File:Rocket Engine TD-339.jpg

English: Lunar Surveyor Rocket Engine TD-339: Biprop engine for Surveyor, the first American spacecraft to achieve a soft landing on the moon (or anywhere off-world for that matter). Part of the FV Space Collection.

It is resting on its transfer case as JPL would have received it, with the desiccant plugs installed (upper right corner) for shipping.

This Vernier engine was of critical importance to provide propulsion for trajectory correction maneuvers, attitude & velocity control before & during landing. Surveyor-2 crashed hard into the moon after an improper mid-course correction burn of a vernier engine put it into a tumble.

Each craft was planned to slow to about 110 m/s (4% of speed before retrofire) by a main solid fuel retrorocket, which fired for 40 seconds starting at an altitude of 75 km above the Moon, and then was jettisoned along with the radar unit at 11 km from the surface. The remainder of the trip to the surface, lasting about 2.5 minutes, was handled by smaller doppler radar units and three vernier engines running on liquid fuels fed to them using pressurized helium. Each engine's thrust could be throttled over a range of 30 to 104 lb. Two of the engines were fixed, while the third could gimbal. Heroicrelics has a detailed description of this TD-339 rocket engine.

Developed by Reaction Motors Division of the Thiokol Chemical Corporation and first used operationally on Surveyor 1, which soft landed on the Moon June 1, 1966. Roughly 9"x6"x7" Serial #1. You can see the bright gold of it on the extra Surveyor spacecraft at the Smithsonian NASM, bottom right. They also have a spare TD-339 vernier engine in their collection.

It used a regeneratively-cooled combustion chamber with a silicon carbide throat insert. The engine employed a somewhat unusual vortex injector. Each engine had a dedicated pair of propellant tanks, using mixed oxides of nitrogen (90% nitrogen tetroxide with 10% nitric oxide) as an oxidizer and monomethyl hydrazine monohydrate (MMH with water) as fuel; this combination ignites hypergolically when mixed in the thrust chamber.