RD-191

The RD-191 is a Russian rocket engine that burns kerosene and liquid oxygen to lift Russia’s Angara rockets off the launch pad. It is the youngest member of one of the most celebrated engine families in spaceflight history.

Quick facts

• Maker: NPO Energomash in Khimki, Russia, with serial production at Proton-PM in Perm.
• Fuel: RP-1 (a highly refined kerosene, similar to jet fuel) burned with liquid oxygen (LOX), which is oxygen chilled until it becomes a liquid.
• Combustion chambers: 1 (a single chamber where the burning happens).
• Thrust: about 1,920 kN (196 tonnes-force, or roughly 430,000 pounds of force) at sea level; about 2,090 kN in the vacuum of space.
• Chamber pressure: about 25.8 MPa (around 3,800 psi) — among the highest of any operational engine.
• Steering: the chamber can swivel up to 8 degrees on two axes.
• Throttle range: roughly 27% to 105% of normal thrust.
• First flight: July 2014; development declared complete in May 2011 after 120 firings.

What it is and how it works

An engine’s job is to push a rocket upward by burning fuel and oxygen and hurling the hot gas out the back. The RD-191 does this with an oxidizer-rich staged-combustion cycle, a clever design that wastes very little.

Here is the idea. A small chamber called a preburner (or gas generator) burns a little fuel with most of the oxygen. This makes a hot, oxygen-rich gas. That gas spins a turbine, which drives the turbopump — the pump that forces fuel and oxygen into the engine at enormous pressure. In simpler engines, that gas would then be dumped overboard and wasted. Here it is routed onward into the main chamber and burned with the rest of the fuel. Because nearly all the propellant ends up burning in the chamber, the engine is highly efficient. Engineers call this a closed cycle.

The tricky part is the oxygen-rich gas, which is chemically aggressive — hot oxygen attacks metal. Energomash mastered this with special alloys and coatings, a challenge Western engineers long avoided. The engine lights using a self-igniting starter fluid, and its powerhead also pressurizes the tanks with helium and supplies hydraulic power for steering.

Why it matters

The RD-191 modernized a legendary heritage. Think of it as one quarter of the RD-170, the most powerful liquid rocket engine ever flown, which used four chambers fed by a single turbopump. Engineers shrank that design down to a clean, single-chamber engine — making it cheaper, simpler, and reusable as a building block. The RD-191 reuses an estimated 70 to 80 percent of RD-170 and RD-180 technology, which sped up development and cut costs. (The two-chamber RD-180, a closer cousin, long powered America’s Atlas V.)

Modularity is the core idea. Angara places one RD-191 in each Universal Rocket Module (URM-1). A light rocket uses a single module; the heavy Angara A5 clusters five — like building different vehicles from the same identical block. The flip side of its record chamber pressure is high performance with little design margin, which has fueled debate about whether it is safe enough to carry crew. One advantage on the safety side: each engine can be fired and acceptance-tested on the ground before flight.

Where it is used and notable examples

• Angara A5 (heavy-lift): five RD-191 engines, one per Universal Rocket Module; first flew in December 2014.
• Angara 1.2 (light-lift) and the Angara 1.2PP suborbital test vehicle, which carried the engine’s maiden flight in July 2014: a single RD-191 in one core.
• RD-181 variant: two engines powered Northrop Grumman’s American Antares rocket on cargo runs to the International Space Station, until Russia suspended deliveries in 2022.
• RD-151 variant: a reduced-thrust (about 170 tonnes-force) version that powered the first stage of South Korea’s Naro-1 rocket, first launched on 25 August 2009.
• RD-193 variant: a lighter, non-gimbaling derivative planned for the Soyuz-2.1v light launcher.
• RD-191M: an uprated version (about 110% thrust) for the future Angara-A5M, with tune-up tests completed around 2024 to 2025.