LM APS question

[Allan F]

Was the APS designed to be fired more than once? I think at least 2 burns with the APS was needed - one to get off the ground and up to altitude and speed for orbital insertion, and one to circularize the orbit. And maybe a few to change the orbtial plane.

Or was the RCS able to handle those burns?

[bknight]

I believe it was only fired once, since it had to be rebuilt each time it was fired one Earth, corrosive fluids.

https://en.wikipedia.org/wiki/Ascent_propulsion_system

ETA:  However it was successfully fired twice during A9
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730010173.pdf

Page 22

[QuietElite]

There was at least another additional burn with the APS for Apollo 14-17 since these missions used the Direct Rendezvous approach instead of the Coelliptic Method like earlier missions.
The deltaV change of the TPI burn is higher for Direct Rendezvous since it missed additional burns that would have already raised the Periapsis of the Orbit. Therefore they used the APS instead of RCS otherwise the burn would take very long.

[Allan F]

Thank you for your input.

[VQ]

I believe it was only fired once, since it had to be rebuilt each time it was fired one Earth, corrosive fluids.

No, it could be fired multiple times. They just didn't want to have it an anything other than pristine condition at the start of a mission. Per the wiki you linked, the APS's immediate predecessor, the Bell 8247 (which used the same fuel) could be restarted at least 15 times.

The LM RCS and APS tanks were crossfed, so after the first (main) burn all dV maneuvers could be performed with either system. So the APS was only life-safety critical for the first burn.

I believe the APS was re-fired a final time on most missions after the crew left it, to deorbit the ascent stage (either back to the lunar surface or to heliocentric orbit).

[Jason Thompson]

The LM APS is about the simplest rocket engine it is possible to build. Fixed thrust, no gimablling, and hypergolic propellants fed by pressure. It's moving parts amount to one valve on the fuel inlet and one on the oxidiser inlet. Opening those will inevitably result in engine firing as the fuels ignite on contact.

As such, it does not have to be 'designed' to fire more than once. Any time those valves are opened and there is fuel and oxidiser flowing the engine will fire. There is no way to prevent it from doing so, and very little that could go wrong to stop it. This is true of the DPS and the SPS as well, and of course the RCS systems. The corrosive nature of the propellants, combined with the abaltive cooling used, meant that each engine flown was previously unfired to ensure it was in good enough condition to surivive essentially the complete buring of the entire fuel load of the LM.

[Obviousman]

And, of course, it was not throttleable.

[ka9q]

I think the simplicity of the hypergolic rocket is a little overstated. It still requires a pressurization system to force the propellants into the combustion chamber. It consists of helium tank(s), pyro valves, regulators, gauges, transducers, check valves, manifolds, and so forth. If anything leaks or if the valves fail, you're still in trouble.

[VQ]

I think the simplicity of the hypergolic rocket is a little overstated. It still requires a pressurization system to force the propellants into the combustion chamber. It consists of helium tank(s), pyro valves, regulators, gauges, transducers, check valves, manifolds, and so forth. If anything leaks or if the valves fail, you're still in trouble.

Sort of... I mean, valves can fail, so both the check valves and ball valves were parallel-redundant. The check valves were also series-redundant. The helium on the ascent stage was stored in redundant gas-phase tanks, with no heat transfer/pressure spike concerns to address as there were on the descent stage. Are you saying writing off the redundancy sounds like hubris? It certainly was a mechanically simpler system than most of the engines associated with Apollo.

Related question - does anyone know if it would have been possible for the astronauts to access the valve package assembly from inside the cabin, while on the moon?

[ka9q]

There have been failures in these systems. One (probable) example that comes to mind was the Mars Observer spacecraft. It went silent as the propellant tanks were being pressurized. The most likely cause was that oxidizer had slowly leaked past the check valves in the pressurization lines. It reacted in the lines with fuel when the valves were opened to pressurize the tanks, blowing the lines open and spilling fuel and helium.

[Jason Thompson]

I think the simplicity of the hypergolic rocket is a little overstated. It still requires a pressurization system to force the propellants into the combustion chamber. It consists of helium tank(s), pyro valves, regulators, gauges, transducers, check valves, manifolds, and so forth. If anything leaks or if the valves fail, you're still in trouble.

Of course, but as rocket engines go it can't get much simpler. Hypergolics always carry the risk that any kind of leakage that bring the fuel and oxidiser into unplanned contact is a big problem. The relative simplicity of the design, though, makes the number of potential failure points much lower. In terms of the original question, a hypergolic engine is, as I understand it, harder to engineer so it can't be fired multiple times than so it can. Contrasted with the incredibly complex plumbing of the F-1 or the SSME, for example, there's very little that can go wrong. When 250,000 miles away with a successful burn being the deciding factor in making it through the mission or being marooned in space, a low risk of catastrophic failure is outweighed by the near certainty that the engine will fire when you need it to.

[ka9q]

I can think of a much simpler rocket engine: a solid fuel engine. Of course, that might not meet the mission requirements either.

[Jason Thompson]

Yes, I was working on the understood context of liquid fuelled engines. Solid fuel engines are of course simpler, but since that simplicity extends to not having an off-switch, that doesn't really meet mission safety requirements for a scenario in which you are completely dependent on it....

[Zakalwe]

I can think of a much simpler rocket engine: a solid fuel engine. Of course, that might not meet the mission requirements either.

The APS was required to be restartable, so that rules out solids.