Cooling Methods Regenerative and Dump Cooling

Baffles usually are cooled with the rocket fuel, although oxidizer and bipropellant cooling occasionally have been used. A typical design, showing coolant passages used on the F-! qualification injector, is presented in figure 15.

Coolant flowrate through regenerative or dump-cooled baffles is established to satisfy two requirements:

(1) The total heat absorbed by the coolant must be equal to the integrated gas-side heat flux, and if the coolant is liquid this heat must be absorbed without producing net vaporization of any of the coolant. In addition, if the coolant temperature is allowed to exceed its decomposition temperature, deposits (e.g.. carbon deposits from kerosene) may clog the coolant passages. Both vaporization and decomposition of coolant occurred during development of the F-l engine.

(2) At the same time, the liquid velocity in the coolant passages must be kept sufficiently high to prevent the peak heat flux to the coolant, at any local point where the gas-side heat transfer is highest, from exceeding the nucleate-boiling heat flux. Calculations of allowable nucleate-boiling heat fluxes follow procedures outlined in references 3() and 40.

Fully regenerative cooling with baffle coolant flow re-entering the main propellant flow and being injected by the main injector is not often used; one successful example is the Titan Transtage engine (ref. 35). This cooling method was tried during the development of the F-l engine as an aid to performance and stability; however, limited testing showed no effect on cither. Because of its complexity, the system was dropped in favor of fuel dump cooling.

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OUTER RING BAFFLE

INNER RADIAL. BAFFLE (OUTER RADIAL SIMILAR IN DESIGN)

INNER RING BAFFLE

OUTER RING BAFFLE

INNER RING BAFFLE

INJECTOR ^

Baffles Injectors Liquid Rocket Engine

INJECTOR ^

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Figure 15. - Coolant passages in baffle on F-1 engine injector.

Most cooling methods involve axial (or more complicated) flow of propellants through the baffle assembly, the coolant flow being injected from the downstream tip of the baffles into the combustion space. A baffle assembly of this type is illustrated in figure 16. Several

Liquid Rocket Engine Regenerative
Figure 16. - Bolton through flow copprr baffle assembly.

techniques have been used to promote efficient combustion of baffle coolant thus injected: small-diameter closely spaced injection orifices to effect uniform distribution: impinging stream orientations to hasten atomi/ation and aid in distribution; moderately high injection velocities to produce fine sprays; and spatial distribution of mixture ratios (produced by the main propellant injector) biased near the baffles to offset the tendency toward persistence of fuel-rich streaks downstream of the cooled baffles.

Although dumping at the baffle tip of the propellant used for baffle cooling initially was thought to influence combustion stability, detailed investigation showed that this assumption was untrue. In fact, baffle configurations with no propellant tip injection, single-propellant tip injection, and bipropellant tip injection - all exhibited the same stability characteristics. An interesting example of the result of excessive heating of the dump coolant is shown in figure 17. The enlargement of the passage was attributed to

PASSAGE ENLARGED BY EROS I ON -

^COOLANT PASSAGE

SECTION THROUGH BAFFLE ORIFICE

Figure 17. — Coolant-passage enlargement caused by erosive cavitation of heated coolant.

erosive cavitation. An increase of pitting along the port is characteristic of cavitation, and is probably caused by coolant exit temperature near the boiling temperature. The condition depicted in figure 17 occurs after extensive testing on all baffles exposed to high temperature.

Occasionally, difficulties in fabrication and assembly of the hardware involved in dump cooling may introduce problems. For example, the Bell LMAE incorporated bipropellant (N204/50:50) dump-cooled baffles. These aluminum baffles had relatively complicated cooling manifolds machined into them. Because of the complex passages, it was necessary to machine from the side of the baffle and subsequently cover these passages with a cover plate. The cover plate was in turn electron-beam welded in place. At one time in the development, an oxidizer leak through a faulty weld apparently triggered an instability. In subsequent tests with leaks intentionally drilled into the oxidizer passages, the engine did indeed run unstably.

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