.

Pressure drops

Any fluid flow that is constricted or restrained in some way has an associated pressure drop in the fluid at the point of constriction. For injectors, this also holds true, since the injector represents a point of constriction for the flow of the fuel and oxidizer. The pressure drop can be altered by changing the orifice size, and it is important to understand how to properly size the orifices so as to achieve an appropriate pressure drop.

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