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)
Diameter	2.5 m (8 ft 2 in)
Gross mass	76.2 t (168,000 lb)
Propellant mass	67.2 t (148,000 lb)
Maximum thrust	2,300 kN (520,000 lbf)
Specific impulse	283.6 s (2.781 km/s)
Burn time	110 seconds

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.

Design

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.

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. By contrast, the SRB-3 uses a fully domestic motor case design developed in Japan.

SRB-A series

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.

Variants

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.
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.

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

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.

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.

References

1 2 3 4 5 6 "Rocket Launch Plan Quasi-Zenith Satellite No. 7 (QZS-7) / H3 Rocket No. 9 (H3-F9)" (PDF). Japan Aerospace Exploration Agency (JAXA). June 2026. p. 11.
↑ "What is SRB-3?". JAXA. Retrieved 2025-09-18.
↑ "JAXA、H-IIB ロケットで宇宙ステーション補給機７号機(HTV7 「こうのとり」)を打上げ". Tokyo Express. 28 September 2018. Archived from the original on 2 October 2022. Retrieved 18 February 2023.
↑ "ATK Composite Technology Supports Maiden Flight of Japan's H-IIA Space Launch Vehicle". ATK. August 29, 2001. Archived from the original on 2013-12-26.
1 2 "宇宙に吼えろ! 新型固体ロケットブースター「SRB-3」燃焼試験取材第2回カギは国産化と簡素化 - 先代から大きく進化を遂げた「SRB-3」". マイナビニュース. 7 September 2018. Archived from the original on 17 February 2023. Retrieved 17 February 2023.
1 2 3 "SRB-A Engine Overview". JAXA. Archived from the original on November 4, 2015. Retrieved 14 February 2016.
↑ Stephen Clark; Justin Ray (29 November 2003). "Japanese launch fails". Spaceflight Now. Retrieved 14 February 2016.
↑ "H-IIB Launch Vehicle" (PDF). JAXA. Archived from the original (PDF) on March 26, 2014. Retrieved 14 February 2016.
↑ "Epsilon Launch Vehicle" (PDF). JAXA. Retrieved 14 February 2016.
1 2 "H3プロジェクト前進へ" (PDF). 宇宙航空の最新情報マガジン JAXA's. 74. JAXA: 10. 1 October 2018. Archived from the original (PDF) on 18 March 2022. Retrieved 17 February 2023.

[F9_Launch_Plan-1] 1 2 3 4 5 6 "Rocket Launch Plan Quasi-Zenith Satellite No. 7 (QZS-7) / H3 Rocket No. 9 (H3-F9)" (PDF). Japan Aerospace Exploration Agency (JAXA). June 2026. p. 11.

[SRB-3-2] "What is SRB-3?". JAXA. Retrieved 2025-09-18.

[tokyoex-3] "JAXA、H-IIB ロケットで宇宙ステーション補給機７号機(HTV7 「こうのとり」)を打上げ". Tokyo Express. 28 September 2018. Archived from the original on 2 October 2022. Retrieved 18 February 2023.

[4] "ATK Composite Technology Supports Maiden Flight of Japan's H-IIA Space Launch Vehicle". ATK. August 29, 2001. Archived from the original on 2013-12-26.

[mynavi_srb3-5] 1 2 "宇宙に吼えろ! 新型固体ロケットブースター「SRB-3」燃焼試験取材第2回カギは国産化と簡素化 - 先代から大きく進化を遂げた「SRB-3」". マイナビニュース. 7 September 2018. Archived from the original on 17 February 2023. Retrieved 17 February 2023.

[SRBAOverview-6] 1 2 3 "SRB-A Engine Overview". JAXA. Archived from the original on November 4, 2015. Retrieved 14 February 2016.

[7] Stephen Clark; Justin Ray (29 November 2003). "Japanese launch fails". Spaceflight Now. Retrieved 14 February 2016.

[8] "H-IIB Launch Vehicle" (PDF). JAXA. Archived from the original (PDF) on March 26, 2014. Retrieved 14 February 2016.

[9] "Epsilon Launch Vehicle" (PDF). JAXA. Retrieved 14 February 2016.

[jaxas_no74-10] 1 2 "H3プロジェクト前進へ" (PDF). 宇宙航空の最新情報マガジン JAXA's. 74. JAXA: 10. 1 October 2018. Archived from the original (PDF) on 18 March 2022. Retrieved 17 February 2023.