Raptor Engine

Raptor Engine
Type	Full-flow staged-combustion-cycle engine
Propellant	Methalox
Variants
Previous variant	Merlin Engine
See Also	Raptor 1, Raptor 2, Raptor 3, Raptor 4, Starship

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Raptor is a family of full-flow staged-combustion-cycle (The only one built so far) rocket engines developed and manufactured by SpaceX for use on the in-development SpaceX Starship. The engine is powered by cryogenic liquid methane and liquid oxygen ("methalox") rather than the RP-1 and liquid oxygen ("kerolox") used in SpaceX's prior Merlin and Kestrel rocket engines. The Raptor engine has more than twice the thrust of SpaceX's Merlin 1D engine that powers the Falcon 9 and Falcon Heavy launch vehicles.

Raptor is used in the Starship system in both the super-heavy-lift Super Heavy booster and in the Starship spacecraft which acts as the second stage when launched from Earth and as an independent spacecraft in LEO and beyond. Starship is planned to be used in various applications, including Earth-orbit satellite delivery, deployment of a large portion of SpaceX's Starlink satellite constellation, exploration, Moon landing, and colonisation of Mars.

Raptor 1 engines produce 185 metric tons of force and the Raptor 2 engines, which started production in November or December 2021, "will do 230+ tons or over half a million pounds of force", according to Elon Musk. Raptor 3 engines, which are being tested in 2025, increase it to 280 tons.

The inner engines of boosters and ships can gimbal; Elon Musk shared a video of a test on Booster 4:

A simple, yet in-depth explanation of the engine can be found on Everyday Astronaut's site, including a comprehensive video. Furthermore, his interview with Elon Musk on the Raptor 2 version is also much recommended.

Overview

The Raptor engine is powered by cryocooled liquid methane, and cryocooled liquid oxygen in a full-flow staged combustion cycle. This is a departure from the simpler "open-cycle" gas generator system and LOX/kerosene propellants that the current Merlin engines use. Also, the RS-25 engines (which were first used on the Space Shuttle) use a simpler form of a staged combustion cycle, as do several Russian rocket engines, including the RD-180 and the RD-191.

Raptor is full-flow, closed-cycle and stage-combusting:

• It is full-flow, because all of the propellant flows through the "pre-processing machinery", such as the pre-burners.
• It is closed-cycle, because the gas from pre-burners and other stages does not get vented and discarded.
• It is stage-combusting because mixes of CH4 and O2 are partially burned in pre-burner stages, before they reaches the main chamber.

Raptor has two pre-burners, where unequal mixes of CH4 and O2 are burned, which creates hot CH4-rich gas and hot O2-rich gas, respectively. The expanding gases then drive their own turbopumps, which then suck in more liquid methane or oxygen, respectively from the tanks. This way it is a self-driving cycle.

The methane and oxygen-rich gas flows are then guided to the main chamber. They are pre-mixed using swirl injectors, where the oxygen-rich gas comes down in a pipe, and holes for methane-rich gas are drilled tangentially into it. That way, the hot gases are mostly pre-mixed before they reach the main chamber.

The gaseous mix is burned in the main chamber, which operates at a pressure of roughly 300 bar. The reaction of CH4 and O2 is not chemically stoichiometric, instead an excess of O2 (LOX) is used (approx 80% by mass of total propellant). The excess O2 also expands and generates thrust, and absorbs some of the heat. According to Elon Musk, being too close to stoichiometric ratios would "melt the engines" (although it would in theory increase ISP).

The resulting gases then exit through the nozzle, expand, and the partial pressure on the nozzle drives the engine the other way.

Variants

Raptor (Sea Level)

Raptor (Sea Level) is the Raptor with a smaller nozzle optimised for sea level. The nozzle has cooling channels milled into its walls, and cryocooled methane from the tanks flows through it (in a closed loop) before reaching the pre-burner (regenerative cooling).

Raptor Vacuum (RVAC)

Raptor Vacuum (RVac) is a variant of Raptor with an extended, regeneratively-cooled nozzle for higher specific impulse in the vacuum of space. The vacuum-optimized Raptor engine is aiming for a specific impulse of ~380 s (3,700 m/s). A full-duration test of version 1 of the Raptor Vacuum engine was completed in September 2020 at the SpaceX development facility in McGregor, Texas. The first three operational Raptor Vacuum engines were slated to fly on SpaceX's first orbital Starship prototype, Ship 20 (S20), and were installed on 4 August 2021. However, Ship 20 was retired before it flew. The first in-flight ignition of an RVac was on Ship 25 (S25) during the Starship Flight Test 2.

Shutdown of a Raptor vacuum engine in slow motion. The engine’s nozzle is sized for use by Starship in the Earth’s upper atmosphere and outer space, so operation at sea level and low chamber pressures results in flow separation creating visible rings in the exhaust.

