Rockets of Today

H — Japan, 1986


The original H-I was built by Mitsubishi as a derivative of their N series rockets, which were licensed copies of Delta. The first H retained the Delta-based first stage but used an original upper stage which burned hydrogen (hence the use of the letter H) and could re-ignite in orbit. The successor H-II was an all-Japanese design. It was replaced with the cheaper and more reliable H-IIA in 2001, and in 2007 it was privatized, so Mitsubishi has full ownership. The H-IIB arrived in 2009, featuring a fatter first stage with two engines. This has mainly been used for space station service. The A remains in use for lighter loads.

Japan had a lot of smaller rockets as they gradually built up their expertise, mostly named after letters of the Greek alphabet. Now those are all gone, except that they’ve gone back to that in naming the new Epsilon. One odd trait of these earlier rockets is that their lower stages were completely innocent of any form of active guidance. They were made this way so as to prove they would be useless as potential ICBMs. They had to be launched from a leaning tower to aim them toward orbit.

The core stage of the H-II burns hydrogen, and is augmented with either two or four stubby solid boosters, sometimes with even smaller booster-ettes tucked in between. The H-IIB always uses four, as its whole point is for heavier lifting than the A. The second stage also burns hydrogen. The original H-I upper stage engine used a conventional gas generator cycle, but for the II they switched to an expander bleed cycle — the first engine with that cycle to enter service. (Blue Origin is also planning to use that cycle for their BE-3U upper stage engine on the New Glenn.)

There were a couple of different plans to launch human beings on an H-IIA, with both capsule and spaceplane designs existing on paper, but none were built.

They are now working on an H-III. (They’re also working on saying “H3” instead of using roman numerals.) For this they’re replacing the dual LE-7A hydrogen engine with either two or three new engines called LE-9. The old engine used staged combustion, but for the new one they are building the world’s biggest expander-cycle engine, apparently on the belief that higher reliability matters more than higher specific impulse. Even with just two engines, this will increase the stage’s thrust considerably. The LE-5B expander on the second stage is getting a small upgrade. The III will have smaller solid boosters than the IIB used. Otherwise, not much else has changed, so the cost is not likely to be reduced all that much. As this neared readiness, the IIB was retired, while the IIA remains active.

H-IIB: mass 531 t, diam 5.2 m (10.3 at base), thrust 8146 kN, imp 2.8 km/s, type ZFh+S, payload 19 t (3.6%), cost $6M/t, record 57/0/3 through 2021.
Stage name Nissan L-178 L-17
Role (pos) count booster (S) ×4 core (1) upper (2)
Diameter (m)   2.50   5.20   4.00
Liftoff mass (t) 77   203    20  
Empty mass (t) 10.9 24.9  3.9
Fuel mass (t) ~20    ~26    ~2.7
Oxidizer mass (t) ~46    ~152     ~13.4 
Fuel type HTPB hydrogen hydrogen
Engine SRB-A3 LE-7A ×2 LE-5B-2
Power cycle solid staged (ZF) expander
Chamber pres. (bar) 127    38  
Ox./fuel ratio   2.3?   5.90   5.00
Thrust, vac max (kN) 2305     2196     137   
Thrust, SL initial (kN) 1615     1685    
Spec. imp, vac (km/s)   2.78   4.30   4.39
Total imp, vac (t·km/s) 734    770    74.2