Core stage: one or two sections?

On 30 July 1976 Dmitriy Ustinov officially appointed GUKOS as the military organization in charge of the program. Ustinov had been promoted to Minister of Defense in April 1976, but continued to serve as Central Committee Secretary for Defense Matters until October 1976, when he was replaced in that capacity by Yakov Ryabov. In co-operation with NPO Energiya, GUKOS worked out updated tech­nical requirements for the Soviet shuttle system throughout the year. These were approved by Ustinov himself on 8 November 1976, reportedly the first time this had been done at such a high level and again underscoring the military objectives of the Soviet shuttle program. Nothing changed to the basic requirement mentioned in the February 1976 decree, namely the capability to place 30-ton payloads into 200 km orbits inclined 51.6° to the equator and return 20-ton payloads back to Earth. Added to the requirements was a payload capacity of at least 16 tons for missions into 97° inclination orbits, roughly matching the payload capacity of the Space Shuttle from Vandenberg. The orbiter was supposed to be able to fly a total of 100 missions, another requirement identical to that for the US orbiter. The strap-on boosters were expected to fly at least ten times each, creating an inventory of boosters that would help the system achieve a wildly ambitious minimum turnaround time of 20 days [68].

On 12 December 1976 Glushko placed his signature under the “draft plan” for the Soviet shuttle system, but he did not have the final say. The draft plan was reviewed by an Interdepartmental Expert Commission chaired by TsNIIMash director Yuriy Mozzhorin. Even at that point, almost a year after the February 1976 decree, there was no consensus on the need to build a heavy-lift shuttle. Several members of the commission spoke out in favor of Chelomey’s 20-ton spaceplane, which could solve practical tasks like servicing space stations. Others called for the development of both a small and a large shuttle. In the end though, the commission’s recommendation was to press ahead with NPO Energiya’s big orbiter, mainly in order to have a deterrent to the US system in the long run [69].

The findings of Mozzhorin’s commission were discussed at a joint meeting of the Ministries of Defense, the Aviation Industry, and General Machine Building in March 1977, which recommended making some amendments to the draft plan. These were approved by the Council of Chief Designers in July 1977 and called for some significant changes to the core stage. In the original design the core stage was a single element with one LOX and one LH2 tank and a diameter of 8.2m (comparable with the Space Shuttle External Tank’s 8.4m diameter). Now its diameter was reduced to 7.7 m and it was lengthened by 7.9 m. More importantly, the core stage was now to consist of two separate sections, each having its own LOX and LH2 tanks (four tanks in all). This was mainly dictated by the fact that the carrier aircraft being studied at the time (a modified Myasishchev bomber) could not transport the stage in one piece. It was also supposed to improve the stability of the rocket by keeping its center of gravity as high up as possible. During the initial stages of ascent the engines would consume the propellants in the lower section and as the tanks emptied they would gradually be refilled with propellants from the upper section through a special cross­feed system. Once the upper section ran out of propellant, it would be jettisoned, as a result of which the rocket shed a significant amount of dead weight during the final phase of the launch [70].

The amendments formed the basis for a new government/party decree (nr. 1006­323) on 21 November 1977, which gave the go-ahead for the next step in the design phase, namely the completion of the so-called “technical plan” in the first quarter of 1978. This was to be followed by the release of “design documentation” for the rocket in 1978 and for the orbiter in 1980, usually the last step before the construction of actual flight hardware begins. The first flight remained optimistically targeted for 1983.

In 1978 it was decided to return the core stage to its original configuration, although it retained the 7.7 m diameter and was now even a bit shorter than the version originally proposed in 1976. Disadvantages of the dual-element design had been the need to develop a complex propellant cross-feed system and the requirement to find safe impact zones for the upper section, which imposed further restrictions on the rocket’s possible trajectories. However, the return to the single-element design did not really solve the transportation problem. The core stage still had to be flown to the cosmodrome in two sections, with the LH2 and LOX tanks being ferried separately before being joined together at the launch site. The later An-225 Mriya carrier aircraft was capable of carrying the core stage (and the strap-ons) in one piece, although it was never used in that capacity. The final amendments to the “technical plan’’ were completed in June 1979, which can be considered the date that the design of the Energiya-Buran system was frozen [71].