Epsilon (rocket)

E-X

The development aim is to reduce the US$70 million launch cost of a M-V;^[9] the Epsilon costs US$38 million per launch.^[10] Development expenditures by JAXA exceeded US$200 million.^[10]

To reduce the cost per launch the Epsilon uses the existing SRB-A3, a solid rocket booster on the H-IIA rocket, as its first stage. Existing M-V upper stages will be used for the second and third stages, with an optional fourth stage available for launches to higher orbits. The J-I rocket, which was developed during the 1990s but abandoned after just one launch, used a similar design concept, with an H-II booster and Mu-3S-II upper stages.^[11]

The Epsilon is expected to have a shorter launch preparation time than its predecessors;^[12][13][14] a function called "mobile launch control" greatly shortens the launch preparation time, and needs only eight people at the launch site,^[15] compared with 150 people for earlier systems.^[16]

The rocket has a mass of 91 t (90 long tons; 100 short tons) and is 24.4 m (80 ft) tall and 2.5 m (8 ft 2 in) in diameter.^[17][18]

Enhanced Epsilon

After the successful launch of the Epsilon first flight (demonstration flight), the improvement plan was decided to handle the planned payloads (ERG and ASNARO-2).^[19]

Requirements for the improvement:^[19]

• Apogee ≧ 28700 km (summer launch), ≧ 31100 km (winter launch) of a 365 kg payload
• Sun-synchronous orbit (500 km) of a ≧ 590 kg payload
• Larger fairing

Planned characteristics:^[19]

• Height: 26.0 m
• Diameter: 2.5 m
• Mass: 95.1 t (Standard) / 95.4 t (Optional 4th stage (Post Boost Stage))

Catalog performance according to IHI Aerospace:^[20]

• Low Earth orbit 250 km × 500 km for 1.5 t
• Sun-synchronous orbit 500 km × 500 km for 0.6 t

Final characteristics:^[8][21]

• Height: 26.0 m
• Diameter: 2.6 m (max), 2.5 m (fairing)
• Mass: 95.4 t (standard) / 95.7 t (optional)

Epsilon S Block1

The Epsilon rocket's first stage has a common design with the SRB-A3, the solid rocket booster of H-IIA/B. As the H-IIA family has retired in 2025, continuing the production of SRB-A3 solely for Epsilon would lead to a significant rise in production cost.^[22] Thus a new Epsilon variant, the Epsilon S Block1 which incorporates commonized parts with the H3, the H-IIA's successor, has been in development. The Epsilon S Block1's first stage will have a common design with SRB-3, the H3's solid rocket booster.

Major changes from Enhanced Epsilon to Epsilon S Block1 are:^[23]

• The first stage is based on SRB-3, the strap-on solid rocket booster of H3.
• The third stage designated the E-31 is a new design, whereas Enhanced Epsilon's third stage was based on the M-V's third stage. New third stage is three-axis stabilized, whereas Epsilon's third stage was spin-stabilized. Also the third stage is outside the fairing, whereas Enhanced Epsilon's fairing covered the third stage.
• The Epsilon S Block1's forth stage, the Post Boost Stage (PBS) is mandatory, whereas Enhanced Epsilon's PBS was optional.

Epsilon S Block1's second stage, the M-35a will have the same performance as Enhanced Epsilon's M-35, but with changes in propellant and insulation.^[2]

In June 2023, a ground test of the E-31 was conducted at JAXA's Noshiro Rocket Testing Center.^[24] Ground testing of the M-35a is scheduled to be conducted in 2026 at the Tanegashima Space Center.^[2]

Epsilon S

In order to further increase the payload capacity of Epsilon, the Epsilon S Block1's second stage M-35a is planned to be replaced with a larger solid-rocket motor designated E-21.^[2]

Initially, the E-21 was to be introduced simultaniously with the SRB-3 first stage and E-31 third stage. However, on 14 July 2023, a test firing of the E-21 failed.^[25] The root cause was determined to be the "melting and scattering of a metal part from the ignition device", which damaged the propellant and insulation.^[26] Corrective measures were implemented and the rocket motor was tested again on 26 November 26 2024; however, the second test also resulted in a failure 49 seconds after ignition.^[27][28][29]

In July 2025, NHK reported that JAXA was reviewing its development plans for the Epsilon S. In order to move up plans for a return to flight, the decison was made to defer planned improvements to the second stage and revert to using the proven, existing design of Enhanced Epsilon. While this would entail performance being lower than originally planned, it could enable a launch as early as late 2026.^[30]

Launch outcomes