Armour in Space?

[Ginnie]

I've come across something in a book that I don't quite understand:

"According to the current data, for a two weeks' voyage, such as would be needed to circumnavigate the moon, the total radiation dose due to primary cosmic radiation and that of the natural and artificial belts surrounding the Earth should not be more than 10 rem with an armour of 2 to 4 lb/sq.ft."

"The protons produced by solar flares, on the other hand, are a real menace to the health and even the life of an astronaut. For greater safety, it would therefore be disirable to increase the armour to 6 lb./sq.ft. "
The Space Encyclopedia (1969)

What does the term 'armour' refer to?

[JayUtah]

To shielding, obviously -- but "armour" is a pretty non-technical name for it.

Shielding density is commonly given in terms of mass per unit area. It indicates the amount of mass that particles impinging on that unit area of outer skin would have to traverse and/or be absorbed in. Think of the unit area as a "window" and the mass figure as the amount of shield mass behind that window. The Apollo spacecraft provided considerably more than 6 lbm (pounds-mass) per square foot.

[Ginnie]

Aug 11, 2008 21:07:08 GMT -4 JayUtah said:

To shielding, obviously -- but "armour" is a pretty non-technical name for it.

Shielding density is commonly given in terms of mass per unit area. It indicates the amount of mass that particles impinging on that unit area of outer skin would have to traverse and/or be absorbed in. Think of the unit area as a "window" and the mass figure as the amount of shield mass behind that window. The Apollo spacecraft provided considerably more than 6 lbm (pounds-mass) per square foot.

...well, not that obvious, obviously... ;D

Now, lets add this in to the picture...
Do you have a few examples of materials and their lbs/sq.ft rating?
Like, what would be the rating of paper, or tin foil etc. Even if someone could point me to a website that has than information.

And I still have a hard time understanding mass vs. density/weight. Even Bob B. PM'd me with info on that and I still can't get my head around it.

[JayUtah]

Like, what would be the rating of paper, or tin foil etc. Even if someone could point me to a website that has than information.

Well, we typically measure it in grams per square centimeter these days. This is because flux values given by AE8 and AP8 are in particles per square centimeter per second. AE8 and AP8 are the mathematical models that describe the radiation environment in cislunar space.

Take a piece of paper and cut a square hole in it, one centimeter to a side. Think of it as making a little window. Now count how many radiation particles go through that little window in a second. That's "flux." Lots of particles through the window means lots of radiation.

Now tape your paper to the side of a spacecraft. Flux still describes how much radiation goes through that window in one second. But in order to affect anything inside the spacecraft, it has to get through the spacecraft skin, structure, or whatever is under that spot on the spacecraft.

Take your magic cutter and cut straight into the spacecraft exactly along the outlines of your square-centimeter window and pull out that little square core sample of the spacecraft -- whatever gets pulled out. Now find out how many grams it weighs. That's the mass of material at that spot in the spacecraft, through which a radiation particle would have to pass in order to get to the interior of the spacecraft and wreak some havoc. So if your core sample weighs 1,500 grams, that would be a shielding factor of 1,500 grams per square centimeter -- the square centimeter of your paper window taped to the skin.

A certain amount of mass behind "that little window" corresponds to a certain reduction factor in the radiation. So if you expect 100 units of radiation per second, and you need to attenuate that down to 10 units per second in order to keep your crew alive, you work through some equations to get the amount of mass you need behind that window.

Yes, it matters a bit what material you use for that mass, but the ballpark engineering is done with just raw mass. If for our toy problem that 1:10 attenuation factor means 2,000 grams of mass behind each square centimeter of skin area, then you can achieve that 2 kg with various materials. That is, you have to think of that 1 cm square core sample, built out to whater length of whatever material adds up to 2 kg.

If you choose lead, you won't need that much length in your core sample to get there. If you choose a much less dense material such as marshmallow, you may need several centimeters in length of that to get 2 kg of mass behind your square centimeter of skin area. Aluminum alloy is a common choice because the shielding and skin are essentially the same thing. You can also use a combination of lead, aluminum, and marshmallow to get the combination of structural strength and shielding, although engineers commonly substitute high-density polyethylene (the stuff hard hats are made from) for the marshmallow.

So that's the best way to think of the "mass per unit area" phenomenon. Think if it as an imaginary core sample drilled out of a one-centimeter area of skin space, and then weighed on a scale. So the overall shielding design has to put enough mass behind each square centimeter of skin space.

And I still have a hard time understanding mass vs. density/weight.

Yes, shielding measurements seem silly.

