Thank you Jrkeller...[Johnny Carson voice]I did not know that[/Johnny Carson voice]

Stainless is typically not used for outer space applications since it has extremely poor surface optical properties.

Which is why, when we are forced to use it for structural reasons, we cover it with aluminized Kapton or Mylar to improve the absorption. The classic example is the Apollo command module. Inconel, another common aerospace alloy, also has attrocious absorption properties. But it has wonderful thermal and structural capacity, so we have to use it.

Due to its low density, aluminum is the material of choice for most aerospace applications.

Yes, and we use it in various alloys, some of which were developed specifically for space applications.

If my strength of materials book was at home with me right now I'd give you an simple structural analysis of the visor.

That would be a fun exercise, but let's keep in mind that it's SaturnV's task to either take or shirk intellectual responsibility for his claims. I want to see him do the structural analysis.

I think I know where this argument came from. From Kaysing, yes, but I think I see now what's going on inside his head on this point. I think everyone has had the experience of heating up a glass bottle and then cooling it rapidly in cold water to watch it crack and/or shatter. If you mistakenly thought the helmets were made of glass, and if you didn't understand the thermal ramp rates associated with exposure to sun in space, you might think the helmets would shatter.

But the helmets are not made of glass, and something alternately exposed to and shaded from the sun does not swing instantly from -200 F to 250 F. The thermal contraction failure of glass is simply irrelevant to the whole point. But it's what someone relying on "common sense" might misunderstand.

That's stainless steel I'm talking about the real stuff actual steel fully stainable it does have a 100,000 PSI tensile strength.
I read a book about steel and it's alloys. If you had read it you would know that certain types of steel were developed for the apollo program in the 1960's that yielded over 150,000 PSI tensile strengh.


<font size=-1>[ This Message was edited by: SaturnV on 2002-11-10 16:55 ]</font>

He has no experience but has had the concept verified by a couple of pilots. The pilots confirmed that the thick plastic used on aircraft windshields have to be carefully borught down to temperature in order to keep them from cracking.
Even I don't know what aircraft windshields have to do with helmets in the apollo space suits. Kaysing does use some strange science to explain what he is not sure of.

I got the point. What would he have to gain by lying? Thousands of dollars in book sales and fame.

We've spent a lot of time in this area trying to guess why hoax believers believe in the "Apollo Hoax."

A very few are seeking money.

But most, I think we've all agreed, are looking for truth, but are using the wrong philosophical tools. They focus on trivia (shadows in photographs) without giving proper weight to the overwhelming preponderance of evidence, and the overwhelming difficulty of anyone contriving such a conspiracy.

Americans are able to accept (provisionally) conspiracies involving dozens of people. (e.g., that the LAPD had planted the bloody gloves behind O.J. Simpson's house.) And we have historical evidence of such ugly conspiracies, such as the Tuskegee syphillis experiments or the exposure of military personnel to radioactive substances.

But how do we accomodate a conspiracy involving millions of people?

By the way, have you (or you-all) seen the text of the speech Richard Nixon was prepared to give if the Apollo mission had failed and left Armstrong and Aldrin stranded on the moon? Eerie. Haunting. A great speech, and a terrifying one. It is echoed by Ronald Reagan's great speech commemorating the Challenger astronauts.

(About the same time, the speech was uncovered that General Eisenhower was prepared to issue if the D-Day invasion had failed.)

I look up at the moon (well, not tonight, as it hasn't risen yet) and absolutely beam with pride and glee. We've Been There!

Silas

He has no experience but has had the concept verified by a couple of pilots. The pilots confirmed that the thick plastic used on aircraft windshields have to be carefully borught down to temperature in order to keep them from cracking.

That's to prevent microfractures, which is a problem when the windshield is subjected to high aerodynamic loads. In that case it is the aerodynamic loading which would fail the windshield, not simply thermal cycling.

Of course I don't think Bill Kaysing knows what a microfracture is, so he's probably just as confused. His real problem is passing his confusion on to his readers in the guise of understanding and erudition.

Then you have to consider that at low altitude, air friction at typical airliner speeds will actually heat up the window. So you go from aerodynamic heating to cruise flight cold soak and back to aerodynamic heating again. This type of cycling would not occur in an Apollo space helmet because the helmet is not hurtling through the air at .8 Mach.

Even I don't know what aircraft windshields have to do with helmets in the apollo space suits.

Next to nothing.

