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Post by porphyry on Feb 15, 2010 14:15:37 GMT -4
I was discussing with another friend inclined toward the "HB" persuasion, and he thinks the biggest problem with the Apollo photographic record is the lack of disturbance of the lunar regolith under the LM engine. Also, McGowan is going to be calling me, and I'm sure he'll ask about the pristine lunar surface under the LM.
This doesn't bother me so much, because I don't feel any definite conviction about whether there should be a crater under the LM, or not. It depends on whether the blast force of the LM engine exceeded the bearing strength of the regolith. My usually infallible intuition fails me here :-)
I did see some of the discussion of this at BAUT, where it was suggested (1) that the LM throttle might have been cut at a high altitude, and (2) that similar-sized engines on the Harrier jet don't create craters.
I don't want to try to support the throttle-cut idea because I don't see the benefit to it -- why would they want to cause any unnecessary mechanical shocks to the system? Besides, McGowan found an astronaut who said the opposite.
About the Harrier jet, it would help to have photos or other evidence showing Harrier operations on sandy, crumbling soils of similar bearing strength to the lunar regolith.
Or if anyone has done any calculations about the force per square meter of the LM engine ejecta vs. the crush strength of the regolith, that might also be an interesting approach to answer this objection.
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Post by echnaton on Feb 15, 2010 14:48:02 GMT -4
First it is important tot get a few things straight.
The Photos showing that are under the LM do not show a "prisine" surface. The show a surface that has been swept of dust and small debris. Find some and compare the look under the LM to the dusty areas where the boot prints are.
The LM full throttle was used for slowing the LM while in orbit. While landing the engine was producing a much lower level of thrust, just enough thrust to bring the LM, with most of the lower stage fuel depleted, slowly down. It was cut after probe contact with the surface and either before or after the actual landing, depending on the mission. Either way the LMs were generally drifting some until landing. That is they did not hover at high elevation over a point then descend to it. The LMs landed with the sun at their "back" and there is evidence in some photos of a sweeping of the surface on the sunward side of the LM.
Anyone that claims there should have been a crater is ignoring the facts and should be asked to prove the assertion before proceeding to further discussion.
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Post by JayUtah on Feb 15, 2010 14:49:34 GMT -4
...I'm sure he'll ask about the pristine lunar surface under the LM.The photographic evidence from Apollo 11 shows it to be anything but pristine. There is clear evidence of fluid scouring and sweeping. My usually infallible intuition fails me here :-)I don't want to try to support the throttle-cut idea because I don't see the benefit to it -- why would they want to cause any unnecessary mechanical shocks to the system?The LM was designed to be operated that way. The struts were fitted with crushable shock absorbers that were designed to absorb the impact of the LM falling from a height of about 2 meters in lunar gravity. I've had the pleasure of inspecting LM-2's struts myself, and I'm familiar with the design. The long probes extending from the bottom of three of the four LM footpads are designed to sense the lunar surface while the vehicle is still high above it, so that the pilot can cut the engine at the designated height. The principal concern was the effect of the engine exhaust on the LM as it interacted with the lunar surface. There was the fear that back pressure waves would form and damage the LM. This is a real fear in rocketry, which is why on Earth we have elaborate systems for damping the fluid effects and shunting away exhaust gases at liftoff. Early rockets suffered damage from exhaust blow-back. Besides, McGowan found an astronaut who said the opposite.Indeed, we know from the film that Apollo 11 touched down with the engine running at approximately hover thrust. Armstrong did not hear Aldrin call "contact light." We know from a couple other pilots that they didn't trust the mandated landing mode and cut their engines much lower than 2 meters. Landings varied. But if you want to talk about how the LM was designed to work, then the final 2-meter unpowered drop is a highly-documented feature. About the Harrier jet, it would help to have photos or other evidence showing Harrier operations on sandy, crumbling soils of similar bearing strength to the lunar regolith.Sure, but the Harrier's combined thrust is about ten times that of the hovering lunar module. Have you seen a Harrier dig a hole anywhere? Or if anyone has done any calculations about the force per square meter of the LM engine ejecta vs. the crush strength of the regolith, that might also be an interesting approach to answer this objection.www.clavius.org/techexhaust.html but this is a work in progress. The first-order version of those computations is commonly done every time the question is brought up. It comes down to about 1 psi, or about 1/100 the pressure exerted by a car resting on its tires. Intuition fails here for a number of reasons. First, laymen are inclined to think of "rocket engines" as invariably powerful. A hover thrust of 2,500 lbf is simply not that much. Second, just under half the thrust of the LM in a vacuum is pressure thrust, not momentum thrust. That is, it's generated by static pressure at the exit plane, not conservation of momentum. Third, in a vacuum the plume spreads out rapidly from the nozzle. 90 to 95 percent of the exhaust plume mass is in a cone with a half-angle of about 45 degrees, so it really spreads out. Fourth, first-order approximations of fluid impingement pressure can be done easily, but the problem is actually a complex one in fluid dynamics. Sweep and scour are the principle erosion modes here, and they represent a fundamentally different nature of force than simple dynamic gas pressure. Sorry, but again the conspiracy theorists have the burden to prove that their expected effects are based in physics, not mere layman's intuition.
