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.


Until ISAS and NASDA were merged as JAXA in 2003, they divided roles to work in tandem for space development, with ISAS taking charge of scientific satellites and NASDA overseeing applications satellites.
As the launch vehicle number is determined when the development plan for the satellite is fixed, the actual order of launches can differ from the vehicle number. With the H-II series, for instance, vehicle No. 6 was launched before vehicle No. 5.
The next-generation principal rocket will be named something like H-III once the development plan has been fixed. For the purpose of this article, the provisional name H-X is used.
The contents of the article is at the time of announcement.


Minoru Otsuka

Minoru Otsuka is a technical writer specializing in such fields as personal computers, robotics, and space development. Otsuka started his career as a system engineer for an electric power company group, and became a freelance writer after a stint as a magazine editor. Recent published works in Japanese to which he contributed include The Science behind Satellites (Ark Publishing Inc.), Super technology behind Asteroid Explorer Hayabusa (Kodansha [Bluebacks series]), and Space Industries of Japan: Vol. 1 (Opening Up Space to Develop Industries), Vol. 2 (Using Space to Change Lifestyles), Vol. 3 (Cultivating Technologies to Train Workforces) (in Japanese, Nikkei BP Consulting). Otsuka is a member of Space Authors Club.
Twitter account: @ots_min