H3 (rocket)

MHI was responsible for overall development of the H3 and oversees final assembly of the launch vehicle and its liquid-fuel engines. Other major Japanese contributors include IHI Corporation, which produces the liquid-fuel engine turbopumps and solid-fuel boosters; Kawasaki Heavy Industries, which builds the S- and L-type payload fairings; and Toray Industries, which supplies the carbon fiber and synthetic resin used in the booster motor cases and fairings.^[12][13][14] Switzerland-based Beyond Gravity manufactures the W-type fairing, based on its standard 5.4-metre-wide (18 ft) design also used on Ariane 6 and Vulcan Centaur.^[15]

Detailed development of the H3 began in 2014 following government approval of the program on 17 May 2013,^[16] with MHI serving as the prime contractor and developing the propulsion system jointly with JAXA.^[3] The H3 was designed to serve government and commercial launch markets. Compared with the H-IIA, the H3 was designed with simpler and less expensive engines to reduce manufacturing time, technical risk and overall cost. JAXA and MHI were responsible for preliminary design work, development of new technologies, manufacturing and preparation of ground facilities.^[17]

As of 2015, the first H3 launch was planned for Japanese fiscal year (JFY) 2020 in the H3‑30 configuration, which lacks solid rocket boosters, followed by a booster-equipped version in JFY21.^[a][4]

The newly developed LE-9 engine was the key to cost reduction, while increasing thrust and improving safety margins. The engine employs an expander bleed cycle, a combustion method previously used on the upper-stage LE-5A engine, and never before used on a first-stage.^[18] While such cycles typically cannot produce high thrust, the LE-9 was designed to reach 1,471 kN (331,000 lb_f), making its development one of the most significant challenges of the program.^[19]

Ground tests of the LE-9 began in April 2017,^[20] and the first solid rocket booster tests were conducted in August 2018.^[21]

During qualification tests of the LE-9 in May 2020, engineers discovered an opening in the combustion chamber wall and a fatigue fracture in the liquid-hydrogen fuel turbopump turbine. Resolving these issues delayed the inaugural H3 launch from its planned 2021 schedule to 2023.^[3][22]

The first launch attempt on 17 February 2023 was aborted shortly before ignition of the SRB-3 boosters, although the main engines had ignited successfully.^[23][24][25] The second launch attempt occurred on 7 March 2023 at 01:37:55 UTC. Approximately five minutes and twenty-seven seconds after launch, the second-stage engine failed to ignite. Because the vehicle could not attain the required velocity, JAXA issued a flight termination command 14 minutes and 50 seconds after launch, destroying both the launch vehicle and the ALOS-3 satellite.^{[26][27][28][29]}

After an investigation into the failure of the first test flight and implementation of corrective measures, the second test vehicle was launched on 17 February 2024.^[3] During the mission, the second stage reached its intended orbit, marking the first fully successful H3 flight.^[30]

The first flight of the H3-24 configuration occurred on 26 October 2025, carrying the HTV-X cargo spacecraft on its inaugural mission to the International Space Station.^[31][32]

A H3‑22S failed during flight on 22 December 2025. JAXA stated that it suspected the Payload Support Structure failed shortly after fairing separation, causing damage to the second-stage liquid hydrogen tank. The initial second-stage burn lasted 27 seconds longer than planned, consistent with abnormal tank pressurization, and the engine shut down one second after the start of the second burn, consistent with fuel depletion. Video footage showed a large object separating from the vehicle shortly after fairing separation, which investigators believed was the payload.^[33]

The H3 returned to flight on 12 June 2026 with the first launch of the H3-30 configuration, successfully placing a test payload and several rideshare payloads into orbit.^[34]

The H3 is a two-stage launch vehicle. The first stage uses two or three LE-9 engines fueled by 225 tonnes (496,000 lb) liquid oxygen and liquid hydrogen (hydrolox) propellants. The first-stage can be fitted with zero, two, or four strap-on SRB-3 solid rocket boosters (SRBs) derived from the SRB-A and fueled with polybutadiene. The second stage is powered by an upgraded LE-5B-3 engine and carries 23 tonnes (51,000 lb) of hydrolox propellant.^[35][11][36]

H3 has dual-launch capability, but MHI has said it is focused on dedicated launches in order to prioritize schedule assurance for customers.^[37]

H3 configurations are identified by a two-digit number and a letter. The first digit indicates the number of LE-9 engines on the core stage (two or three), while the second digit indicates the number of SRB-3 solid rocket boosters (zero, two, or four). The final letter specifies the payload fairing: short ("S"), long ("L"), or wide ("W"). For example, the H3‑24L has two LE-9 engines, four SRBs, and a long fairing, while the H3‑30S has three engines, no SRBs, and a short fairing.^[38][39]

As of 2026^[update], five configurations were offered: H3‑30S, H3‑22S, H3‑22L, H3‑24L, and H3‑24W.^[6]

The H3‑32, a proposed variant with three engines and two SRBs, was cancelled in late 2018 after tests showed that the H3‑22 offered better-than-expected performance, reducing the need for the more powerful version. JAXA cited commercial precedent, noting that SpaceX's Falcon 9 frequently launched satellites into a low geostationary transfer orbit (GTO), leaving the satellites to raise themselves to a geostationary orbit. Since commercial clients appeared willing to accept this trade-off, JAXA concluded that customers would prefer the less expensive H3‑22 even if it required additional onboard satellite propellant.^[38]

The H3‑30 configuration is designed as a low-cost variant, expected to be used primarily by government customers. It can carry a payload of up to 4,000 kg (8,800 lb) into sun-synchronous orbit (SSO) for about ¥5 billion (equivalent to ¥5.13 billion or US$33.92 million in 2024)^[2], about half the cost of the H-IIA and intended to be on par with SpaceX's Falcon 9.^[4][37]

The most powerful H3‑24 variant can deliver more than 6,000 kg (13,000 lb) of payload to trans-lunar injection (TLI) and 7,900 kg (17,400 lb) of payload to GTO.^[8]

MHI has stated that it plans to address growing demand for low-cost small-satellite launches by introducing rideshare missions capable of carrying multiple payloads on a single H3 launch.^[3]

As of October 2019, MHI was studying several upgrades for the H3, including an extended second stage and a heavy-lift variant.^[8] Proposed second-stage improvements included increasing propellant capacity and developing a new upper-stage engine.^[3]

The proposed heavy-lift configuration would consist of three H3 core stages operating in parallel, similar to the Delta IV Heavy and Falcon Heavy.^[40] The vehicle was projected to place up to 28,300 kilograms (62,400 lb) into low Earth orbit.^[8]