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Author
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Topic: Apolllo 15 lunar module landing attitude
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LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-10-2011 10:24 PM
  
The Apollo 15 Lunar Module landed with its rear (-Z) footpad in a five-foot deep crater. As a result, it came to rest at a pitch-up attitude that is very evident in some of the EVA photos taken. When Jim Irwin came down the ladder, he spun around on the front (+Z) footpad and almost lost his balance because that footpad was barely touching the lunar surface.The LM was pitched up about 7 degrees. What was the tilt angle limit for liftoff? How close did Scott and Irwin come to that limit? |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 06-10-2011 10:51 PM
In one sense they were somewhat fortunate that the quadrant of the LM with the ALSEP package was not the quadrant farthest up in the air. That was one of the major concerns if the LM landed on a slope, that the ALSEP would be too high up for the astronauts to unstow it. The other concern was the engine bell coming into contact with the surface while the engine was still running.The official stats from the Mission Summary were: The vehicle contact velocity and attitude data at touchdown show that the landing was very stable in spite of the relatively high lunar surface slope at the landing point. The plus-Z and plus-Y footpads contacted the lunar surface nearly simultaneously, providing a nose-up pitch rate of 17 deg/sec and a roll rate to the left of 15 deg/sec. Final spacecraft settling occurred 1.8 seconds later. The vehicle at-rest attitude, as determined from the gimbal angles, was 6.9 degrees pitch up and 8.6 degrees roll to the left, resulting in a vehicle tilt angle on the lunar surface of approximately 11 degrees from the horizontal. |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-10-2011 11:31 PM
Interesting statistics. The roll was even more than the pitch. |
jasonelam
Member Posts: 691
From: Monticello, KY USA
Registered: Mar 2007
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posted 06-11-2011 05:11 AM
The maximum tilt that could occur and allow for a safe liftoff was 12 degrees. |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-11-2011 07:38 AM
So the LM tilt angle on the lunar surface was 11 degrees from the horizontal, and the maximum angle for a safe liftoff was 12 degrees. That's cutting it a little close! |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 06-11-2011 04:59 PM
Makes you wonder what would happen if they had tilted just one or two more degrees. You have to imagine that they would still try to take off anyway. What else could they do? |
SpaceAholic
Member Posts: 4494
From: Sierra Vista, Arizona
Registered: Nov 1999
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posted 06-11-2011 05:32 PM
12 degrees was not the "make or break", it was a self imposed limit based on toppling stability (relative to the LM's "X" axis and local horizontal plane). There was added conservatism and actual envelope varied with respect to true center of gravity (based on liquid load and position of pax/payload). |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 06-11-2011 08:51 PM
I used to enjoy testing the tilt limits in EagleLander3D by landing on the interior slopes of Hadley Rille and seeing how cockeyed I could get and still take off. |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-12-2011 07:33 AM
Even if the astronauts found a crater-free area to touchdown, wouldn't it be difficult to see if they were landing on a slope because of all the dust kicked up by the descent engine? |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 06-12-2011 09:00 AM
During P64, when the computer was targeting the general landing site, the Commander scanned the area and clicked the controller to adjust the computer targeting to an area that was as free of craters and boulders as possible. Ideally, the CDR had this area picked out before he reached an altitude of 400 feet. Left to its own devices, when program P66 kicked in at 400 feet, the LM guidance system would then bring the ship down on the targeted spot. As we know, all of the CDR's flew their final approaches manually. The goal was to have killed all forward, backward, and lateral movement by the time he was 100 feet above the surface. That was before any sizeable amount of dust was being kicked up. They would try to bring the LM straight down from 100 feet. It wasn't all that easy, as you'll hear the LMP calling out lateral velocities during final descent in almost every recording of a lunar landing ("four forward, drifting to the right a little"). What you REALLY wanted to avoid was drifting backward. |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-12-2011 10:49 AM
The Moon is such a fascinating place. What a unique challenge that must have been for the astronauts to make a level landing on such a rugged terrain. Thanks for all the detailed information ilbasso.The view out the LM windows during the landing phase was facing west and down-sun. That has a tendancy to wash out lunar surface details, making it more difficult to see craters, boulders and slopes. From the Apollo Lunar Surface Journal:
In a 1995 letter, Dave commented that he may not have been aware of the crater in which he put the rear pad because "it was shallow and probably had no shadow". |
nasamad
Member Posts: 2141
From: Essex, UK
Registered: Jul 2001
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posted 06-12-2011 12:54 PM
If I remember correctly from reading the Apollo Lunar Surface Journal, the angle of the LM also caused an issue with the LRV deployment,so one or both of the astronauts had to jiggle the deployment/release mechanism to get it out. |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 06-12-2011 05:07 PM
The question got me to thinking about what would happen on liftoff for the ascent module if there was serious tilt. There might be a slight lateral movement of the ascent stage when the pyros were fired to sever the connections to the descent stage, in the brief moment between the firing of the pyros and the ignition of the ascent engine. The bottom thruster in one or more RCS quads could conceivably impact its plume deflector and the bottom of the ascent engine bell might hit part of the top of the descent stage.The Soviet lander might suffer real problems in the event of a severely tilted angle on the surface. Since the ascent engine bell went all the way through the descent stage, might a bad tilt have caused the ascent engine to impact or hang up on the descent stage during liftoff? |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-16-2011 09:41 AM
Dave Scott made some interesting comments about the LM landing shortly after he stepped onto the lunar surface. From the Apollo 15 Mission Transcripts: "Well, I see why we're in a tilt. We've got - (laughter) that's very interesting. There's so man -- so much hummocky ground around here, we're on a slope of probably about 10 degrees. And the left-rear footpad is probably about 2 feet lower than the right-rear footpad. And the left-front's a little low too. But the LM looks like it's in good shape. The rover is in good shape. Tell the Program Manager I guess I got his engine bell. (Laughter) It's a little rise right under the center of the LM. The rear leg's in a crater and - and the rim of the crater is right underneath the engine bell." See photo AS15-87-11839. |
Blackarrow
Member Posts: 3160
From: Belfast, United Kingdom
Registered: Feb 2002
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posted 06-17-2011 04:03 AM
Many of these issues are dealt with in the Apollo 15 chapter in "Footprints in the Dust." For instance, there was a risk that if the enlarged LM engine bell contacted the surface while still firing, "blow-back" could cause an explosion. The instructions to Dave Scott were to cut the engine as soon as the contact light came on. Just as well he did -one side of the engine bell came down on the edge of a small crater and was crumpled. Essentially, it was a "five-point landing" which probably explains why the landing felt very "hard" with some of the shock transmitted up through the engine and descent stage structure. |
LM-12
Member Posts: 3324
From: Ontario, Canada
Registered: Oct 2010
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posted 06-17-2011 03:22 PM
Dave Scott talks about that in the technical debriefing: SCOTT: It gave us a tilt of about 10 degrees left and 10 degrees up, which was subsequently no problem. There was a rumble when we landed. I think all the equipment on board rattled. It seemed as if I could hear it all when we landed, like you would shake the vehicle. Couldn't you hear that?IRWIN: Yes, I agree. |
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