The Lunar Ascent Engine was used during the Apollo missions to ascent from the lunar surface back to the Command and Service Module. There was one risky aspect of that engine: it was not possible to fire it repeatedly. Each ascent engine used on the Apollo missions was fired for the first time on the Moon. //
Why was the engine manufactured like that? //
According to a Boeing document titled "Apollo Spacecraft Engine Specific Impulse Part II dated October 1968 (bold mine):
the injector and valve assembly are first calibrated and then acceptance test fired in a water cooled steel chamber with an ablative liner. A single compatibility test of 460 second duration is then carried out. Finally, the injector and valve assembly are then assembled with their flight chamber and the complete engine is acceptance tested...
The description of the acceptance tests for the complete engine includes:
The engine and its thrust measuring rig are mounted horizontally in a capsule. Altitude pressure is obtained by a steam ejector and maintained during engine firing by an exhaust driven diffuser. Propellant tanks are pressurized with helium. There is provision for both temperature conditioning and helium saturating the propellants. A minimum of two satisfactory engine acceptance tests of 15 second duration have to be carried out. ... Throat and exit area measurements are taken prior to the first test and after the last in each test series.