Versions

See also: Versions

As of August 2024, there are three known versions of Raptor, Raptor 1, Raptor 2 and Raptor 3:

• Raptor 1 was the first version, which were installed on the early "SN"-Ships which were flight-tested at Starbase. It was not used on any Integrated Flight Test.
• Raptor 2 had many of the outer cables, pipes, and the torch igniters of the main chamber removed, along with improving performance. It was used on Integrated Flight Tests 1-8 and is as of February 2025 the operational engine used.
• Raptor 3 is currently undergoing early testing (Feb 2025). Almost all of the outer connections were removed and integrated into the engine, eliminating the need for a booster engine heat-shield (shrouds).

Elon Musk explained the upgrades on Version 3:

"The amount of work required to simplify the Raptor engine, internalize secondary flow paths and add regenerative cooling for exposed components was staggering. As a result Raptor 3 doesn’t require any heat shield, eliminating heat shield mass & complexity, as well as the fire suppression system. It is also lighter, has more thrust and has higher efficiency than Raptor 2. Truly, a work of art."

Gallery

The 3 current versions. c SpaceX

Versions of the Raptor engines (Presentation April 2024)

Overview on versions as of 3 August 2024

Version	Thrust	Specific impulse	Engine mass	Engine + vehicle-side commodities and hardware mass	Notes
R1 sea level	185tf	350s	2080 kg	3630 kg
R2 sea level	230 tf	347s	1630 kg	2875 kg
R3 sea level	280 tf	350s	1525 kg	1720 kg	Raptor 3 is designed for rapid reuse, eliminating the need for engine heatshields while continuing to increase performance and manufacturability; 300 tons of thrust according to Elon Musk

Updates

15 October 2024

According to Elon Musk, "Raptor 3 will aim for 300 tons of thrust at liftoff, so 10,000 tons of combined thrust for 33 engines".

08 August 2024

Gwynne Shotwell and Elon Musk shared pictures of the first static fire test of Raptor V3 SN1.

First Raptor V3 firing. c Gwynne Shotwell

c Elon Musk

c Elon Musk

03 August 2024

SpaceX provided an update on the stats of Raptor versions 1 and 2 while mentioning that "Raptor 3 is designed for rapid reuse, eliminating the need for engine heatshields while continuing to increase performance and manufacturability".

Overview on versions 3 August 2024

Version	Thrust	Specific impulse	Engine mass	Engine + vehicle-side commodities and hardware mass	Notes
R1 sea level	185tf	350s	2080 kg	3630 kg
R2 sea level	230 ft	347s	1630 kg	2875 kg
R3 sea level	280 ft	350s	1525 kg	1720 kg	Raptor 3 is designed for rapid reuse, eliminating the need for engine heatshields while continuing to increase performance and manufacturability

Elon Musk added: "Important to note that the induced mass (mass required by a given engine design that is not the engine itself) of Raptor 3, while far better than Raptor 2, still has a lot of room for improvement. Thrust will exceed 300 tons with Raptor 3.x (thrust/mass>200), enabling 10,000 tons of thrust at liftoff and there might be up to 5 sec of Isp gain over time. Getting close to the limit of known physics." He also gave an outlook: "In a few years, we will finally have a Raptor 3/4 vacuum version (giant nozzle) that has an Isp of 380"

Elon Musk furthermore explained V3: "The amount of work required to simplify the Raptor engine, internalize secondary flow paths and add regenerative cooling for exposed components was staggering. As a result Raptor 3 doesn’t require any heat shield, eliminating heat shield mass & complexity, as well as the fire suppression system. It is also lighter, has more thrust and has higher efficiency than Raptor 2. Truly, a work of art."

22 June 2024

Answering to Everyday Astronaut in the context of a new Starbase tour, Elon Musk wrote on X:

"We could build a lot more [add: Raptor 2s engines], but the next version [add: V3] of Raptor is really the one to scale up production. We begin testing it in McGregor within a week or so. Regenerative cooling and secondary flow paths have been made integral to the whole engine, thus no heat shield is required. Nothing quite like this has ever been done before. Taking away the engine heat shields also removes the need for 10+ tons of fire suppression behind the engine heat shield, as any gas leaks simply enter the already super-heated plasma surrounding the engines, rendering the leaks irrelevant. Raptor 3 also has higher thrust and Isp."

06 April 2024

Elon Musk announced on 6 April 2024, that Raptor 3 will be massively simplified compared to Raptor 2 with the heat shield to be removed and much of the plumbing integrated into the pump and chamber jacket.

28 February 2024

Elon Musk answered on a tweet by Everyday Astronaut: "Raptor start is now reliable on the test stand under most conditions. Now we’re working on dynamically adapting the start sequence based on increasingly difficult propellant inlet pressures & temps. Operating with low pressure, “warm” liquid oxygen is particularly important."