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Post by Data Cable on Feb 15, 2010 14:51:25 GMT -4
I don't want to try to support the throttle-cut idea because I don't see the benefit to it -- why would they want to cause any unnecessary mechanical shocks to the system? The throttle wasn't sharply "cut" (other than shutting the engine off at probe contact with the surface) but rather was steadily throttled back during the descent due to the fact that the LM was continuously getting lighter due to expending fuel. [Edit to add: I interpreted "cut at high altitude" to mean more than the designed-for 2m drop distance. Is that what you meant?] The loose lunar regolith is only a few inches deep, beneath which is tightly compacted material, both of which are largely the same color. This makes it difficult to see where the loose "topsoil" has been swept away from the more dense "bedrock" beneath. |
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Post by laurel on Feb 15, 2010 15:25:07 GMT -4
I think "undisturbed" is too strong a word. In the documentary In The Shadow of the Moon, Charlie Duke said, "We were blowing lunar dust everywhere. It was like landing through the fog." Or if you read the Apollo 15 ALSJ: [Jones - "In the Tech Debrief, you said you lost the surface at about 60 feet. John (Young, Apollo 16) and Pete (Conrad, Apollo 12) talk about being able to see a few rocks through the dust to give them a little judgment on left/right, forward/back. There weren't very many rocks around your site."]
[Scott - "There weren't many rocks around, and I don't remember seeing anything. It was just a white-out. www.hq.nasa.gov/alsj/a15/a15.landing.html |
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Post by PhantomWolf on Feb 15, 2010 15:58:32 GMT -4
Doesn't look very "pristine and undisturbed" to me. AS11-40-5864AS11-40-5918AS12-47-6910AS14-66-9266AS15-87-11842Note that Apollo 15 turned its engine off quite high and so achieved the hardest landing of any of the missions, something the caused a lot of ribbing of the all Airforce Crew. This is why the "sweeping" is a lot less in this image, but it is still visible. I am gathering that by Apollo's 16 and 17 the surface area under the bell wasn't considered worth more study as there don't appear to be any colour images of it. |
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Post by porphyry on Feb 15, 2010 17:32:51 GMT -4
Thanks for this info.
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Post by cos on Feb 15, 2010 19:38:44 GMT -4
In all the supposedly elaborate things done to create a hoax I would suggest that digging a crater under the LEM would have been one of the simpler things if they thought it was required. And if the set designer forgot to dig said crater the last thing you would do is draw attention to it. So why would Armstrong describe the blast crater (lack of) as virtually his first act after stepping onto the moon? www.youtube.com/watch?v=d73jCthcAok&feature=related(5.48 in) Even before you have to do a bit of technical research to discover the actual downforce, the hoax proposition is a non sequitur. P.s I have seen 3 Harriers take off (vertically) from a grass strip and they never left a crater. If the hoax proponents say there should be a crater I want to see their homework, just saying it should is not an argument. |
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Post by JayUtah on Feb 15, 2010 19:43:30 GMT -4
I am gathering that by Apollo's 16 and 17 the surface area under the bell wasn't considered worth more study as there don't appear to be any colour images of it. Correct. In a quirk of history, Apollo 11 was classed as a test flight. Its only written objective was, "To perform a manned lunar landing and return." They didn't even have to get out of the LM in order for it to have been judged successful. But once that objective was secure, the secondary objectives came into play. The scientific objectives got lots of attention in the publications, but the lesser-known engineering objectives included assessing the performance of the lunar module. That meant lots of photographs of the various parts of the lander, including the area underneath. Once the various design hypotheses had been either confirmed or refuted, there was little need to gather further information. LM-5 ( Eagle) was the last test article; LM-6 ( Intrepid, Apollo 12) would be the first item considered operational. But that's mainly why we have so many photos of Eagle and the ground around it. It wasn't meant to set a precedent for LM photography. |
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Post by randombloke on Feb 15, 2010 19:46:27 GMT -4
Also, it should be remembered that the lunar soil is basically grey all the way down; without a pre-landing reference shot to establish the precise shade of the "pristine" soil, it becomes very difficult to determine that the subsoil has been exposed or disturbed.
