posted 01-04-2015 06:06 PM
quote:
Originally posted by Blackarrow:
I specifically disagree with the comment quoted...
To recap, the issue is whether damage to the Apollo 15 lunar module's engine bell was caused by contact with the lunar surface or engine-exhaust back-pressure.At the outset, I should declare an interest: I wrote the Apollo 15 chapter in "Footprints in the Dust" and in October, 2008 when I interviewed Dave Scott, I discussed this very issue with him. During the interview and in follow-up emails he agreed that the damage was probably caused by the engine bell impacting the rim of a small crater which had been completely obscured by dust during the final descent.
I am confident that this is the correct conclusion, and that it is supported by (at least) 2 Apollo commanders and a MIT engineering professor.
Strange as it may seem, Apollo 16 is the starting point to understand this issue. When John Young and Charlie Duke were on the surface, Duke is reported in the ALSJ at 119:17:36 as saying: "...the engine bell is about 6 inches off the ground and it's not crushed at all."
There are no Apollo 16 photographs showing close-up views of the underside of "Orion" but a decent enlargement of part of AS16-107-17434 shows the engine bell clear of the surface. I estimated about 4-5 inches, but I will not quibble with Duke's figure of 6 inches: he was there. However, the key point is that "Orion" is resting on flat ground, and in spite of that, the clearance under the engine bell is barely the gap between thumb-tip and forefinger-tip when stretched apart.
Young and Duke both noted with some concern that "Orion" was only 10 feet from a fairly steep-sided crater. In the "Apollo 16 Technical Crew Debriefing" John Young said (page 9.17): "If the rear strut had been over that hole, we'd have been just like the Apollo 15 guys. We would have been landing on the first 3 and the engine bed."
I suggest in the context, the word "bed" is a typo and he actually said "bell" but either way it is clear what he is saying.
Referring back to the Apollo 16 photograph, consider the effect on the clearance between the engine bell and the ground if "Orion's" rear legs had been in a crater or a depression leaving "Orion" tilted 11 degrees from the horizontal (as in the case of "Falcon.")
Returning now to Apollo 15, on inspecting the exterior of "Falcon", Scott reported (at 119:57:14): "Tell the Program Manager [Jim McDivitt] I guess I got his engine bell. It's a little rise right under the centre of the LM. The rear leg's in a crater and the rim of the crater is right underneath the engine bell."
In the "Apollo 15 Technical Crew Debriefing" (at p. 10.3) both Scott and Irwin agreed that the front landing pad was, at most, only lightly in contact with the ground. The landing pad actually pivotted round when Irwin stepped on it.
During my 2008 interview with Dave Scott I asked whether it was possible that "Falcon" was actually resting on three legs and the engine bell. He agreed that this was probable. In a long follow-up email after the interview (which actually includes the long quote from the Apollo 15 Mission Report which LM-12 quoted in his 2nd January post) Scott wrote:
Your comment about the engine bell being in contact with the surface, yet the front footpad was 'loose' seems to indicate that we may have made contact with the bell at about the same time as the footpads, hence a 5-point landing. If so, the LM struts would not have absorbed as much of the impact as designed, and maybe that's why the 'bump.'
This is an important point: the landing-legs were designed to absorb much of the landing forces, but contact between the engine bell and the ground would have transmitted impact forces directly through the structure of the LM, explaining the jarring landing which rattled everything in the LM cabin.Let's now turn to the photographic record. Kipp Teague provided the ALSJ with an enhanced image taken by Scott of the underside of "Falcon." The Apollo 15 Mission report refers to the engine bell damage as "buckling." I am not an engineer, but I must suggest that the visible damage looks entirely consistent with a direct impact with the rising ground under the LM. It crumples inward, and does not seem consistent with an internal pressure build-up caused by reflected exhaust gas.
I accept that the closest part of the bell may be an inch or two off the surface, but it definitely appears to me that the rest of the bell is imbedded into the rising crater wall. Furthermore, the large white object, apparently the "jettison bag" is resting on the ground beside the engine bell. This provides scale, and strongly suggests that the part of the bell beyond the bag is in contact with the ground.
I said I am not an engineer, but David Mindell is. The author of "Digital Apollo", Mindell is Professor of Engineering Systems at M.I.T. At page 255 of his book, discussing the jarring impact, he writes:
"Indeed, there had been a bump: the LM was overlapping the edge of a small crater, with the bell of the descent engine dropping directly on its rim, buckling it."
In conclusion, I suggest that when careful consideration is given to the very limited engine bell clearance on flat ground, it is almost impossible to see how contact between the ground and the engine bell could be avoided if the rear of the LM drops into a crater, leaving the LM 11 degrees off the horizontal (as alluded to by John Young). I'm sure a scale diagram could show this clearly, but I suggest that the photographic evidence is sufficient to put the issue beyond dispute.
I will add one last comment: the Apollo 15 Mission Report states: "The crew also reported that the buckling seemed to be uniform around the skirt periphery and that the exit plane height above the surface was uniform." I would be interested to learn if anyone can find any relevant dialogue or comment in the ALSJ, or the Debriefing Report, or elsewhere. I can find no corroboration for that statement, and I must suggest that the photographic evidence suggests otherwise.
posted 01-03-2015 03:47 PM
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