Raptor Vacuum

The Raptor Vacuum, or “RVac,” is a rocket engine built for the emptiness of space. It is the same powerful engine SpaceX uses near the ground, but fitted with a giant nozzle so it can squeeze more push out of every drop of fuel once it leaves the thick lower air.

Quick facts

• Type: Vacuum-optimized variant of the SpaceX Raptor engine.
• Propellants: Liquid methane and liquid oxygen (“methalox”), mixed at roughly 3.6 parts oxidizer to 1 part fuel. The propellants are densified (chilled extra-cold, below their normal boiling point, to pack more into each tank).
• Cycle: Full-flow staged combustion, the same advanced engine cycle as the sea-level Raptor.
• Thrust in vacuum: about 200 tonne-force (tf) on Raptor 1, about 258 tf (2.53 MN / 569,000 lbf) on Raptor 2, and a targeted 275 tf (2.70 MN / 606,000 lbf) on Raptor 3.
• Efficiency: specific impulse of about 380 seconds, versus roughly 327-350 seconds for the sea-level Raptor.
• Nozzle expansion ratio: roughly 80:1, versus about 34:1 for the sea-level engine.
• Used on: the Starship upper stage (the “Ship”).

What it is and how it works

A rocket nozzle is the flared, bell-shaped exit where hot gases rush out. It works best when the exhaust expands to roughly the same pressure as the surroundings, called ambient pressure. Near the ground, air pressure is high, so a sea-level engine needs a relatively short nozzle to keep the exhaust from over-expanding. In space there is almost no outside pressure, so a much longer, wider nozzle can keep accelerating the exhaust gases, pulling more thrust and efficiency from the very same fuel.

RVac is exactly that. It shares Raptor’s core machinery, including the turbopumps (pumps, spun by the engine’s own gases, that force fuel and oxygen into the chamber) and its full-flow staged combustion cycle. In that cycle, both the fuel-rich and oxygen-rich gas flows are routed into the main chamber, so no propellant is wasted overboard. Onto that core, RVac bolts a large extended bell nozzle with an expansion ratio (the ratio comparing the nozzle’s wide exit to its narrow throat) of about 80:1.

That extension is regeneratively cooled: propellant flows through brazed steel tubes in the nozzle wall to carry away heat before being burned. Think of it like running cool water through the walls of a furnace so the walls survive the fire. This keeps the nozzle lightweight yet able to withstand the searing exhaust. The payoff is a specific impulse near 380 seconds, a measure of fuel efficiency, about 30 to 50 seconds better than the sea-level engine.

Why it matters

RVac is what makes Starship’s upper stage efficient enough to reach orbit and maneuver in space economically. By fitting a high-expansion vacuum nozzle on three of the Ship’s six engines, SpaceX gains a big jump in efficiency exactly where the vehicle spends most of its energy: high in the atmosphere and in vacuum. The smaller sea-level engines are kept for steering and landing.

It is also notable as one of the few operational full-flow staged combustion engines ever flown. Because it shares production tooling and the methalox cycle with the sea-level Raptor, it supports SpaceX’s goal of mass-producing cheap, reusable engines. As Raptor advances from version 1 to 2 to 3, the RVac variant scales in thrust alongside it, directly increasing how much Starship can carry to orbit.

Trade-offs worth knowing

RVac’s large nozzle cannot be safely fired at full thrust at sea level. High ground-level air pressure would cause flow separation, where the exhaust peels away from the nozzle wall and shakes it violently, so the engine is purpose-built for high-altitude and vacuum operation. The three RVacs are also fixed, meaning they do not gimbal (swivel to steer). Steering and the landing burns are handled by the three gimbaling sea-level engines, leaving RVac to provide efficient cruise and ascent thrust.

Where it is used and notable examples

• SpaceX Starship upper stage: the primary user. On Blocks 1-3, the Ship flies 3 Raptor Vacuum engines plus 3 sea-level Raptors, with future plans adding more RVacs.
• Sea-level Raptor: the companion engine RVac is derived from, sharing the same methalox full-flow staged combustion core but with a short nozzle for atmospheric flight, steering, and landing.
• Raptor 2 RVac (~258 tf): the production version flying on operational Starship test flights.
• Raptor 3 RVac (~275 tf targeted): the next-generation upgrade, raising thrust and simplifying the engine.
• Development milestones: the first full-duration test fire of RVac version 1 happened at SpaceX’s McGregor, Texas site in September 2020, and the first in-flight ignition came on Starship’s second integrated flight test in November 2023.