The R-4D is a small hypergolic rocket engine, originally designed by Marquardt Corporation for use as a reaction control system thruster on vehicles of the Apollo crewed Moon landing program.

[2] Post-Apollo, modernized versions of the R-4D have been used in a variety of spacecraft, including the U.S. Navy's Leasat, Insat 1, Intelsat 6, Italsat, and BulgariaSat-1.

The original R-4D's combustion chamber was formed from an alloy of molybdenum, coated in a layer of disilicide.

the design was changed again to use an iridium-lined rhenium combustion chamber, which provided greater resistance to high-temperature oxidization and promoted mixing of partially reacted gasses.

It is rated for up to one hour of continuous thrust, 40,000 seconds total, and 20,000 individual firings.