Restarting Eagle's descent engine

[wd40]

What was the total burn-time of the LM descent engine?

If the Eagle had landed on a totally incorrect location, or at a bad angle, or if the astronauts wanted to take off to hover off the surface for a few minutes in an emergency eg to wait for a rumbling moon-quake to pass and then re-land, did the LM descent engine have the ability to be restarted, shut off and then restarted? And if so, for how many times?

[NEOWatcher]

What was the total burn-time of the LM descent engine?

I don't know the total burn time, but...

If the Eagle had landed on a totally incorrect location, or at a bad angle, or if the astronauts wanted to take off to hover off the surface for a few minutes in an emergency eg to wait for a rumbling moon-quake to pass and then re-land, did the LM descent engine have the ability to be restarted, shut off and then restarted? And if so, for how many times?

The Eagle landed with only 25 seconds of fuel left which was already close to the decision of an abort. It was about 40 seconds later than expected. So, there is no way they could have hovered for even a minute if they landed at the expected point of consumption.
Besides, the engine bell was designed in such a way that it could have collapsed on landing. So; I'm not sure anyone would be confident that integrity of the bell if it needed to take off again without an inspection.

[wd40]

The Eagle landed with only 25 seconds of fuel left which was already close to the decision of an abort.

Judging by this description, an abort sounds like it would have been a hairy event.

[schlaugh]

As for restarting, Apollo 13 restarted the descent engine several times. But i'm not sure if the descent engine had enough thrust to lift the LEM from the surface.

[NEOWatcher]

As for restarting, Apollo 13 restarted the descent engine several times. But i'm not sure if the descent engine had enough thrust to lift the LEM from the surface.

If I remember correctly, the operation of the LM was limited to 65% power to avoid damaging the engine.

Damaging the engine in an abort maneuver probably isn't much of an issue. You just need to push the craft back up and have it last long enough so the ascent module can do its job.

[VQkr]

If I remember correctly, the operation of the LM was limited to 65% power to avoid damaging the engine.

Damaging the engine in an abort maneuver probably isn't much of an issue. You just need to push the craft back up and have it last long enough so the ascent module can do its job.

If a LM crew had executed a powered descent and then hovered just above the surface until their fuel ran out, the total DPS burn time would have been about 12.5 minutes. There was no hard limit on the number of restarts available to the engine.

Available settings were 100% on, and variable thrust from 10% to 65%. The descent engine ran at 100% for the first ~5 minutes of powered descent. Around the time of touchdown, only ~10% thrust was needed to hover so there was plenty of thrust available for the descent stage to lift off. Because there was so little fuel left and the descent engine was not designed to be usable after touchdown, the contingency for "teetering at the edge of a cliff" was to abort. Because of the low surface gravity a tilt-over would have developed slowly, giving the commander several (>4) seconds to hit the abort button before the craft fell over irrecoverably. Upon abort the ascent stage was designed to recover from a 60° angle tilt automatically (70° if the pilot overrode the angular acceleration limits).

In an ideal landing in which the descent module still had ~2 minutes of hover time and the landing did not damage the descent engine, and the crew had not depressurized the descent propulsion system yet, yes they could have theoretically fired the engine again, hovered, and touched back down.

[wd40]

Without starting a new thread, for how many minutes could the Ascent Module's motor maintain 100% thrust, and once ignited, could it also be shut down and restarted?

Aside from a total failure of its motor to ignite, what was the next worse scenario involving the AM that would have compelled the Command Module pilot to return to Earth alone eg a failure of the thrusters, or the radars and computers failing to locate eachother, lock on and dock?

How long could the AM by itself remain in lunar orbit and at that stage of the mission how long were its life support supplies: hours or days?

[schlaugh]

My recollection may be wrong but I believe LEM abort scenarios all used the ASCENT engine with the lower section being jettisoned, even if functional.

A great deal of information on LEM landing scenarios is contained in the Spaceref.com site. See especially 4.0 ABORT AFTER TOUCHDOWN.

