Inspired by a show with James May where he visited the old Airfix factory - ye olde english brand of plastic model kits - I ordered a kit from them that I remember building some thirty years ago (now long gone…).
It’s a model of the lunar landing module. Opening the box was really a trip down memory lane. They’ve added a few bits, since this was an anniversary edition, but the main kit is still the exact same as it was when I got it for christmas when I was 11 or 12.
Anyway. In preparing to build this thing again, I looked at a few pictures of the actual LM’s. There’s quite a lot of foil there, especially on the descent stage. Here’s a pic from Apollo 13’s LM.
What was the purpose of that foil? Were there different kinds of foil (colors seem to differ)?.
The wrapped section of the LM contained several tanks of fuel, oxidizer and water, as seen in this cutaway. It’s unclear whether NASA sought to keep these things from freezing as the LM radiated heat away to the blackness of space, or overheating as it absorbed incident sunlight. Probably the latter, as the LM sat on the illuminated surface of the moon for a long time and would have been radiating away a lot less in that circumstance.
It would be wrinkly because it’s fitted loosely over the items it covers; this minimizes heat transfer via direct conduction from the LM to the blanket.
That’s what it sounds like. I know that “space blankets” which are used in camping, at marathons, etc. are rather like Mylar balloons. And, the Wiki article on Kapton, the material on the LEM, describes it as a “polyimide film”.
In a vacuum, increasing the number of layers will reduce the rate of heat transfer. But there diminishing return as the number of layers increase. Different kinds of blankets are available which can have transparency or reflection to a specified band of EM (IR and visible light, e. g.).
Unless the tanks are contained internally (and even these still have minimal blanketing), within the spacecraft structure, you can bet that they will insulated like hell if mounted externally. A frozen tank is useless, an overheated one is dangerous.
FWIW, I was told the “wrinkleness” was the result of the plastic sheeting and coating acting as a laminated surface and elongate beyond the tensile strength of the interface.
OK, I’ll steal a line from Apollo 13, where it mentioned a difference in temperature of 400 degrees F between sun side and darkness. Lunar and orbital conditions aren’t very different.
So, on the sunward side, it’d be REAL hot. Blowing tanks into next Tuesday hot. On the shaded side, it’d be cold, MOST gasses freezing cold.
Hence, why orbiters rotate, to reach the “happy medium” of heating and cooling AND “stirring” the tanks.
The thermal blankets are an attempt to control thermal INPUT, whist also balancing output/input, to keep the element/compound at a certain temperature/pressure.
As for wrinkles, THEN, it’d largely be due to material constraints, overall.
NOW, more balanced, as computer modeling would give rather good information as to reflect/shadow per wrinkled area.
In short, comparing 20th century practice against a 21sth century mental filter…
Having the insulation sheets wrinkled actually does have a functional reason. The only way heat can transfer between layers of insulation in a vacuum is through thermal radiation or direct contact. Thermal radiation you control by coating the insulation foil in a material with very low emissivity. Direct contact between the sheets can be minimized by making the sheets wrinkled. If the sheets were smooth, they’d be able to lay flat and make physical contact over large areas of surface. Wrinkled insulation sheets will only touch each other at points, and so are more effective as insulation. The insulation foil on the LEM was wrinkled deliberately.
oooh, I built one of those as a kid. I was probably only like 6 or so. It was part of some sort of sciencey book club. The books came with the illustrations as separate stamps that you had to lick and paste into the books, and the model was a premium to get you to sign up for the books.
A shorter answer would be, INCREASED SURFACE AREA.
Wrinkle the surface, present irregular and LARGER surface area, which can radiate energy.
Vacuum is REALLY different from our air filled environment, as air can carry heat. In a vacuum, heat can only be inefficiently radiated, if energy of higher input isn’t radiating on it (like some Sun thingie)..
On a similar note, one of the major issues during the problematic launch of the Skylab mission was that the sunshield got torn off during launch (a spine of metal from which prevented deployment of one of the solar panels). NASA therefore improvised and sent up a huge sheet of aluminized mylar that the astronauts unfurled over the sunward side of the space station during a pre-inhabitation spacewalk, in order to replace it. Note that the original shield was smooth and not wrinkled.
The intent of the blankets, as described upthread, was to reduce heat transfer, not increase it.
Moreover, as regards radiative heat transfer, I don’t believe wrinkles would enhance the overall view factor between the vehicle and deep space; faces of adjacent wrinkles would essentially be radiating heat toward each other instead of out toward space. Radiation toward deep space would happen only from the bottoms of the gaps between wrinkles, and from the tips of the wrinkles - basically the same surface area you would have if the wrinkles didn’t exist in the first place.
Right. The purpose of the wrinkles is not to increase surface area. The purpose of the wrinkles was to decrease physical contact between layers of insulation. Less physical contact between layers equals less heat transfer through conduction from layer to layer.
Close, but you forget reflection. In THAT case, the “valleys” would reflect between themselves and reflect outward (at a slight loss).
There are TWO factors present: Radiation outward and reflection. Hence, the use of metal coated mylar.
For reflection, the oblique reflection would be a good thing due to scattering and perhaps a bit of destructive (though unlikely) interference. AND there’d be a LOT of room for thermal expansion and contraction, compared to a taut materiel.
What’s not definitive about these answers? To reiterate what others have said: the “foil” is called multi-layer insulation (MLI), and used on most satellites and spacecraft. The purpose is insulation; it avoids excessive heating when the surface is facing the sun, and avoids excessive cooling when it’s facing away from the sun.
Not sure what you’re trying to say. If anything, the wrinkles reduce the radiation slightly, because some of the radiated energy will hit the wrinkles rather than being radiated straight out. Given the same surface properties, a rough, textured surface has higher emissivity than a smooth surface. But I think the slight amount wrinkling you see on a typical MLI has negligible effect on its radiative properties.
It’s just the nature of the material - it’s not an elastic (stretchy) film. And there’s no reason to try to make it perfectly smooth and flat; and some reason it may be undesirable (e.g. if it’s stretched taut, it’s more likely to rip due to mechanical stresses, vibration, thermal expansion/contraction, etc).
.jpg){kind=link}