Contact light and engine cutoff...

[gonetoplaid]

From this link:

www.hq.nasa.gov/office/pao/History/SP-4009/v3o.htm

****************

December 9-16
Preliminary results of the "fire-till-touchdown" study by Grumman indicated that this maneuver was not feasible. The engine might be exploded by driving the shock wave into the nozzles. The base heatshield temperature would exceed 1,789K (5,000 degrees F), which was high enough to melt portions of the structure, possibly causing destruction of the foot pads. The allowable pressure on the nonstructural elements of the base heatshield would be exceeded; and the descent engine flow field would tend to cause a "POGO" effect which would cause landing instability and could prevent engine cutoff.

As an outgrowth of the study, the landing probes would have to be made longer (137.1 to 187.9 cm [54 to 74 in] with automatic cutoff, 228.6 to 304.8 cm [90 to 120 in] with manual cutoff). The probe switches would be moved from the tip of the probe to the base, which was objectionable from the standpoint of a possible false reading due to probe dynamics.

MSC, "ASPO Weekly Management Report, December 16-23, 1965."

****************

Well, knowing that the length from edge-to-edge of the footpads for opposite landing gear legs is 31 feet, I examined the Eagle LM in photo AS11-44-6598 and calculated that the length of the landing probes is 5.5 feet or roughly 66 inches. That seems to be right in the middle of the recommended probe length for using automatic engine cutoff.

My question is, does anybody know "for sure" whether the LM's used auto engine cutoff on probe surface contact or did the astronauts have to manually kill the engine within 0.7 seconds of getting the blue contact light? Some stuff that I have read says that ideally the engine had to be killed within 0.4 seconds.

I ask because some sources say that engine cutoff was automatic yet other sources say that engine cutoff was manually executed.

[homobibiens]

Aug 19, 2009 21:26:03 GMT -4 gonetoplaid said:

My question is, does anybody know "for sure" whether the LM's used auto engine cutoff on probe surface contact or did the astronauts have to manually kill the engine within 0.7 seconds of getting the blue contact light? Some stuff that I have read says that ideally the engine had to be killed within 0.4 seconds.

I ask because some sources say that engine cutoff was automatic yet other sources say that engine cutoff was manually executed.

I have no idea what the answer is, but the reaction time doesn't sound unreasonable to me. It's not something that could happen at any time, they would know about when it should happen, and be watching for it.

[Czero 101]

Engine shut-off was manual, based on indication of probe contact ("Contact Light") and altitude data from PNGS.

From the Apollo 11 Crew Technical Debrief, page 9-25 onward:

9.2.28 Touchdown

ARMSTRONG
We continued to touchdown with a slight left translation. I couldn't precisely determine touchdown. Buzz called lunar contact, but I never saw the lunar contact lights.

ALDRIN
I called contact light.

ARMSTRONG
I'm sure you did, but I didn't hear it, nor did I see it. I heard you say something about contact, and I was spring loaded to the stop engine position, but I really don't know whether we had actually touched prior to contact or whether the engine off signal was before contact. In any case, the engine shutdown was not very high above the surface. The touchdown itself was relatively smooth; There was no tendency towards tipping over that I could feel. It just settled down like a helicopter on the ground and landed.

Cz

[gonetoplaid]

Czero 101, thanks for digging that up. That seems to conclusively prove that engine cutoff was manually performed. I needed this info to address a HB question on YouTube.

Anyway, while we are at it, does anybody know exactly how long the landing probes were? I calculated 66" from the photo of the Eagle.

[Czero 101]

According to the Apollo Operations Handbook, Lunar Module (LM 10 and Subsequent), Volume One. Subsystems Data, page 1-4, LM Overall Dimensions, the LSSP (Lunar Surface Sensing Probe) extends 5' 7.2" from the bottom of the LM Landing Gear Footpad, as seen in the following Apollo 11 image:


^{Click for hi-res image}




Cz

[gonetoplaid]

He he. So my 66" probe length measurement was only 1.2" off from the actual probe length. Cool.

[Czero 101]

Some other documentation and blueprint schematics I have seen also mention 66 inches as well. I think the 67.2" is the entire length of the probe assembly including the mounting and the 66" is how far the probe hangs below the bottom of the footpad despite what the plan drawings I referenced earlier specify.


Cz

[homobibiens]

Manual seems to me like it would make sense. Either a false positive or a false negative would be really, really bad - having the astronauts do it seems to provide a certain level of error correction. Just my thoughts . . .

[gonetoplaid]

Aug 20, 2009 16:00:56 GMT -4 @homobibiens said:

Manual seems to me like it would make sense. Either a false positive or a false negative would be really, really bad - having the astronauts do it seems to provide a certain level of error correction. Just my thoughts . . .

I think that this is exactly why NASA overruled Grumman's recommendations and decided to stick with manual engine cutoff combined with excellent astronaut training, especially since concerns were expressed about the reliability of the contact probe switches.

[macapple]

Yes it was very manual as Pete Conrad found out on Apollo 12. He hit the engine stop button when he first saw the Lunar Contact light come on which switched the engine off whilst it was still space bourne.

I think they dropped about 1.5m to the surface whereas the other missions switched off when they were on the surface.

[laurel]

Apollo 15 switched off their engine as soon as the Contact Light came on. From page 413 of A Man On The Moon by Andrew Chaikin: "When Scott saw the blue glow of the Contact Light, his hand went immediately to the Engine Stop button. Several thousand pounds heavier than previous landers, Falcon fell to the Moon with a firm thud that rattled every piece of gear in the cabin and brought an exclamation of surprise from Irwin."

[gonetoplaid]

Sept 11, 2009 15:13:06 GMT -4 laurel said:

Apollo 15 switched off their engine as soon as the Contact Light came on. From page 413 of A Man On The Moon by Andrew Chaikin: "When Scott saw the blue glow of the Contact Light, his hand went immediately to the Engine Stop button. Several thousand pounds heavier than previous landers, Falcon fell to the Moon with a firm thud that rattled every piece of gear in the cabin and brought an exclamation of surprise from Irwin."

As usual, Laurel provides a tidbit of information that I didn't know about. Now I know why some of the metalized Mylar on the side of the Apollo 15 descent stage tore off and fell on the ground and atop the -Z LM footpad. Two Mylar sheets atop the -Z LM footpad of course lead Jack White to produce an absurdly wild hoax theory about those two Mylar sheets. A few days ago I had fun creating a video which debunks the "expert" photo analyst one eyed Jack's hoax claim:

www.youtube.com/watch?v=S6aiWV8gdgU

[echnaton]

Nice vid. It make much more sense than JW silly claims.

[scooter]

In the Apollo 16 debrief, John Young talked about the "one patato, two potato" before engine shutdown...seems the CDRs had discussed the "drop" after shutdown amongst themselves, trying to figure out a "smooth landing".
In retrospect...they all got the job done.