The Long March 3B is the second variant of the Long March 3A family, designed to deliver a single heavy communications satellite or multiple satellites to GTO. With four strap-on liquid boosters, the launch vehicle’s payload capacity was increased to 5,200 kg (5,500 kg on later variants) to GTO, making it (at the time of its introduction) the most powerful space launch vehicle in China, and the second most capable in the world, only after the Russian Proton.
The 3B model was developed as the second phase of the Long March 3 upgrade programme. The focus of the development was to add the 3A vehicle with four strap-on boosters, each powered by a single DAFY5-1 liquid engine burning the N2O4/UDMH bi-propellant. Other onboard systems were largely unchanged. The development of the launch vehicle began in July 1989, with the first flight scheduled for 1996.
On 15 February 1996, a Long March 3B launch vehicle carrying a U.S. communications satellite Intelsat 708 was launched from Xichang Satellite Launch Centre. However, the vehicle began to veer off course only 2 seconds after lift-off, and completely lost control afterwards. The launch vehicle hit a hill 1.2 km away from the launch pad at T+22 seconds, destroying the US$125 million satellite onboard.
Chinese official media confirmed that the impact and violent explosion of the rocket killed six people and injured another 57 on the ground, and destroyed over 80 buildings in a nearby village, though some observers present at the launch site claimed the damage to be bigger.
The cause of the failure was traced to the launch vehicle’s guidance and control system. A gold-aluminium solder joint in the output of one of the gyro servo loops failed, cutting electrical current output from the power module and causing the inertial reference platform of the vehicle’s guidance and control system to slope. This led to the onboard computer sending the vehicle veering off the planned trajectory shortly after lift-off.
The Long March 3B resumed flight 18 months later, sending the Philippine communications satellite Agila 2 MABUHAY into orbit on 20 August 1997. Since then, the launch vehicle has provided a reliable service for lofting Chinese and foreign satellites to orbit.
The improved Long March 3B/E was introduced in May 2007 for the launch of the Nigerian communications satellite NIGCOMSAT 1. In order to carry additional propellant, the vehicle’s first-stage was stretched by 1.5 m, and the four strap-on boosters by 0.8 m, increasing its payload capacity by 300 kg.
The Long March 3C was the third variant in the Long March 3A family, developed in the late 1990s to fill the payload capacity gap between the 3A and 3B model. The launch vehicle is capable of delivering 3,700 kg payload into GTO. The 3C model is almost identical to the 3B model in design, only fitted with two strap-on boosters instead of four. A minor modification on the 3C model is the lack of the stabilising fins found at the bottom of core stage on the 3A and 3B.
Development of the Long March 3C began in March 1995, and the launch vehicle was almost ready by early 1992. However, following the Long March 3B disaster in February 1996, the 3C development was suspended so that CALT could focus on the improvement of the 3B model. The two satellites originally scheduled to be launched by the Long March 3C were launched on the Long March 3B instead. As a result, the debut of the 3C model was postponed until 2008.