The design goal: a simple and non-explosive engine

The LE-7A engines are highly reliable and have never had a launch failure. But the two-stage combustion cycle requires very precise control because pressurized high-temperature combustion gases go through its piping. If the mixture ratio of the propellant fed into the sub-combustion chamber is even slightly off, the temperature could easily rise past the melting point of the chamber walls to cause an explosion.

In comparison, the expander bleed cycle does not expose any component to such high temperatures or high pressures, because the turbine is rotated by the fuel gases that are warmed in exchange for cooling the combustion chamber. This design is inherently safer because it makes the engine less prone to explode even in case of trouble. As the expander bleed is schematically the simplest of the four cycles, it could easily reduce costs, too. Whereas an LE-7A engine costs nearly 1 billion yen to build, the LE-X team hopes to cut the cost by half.

The setback is that an engine without a sub-combustion chamber does not usually deliver high thrust. That is why this type of engine has never been used for the large first-stage engine. The LE-X could change all that, as it has the potential of producing high thrust. This was made possible through the latest machining technology for carving fine grooves on the inside of the large combustion chamber, allowing the flow of fuel gases through the piping around the chamber to absorb heat more efficiently and turn the turbine powerfully with smaller amounts of exhaust gases.

The LE-X project is scheduled to be completed in fiscal year 2013, so tests of an actual size combustion chamber and a turbopump will be separately conducted in fall 2013. Although the combustion chamber and the turbopump will not be tested as a complete engine at this time, the separate tests will provide sufficient data for technology evaluation.

Safer engines are not only suitable for satellite carrying missions but also for crewed missions. Although Japan does not have plans to develop rockets for crewed flights as yet, it is important to be prepared for such an option in the future. The LE-X engine can be an important step in that direction, too, although there are other technologies that need to be developed for realizing crewed spaceflights.

Someday, hopefully in the not too distant future, a rocket carrying Japanese astronauts may be launched into space from the Tanegashima Space Center.