[Ginnie]

Okay, okay. Can two different materials have the same 'mass' but different shielding properties? By what you said before, the answer seems to be 'no', but...

[JayUtah]

Sure, a gram of marshmallow, a gram of lead, and a gram of polyethylene have different absorptive properties that affect whether we choose them for shielding in some application. But they're largely second-order effects, and depend on other factors such as radiation energy, radiation type, and side effects: things I left out of the example.

[Bob B.]

Jay, I assume you're just pulling numbers out of the air to illustrate your point? For instance, if 2,000 grams per square centimeter of shielding were really required, that equates to nearly 6 feet of lead (yikes, there's that dreaded number!). I don't want anyone to get the mistaken impression that you're using real numbers. In fact, the 6 lb/ft^2 number given in the initial post converts to just 2.9 g/cm^2 (the equivalent of just over 1 cm of aluminum).

edit spelling

[JayUtah]

I am totally pulling random numbers out of the air.

[BertL]

You explained before that a gram of marshmellow, lead or whatever (I assume here that the material here are manipulated to have the same density) have different absorptive abilities that affect their effectiveness against radiation. To what degree would the material matter, taking away the density factor?

EDIT: I'll be getting the subject of radiation in school in a few months, but it can't do much harm to do some research beforehand.

[JayUtah]

Different kinds of materials absorb different kinds of radiation better because of their atomic makeup. And different materials also respond to different energies better. When absorption entails a response by creating ions, the side-effects of that creation are important. Other side effects occur too, depending on the atomic behavior of the shielding material.

This includes the Brehmsstrahlung effect, whose side effect for some absorptions is x-rays, which can be as hazardous as the primary radiation. Yes, the material absorbs the incoming radiation, but you don't want the effect of that absorption to be a different problem you now have to handle. If your shielding is also part if your structure, then you have to know about, say, the mechanical effects of radiation absorption (e.g., metals become more brittle).

[Bob B.]

Aug 12, 2008 15:43:56 GMT -4 JayUtah said:

If your shielding is also part if your structure, then you have to know about, say, the mechanical effects of radiation absorption (e.g., metals become more brittle).

That is the research my father was involved in back in 1960-70s at NASA's Plum Brook Station. NASA ran a research reactor in which materials were subjected to high neutron fluxes and cryogenic temperatures to study the effects. The work was in support of the nuclear propulsion program. My father worked in the health physics office.

Speaking of neutron radiation, this is a good example of how the properties of the shielding material has to match up with the type of radiation. Protons, electrons, and alpha particles all have an electrical charge and, therefore, interact electrically with the atoms in the material through which they pass. These interactions reduce the particles' energy, and the more material through which the particles pass, the more energy they lose. Neutrons, on the other hand, have no electrical charge. They'll pass right through a material unimpedded until they smack into an atom. You therefore want to use a shielding material that has a bunch of atoms to get in the neutrons' way. Neutrons are not an issue with naturally occuring cosmic radiation, but for a nuclear reactor, they are a major concern.

[JayUtah]

...that equates to nearly 6 feet of lead (yikes, there's that dreaded number!).

Indeed, yikes. If I had bothered to think real-world, I would have specifically avoided anything close to Six Feet of Lead in my made-up numbers.

(the equivalent of just over 1 cm of aluminum).

Which, as long as we're carefully distinguishing toy examples from real-world figures, is about what is typically used. My brother designs rad-hard electronics and their enclosures for space, and his computer enclosures for long-term Van Allen belt soaks are 2 cm plus some change.

[Grand Lunar]

Jay, you are made of awesome.

Your brother works at making space-rated electronics? Whoa!

Is it true that this process makes space rated computer systems not as capable as Earth based ones, like those in a PC? But this also makes them less prone to crash?

When it comes to shielding electronics, is it nearly equal to providing protection for humans, or do electronics need less protection than the human body?

[Count Zero]

Aug 11, 2008 21:56:42 GMT -4 Ginnie said:

Do you have a few examples of materials and their lbs/sq.ft rating?
Like, what would be the rating of paper, or tin foil etc. Even if someone could point me to a website that has than information.

I googled "aluminum properties density" and found this site, which lists densities of various metals in lbs/square inch. Multiply by 144 to get lbs/ft^2 (note that your resulting weight will be for a 1-inch thick plate). Thus a 1/8th inch plate of aluminum 1-ft square would weigh 1.764 lbs. The same-sized plate of lead would weigh 7.38 lbs.

[Ginnie]

I just checked out that link CZ. I've bookmarked it and will take a good look over.
Thanks!