This blog post (and other resources) states the Polyus was mounted engines facing the front and this stage must turn 180° before firing its engine.

I easily imagine that staging outside the atmosphere does not need the upper stage to be mounted directly with the right angle. When no engine is running (e.g. during the staging), rotations can be performed without loosing mechanical energy (no aerodynamic forces) and thus rotation can be performed before firing the next stage engines.

For me, it seems that this mechanism add complexity. To decide to implement such mechanism, engineers might have found advantages; or they found that a specific problem was easier to solve that way. I fail to find the reason why they decided to mount the Polyus backward (NASA programs' documentation are easier to find and in a language I can understand).

Why was this stage mounted backward? What was the problem they tried to solve? Was it the only solution?

  • $\begingroup$ It pretty much depends on which side is stronger hand heavier. $\endgroup$ May 20 '20 at 21:28
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    $\begingroup$ I don't know why the cited blogpost translates "Polyus" ("Полюс") as "Staff". :) It's just incorrect. "Polyus" translates to English as "pole" (exactly Noth or South pole, but not appropriate for other meanings, like "pole vault" sport). It's the same Latin root. $\endgroup$
    – Heopps
    May 21 '20 at 11:04

The service module of Polyus was too fragile to withstand heavy laser payload above it during the launch.

I've found the answer at buran.ru (in Russian).


According to Constantin Lantratov's article Polyus was designed of two parts - Skif-DM laser payload and service module. Service module was a ready TKS spaceship. It had propulsion, navigation, communication and other standard spaceship subsystems.

This solution was cheaper and faster. TKS already flown to space several times before. But of course TKS wasn't designed to launch with several dosens tonnes of extra payload above it, it would be cracked. So the solution was to place service module at the top for the launch and to rotate Polyus after separation.


Конструктивно первый "Скиф-Д" (бортовой номер 18101) состоял из двух жестко соединенных между собой модулей: функционально-служебного блока (ФСБ) и целевого модуля (ЦМ). ФСБ, разработанный на базе функционально-грузового блока 11Ф77 корабля 11Ф72 ТКС, использовался для доразгона "Скифа-Д" после его отделения от РН: блок добавлял необходимые 60м/с для выхода КА на опорную низкую орбиту. В ФСБ также располагались основные служебные системы аппарата. Для их энергопитания на ФСБ устанавливались солнечные батареи от ТКС.


Structurally, the first "Skif-D" (number 18101) consisted of two modules rigidly interconnected: a functional-service module and a laser module. The service unit, developed on the basis of the 11F77 functional cargo block of the 11F72 TKS spacecraft, was used for the Skif-D orbital push after its separation from the launch vehicle: the block added the necessary 60m s for the spacecraft to enter low earth orbit. The service module also housed the main service systems of the apparatus. Solar panels installed from the TKS to power them.


Так как функционально-служебный блок "Скифа-ДМ" брался уже практически готовым, то нагрузки, действующие на него при запуске на РН "Энергия", должны были быть аналогичными нагрузкам, действующим при запуске на РН 8К82К "Протон-К". Поэтому, чтобы соблюсти расчетный режим нагружения при запуске, ФСБ располагался в головной части "Скифа-ДМ". Однако такая компоновка приводила к тому, что после отделения аппарата от РН он летел двигателями вперед. Поэтому требовался разворот "Скифа-ДМ" на 180º вокруг поперечной оси Z, чтобы лететь двигателями назад. Перенос же двигателей ДКС на целевой модуль усложнял компоновку аппарата и увеличивал массу трубопроводов подачи топлива. К этому развороту на 180º из-за особенностей системы управления ФСБ требовался еще "доворот" вокруг продольной оси X на 90º. Только после такого маневра, прозванного в КБ "Салют" "перевертоном", можно было доразгонять "Скиф" для выхода его на орбиту.


Since the Skif-DM service module was taken almost ready, the loads acting on it when launching on the Energia should have been similar to the loads acting on launch on the Proton-K 8K82K rocket. Therefore, in order to comply with the calculated loading mode at startup, the service module was located in the Skiff-DM head. However, this arrangement led to the fact that after the separation of the apparatus from the rocket it flew forward with the engines. Therefore, it was necessary to turn the Skif-DM 180º around the transverse axis Z to fly backwards with the engines. The transfer of engines to the target module would complicate the layout of the apparatus and increased the mass of the fuel supply pipelines. Due to the peculiarities of the service module control system, this 180 ° turn required another 90 ° turn around the longitudinal axis X. Only after such a maneuver, nicknamed in the Design Bureau “Salyut” “pereverton”, it was possible to accelerate “Skif” to orbit.

Bold is myne.

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    $\begingroup$ It's really great to have some people here who can access these Russian sources and provide that information. $\endgroup$ May 21 '20 at 17:44

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