The sources of heat and cold that affect aircraft windshields are not a factor for Apollo space helmets. The sources of mechanical stress on aircraft windshields are not a factor for Apollo space helmets.

Kaysing does use some strange science to explain what he is not sure of.

Bill Kaysing has carefully cultivated the image of himself as a "former NASA engineer", and so he must have a putatively scientific explanation for his "findings". The general public might be fooled, but the genuine scientists and engineers are not. Bill Kaysing is an English major and a professional conspiracy theorist.

Bill Kaysing does not know what he's talking about, but that doesn't stop him from making up whatever he needs in order to retain a semblance of credibility. His livelihood depends on people believing his theories.

There is an interesting book called "Almost History," which contains the texts of a few dozen documents like that. Much of it is very moving material, and the rest of it is just really cool.

Aporetic

Using some simple structural analysis methods, an operating pressure of 5.5 psia, and an ultimate tensile strength of 9500 psi, I got a factor of safety for the lexan visor of approximately 70. So in other words, the helmet is 70 thicker than it needs to be to hold the pressure within the suit.

I think the reason the helmet visor so much thicker is that it needs to take the impacts of bumping into things and of course falling down. If you have the opportunity to view the helmet cam video from the ISS assembly missions, you will see that the astronauts do bump into a lot of things.

S-V what's your number and how did you get it.

<font size=-1>[ This Message was edited by: jrkeller on 2002-11-11 11:05 ]</font>

Here's a link to Nixon's unused speech, prepared in case of the failure of the Apollo mission.

http://news.bbc.co.uk/1/hi/sci/tech/390933.stm

I'm still looking for a link to Eisenhower's unused speech...

Silas

I think the reason the helmet visor so much thicker is that it needs to take the impacts of bumping into things and of course falling down.

Obviously correct. In deciding how much of a factor or margin to employ, engineers must consider the consequences of failure. Because helmet failure would cause near immediate astronaut incapacitation and death it is wise to provide a healthy -- even ridiculous -- margin.

If there were flaws in the space suits (helmets that crack, or suits that lose pressure when exposed to sunlight) wouldn't that have caused all earth orbit spacewalks to end in catastrophe?

I've seen the Hubble repair missions on TV. Seems like the suits worked just fine.

I remember spacewalks back in the Gemini program that were successful.

I remember spacewalks back in the Gemini program that were successful.

Some were and some weren't. And among those that weren't, deficiency of the space suit was sometimes a factor. The point is that Gemini fulfilled its role by developing the technology to make Apollo possible.

The first step in designing a successful visor is determining what that visor must accomplish. It must resist static gas pressure against a vacuum. It must resist impact from projectiles and from falling down. It must do this in an environment of unfiltered sunlight. (Polymers, for example, are degraded over time by ultraviolet.) And it must do this in a thermal environment consistent with space.

Without training or experience, it is hard for the layman to understand the peculariaties of thermodynamics in space. Human intuition is almost hard-wired to deal with the presence of an atmosphere as a convective medium. That makes it hard to deal with helmet design using only common sense.

The interior surface of the visor will be in contact with air which is kept at 68 F or so by the addition of energy as necessary. We expect a thermal gradient from the inner surface to the outer surface as heat is conducted away from the interior and radiated away from the outer surface. Sunlight falling on the visor will be transmitted in most wavelengths, and reflected in a few. Absorption is minimal. Therefore the thermal cycles arising from alternating exposure and shielding from the sun are neglible in the engineering sense.

Bill Kaysing wants to compare all that to airliner windshields, which is a completely different animal. Standard airliner cabin pressure is ironically about the same as an Apollo spacecraft -- about 5 psig, or 720 psfg. Fairly substantial, but within the capacity of today's polycarbonates. There is about a 40X factor on airliner windows.

Dynamic loading varies. At 175 kts at sea level (e.g., take-off and landing) the dynamic load on the cockpit windshield is about 100 lbf/sqft. At cruise altitude (35,000 ft @ 0.8 Mach) it's about 170 lbf/sqft. This acts contrary to the static load and is therefore somewhat mitigated by it.

But the thermal cycling is the problem. A 0.8 Mach slipstream at -60 F provides a tremendous source of forced convection. Thermal energy is lost very rapidly to such a force, and so the windshields must generally be heated to mitigate the effects. A typical ascent or descent is 20 minutes, during which time the slipstream can vary in temperature by 100 degrees F or more, in velocity by hundreds of feet per second, and in density by an order of magnitude. These thermal cycles, in combination with varying aerodynamic stress, can produce microfractures in the material which systematically and uniformly weaken it over a reasonably lengthy time. Invisibly too. Thus airliner pilots are wise to use methods to control the thermal cycling of the windshields.

