The Last Flight of the NK-33 Engine

On Feb. 5, 2025, at 04:00 UTC, powered by the last NK-33 lunar engine, a Soyuz 2.1v rocket lifted off from the Plesetsk Military Cosmodrome for the last time, carrying a satellite identified as Kosmos 2581. The satellite was set to be placed into a polar orbit inclined at 82.4°.

NORAD observations revealed that the payload separated into three distinct orbital objects. Since the Russian Federation’s Ministry of Defense has not issued an official statement, it is assumed that these are three different satellites.

Sojuz 2.1v on its way to the launch pad at Plesetsk Cosmodrome. Credit: Russian Ministry of Defence — *Sojuz 2.1v on its way to the launch pad at Plesetsk Cosmodrome. Credits: Russian Ministry of Defence*

This was the final launch of the “light” version of the long-serving Soyuz rocket, a descendant of the legendary R-7. The Soyuz 2.1v had a unique feature: it lacked the four characteristic lateral boosters. Its first stage was powered by a single NK-33 closed-cycle liquid-fuel engine (LOX and RP-1), an engine with a long and storied history.

Born for the Moon

During the height of the Moon race, Sergei Pavlovich Korolev faced a major problem: for his N-1 lunar rocket, he needed a more rational solution than the 30 NK-15 engines in the first stage. In theory, the solution was within reach: he could have turned to Design Bureau No. 52 (OKB-52), led by Vladimir Chelomei, and adopted a first stage powered by hypergolic-fueled engines developed by Valentin Glushko.

However, there was a significant problem: the two design bureaus were competitors in the race to develop a lunar rocket. Even more problematic was the personal feud between Glushko and Korolev. Their animosity dated back to the Stalinist era, during which Korolev was sentenced to eight years in a gulag—allegedly due to a denunciation from Glushko himself.

Thus, Korolev turned elsewhere and chose the Kuznetsov Design Bureau, which developed a revolutionary closed-cycle combustion engine named NK-33 (after its creator, Nikolai Kuznetsov). Despite its great power, the engine suffered from severe development delays due to the extreme temperatures its combustion chamber had to withstand. As a result Korolev never saw the NK-33 in action.

The NK-33 (AJ-26) Engine. Credit: Aerojet — *The NK-33 (AJ-26) Engine. Credits: Aerojet*

By the time they were ready, the N-1 program had already been canceled. Under orders from Glushko—who had become the new director of the Soviet space program—the engines were supposed to be destroyed, just like the unfinished N-1 rockets. However, 80 NK-33 engines were secretly stored in an industrial warehouse and miraculously rediscovered in the late 1990s. A group of engineers proposed to American investors the idea of forming a joint venture to use these engines in the commercial launch market.

This led to the creation of Aerojet, and the engines originally meant for the Moon—renamed AJ-26—were used in the first version (100 series) of the American Antares rocket.

The Light Soyuz Version

However, the Antares rockets equipped with AJ-26 engines did not have much luck. A series of failures delayed resupply missions to the ISS for the Cygnus cargo spacecraft, the primary user of these rockets. Eventually, Antares’ first stage was modified to use RD-180 engines, a simplified version of the RD-171 engines from the powerful Energia rocket, designed by Valentin Glushko.

Nonetheless, the NK-33 engines still found their way into space. With the development of the Soyuz-2 family—the final version of the venerable R-7-derived launcher—a light variant was envisioned to place small payloads into low polar orbits, particularly for the Russian Aerospace Forces operating from Plesetsk Cosmodrome.

A Sojuz 2.1 in Plesetsk. Credit: Russian Ministry of Defence — *A Sojuz 2.1 in Plesetsk. Credits: Russian Ministry of Defence*

Thus, the Soyuz 2.1v was born. This two-stage rocket, lacking the four lateral boosters, featured a single NK-33 engine in its central core. It debuted in December 2013, launching from Plesetsk in Russia’s Arkhangelsk region. Throughout its service, it completed 13 launches—12 successfully and one failure due to an upper-stage issue.

Capable of carrying up to 3,000 kg to low polar orbit, the Soyuz 2.1v remained in service until the available NK-33 stock was exhausted. Since these engines were no longer in production, their depletion marked the end of this particular Soyuz 2 variant. It is set to be replaced by the Angara 1.2, which offers slightly higher payload capacity, and the Rokot-M, which will be used for small payloads up to one ton.

However, the story may not be over yet. In one of its 13 launches, the first stage was fitted with an RD-193 engine—another modified and simplified version of the legendary RD-170 from Energia.

A relay race destined to continue…