SRB-A

SRB-A / SRB-3

Closeup of the SRB-A attached to the H-IIA rocket which launched the Global Precipitation Measurement spacecraft
Manufacturer	IHI Corporation
Country of origin	Japan
Used on	SRB-A: H-IIA, H-IIB, Epsilon SRB-3: H3, Epsilon S
SRB-A
Height	14.6 m (47 ft 11 in)
Diameter	2.5 m (8 ft 2 in)
Gross mass	75.5 t (166,449 lb)
Propellant mass	66.8 t (147,269 lb)
Maximum thrust	2,300 kN (517,061 lbf)
Specific impulse	283.6 s (2.781 km/s)
Burn time	110 seconds
SRB-3
Height	15 m (49 ft)1
Diameter	2.5 m (8 ft 2 in)1
Gross mass	76.2 t (168,000 lb)1
Propellant mass	67.2 t (148,000 lb)1
Maximum thrust	2,300 kN (520,000 lbf)1
Specific impulse	283.6 s (2.781 km/s)2
Burn time	110 seconds1

The SRB-A and SRB-3 are a family of Japanese solid-fueled rocket boosters developed and manufactured by IHI Corporation for use on the H-IIA, H-IIB, Epsilon, H3, and Epsilon S launch vehicles. The earlier versions, designated SRB-A, flew from 2001 to 2025, while the successor SRB-3 made its first flight in 2023.

All SRB variants have a composite motor case constructed from carbon-fiber-reinforced polymer materials, with thrust vector control provided by electrically actuated nozzles. IHI led development and production, with contributions from other Japanese manufacturers: the composite propellant (BP-208) was developed by NOF Corporation, and the T1000GB carbon fiber used in the motor case was developed by Toray.³

The SRB-A motor case incorporated foreign technology licensed from Alliant Techsystems (ATK), based on the Castor 120 motor used in the LGM-118 Peacekeeper ICBM.⁴ By contrast, the SRB-3 uses a fully domestic motor case design developed in Japan.⁵

The SRB-A was developed for the H-IIA rocket and first flew in 2001. It was later used on the larger H-IIB and as the first stage of the smaller Epsilon rocket. It measured 2.5 metres (8 ft 2 in) in diameter and 15.1 metres (50 ft) in length, with a filament-wound composite casing. It replaced the SRBs used on the earlier H-II rocket.⁶

• SRB-A — The initial version, used on the first six H-IIA launches. A separation failure on the sixth launch in 2003 led to loss of mission.⁶⁷
• SRB-A2 — A planned upgrade canceled after the 2003 failure; its design changes were merged into the SRB-A Improved.
• SRB-A Improved — Incorporated a redesigned bell-shaped nozzle to reduce erosion, with slightly lower thrust and longer burn time. Used on the seventh through thirteenth H-IIA flights, but nozzle erosion persisted.
• SRB-A3 — Final SRB-A version, with further improvements to reliability and performance. Available in a high-thrust or long-burn variant. Used on all H-IIA launches after the thirteenth flight, as well as on the H-IIB and as the first stage of the Epsilon rocket.⁶⁸⁹

The SRB-A series was retired in 2025 and replaced by the SRB-3.

The SRB-3 is the current generation of Japanese solid rocket boosters, developed for the H3 and Epsilon S launch vehicles. Compared to the SRB-A, it carries 1 tonne (2,200 lb) more propellant, uses a fixed nozzle, and incorporates a simplified separation system to reduce cost and increase reliability.⁵¹⁰

Unlike the SRB-A series, which required different burn patterns depending on configuration and vehicle type, the SRB-3 uses a unified burn pattern whether flown in pairs or quartets on the H3, or as the first stage of the Epsilon S.¹⁰