Excepting, of course, the obvious radial striations caused by scouring etc.
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Post by carpediem on Feb 15, 2010 19:47:31 GMT -4
Or if anyone has done any calculations about the force per square meter of the LM engine ejecta vs. the crush strength of the regolith, that might also be an interesting approach to answer this objection. The closest I've seen is some excellent Youtube videos by Philwebb59, although they were created to debunk a specific hoaxer I think he covers most of the missing crater controversy. www.youtube.com/watch?v=BueSyXC8cf8www.youtube.com/watch?v=g2-mqEc0PFIIn the last video he considers the amount of missing material www.youtube.com/watch?v=yDs6-3Yt0Yk |
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Post by ka9q on Feb 15, 2010 19:51:08 GMT -4
Doesn't look very "pristine and undisturbed" to me. Note that Apollo 15 turned its engine off quite high and so achieved the hardest landing of any of the missions, something the caused a lot of ribbing of the all Airforce Crew. You can also see in that picture that the engine nozzle struck the surface and buckled. The J-series LMs (15,16,17) had longer DPS nozzle extensions for improved engine performance. This was necessary to land the extra payload mass for the extended surface missions. My understanding of rocketry is that a longer nozzle is especially helpful in a vacuum because plumes otherwise diverge very rapidly when not constrained by atmospheric pressure. The longer nozzle also converts more of the heat energy in the exhaust to linear kinetic energy much as increasing the expansion ratio in a gasoline or diesel engine increases efficiency. The exhaust is cooler and the engine is more efficient. |
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Post by JayUtah on Feb 15, 2010 20:15:26 GMT -4
[Edit to add: I interpreted "cut at high altitude" to mean more than the designed-for 2m drop distance. Is that what you meant?]We have to recall that Porphyry has a degree in physics. It's fair to assume he has a working knowledge of Newtonian dynamics. The loose lunar regolith is only a few inches deep, beneath which is tightly compacted material...Yes, I would consider sand to be a poor analogue of lunar regolith. This was part of my interview with History Channel. First, as we've discussed at length, the non-spherule particles are jagged and form a tight matrix when compressed or made to settle. We also have chemical cementation. All that's covered in the Preliminary Science Report and is well accepted by soil mechanics specialists. Sand is composed of rounded particles that do not cohere very well. Second, the regolith particle sizes vary greatly. Beach sand, in contrast, is more uniformly sized. When you have an aggregate composed of different sized particles, you create conditions for dry macadam concretion. The macadam principle (John MacAdam, a Scottish roadway engineer) uses large aggregate particles to fill the required volume quickly, then progressively smaller particles to fill the gaps between larger ones. The resulting concrete may not require any chemical cement to achieve the desired rigidity. This is the principle by which cobblestone streets are still built. The lunar surface naturally attains this condition as one progresses below the surface regolith. Loose regolith was estimated at 1-7 cm depending on landing site, and beneath this the regolith had tightly compacted. This will occur through thermal epansion and contraction cycles, electrostatic effects built up through solar radiation, impact from incoming meteorites, moonquakes, and gravitational fluctuation. ...both of which are largely the same color.Yes, that's key to remember. The regolith is uniformly dry regardless of depth, unlike Earth soils which generally acquire groundwater with depth and take on a darker appearance. What do you get when you scrape away the regolith? More regolith. Based on Armstrong's inspection of West Crater, the bedrock is several meters below the surface.
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Post by JayUtah on Feb 15, 2010 20:33:24 GMT -4
The J-series LMs (15,16,17) had longer DPS nozzle extensions for improved engine performance. This was necessary to land the extra payload mass for the extended surface missions.In addition to more massive payloads (expanded consumables, ALSEP, LRV), the J-type lunar modules were meant to fly steeper approaches and landings than their predecessors, so that they could clear the mountains in the lunar highlands. That meant an approach that was no longer fuel-optimal, therefore greater fuel efficiency was desired. My understanding of rocketry is that a longer nozzle is especially helpful in a vacuum because plumes otherwise diverge very rapidly when not constrained by atmospheric pressure.Thrust chamber and nozzle design is a black art. We are just reaching the point where we can simulate enough of the thermal and flow regimes computationally in order to avoid extensive empirical trial and error. In general the de Laval nozzle wants to be long and skinny in order to best collimate the thrust and maintain laminarity. However, long skinny nozzles increase flow drag. So there's a competing engineering variable. Further, the smaller the exit plane area, the greater the exit-plane static pressure. That static pressure causes plume expansion too.
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Post by trebor on Feb 15, 2010 20:34:54 GMT -4
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