SRB-A is 2.5 meters in diameter, and 15.1 meters in length. Its casing is a Carbon-fiber-reinforced polymer filament wound composite. Two-axis attitude control is provided by electrically-actuated thrust vectoring. The composite motor case design is based on technology used on the Castor 120 motor by Alliant Techsystems.
The original SRB-A was developed for the H-IIA rocket, and was used on its first 6 flights. It was derived from the SRB used on H-II. During the sixth launch of an H-IIA, one of the boosters failed to separate due to a leak of hot gasses eroding the detachment points, causing the rocket to fail to reach orbit.
SRB-A2 was a planned upgrade, intended to replace SRB-A on H-IIA. Following the 2003 failure, it was cancelled and its design improvements were merged into the SRB-A Improved.
An improved version of SRB-A was developed following the 2003 incident. The nozzle was changed from a conical to a bell shape, to reduce thermal loading and erosion. Its thrust was also reduced slightly, and its burn time lengthened, to further decrease heating. This version was flown on the seventh through the thirteenth H-IIA. However, the nozzle erosion problem was still not fully solved, leading to the development of the SRB-A3
SRB-A3 is the current version, redesigned to provide higher performance and improve reliability. It is available in two variants, one producing high thrust at a short duration burn, and the other with a longer duration lower thrust burn. It has been used on all H-IIA flights past the thirteenth mission, as well as the H-IIB and as the first stage of Epsilon.