FIGURE 9-5. Graphical representation of a series of 40 superimposed frequency-amplitude diagrams taken 0.200 sec apart during the start phase (for the first 8 sec) of the Vulcain HM 60 thrust chamber. In this static hot-firing test the thrust chamber was operating at 109 bar chamber pressure and an oxidizer-to-fuel mass flow mixture ratio of 6.6. (Copied with permission from Ref. 9-13).
frequency (up to 500 Hz) during the first few seconds and a natural frequency around 1500 Hz is attributed to the natural resonance frequency of the oxygen injector dome structure where the high-frequency pressure transducer was-mounted. The continued oscillations observed at about 500 and 600 Hz are probably resonances associated with the feed system.
Semi-empirical techniques exist for artificially disturbing combustion in a rocket thrust chamber during test operation and evaluating its resistance to instability (see Ref. 9-14). These include: (1) nondirectional "bombs" placed within the combustion chamber; (2) oriented explosive pulses from a "pulse gun" directed through the chamber sidewall; and (3) directed flows of inert gas through the sidewall into the chamber. Often heavy prototype thrust chambers are used because they are less expensive and more resistant to damage than flight-weight engines. Other techniques used less widely but which are important, especially for small engines, include: (1) momentary operation at "off-mixture ratio;" (2) introduction of "slugs" of inert gas into a propellant line; and (3) a purposeful "hard start" achieved by introducing a quantity of unreacted propellant at the beginning of the operation.
The objective of these rating techniques is to measure and demonstrate the ability of an engine system to return quickly to normal operation and stable combustion after the combustion process has intentionally been disturbed or perturbed.
All techniques are intended to introduce shock waves into the combustion chamber or to otherwise perturb the combustion process, affording opportunity for measuring recovery time for a predetermined overpressure disturbance, assuming stable combustion resumes. Important to the magnitude and mode of the instability are the type of explosive charge selected, the size of the charge, the location and direction of the charge, and the duration of the exciting pulse. The bottom curve in Fig. 9-2 characterizes the recover of stable operation after a combustion chamber was "bombed." The time interval to recover and the magnitude of explosive or perturbation pressure are then used to rate the resistance of the engine to instability.
The nondirectional bomb method and the explosive pulse-gun method are the two techniques in common use. The bomb that can be used in large flight-weight thrust chambers without modification consists of six 250 grains of explosive powder (PETN,RDX,etc.) encased in a Teflon, nylon, or micarta case. Detonation of the bomb is achieved either electrically or thermally. Although the pulse gun requires modification of a combustion chamber, this technique affords directional control, which is important to tangential modes of high-frequency instability and allows several data points to be observed in a single test run by installing several pulse guns on one combustion chamber. Charges most frequently used are 10, 15, 20, 40, and 80 grains of pistol powder. Pulse guns can be fired in sequence, introducing successive pressure perturbations (approximately 150 msec apart), each of increasing intensity, into the combustion chamber.
The control of instabilities is an important task during the design and development of a rocket engine. The designer usually relies on prior experience with similar engines and tests on new experimental engines. He also has available analytical tools with which to simulate and evaluate the combustion process. The design selection has to be proven in actual experiments to be free of instabilities over a wide range of transient and steady-state operating conditions. Some of the experiments can be accomplished on a subscale rocket thrust chamber that has a similar injector, but most tests have to be done on a full-scale engine.
The design features to control instabilities are different for the three types described in Table 9-2. Chugging is usually avoided if there is no resonance in the propellant feed system and its coupling with the elastic vehicle structure. Increased injection pressure drop and the addition of artificial damping devices in the propellant feed lines have been used successfully. Chugging and acoustical instabilities sometimes relate to the natural frequency of a particular feed system component that is free to oscillate, such as a loop of piping that can vibrate or a bellows whose oscillations cause a pumping effect.
With the choice of the propellant combination usually fixed early in the planning of a new engine, the designer can alter combustion feedback (depressing the driving mechanism) by altering injector details, (such as changing the injector hole pattern, hole sizes or by increasing the injection pressure drop), or alternatively by increasing acoustical damping within the combustion chamber. Of the two methods, the second has been favored in recent years because it is very effective, it is better understood, and theory fits. This leads to the application of injector face baffles, discrete acoustic energy absorption cavities, and combustion chamber liners or changes in injector design, often by using a trial and error approach.
Injector face baffles (see Fig. 9-6) were a widely accepted design practice in the 1960s for overcoming or preventing high-frequency instability. Baffle design is predicated on the assumption that the most severe instability, oscillations, along witht he driving source, are located in or near the injector-atomi-zation zone at the injector end of the combustion chamber. The baffles minimize influential coupling and amplification of gas dynamic forces within the chamber. Obviously, baffles must be strong, have excellent resistance to combustion temperatures (they are usually cooled by propellant), and must protrude into the chamber enough to be effective, yet not so far as to act like an individual combustion chamber with its own acoustical characteristics. The number of baffle compartments is always odd. An even number of compartments enhances the standing modes of instability, with the baffles acting as nodal lines separating regions of relatively high and low pressure. The design and development of baffles remains highly empirical. Generally, baffles are designed to minimize acoustical frequencies below 4000 Hz, since experience has shown damaging instability is rare at frequencies above 4000 Hz.
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