order to extend landing payload weight and lunar surface stay times, the last three Apollo Lunar Modules were upgraded by adding a nozzle extension to the engine to increase thrust. The nozzle exhaust bell, like the original, was designed to crush if it hit the surface. It never had on the first three landings, but did buckle on the first Extended landing, Apollo 15. TR-201 in Delta second stage After the Apollo program, the DPS was further developed into the TRW TR-201 engine. This engine was used in the second stage, referred to as Delta-P, of the Delta launch vehicle (Delta 1000, Delta 2000, Delta 3000 series) for 77 successful launches between 1972–1988. References External links NASA Technical Note: Apollo Experience Report – Descent Propulsion System. March 1973 31 pages, much detail on both designs, and the testing. Apollo Lunar Module Propulsion Systems Overview, NASA Rocket Propulsion – The Apollo Mission, Don Harvey. Mr. Harvey worked at STL designing the LMDE Rocket engines Rocket engines using hypergolic propellant Apollo program hardware TRW Inc. Rocket engines using the pressure-fed cycle