[NEOWatcher]

Without starting a new thread, for how many minutes could the Ascent Module's motor maintain 100% thrust

Probably just enough to get it into orbit for rendezvous with some safety margin. The reaction control system and the CM/SM did the maneuvering for re-docking.

and once ignited, could it also be shut down and restarted?

Most likely. It was hypergolic and fixed thrust.

How long could the AM by itself remain in lunar orbit and at that stage of the mission how long were its life support supplies: hours or days?

Probably not much longer than A17 was on the moon (over 3 days) or was used as a lifeboat in A13 (again, over 3 days)

[VQkr]

My recollection may be wrong but I believe LEM abort scenarios all used the ASCENT engine with the lower section being jettisoned, even if functional.

A great deal of information on LEM landing scenarios is contained in the Spaceref.com site. See especially 4.0 ABORT AFTER TOUCHDOWN.

No, the LM could abort without staging during the descent. For any abort scenario during the descent in which the DPS was still working properly, it was safer to build up more altitude before performing the in-flight staging sequence. The LM control panel included two abort buttons - "Abort" would switch the guidance system back to targeting a rendezvous orbit using the DPS (with possible staging later depending on how far into the descent they were), and "Abort stage" would immediately drop off a malfunctioning descent stage and switch to APS for the return to the CSM.

[schlaugh]

Ah, very good. And that makes more sense to use the descent for altitude and stability until it must be cut loose.

[joema]

No, the LM could abort without staging during the descent. For any abort scenario during the descent in which the DPS was still working properly, it was safer to build up more altitude before performing the in-flight staging sequence....

Correct. Apparently the author of "The Eagle Has Crashed" did not understand this. He equated an abort solely with staging. One key reason for a 20 sec propellant margin on the descent stage was to reach a safe staging altitude using the Descent Propulsion System (DPS) engine. Near touchdown the LM was very light relative to the DPS maximum thrust.

LM thrust-to-weight ratio using DPS was about 3.7-to-1 at lunar landing, producing about 34.8 meters/sec^2 vertical acceleration at max thrust. Even a 2 second burst near touchdown would provide adequate altitude for safe staging. This explains why a non-staged abort was often preferable before landing: higher thrust/weight ratio and better abort margins.

Re the original question about landing at a bad angle, moonquake, etc. then taking off, there are several issues (some already covered):

(1) Yes the DPS engine could be ignited after landing, although this wasn't intended due to possible crushing of the engine bell and backpressure if it were obstructed. After landing a staged "abort" or ascent stage liftoff would be the normal procedure.

(2) The normal post-landing procedure was to "safe" and depressurize the descent propulsion tanks, so any hypothetical DPS liftoff would have to be before this.

(3) The risk of landing at tilt angles precluding liftoff was very low. The nominal tilt limits are often stated as +/- 12 degrees from vertical, but in fact a safe ascent stage liftoff was possible at extreme angles -- apparently up to 62 degrees from vertical (maybe more). The LM stability limit was 43 degrees from vertical, beyond which it would fall over. So even if it hooked a leg over a crater and the entire vehicle was nearly sideways at 60 deg. from vertical, the ascent stage could still safely lift off. This was due to the low lunar gravity and the rapid pitch response on the ascent stage from the RCS thrusters (sized to control the larger two-stage vehicle).

(4) As VQkr mentioned, if the vehicle was unstable after landing and started toppling over, there was a several second margin where the ascent stage could have theoretically staged and aborted. In lunar gravity it fell over fairly slowly. This scenario was considered extremely unlikely due to the above factors -- the LM could land on almost any conceivable surface even if this wasn't "in the spec". However the tiltover case was studied including probable pilot reaction time and the conclusion was it would have been possible.

We had an extensive discussion of this on this thread, including contributions by Dr. David Baker who wrote "History of Manned Space Flight": http://forum.nasaspaceflight.com/ind...?topic=32246.0