"After analyzing the sanctions against our space agency, I suggest to the USA to bring their astronauts to the International Space Station using a trampoline" - Dmitry Rogozin, 2014

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It's funny how things come full circle sometimes. When Russia first annexed Crimea in 2014, I was a freshman at UT Austin starting to get involved in one of the stock-trading student orgs in the business school. Congress had just mandated that the ULA phase out the import of Russian rocket engines - our fund happened to own shares of Aerojet Rocketdyne, which at the time was vying to supply ULA with a domestically manufactured replacement, and one of the upperclassmen was giving a really neat presentation on the Russian RD-180 engine still used on the Atlas V today. Back then, I wasn't nearly as familiar with the aerospace industry. Eight years later, with me now working at SpaceX, a full-blown invasion of Ukraine underway, and the Russians refusing to sell any more rocket engines, the question still remains: why on Earth does the Atlas V use a Russian engine? 

Check it out! Here's the cover page of the portfolio review I wrote on Aerojet Rocketdyne for my college student investing org, back in 2016. Alas, ULA didn't pick Aerojet Rocketdyne's AR1 engine, they went with Blue Origin's BE-4 instead

Believe it or not, to answer this question we have to go back all the way to the heyday of the Space Race, when both the Americans and the Soviets were investing a ton of R&D into creating the best possible rocket engine. In the 1950s, the Air Force had commissioned the development of the enormous F-1 Rocketdyne engine, only to realize a few years later that it had absolutely no use for an engine that huge. Fortunately, NASA had just been created around that time, and they picked up Rocketdyne's tab for the F-1's development because they knew they'd need it. The F-1 was the perfect engine to power the first stage of the Saturn V: it was big enough to get Saturn V off the pad, but also relatively simple, using a gas-generator cycle and burning kerosene (RP-1). For the first stage, thrust was the more important factor; then for the second and third stages where efficiency mattered more, NASA leveraged prior work done by Lockheed to develop the hydrogen-burning J-2 engine, since hydrogen has better specific impulse than RP-1. In short: powerful kerosene-burning engines for the first stage + efficient hydrogen-burning engines for the second and third stage

5 kerosene-burning F-1 engines (left) powered the Saturn V first stage 5 hydrogen-burning J-2 engines (right) powered the second stage, and 1 powered the third stage

So what did the Soviets pursue? They opted to stay away from hydrogen, because working with hydrogen as your fuel is an absolute pain: super low density, insanely cold, highly flammable and explosive, embrittles metal, leaks easily, you name it. So left using kerosene for the upper stages, they needed a way to improve the efficiency of their engines to compensate for the lower specific impulse that kerosene provides versus hydrogen. To that end, the Soviets did something the Americans thought impossible - they developed a staged combustion cycle engine, the NK-15, for their N1 moon rocket (the Soviet counterpart to the Saturn V). What does that mean? 

Rocket engines need a turbopump to draw propellant into the combustion chamber, and to power that turbopump, some propellant has to be diverted and combusted on the side to generate exhaust. In a gas-generator engine like the F-1, the exhaust that powers the turbopump is then dumped overboard, essentially wasting that propellant. A staged combustion cycle engine like the NK-15 is way more complex, but it manages to recycle the exhaust from the turbopump and redirect it back into the combustion chamber, so that no propellant gets wasted. Pretty neat!

So now the Soviets have a revolutionary staged combustion cycle engine that burns kerosene and is highly efficient. But it's showing up pretty late in the race to the moon, and it's not very big (only 1/5th the thrust of the F-1), so the Soviets need to stick a ton of them (30) on the first stage of the N1 rocket. Having lots of engines on a stage is fine in theory if your quality assurance is good (SpaceX's Super Heavy plans to have 33 Raptor engines), but unfortunately the Soviets proved better at design than manufacturing; all 4 launches of the N1 rocket suffered anomalies in flight and exploded

Lots of NK-15 engines at the base of the N1 rocket! The diagram on the right shows the propellant flow of a staged combustion cycle engine. Where I've put the green circle that says "HERE" shows the recycled exhaust from the turbopump being redirected back into the combustion chamber

Now jump forward a few years: the Space Race is over, no one's going to the moon anymore, which means most of the payloads being launched now are only going to Earth orbit. With less thrust required, there's no longer any need for a supermassive rocket engine or a boatload of engines strapped together; a medium-sized, highly-efficient engine makes for a perfect choice for a first stage booster. The Soviet Union is now the global leader in staged combustion, kerosene-burning engines, and new engines like the NK-33 and the RD-180 pop up in the succeeding decades (if you want to learn more about the lineage of Soviet rocket engines, this video by The Everyday Astronaut is the best source)

But then something unexpected happened - the Soviet Union collapses! Flush with our victory in the Cold War and convinced of the inevitability of global democracy, yet fearful of Russian rocket scientists and nuclear experts dispersing and lending their expertise to unsavory third parties, the US effectively pumps money into Russia's space program through various partnerships (like the ISS) to prop it up. The hope in the 1990s was that if Russia can be free and democratic, then surely we can do business with them!

Maiden launch of the Atlas III with its RD-180 in 2000, the first American rocket to launch using a Russian engine

It was around this time that General Dynamics (and later Lockheed Martin after acquiring GD's space division) was developing the next iteration of its storied Atlas line of rockets, the Atlas III. Rather than trying to develop a new engine internally, which was sure to be an exorbitant effort, it was just so much easier to buy reliable, high-performance Russian engines right off the shelf, now that they're available. A joint venture was formed between Russia's NPO Energomash and Pratt & Whitney, whereby Pratt & Whitney received the license to manufacture the RD-180 for Lockheed Martin. From there, for reasons that continue to escape me to this day (as this Quora user aptly put it), the US government approved the partnership, and the Atlas III got the RD-180. Today's Atlas V still uses it

An Antares rocket being transported, with its Russian engines and Ukrainian first stage

Finally, it's worth noting that the Atlas V isn't the only American rocket with a Russian engine. While Atlas V and its RD-180 engine is more flown and more noteworthy, Northrop Grumman's Antares rocket is even more bizarre. Not only does it use a Russian engine, either the NK-33 or the RD-181 depending on the variant, but Northrop Grumman also subcontracts the construction of the entire first stage core to the Ukrainian Yuzhmash state-owned factory! Obviously Russia and Ukraine are at war with each other now, and there were unconfirmed reports that Yuzhmash had been damaged or destroyed in the fighting, so I have no idea what will become of the Antares rocket

One thing's for sure though, domestic manufacturing of high-performance rocket engines is a matter of national security. Thank goodness SpaceX has Merlin and Raptor!