But these effects are due to the atmosphere, not heating or cooling from the sun. Take away the atmosphere and you have no such difficulty. So the comparison of Apollo helmets to airliner windshields is apples to oranges. If the helmets had been subjected to the same thermal cycling as airline windshields, then there might be a cause for concern. But they were not, so there isn't. Bill Kaysing's argument is just a big, distractive red herring.

Bill Kaysing is a red herring...

I hope those better informed than I will excuse a (probably) stupid question from a neophyte but I was thinking about this. Is the following logic correct? Since the area above lunar surface is a vacuum, surely it hasn't got a temperature - there's nothing there to be hot. The surface underneath may be hot (since its heated by the thermal input from the sun) and the space suit/helmet may be heated the same way but there is no temperature differential to cause cracking. What there is, is the space suit getting hotter. Now, if the wearer of the space suit steps into the shadows, its not as if he is stepping into cold at all. There isn't any cold to step into; there is just an absence of thermal input from the sun. All that happens is that the sun stops heating the helmet and it starts to cool down as the heat radiates away. The only temperature differential that exists is between the outer surface of the helmet (being cooled by radiation) and the inner surface. That differential will be determined by the thermal conductivity of lexan and the thickness of the helmet. The higher the conductivity and the thinner the helmet, the less marked the temperature differential across its thickness and the less likely it is to crack. That being the case, isn't the suggestion that the helmets would crack rather foolish since companies have been making oven-safe glasswear for decades?

<font size=-1>[ This Message was edited by: Stuart on 2002-11-12 13:59 ]</font>

That's about 30kPa, isn't it? What's the composition? Isn't there a risk of nitrogen narcosis at pressures that low?

Next time I fly, I'll have to remember we're breathing Apollo air. [img]/phpBB/images/smiles/icon_cool.gif[/img]

That's about 30kPa, isn't it?

That sounds about right for the conversion, but remember it's a pressure gradient. That's the difference between "psig" and "psia". The 'a' is for "absolute" (i.e., kinetic-molecular interests) and the 'g' is for "gradient" or "difference in pressure" (for structural engineering interests).

Now that I've looked at some standard charts, the pressure difference may be a bit more. You typically want a cabin pressure altitude of 9,000 feet (2,700 m) or so. Normal air pressure at that altitude is about 10 psia (ca. 70 KPa). At 35,000 feet (10,700 m) normal air pressure is about 3.5 psia (ca. 25 KPa).

So the cabin will be pressurized to about 10 psia and that will be carefully controlled to alleviate the effects of altitude such as dehydration, fatigue, and in extreme cases nitrogen narcosis. Outside the pressure will be around 3.5 psia -- not sufficient to sustain life or consciousness. But the difference in pressure will be about 6.5 psig, and that's what the structural engineer of the airliner has to worry about. He or she will specify that the airliner's pressure vessel be made to withstand differences in pressure of up to a certain amount, plus a margin for safety.

What's the composition?

Oxygen and nitrogen.

Isn't there a risk of nitrogen narcosis at pressures that low?

No, because from your point of view you've only ascended to 9,000 feet and you've done so slowly over a period of 20 minutes or more. Boeing airliners have a panel above the copilot's head which controls the rate at which the cabin pressure changes. Like everything else on a modern airliner, it's quite a sophisticated system.

From the structure's point of view, it has gone from an environment of zero pressure difference across its fuselage to one of 6.5 psig, which is fairly substantial.

Next time I fly, I'll have to remember we're breathing Apollo air. [img]/phpBB/images/smiles/icon_cool.gif[/img]

No, not really. But you're seated in a pressure vessel that's engineered to a greater degree of strength and reliability than an Apollo command module.


<font size=-1>[ This Message was edited by: JayUtah on 2002-11-12 14:21 ]</font>

Here's some data on the LEXAN that is used for the helmets.

From http://www.geplastics.com (you've got to register to get the data sheets)

The ultimate tensile strength is 9600 psi

From measured data here at Johnson Space Center

Solar Absorptance - 18%
Solar Reflectance - 9%
Solar Transmittance - 73%

Infrared Emittance - 86%