The R-4D is a small hypergolic rocket engine, originally designed by Marquardt Corporation for use as a reaction control thruster on vehicles of the Apollo moon program.The engine runs on self-igniting components: monomethylhydrazine and nitrogen tetroxide. The R-4D is a fuel-film cooled engine. Some of the fuel is injected longitudinally down the combustion chamber, where it forms a cooling film.It is rated for up to one hour of continuous thrust, 40,000 seconds total, and 20,000 individual firings.

The model was recreated from photographs and videos from the Internet. A model has been created in the Fusion 360 program.