Source: NASA Technical Note D-7082 Apollo Experience Report: Ascent Propulsion System
The ascent engine itself was designed to be restartable. (In fact, a second burn was how the empty ascent stages were crashed into the moon.) It was the helium pressurization system that fed the fuel and oxidizer which had problems restarting.
The ascent engine, shown in figure 2, is a fixed-injector, restartable, bipropellant rocket engine that has an ablatively cooled combustion chamber, throat, and nozzle extension.
In the engine itself, there were problems with the injector, and ultimately it was re-designed and manufactured by a second vendor. In contrast, the helium pressurization system had many problems:
- Ten helium tanks were manufactured 0.1 inch too small. These were used as test articles.
- The original vendor of the helium solenoid latching valve could not meet pressure or leakage requirements. Under a new vendor, the O-rings showed chafing when the valves were repeatedly cycled. Testing of this valve at White Sands also showed problems with this valve remaining open when rapidly cycled. There were also problems brazing this valve into several of the LMs.
- The primary and backup vendors of the helium pressure regulator both had problems with lockup. The primary vendor's regulator locked up repeatedly. The backup vendor's regulator locked up once and had oscillations during testing, but were deemed within an acceptable range and used.
- Some of the squib valves had faulty brazing joints.
The ascent engine was tested at White Sands on the PA-1 test rig. Five "flight-qualified" engines and six thrust chambers were used among all the tests. I find no evidence that these test engines were re-used for actual flights, nor that hot-fire testing of production engines or their components was done. Test series #4 was 45 runs of "LM-1 mission duty cycle, restarts, engine stability, propellant depletion, off-nominal performance".
The unmanned Apollo 5 mission fired the ascent engine twice. The first firing was 60 seconds in duration. The second firing occurred 1-1/2 hours later, and continued to propellant depletion. The firing time was 40 seconds less than predicted. Later analysis concluded that at least 10 seconds of this missing time was attributable to problems in the helium system.
Apollo 9 was the first manned mission with the lunar module. It ran one manned 3 second firing of the ascent stage, followed by an unmanned firing of the ascent stage to depletion. During the first 290 seconds of the 362.3 seconds of the second firing, the system was significantly under-pressure. This was attributed to a malfunction of the helium pressure regulator.
Apollo 10 had an unexpected low fuel warning light, during the ascent from the near-touchdown. This was attributed to an ullage issue with the tanks that were deliberately filled half-full; not an issue with the helium system.
Remaining flights had no problems with the ascent propulsion system.
Update: The existance of time limits on pressurization of the LM propulsion systems is confirmed by the Apollo Operations Handbook: Lunar Module vol. 1, section 2.8.6:
ASCENT He PRESS switch should not be actuated longer than 24 hours before termination of ascent engine operation. The ascent pressurization valves are designed to operate for only 24 hours after exposure to propellant vapors. Exceeding this limit may cause ascent valve failure.
DES START He PRESS and
DES PRPLNT ISOL VLV switches should not be actuated longer than 3.5 days before termination of descent engine operation. The descent pressurization valves are designed to operate for only 3.5 days after exposure to propellant vapors. Exceeding this limit may cause descent valve failure.