On the Atlas 3 and its successor, the Atlas 5, the RD-180 carried at least 16 American spy satellites to orbit, along with 13 military communications satellites, a half-dozen GPS satellites, two military weather satellites, and three missile warning satellites, designed to detect rocket launches from, among other countries, the one where it was built. It launched four American Mars missions. NASA’s launch of New Horizons to Pluto in 2006 and Juno to Jupiter in 2011 were both made on the back of the RD-180.
The RD-180 is remarkable not only for the geopolitical peculiarities of its rise to prominence, but because it was in many ways simply better than any other rocket engine of its time. When, in February 2019, Elon Musk announced a successful test of SpaceX’s Raptor engine, which is intended to power the company’s next-generation rocket Starship, he bragged of the high pressures reached in the Raptor’s thrust chamber: over 265 times atmospheric pressure at sea level. Raptor, he said on Twitter, had exceeded the record held for several decades by the “awesome Russian RD-180.”
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After Russia annexed the Crimea in 2014, the RD-180’s days as a staple of American rocketry were numbered. Defense hawks had long been uncomfortable with the arrangement, but the engine was both very good and, given its capability, cheap—and so it stayed. But as relations with Russia frayed, congressional opponents of the engine, led by Senator John McCain, succeeded in passing a prohibition against the engine’s use in American rockets after the end of 2022. This has forced the Air Force to find a new rocket to succeed the RD-180-powered Atlas 5.
All of which raises a question: How did a decades-old Russian engine become the bar against which America’s best rocket scientists measure themselves?
If you want to understand what made the RD-180 such a good engine, it helps to understand that there is a great deal of craft involved. Though hundreds of people collaborate on rocket engines, having someone with an instinct for good design in charge is vital: the trade-offs are too complex to be figured out by brute force or by committee. In the case of the RD-180, that someone was named Valentin Glushko.
After the USSR lost to America in the race to the moon, designing the best possible rocket engine became “a national priority,” according to Vadim Lukashevich, an aerospace engineer and Russian space historian. Soviet leaders wanted to build the world’s most powerful rocket, the Energia, to sustain their space stations in Earth orbit and to lift the Buran, a would-be Russian space shuttle. Glushko was given resources to build the best engine he could, and he was good at building engines. The result was the RD-170, the RD-180’s older brother.
The RD-170 was among the first rocket engines to use a technique called staged combustion. The US space shuttle main engine, also developed in the 1970s, was another. By contrast, the F-1 engines in the first stage of the Saturn V rocket, which launched Apollo to the moon, were of an older, simpler design called the gas-generator engine. The key difference: staged-combustion engines can be more efficient, but they’re at greater risk of exploding. As William Anderson, who studies liquid-fueled rocket engines at Purdue University, explains, “The rates of energy release are just extreme.” It takes someone with a really astute imagination, Anderson says, to understand the crazy stuff that’s going on inside rocket engines’ combustion chambers. In Russia, that astute person was Glushko.
To understand why Glushko’s engines were such an engineering achievement, we need to get a little bit technical.
There are two key measures of a rocket’s performance: thrust, or the amount of force a rocket exerts, and specific impulse, a measure of how efficiently it uses its propellants. A rocket with high thrust but low specific impulse won’t reach orbit—it would have to carry so much fuel that the weight of the fuel would necessitate more fuel, and so on. Conversely, a rocket with high specific impulse but low thrust would never leave the ground. (Such rockets work well in space, though, where a steady push suffices.)
A rocket engine, much like an aircraft jet engine, burns fuel together with an oxidizer—often oxygen—to create hot gas that expands down and out of the engine nozzle, accelerating the engine the other way. Unlike jet engines, which get oxygen from the air around them, rockets need to carry their own oxygen (or other oxidizer), since in space, of course, there isn’t any. Like jets, rockets need a way to force the fuel and oxygen into the combustion chamber at high pressure; all else being equal, higher pressure means better performance. To do that, rockets use turbopumps that spin at hundreds of rotations per second. The turbopumps are driven by turbines, and they, in turn, are powered by pre-burners, which likewise burn some fuel and oxygen.