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'limited only by available propellant. Sources: Companies listed above and NASA.

The thrust for the thrust chamber is usually slightly less than the thrust of the engine for open cycles, such as a gas generator cycle; the thrust chamber specific impulse is actually slightly higher (about 1 %) than the engine specific impulse. For closed cycles such as the staged combustion cycle, the F and /, values of the engine and thrust chamber are the same.

Injection holes

Injection holes

Impingement points

Face of injector

Doublet impinging stream pattern

Doublet impinging stream pattern

Impingement points

Face of injector

Impingement points

Face of injector

Fuel manifolds

Triplet impinging stream pattern

Fuel manifolds

Impingement points

Face of injector

Triplet impinging stream pattern f

Fuel manifolds Oxidizer manifolds

Self-impinging stream pattern

Typical impingement point

Self-impinging stream pattern

Typical straight propellant stream

Shower head stream pattern

Typical straight propellant stream

Shower head stream pattern

Inner tube Outer

Gaseous or post Spacer sleeve hydrogen

Inner tube Outer

Gaseous or post Spacer sleeve hydrogen

Hollow post and sleeve element

Variable injection area concentric tube injector

FIGURE 8-3. Schematic diagrams of several injector types. The movable sleeve type variable thrust injector is adapted from Ref. 8-1.

Hollow post and sleeve element

Variable injection area concentric tube injector

FIGURE 8-3. Schematic diagrams of several injector types. The movable sleeve type variable thrust injector is adapted from Ref. 8-1.

lant flow over a wide range without excessive reduction in injector pressure drop. This type of variable area concentric tube injector was used on the descent engine of the Lunar Excursion Module and throttled over a 10:1 range of flow with only a very small change in mixture ratio.

The coaxial hollow post injector has been used for liquid oxygen and gaseous hydrogen injectors by most domestic and foreign rocket designers. It is shown in the lower left of Fig. 8-3. It works well when the liquid hydrogen has absorbed heat from cooling jackets and has been gasified. This gasified hydrogen flows at high speed (typically 330 m/sec or 1000 ft/sec); the liquid oxygen flows far more slowly (usually at less than 33 m/sec or 100 ft/sec) and the differential velocity causes a shear action, which helps to break up the oxygen stream into small droplets. The injector has a multiplicity of these coaxial posts on its face. This

Self Impinging Doublet

FIGURE 8-4. Injector with 90° self-impinging (fuel-against-fuel and oxidizer-against-oxidizer)-type countersunk doublet injection pattern. Large holes are inlets to fuel manifolds. Pre-drilled rings are brazed alternately over an annular fuel manifold or groove and a similar adjacent oxidizer manifold or groove. A section through a similar but larger injector is shown in Fig. 8-1.

FIGURE 8-4. Injector with 90° self-impinging (fuel-against-fuel and oxidizer-against-oxidizer)-type countersunk doublet injection pattern. Large holes are inlets to fuel manifolds. Pre-drilled rings are brazed alternately over an annular fuel manifold or groove and a similar adjacent oxidizer manifold or groove. A section through a similar but larger injector is shown in Fig. 8-1.

type of injector is not used with liquid storable bipropellants, in part because the pressure drop to achieve high velocity would become too high.

The SSME injector shown in Fig. 9-6 uses 600 of these concentric sleeve injection elements; 75 of them have been lengthened beyond the injector face to form cooled baffles, which reduce the incidence of combustion instability.

The original method of making injection holes was to carefully drill them and round out or chamfer their inlets. This is still being done today. It is difficult to align these holes accurately (for good impingement) and to avoid burrs and surface irregularities. One method that avoids these problems and allows a large number of small accurate injecton orifices is to use multiple etched, very thin plates (often called platelets) that are then stacked and diffusion bonded together to form a monolithic structure as shown in Fig. 8-5. The photo-etched pattern on each of the individual plates or metal sheets then provides not only for many small injection orifices at the injector face, but also for internal distribution or flow passages in the injector and sometimes also for a fine-mesh filter inside the injector body. The platelets can be stacked parallel to or normal to the injector face. The finished injector has been called the platelet injector and has been patented by the Aerojet Propulsion Company.

