|
Post by theteacher on Nov 16, 2011 17:51:34 GMT -4
Looks like Collins has fallen prey to a common misconception. The pupil responds very quickly to light level changes, but it only provides a small range of sensitivities. The real factor is the sensitivity of the retina, which slowly increases in the dark to far larger values than in bright illumination. I should have used the term "narrative" instead of "explanation" and thus have avoided to confuse the two. I merely wanted to convey his experience, as "he was there" :-) A brief explanation is here by the way.
|
|
|
Post by JayUtah on Nov 16, 2011 17:54:26 GMT -4
The stream was directed at the Mylar You mean not directed at the Mylar. When the fluid stream hits a surface perpendicular to the flow, it tends to hug the surface. That's because the right-angle turn for the fluid particles is the least-energy solution, compared to going any which way. Now you still get a bit of spread due to turbulence and static-pressure effects. But the overall trend is to spread out laterally, not billow up in a cloud or bounce back.
|
|
|
Post by JayUtah on Nov 16, 2011 17:56:02 GMT -4
you guys were dead wrong concerning the laws of thermodynamics in a vacuum Nope. Did you read the text on heat transfer that I linked you to?
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Nov 16, 2011 17:57:36 GMT -4
The stream was directed at the Mylar You mean not directed at the Mylar. Sorry, I just caught that mistake and corrected it.
|
|
|
Post by trebor on Nov 16, 2011 17:59:20 GMT -4
trebor really NONE no difference at all False. Heat is a transfer of energy from a high temperature system to a low temperature system. or are you attempting to ask about about temperature vs specific heat? An excellent example; with specific heat being the amount of energy needed to raise the temperature of a system by a certain amount. don't you guys get tired of this? Don't you get tired of being constantly wrong?
|
|
|
Post by gillianren on Nov 16, 2011 18:10:45 GMT -4
Don't you get tired of being constantly wrong? First, he would have to acknowledge that it's ever possible for him to be wrong.
|
|
|
Post by ka9q on Nov 16, 2011 18:26:30 GMT -4
so exhasut gas if, what did you calculate was 1000 degrees at the end of the nozzle, then it will be very close to 1000 degrees period as it expands out into the vacuum. It's more complex than that. When an expanding gas does work (e.g, against a piston or turbine blade), that energy comes from the heat energy of the gas. So the gas always cools. But if you expand an ideal gas past the end of a nozzle into a vacuum without doing any work it will stay at the same temperature. An ideal gas has no forces acting between the individual atoms or molecules. No energy is released or needed to move them apart. Since the gas does no work, conservation of energy keeps the gas at the same temperature. However, real gases are not ideal. They have forces acting between their molecules, such as the Van der Walls force, and this introduces the Joule-Thompson effect. Depending on the specific gas, its starting temperature and other factors it can warm, cool, or stay the same. Air conditioning evaporators get cold because the refrigerant boils when the pressure is lowered. A boiling liquid absorbs heat. But there's no phase change in the hot gases coming out of a rocket nozzle so we can't say what will happen to their temperature without additional information. We can still address the original question about damage to thermal surfaces. The pressure of an expanding gas obviously falls, so even with the engine firing the pressure at the blanket surfaces will be extremely low. Even if the individual gas molecules striking it are hot, so few gas molecules do so that very little heat energy is transferred to the insulation and it remains undamaged. In practice, radiative heating from the hot engine nozzle is a more serious consideration. That's why the LM has a heat shield on its bottom surface around the descent engine. A special shield protects the landing radar antenna also mounted on the underside. By the way, I don't think the LM thermal blankets are even Mylar. Mylar is grey but the blankets appear gold. They are in fact Kapton (polyimide) with a thin layer of aluminum deposited on the back side. Kapton appears orange-gold in color. Kapton is very popular in aerospace applications because it withstands very high temperatures. It doesn't even have a melting point; it decomposes at well over 400 C. So there's even less to worry about here.
|
|
|
Post by dwight on Nov 16, 2011 18:32:11 GMT -4
JayUtah could you demonstrate that principle on a pool table? How bizarre. You play pool with fluid billiard balls?
|
|
|
Post by Apollo Gnomon on Nov 16, 2011 18:34:29 GMT -4
JayUtah could you demonstrate that principle on a pool table? Why would you land a Lunar Module on a pool table? That doesn't make any sense.
|
|
|
Post by ka9q on Nov 16, 2011 18:42:33 GMT -4
A page on the external coatings of the Apollo Lunar Module: home.earthlink.net/~pfjeld/lmdata/index.htmlSome interesting items: Aluminized Kapton is used quite a bit on the Descent Stage. There is no Mylar on the outer surfaces of the LM. The heat shield around the descent engine is nickel foil, obviously intended to reflect infrared. Extra aluminized Kapton was added to the legs of LM-5 (Eagle) shortly before launch precisely to protect the legs against the engine plume if the engine is still firing at touchdown -- as it was during the Apollo 11 landing.
|
|
|
Post by laurel on Nov 16, 2011 19:16:35 GMT -4
JayUtah could you demonstrate that principle on a pool table? Why would you land a Lunar Module on a pool table? That doesn't make any sense. Yeah, because they didn't play pool on the Moon. Landing a LM on a golf course would make more sense.
|
|
|
Post by JayUtah on Nov 16, 2011 19:30:59 GMT -4
could you demonstrate that principle on a pool table? 1. What principle are you talking about? 2. Why a pool table?
|
|
|
Post by ka9q on Nov 16, 2011 19:56:21 GMT -4
Well, we've shown that plume heating simply wasn't a problem. In fact, Kapton was added to the legs of the Apollo 11 lunar module Eagle precisely to protect the legs against the plume if the engine were to run all the way to touchdown.
|
|
|
Post by JayUtah on Nov 16, 2011 20:13:56 GMT -4
the pool table was just to point out, which was pointed out, it is not laminar flow, but can be turbulent flow also, call it a draw. What are you asking to call a draw? I'm still not sure what the pool table has to do with it. Are you suggesting that the felt covering of the pool table enhances turbulence? The problem is that at something on the order of 2,000 meters per second, laminar flow overwhelmingly dominates the behavior. Turbulent flow is at best a second-order effect.
|
|
|
Post by ka9q on Nov 16, 2011 21:05:52 GMT -4
It took a long time to come up with the Aluminized Kapton good job, that gave you another 500 degrees or so, a lot less chance of being damaged by "heat" from exhaust. you win Kapton is rated up to 400° C; that's 750° F! It can go even hotter for shorter periods of time; the graphs show it being tested up to 600° C (1112° F) but then it falls apart in "only" 6 hours or so. The Apollo 11 LM descent engine fired for about 12.5 minutes, and that was unusually long. And that's only if the Kapton on the LM is actually raised to those temperatures. It isn't. In a vacuum, a rocket plume expands so quickly, and its pressure drops so quickly, that even a very hot gas simply won't transfer a significant amount of heat. And finally, 5 mil Aluminized Kapton has a high emissivity in the far infrared (0.86) so what little heat is transferred will be quickly radiated away. So once again, the bottom line is that there was simply no problem. As I said, Apollo engineers actually added aluminized Kapton to the legs of the Apollo 11 LM Eagle when the astronauts expressed concerns about engine plume impingement were the engine to run all the way to the surface, which it did on that flight. I think the case is closed.
|
|