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  Apollo lunar module touchdown dynamics

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Author Topic:   Apollo lunar module touchdown dynamics
LM-12
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posted 06-29-2014 10:15 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
An interesting description of the six Apollo lunar module (LM) lunar landings can be found in this NASA SP-2013-605 An Analysis and a Historical Review of the Apollo Lunar Module Touchdown Dynamics document.
The touchdown dynamic digital simulation of the Apollo 11 indicated that the footpad touchdown sequence was that the +Y footpad made first contact with the lunar surface, followed by the +Z footpad 0.1 second later...
The description for NASA photo AS11-40-5915 includes the following:
At the time of touchdown, the Lunar Module was basically moving away from the point at which this photograph was taken, or 90 degrees to the local slope. The flight path would have been just over the crater, as seen in the lower left-hand side of the photograph, prior to landing...

Headshot
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posted 06-30-2014 07:27 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Thanks LM-12, this is indeed a fascinating document. I believe a similar study was published in the Surveyor Program Results for the four Surveyor spacecraft that made successful unmanned landing, although the report was not quite as elaborate as this.

LM-12
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posted 06-30-2014 11:19 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
You can see in Figure 12 (frame 5858) that the Apollo 11 +Y contact probe dragged across the lunar surface just before touchdown.

Figure 29 (frame 9255) shows how the Apollo 14 +Y footpad plowed in a bit on that crater rim.

LM-12
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posted 07-03-2014 02:31 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The LM descent engine exhaust is mentioned on page 13:

It was noted that the exhaust velocity of the descent engine was on the order of 10,000 feet per second and the orbital velocity for the moon was about 5000 feet per second. The lunar dust, entrained by the descent engine plume, would move over the lunar surface on the order of lunar orbital speed.
Neil Armstrong made a very interesting comment about observing dust particles in the engine plume during touchdown. See the Apollo 11 Lunar Surface Journal (just before 102:45:45).

LM-12
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posted 08-04-2014 10:20 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
How was the descent engine shut down? Was it by button or switch? Where was the button/switch located on the LM control panel?

Lou Chinal
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posted 08-04-2014 02:11 PM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
I would like to know also. I've heard of two schools of thought on this:
  1. the engine was shut down manually with a button on the control panel (shut down DPS)
  2. the engine was shut down automatically when the probes made contact with the lunar surface
The LM was designed to withstand a free fall of up to 12 feet. That was the design limit.

Headshot
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posted 08-04-2014 02:24 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
My memory may be defective, but as I recall, when the LM probes contacted the lunar surface they triggered the Lunar Contact lights (there were two I believe) to illuminate. The DPS was to be shut off manually.

I also recall reading that Neil Armstrong calculated, and confirmed his calculations with Grumman engineers, that the LM could withstand a free fall of 40 feet to the lunar surface.

Robert Pearlman
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posted 08-04-2014 02:27 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
From the Lunar Module News Reference:
The [descent] engine remains on until an engine-off discrete is initiated by the astronauts with either of two engine stop pushbuttons or by the computer. When the LM reaches the hover point where the lunar contact probes touch the lunar surface, a blue lunar contact light is illuminated. This indicates to the astronauts that the engine should be shut down.
On Apollo 11, Armstrong intended to shut down the engine when the contact light lit, but missed seeing it and did not clearly hear Aldrin report the light as turning on.
I heard Buzz say something about contact, and I was spring-loaded to the stop engine position, but I really don't know... whether the engine-off signal was before (footpad) contact. In any event, the engine shutdown was not very high above the surface.

We actually had the engine running until touchdown. Not that that was intended, necessarily.

LM-12
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posted 08-04-2014 02:50 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Apollo 14 post-flight debrief included these comments by Al Shepard regarding touchdown:
For touchdown, we had the habit of waiting about 2 seconds after the lunar contact light came on before shutting the engine down. From the looks of things, we actually were on the ground and stopped before the engine shut off. It must have been a pretty light touchdown.

Lou Chinal
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posted 08-04-2014 03:43 PM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
Okay, just for everyone's general knowledge, how long were the probes?

Headshot
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posted 08-04-2014 03:53 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Virtual LM by Apogee Books show two blue Lunar Contact lights on LM Panels 1 (in the center) and 3 (upper right hand corner). These lights are shown on panel diagrams from other sources also.

The probes extended approximately 5 feet below the base of the LM footpad.

LM-12
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posted 08-04-2014 04:38 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The lunar surface probes were about 67 inches in length, according to the NASA SP-2013-605 document mentioned earlier.

Does the Virtual LM book show where the "two engine stop pushbuttons" are located?

Headshot
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posted 08-04-2014 06:27 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
I have not located either "engine stop pushbutton." Either the print is too small and I cannot make it out or, more likely, the buttons are labeled in NASA-speak as something else.

LM-12
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posted 08-04-2014 06:40 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
I am looking at this diagram, and I don't see any "engine stop pushbuttons" either.

mikej
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posted 08-04-2014 06:40 PM     Click Here to See the Profile for mikej   Click Here to Email mikej     Edit/Delete Message   Reply w/Quote
I have a high-resolution scan of the Lunar Module Controls and Displays from the Apollo Lunar Module News Reference.

I took a quick peek and failed to locate the DPS controls, but all of the labels are quite readable.