Fuel distribution plate with etched cooling passages

Face plate with inclined injection holes (laser drilled)

Fuel distribution plate with etched cooling passages

Alignment pins (2)

Oxidizer distribution manifold plate

Face plate with inclined injection holes (laser drilled)

Cover plate with oxidizer inlet

Four stacked plates (with exaggerated thickness)

Oxidizer manifold

Oxidizer filter Fuel manifold

Valve inlet port

Oxidizer inlet port

Oxidizer manifold

Oxidizer filter Fuel manifold

Valve inlet port

Oxidizer inlet port

Like on like/ Injector face impinging orifices

Cross section through an etched plate

Like on like/ Injector face impinging orifices

Cross section through an etched plate intergra! flange

Enlarged detail segment of one design of like-on-like impinging orifices and feed passage geometry

FIGURE 8-5. Simplified diagrams of two types of injector using a bonded platelet construction technique: (a) injector for low thrust with four impinging unlike doublet liquid streams; the individual plates are parallel to the injector face; (b) Like-on-like impinging stream injector with 144 orifices; plates are perpendicular to the injector face. (Courtesy of Aerojet Propulsion Company.)

Injector Flow Characteristics

The differences of the various injector configurations shown in Fig. 8-3 reflect themselves in different hydraulic flow-pressure relationships, different starting characteristics, atomization, resistance to self-induced vibrations, and combustion efficiency.

The hydraulic injector characteristics can be evaluated accurately and can be designed for orifices with the desired injection pressures, injection velocities, flows, and mixture ratio. For a given thrust F and a given effective exhaust velocity c, the total propellant mass flow m is given by m = F/c from Eq. 2-6. The relations between the mixture ratio, the oxidizer, and the fuel flow rates are givenby Eqs. 6-1 to 6-4. For the flow of an incompressible fluid through hydraulic orifices,

where Q is the volume flow rate, Cd the dimensionless discharge coefficient, p the propellant mass density, A the cross-sectional area of the orifice, and Ap the pressure drop. These relationships are general and can be applied to any one section of the propellant feed system, to the injector, or to the overall liquid flow system. A typical variation of injection orifice flow and pressure drop is shown in Fig. 8-6. If the hole has a rounded entrance (top left sketch), it gives the lowest pressure drop or the highest flow. Small differences in chamfers, hole entry radius, or burrs at the edge of a hole can cause significant variations in the discharge coefficient and the jet flow patterns, and these in turn can alter

Pressure drop, atm

FIGURE 8-6. Hydraulic characteristics of four types of injection orifice.

Pressure drop, atm

FIGURE 8-6. Hydraulic characteristics of four types of injection orifice.

the quality and distribution of the atomized small droplets, the local mixture ratio, and the local heat transfer rates. An improperly manufactured hole can cause local chamber or injector burnout.

For any given pressure drops the injection orifices determine the mixture ratio and the propellant flows of the rocket unit. From Eqs. 6-1 and 8-2 the mixture ratio is r = rhjrhf = [(Cd)0/(Cd)f](A0/Af)j(p0/pf)(Ap0/Apf) (8-3)

The quantities in the preceding equations have to be chosen so that the correct design mixture ratio is attained, even if the total flow is varied slightly. Orifices whose discharge coefficients are constant over a large range of Reynolds numbers and whose ratio (Cd)0/(Cd)f remains invariant should be selected. For a given injector it is usually difficult to maintain the mixture ratio constant at low flows or thrusts, such as in starting.

The quality of the injector is checked by performing cold tests with inert simulant liquids instead of reactive propellant liquids. Often water is used to confirm pressure drops through the fuel or oxidizer side at different flows and this allows determination of the pressure drops with propellants and the discharge coefficients. Nonmixable inert liquids are used with a special apparatus to determine the local cold flow mixture ratio distribution over the chamber cross section. The simulant liquid should be of approximately the same density and viscosity as the actual propellant. All new injectors are hot fired and tested with actual propellants.

The actual mixture ratio can be estimated from cold flow test data, the measured hole areas, and discharge coefficients by correcting by the square root of the density ratio of the simulant liquid and the propellant. When water at the same pressure is fed alternately into both the fuel and the oxidizer sides, Apf = Ap0 and pj = p0 and the water mixture ratio will be

Therefore, the mixture ratio measured in water tests can be converted into the actual propellant mixture ratio by multiplying it by the square root of the density ratio of the propellant combination and the square root of the pressure drop ratio. The mechanism of propellant atomization with simultaneous vaporization, partial combustion, and mixing is difficult to analyze, and performance of injectors has to be evaluated by experiment within a burning rocket thrust chamber. The injection velocity is given by v=Q/A = Cdj2Ap/p (8-5)

Values of discharge coefficients for various types of injection orifices are shown in Table 8-2. The velocity is a maximum for a given injection pressure drop

TABLE 8-2. Injector Discharge Coefficients

Orifice Type

Diagram

Diameter (mm)

Discharge Coefficient

Sharp-edged orifice

Above 2.5 Below 2.5

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