Under the geostationary communications satellite programme (Project 331), the Shanghai-based 8th Academy (SAST) and the Beijing-based 1st Academy (CALT) each submitted their proposal for the launch vehicle. With CALT’s Long March 3 design becoming successful in 1984, the SAST design became redundant but not abandoned. Instead, it was revived a few years later as the Long March 4A under the meteorological satellite programme (Project 771).
Long March 4A
The Long March 4A was based on the two-stage Long March 2, by adding it with a third-stage burning the conventional N2O4/UDMH bi-propellant. The first-stage of the vehicle was stretched by 4 m, allowing it to carry an additional 40 tonnes of propellants. The stage was powered by an improved YF-21B engine, which gave a maximum thrust of 2,942 kN.
The third-stage of the launch vehicle was powered by an YF-40 engine, which consisted of two fully swivelling chamber motors and carried a total of 11.3 tonne of bi-propellant. Other improvements included a computerised guidance system, and an onboard propellant management system.
The development of the Long March 4A began in 1985 and the launch vehicle made its first launch successfully from Taiyuan Satellite Launch Centre on 7 September 1988, placing the meteorology satellite Fengyun 1A into orbit. Two years later, on 3 September 1990, the launch vehicle made its second mission to loft Fengyun 1B satellite and two Daqi 1 balloon satellites.
On 4 October 1990, the third-stage of the Long March 4A launch vehicle for the Fengyun 1B mission exploded on the 895 km orbit, creating more than 80 pieces of trackable space debris. This incident led to a redesign of the subsequent models in the launch vehicle family.
Long March 4B
The Long March 4B was introduced in the late 1990s for the Ziyuan 1 (CBERS 1) mission. The 4B model was generally similar to the 4A model. The most significant modification was a newly-designed payload fairing 8.48 m in length and 3.35m in diameter to house the large remote-sensing satellite.
Other improvements on the launch vehicle included:
- Increased payload capacity;
- Replacing the original mechanical-electrical flight control with a digital electronic control;
- Improved telemetry, tracking & control system and self-destruction mechanism, with smaller size and reduced weight;
- A revised nuzzle design in the second-stage for better high-altitude performance;
- A propellant management system on the second-stage of the rocket to reduce the spare propellant requirement, thus increasing the vehicle’s payload capability;
The Long March 4B made its maiden flight on 10 May 1999, successfully placing Fengyun 1C and Shijian 5 into orbit.
Following the third-stage explosion in 1990, SAST revealed that modifications were to be made to the launch vehicle’s third-stage including a residual propellant venting system to prevent similar incidents from occurring again. However, this system was featured on the early variant of the Long March 4B, due to concerns that it may affect the safety of the payload.
Then another third-stage explosion occurred on a Long March 4B on 11 May 2000, after it had lofted CBERS-1 on 14 October 1999, producing over 300 pieces of trackable space debris on a 735 km orbit. The residual propellant venting system was swiftly introduced on all subsequent Long March 4B missions. No further explosion has occurred since then.
On 9 December 2013, a Long March 4B failed to place CBERS-3 into orbit. During powered flight of the third-stage, one of its two engines shut down prematurely and as a result the payload failed to orbit. The cause was traced to foreign debris that blocked the fuel intake of the third-stage engine.
Long March 4C
The further improved 4C model was introduced in 2006 for the Yaogan 1 mission. The launch vehicle featured an improved third-stage with multiple re-start capability. While the 4B model remained as the primary launch vehicle for Sun-Synchronised Orbit (SSO) missions, the 4C model was used primarily for lofting heavier satellites, or multiple satellites on a single vehicle.
Improvements on the Long March 4C included:
- An improved third-stage powered by an YF-40A engine with multiple restart capability;
- A propellant management system on the third-stage;
- A remotely-operated automated launch control system that integrated various functions previously carried out separately, including launch control, system testing, data transmission, telemetry, and power supply;
- A new flight computer with better calculation performance and a smaller size power supply;
- A new guidance system with GPS input;
The Long March 4C had adopted a different launch checkout procedure to its predecessors. Instead of being tested in a horizontal position before being erected on the launch pad, the vehicle could be assembled and tested vertically on the launch pad at the same time, reducing the launch preparation time by a third.
The launch vehicle made its debut flight on 26 April 2006 from Taiyuan Satellite Launch Centre, lofting the Yaogan 1 reconnaissance satellite into orbit.
On 5 March 2010, a Long March 4C carrying the Yaogan 9 reconnaissance satellite was launched from Jiuquan Satellite Launch Centre. This was the first ever mission of the Long March 4 series from Jiuquan.