The fuel turbopump had an issue using the originally designed inducer where the inducer would itself begin to cavitate and cause an imbalance resulting in excessive vibration. A comprehensive post-flight analysis of the unsuccessful 8th H-II launch, including a deep ocean retrieval of the wreckage, determined that fatigue due to this vibration was the cause of premature engine failure.
LE-7A
The LE-7A is an upgraded model from the LE-7 rocket engine. Basic design is unchanged from the original model. The 7A had additional engineering effort placed on cost cutting, reliability, and performance developments. The renovation was undertaken to mate it with the likewise improved H-IIAlaunch vehicle, with the common goal being a more reliable, more powerful and flexible, and more cost effective launch system.
Changes / improvements
Specific emphasis was placed on reducing or the amount of required welding by allowing for more machined or cast components, and to simplify as many of the remaining welds as possible. This resulted in a substantial rework of the pipe routing. To combat the fuel inducer complications described above, the fuel inducer was redesigned for the 7A. The oxidizer inducer was also redesigned, but this was primarily due to poor performance at low inlet pressures as opposed to reliability concerns. The fuel turbopump itself was also the subject of various durability enhancements. Additionally the combustion chamber/injector assembly underwent a number of small changes, like decreasing the number of injector elements, to reduce machining complexity and improve reliability. While these changes overall resulted in a drop in maximum specific impulse to , the trade off for lower cost and enhanced reliability was considered acceptable.
For the new engine model, a nozzle extension was designed that could be added to the base of the new standard “short” nozzle when extra performance was required. But when the engine was fitted with the nozzle extension, the 7A encountered a new problem with unprecedented side-loads and irregular heating on the nozzle strong enough to damage the gimbalactuators and regenerative cooling tubes during startup. Meticulous computational fluid dynamics work was able to sufficiently replicate and trace the dangerous transient loading and a new one-piece “long” nozzle with full regenerative cooling was designed to mitigate the problem. Before this new nozzle was ready, some H-IIA's were launched using only the short nozzle. The 7A no longer uses a separate nozzle extension in any configuration.
