The Dong Feng-4 (DF-4, or CSS-3 in its U.S. DoD designation) is a two-stage, liquid-propellant, single-warhead intermediate-range ballistic missile (IRBM), which has also been developed into a space launch vehicle under the designation Long March 1.
Following the successful development of the DF-3 (CSS-2) medium-range ballistic missile, the Seventh Ministry of Machinery Industry (Ministry of Astronautics) began to study the feasibility of a more advanced multi-stage missile with longer range. A two-stage rocket designated SDF-4 was proposed as the testbed for demonstrating the relevant technologies. The rocket used the single-stage DF-3 as its first-stage, added with a newly-developed second-stage.
By 1964, the conflicts in Vietnam had escalated into a full-scale war and the Republic of China (ROC) forces in Taiwan were actively planning for an invasion on the China mainland. Threatened by the worsening international situation, the Chinese political leadership ordered to speed up the ballistic missile development. As a result, the SDF-4 concept was turned from a technology demonstrator into an operational missile designated DF-4, with a maximum range of 4,000 km, capable of hitting the U.S. bases in Guam.
The first-stage of the DF-4 was based on the DF-3, powered by a cluster of four parallel YF-1 chamber motors burning a bi-propellant with unsymmetrical dimethylhydrazine (UDMH) as fuel and red fuming nitric acid (HNO3) as oxidiser. A more advanced design that burns the HDMH/N2H4 bi-propellant, designated DF-4A, was proposed in 1966 but was not adopted. The missile was fitted with a cascade compensation inertial guidance package system derived from a similar system used by the DF-3. The three-megaton thermonuclear warhead, codenamed “512”, was also based on the design of the DF-3’s warhead.
The key challenge in designing a two-stage rocket was ensuring that the vehicle’s flight was not interfered by the staging process, which consisted of the separation of the two stages, the jettison of the first-stage that runs out of propellant, and the ignition of the second-stage. The DF-4 adopted a ‘hot’ staging method, where the second-stage ignites while the two stages are still connected, and the first-stage is jettisoned by the hot exhaust gas from the second-stage engine.
This staging method eliminated the need for a separate jettisoning mechanism, which helped reduce the structural weight of the rocket. It also reduced the duration of powerless flight during the staging process, allowing better flight control of the vehicle. To prevent the high-pressure and hot stream from the second-stage engine exhaust from damaging the empty propellant tank of the first-stage during staging, which may ignite any residual propellant inside the tank, a fibreglass heat insulation layer was added on the top of the propellant tank on the first-stage.
A second technical challenge was the development of the YF-3 engine used by the second-stage. The increase in the missile’s range meant that the vehicle’s flight trajectory (>60 km) extends beyond Earth’s upper atmosphere, which meant that the second-stage engine was required to be able to ignite in vacuum. Other technological challenges included protecting the warhead from the re-entry heat, and a guidance system with improved accuracy to compensate the deviation resulted by the extended range.
As the existing “Project 150” optical ground tracking system could not meet the requirements for the flight test of long-range missiles, a new “Project 154” missile telemetry and tracking system was introduced at the Jiuquan missile test centre in 1969. A new missile launch site was constructed in the northeast part of the country to allow west-borne full-range flight tests to the target impact zone in Xinjiang.
Following two unsuccessful attempts in late 1969, the first successful DF-4 test flight using a depressed trajectory (DT) was conducted from the Jiuquan missile base on 30 January 1970. The two stages of the vehicle separated and the second-stage ignited successfully as scheduled, and the warhead hit the target zone with high accuracy. Three months later, on 24 April, a modified DF-4 added with a third-stage, designated Long March 1, successfully placed China’s first artificial satellite Dong Fang Hong 1 into Earth orbit.
In the 1970s, the changing security situation required the maximum range of the DF-4 to be extended to 4,700 km in order to reach targets in Europe Russia when launched from western China. A modified engine with increased thrust and prolonged burn time was developed. The engine passed its first 280-second ground burning test in July 1970, though the missile development was completed until 1976, due to the disruptions of the ‘Culture Revolution’. The improved missile was tested in two depressed trajectory tests in 1976 and five full-range flight tests in 1977 and 1978.
The design of the DF-4 was finalised between 1980 and 1983, and the missile was certified in June 1983. However, the modified “512” warhead was not completed until August 1988. The whole development programme took 18 years to complete, due to changing requirements and political interruptions. The missile achieved the initial operational capability (IOC) in the late 1980s. The first operational fire of the missile carrying a dummy warhead was conducted in 1986.
The DF-4 was China’s first underground-silo-deployed missile, stored vertically in underground silos and raised to surface level before firing, much like the early U.S. Titans and Atlases. The missile was fuelled inside the silos up to 15 days prior to firing. This configuration was first tested on 9 February 1980. A cave-deployed version was later developed, with the missile stored horizontally in a tunnel, and rolled out to a pre-surveyed launch spot immediately outside the tunnel mouth, where it was erected, fuelled, and fired.
Stages:....................2 Overall length (m):........29 Wingspan (m):..............N/A Core stage diameter (m):...2.25 Take-off mass (kg):........82,000 Maximum range (km):........4,700 Payload:...................Single 3 MT thermal nuclear Guidance:..................Inertial First-Stage Length (m):................N/A Diameter (m):..............2.25 Dry mass (kg):.............N/A Propellant mass (kg):......N/A Engine.....................YF-2 (4 x YF-1) Propellant:................Liquid (UDMH/HNO3) Thrust (kN):...............1,020 Second-Stage Length (m):................N/A Diameter (m):..............2.25 Dry mass (kg):.............N/A Propellant mass (kg):......N/A Engine.....................YF-3 Propellant:................Liquid (UDMH/HNO3) Thrust (kN):...............320