Elsewhere on the page, I had already annotated the location of the "Contact" lights.

Robert Pearlman
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posted 08-04-2014 06:54 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
The commander-side push button is on Panel 5, labeled "START" (as it both started and stopped the descent engine). Here's the panel from inside Apollo 16's Orion.

The LMP-side push button is on Panel 6, as seen here.

Both are identified by surrounding yellow and black stripes.

Headshot
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posted 08-04-2014 07:14 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
So, you hit the Start button to turn off the engine? Sounds like this is where Microsoft got the idea for shutting down Windows!

Thanks Robert, we would have been looking all night for these things.

space1
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posted 08-04-2014 08:02 PM     Click Here to See the Profile for space1   Click Here to Email space1     Edit/Delete Message   Reply w/Quote
I'm a little late to this fun discussion. I am going to disagree with Robert on this one.

The photos to which Robert linked showing Panels 5 and 6 show a box assembly (which includes the engine START button) with a latch mechanism attached to the left side of each box. Apparently that latch mechanism was actuated to push a button to enable the engine. The engine would be turned off by releasing this latch mechanism.

Robert Pearlman
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posted 08-04-2014 09:07 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
I wondered about the matching latch mechanisms on both panels, and it makes more sense to have those functions protected.

sev8n
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posted 08-04-2014 10:53 PM     Click Here to See the Profile for sev8n     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
So, you hit the Start button to turn off the engine?
Haha, that's how my Prius works!

LM-12
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posted 08-04-2014 11:10 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
It is interesting to note if the CDR was "inside" or "outside" the cockpit when the blue LUNAR CONTACT light came on.

Armstrong was busy avoiding that boulder field, finding another landing spot and dealing with translational velocities, so he was probably "outside" the cockpit when the blue light came on. Conrad and Scott were seeing a lot of dust.

CONRAD: I had my head in the cockpit when the LUNAR CONTACT light came on and I instinctively hit the STOP button and that's how we got a shutoff in the air.

SHEPARD: Ed was inside the cockpit, mostly, giving me values of velocities, and I was outside the cockpit, mostly.

SCOTT: At about 50 to 60 feet, the total view outside was obscured by dust. It was completely IFR. I came into the cockpit and flew with the instruments from there on down.

YOUNG: From 200 feet on down, I never looked in the cockpit. It was just like flying the LLTV; your reference is to the ground outside ... the ( LM ) shadow was right there to help you with the rate of descent.

CERNAN: I saw the shadow come right on up to me ... When it passed on under me, I was expecting a blue light.

LM-12
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posted 08-05-2014 04:19 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by space1:
Panels 5 and 6 show a box assembly (which includes the engine START button) with a latch mechanism attached to the left side of each box.

The LM diagram I linked earlier shows a switch detailed near the latch mechanism on Panel 5. It is below "not placarded" but I cannot make out the switch label. Considering its location, it might be part of the descent engine shutdown procedure.

space1
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posted 08-05-2014 06:49 PM     Click Here to See the Profile for space1   Click Here to Email space1     Edit/Delete Message   Reply w/Quote
That switch says DPS RATE (Descent Propulsion System rate), +1 FPS (Foot Per Second), CENTER, -1 FPS. That switch was spring-loaded to center, and used for making fine adjustments to the descent rate.

LM-12
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posted 08-05-2014 07:34 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
Okay. Thanks for that explanation, space1.

At 119:57:14 in the Apollo 15 Lunar Surface Journal, Dave Scott talks about shutting down the descent engine. He mentions the blue CONTACT light, then he mentions three buttons and says it is the "blue one" that shuts down the engine.

space1
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posted 08-09-2014 03:19 PM     Click Here to See the Profile for space1   Click Here to Email space1     Edit/Delete Message   Reply w/Quote
Looking at Apollo Operations Handbook Lunar Module LMA790-3-LM5 Volume I Subsystems Data, page 2.3-2, there are only two ways to manually shutdown the descent engine. One is to use a switch on Panels 5 or 6 (as described above), and the other is to use the ABORT STAGE switch on Panel 1. You do not want to use the ABORT STAGE switch because it will also separate the Ascent Stage and fire the ascent engine for an abort.

I don't know what Scott means by the blue button. The CONTACT LIGHT indicators were blue lights (as Scott says), but they were not also switches. As far as I know the Panel 5 and 6 switch latch mechanisms were a bare metal finish and not blue in color, although if we were searching for a way to defend his remark maybe titanium or aluminum could appear to have a bluish cast.

Here are the switch and latch mechanism on LM 10 Panel 5.

LM-12
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posted 08-09-2014 08:45 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
That must be the DPS RATE switch that you described earlier below the latch mechanism.

space1
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posted 08-10-2014 06:41 AM     Click Here to See the Profile for space1   Click Here to Email space1     Edit/Delete Message   Reply w/Quote
Yes that is the DPS RATE switch. And the manual also says that DPS is Descent Propulsion Section, not "system" as I had written.

LM-12
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posted 08-16-2014 10:56 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
Some interesting comments in the Dynamics document regarding Apollo 15:
The most dynamic landing was Apollo 15...

Although the descent engine nozzle did buckle, the cause was not due to contact with the lunar surface during the touchdown maneuver...

The digital simulation also indicated that... the +Z landing gear footpad was off the landing surface in the final at-rest position.

All times are CT (US)

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