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TheLR105 is aliquid-fuel rocket engine that served as the sustainer engine for theAtlas rocket family.^[1][2][3] Developed byRocketdyne in 1957 as the S-4,^[4][5][6] it is called a sustainer engine because it continues firing after theLR89 booster engines have been jettisoned, providing thrust during the ascent phase.^[7]

The LR105 is a liquid-propellant engine usingRP-1/LOX. The engine operates on agas-generator cycle, where a small portion of the propellant is burned in a gas generator to drive theturbopumps, which supply the engine with fuel and oxidizer.

The engine was designed to bethrottleable, meaning its thrust could be adjusted during flight to optimize performance. The LR105 also featuresregenerative cooling, where RP-1 fuel is circulated through cooling channels in the engine's nozzle and combustion chamber before being injected into the combustion process, preventing overheating and improving efficiency.

The LR105 engine underwent several upgrades over its operational life, leading to multiple variants:^{[1][8][9][10][3]}

• Rocketdyne LR89
• Rocketdyne LR101
• SM-65 Atlas
• Rocketdyne

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