Proton's first stage: this is all one thing!
The design of the first stage had forever defined Proton's unique architecture. No other launch vehicle before or after it had the same design and when this architecture started emerging from behind the Iron Curtain, it became a great source of mystery and confusion.
Designated 8S810KM, the first stage of the Proton-M rocket is comprised of a central (core) oxidizer tank, surrounded by six strap-on fuel tanks. Each of stage's six engines are attached to the one of the strap-on tanks, however the entire cluster of tanks remains connected during the flight. Sectioning of the first stage was dictated by the transportation restrictions, not by flight dynamics. The core tank of the stage has a diameter of 4.1 meters, which is an absolute maximum still allowing its delivery by rail.
The first stage is connected to the second stage with a lattice structure manufactured from the aluminum alloy V95. It is designed to let exhaust gas from the engines on the second stage to escape as they fire shortly before the separation of two stages around two minutes after liftoff.
With the introduction of the Proton-M version of the rocket in 2001, the first stage was slightly lightened thanks to improved manufacturing techniques and the reduction in the mass of onboard avionics. In addition, RD-253 engines used on the first stage had been upgraded and, from 1993, they were designated RD-275. Yet another upgrade of the engine became known as RD-276.
Another upgrade on the first stage involved its propellant feed system. It was simplified and redesigned in order to reduce by as much as 50 percent the amount of propellant remaining inside the stage by the time it finishes firing close to two minutes into the flight. A special propellant purge system was added to dump all residuals from the spent first and second stages before they fall back to earth. Although the main purpose of this change was to reduce controversial spills of toxic propellants at the impact sites downrange from Baikonur, the rocket's performance was also improved as a result.
The core tank of Proton's first stage (left) before the installation of strapon tanks (right). Credit: GKNPTs Khrunichev
Technical specifications of the first stage of the original Proton-K rocket:
Technical specifications of the first stage of the Proton-K and Proton-M rocket:
*Varies depending on a particular mission
Page author: Anatoly Zak
Last update: December 24, 2019
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The first stage of the Proton rocket during the assembly. Credit: GKNPTs Khrunichev
A launch pad view of Proton's first stage. Credit: GKNPTs Khrunichev
A business end of the Proton rocket with six RD-275 engines arranged in a circle on the first stage. Click to enlarge. Copyright © 2000 Anatoly Zak
Separation of the Proton's first stage during the launch of the Eutelsat-9B satellite as seen by ground cameras. Credit: ILS