Our third engine (fourth? Are SRB's engines?)! This is an interpretation of the descent propulsion system found on the Apollo LEM, later developed into the upper stage vacuum optimized Delta TR-201. These engines were claimed to be the most technically complicated examples of engineering in the entire Apollo program, due to the challenging mission requirements that they had to meet.
- They had to have a deep throttle - giving astronauts the ability to burn hard and cancel substantial horizontal velocity on approach, as well as being able to throttle down to land on the surface with a lighter, gentler force.
- They had to have thrust vectoring to maximize control alongside the RCS system; allowing for a more responsive craft that might have needed to respond to landing zones that weren't ideal. As the designers of the LEM were worried about the height (and tipping risk) of the lander, the engine was inset within the landing stage with its fuel sitting "astride" it in a low-profile, quad-bay arrangement. This meant the gimbal had to be designed to articulate from a central point, as there was little to no room for a longer engine design.
- The engine had to be reliable, and restart multiple times. Hypergolic fuels were used, substances that were incredibly toxic but combusted on contact with each other. This meant the thrust assembly could be simplified and weight saved by omitting a lot of hardware other engine designs might have required.
