From: email@example.com (George Herbert)
Subject: Re: Hardsuit in Martian Dust Storm
Date: 24 Dec 1995 02:03:50 -0800
Organization: Retro Aerospace
In article <firstname.lastname@example.org>, Sever <email@example.com> wrote:
>Frank Crary <fcrary@rintintin.Colorado.EDU> wrote:
>>You may want to reconsider this. A hardsuit is
>>not viable on Mars. The current designs have a
>>mass of about 200 kg.
>Kerry Mark Joels in his book "The Mars One Crew Manual" suggests a
>hardsuit for Martian EVAs. Perhaps not the most scientific of documents;
>but it did seem well-researched. He gives the weight of the suit to be
>22.7 kilograms (Earth weight); although, he does not say what it is made
>of. I suppose he is assuming a significant advance in metallurgy, as you
>suggest. My story takes place between 2021 and 2023 (opposition
>class--outbound Venus swingby), which should be sufficient time for this
>type of advancement (I hope).
The question isn't one of metalurgy, really. It's one of joint design.
The rotary, pressure-sealed, low-drag joints which let a hardsuit be
relatively mobile are very heavy. Specifically, about 1kg for the
average diameter one on existing hardsuits. The material you make
the rest of the suit out of is minor, compared to the joint weight.
To keep mobility up, especially under planetary surface G's, you need
lots of these joints (so that the suit bends freely no matter what
direction you reach/walk/etc). The existing joints are fine in terms
of mobility but very poor from mass considerations. There are no current
proposals I am aware of on reducing those masses.
Btw, 22.7 kilos is probably lighter than it would need to be.
Our study came up with an optimal mass in the 45kg range (about 100 lb),
which would have a percieved weight on Mars of about 34 lb.
>>Quite correct. Scientific analysis suggests
>>that the winds on Mars would, at most, be able
>>to pick up millimeter-sized dust grains.
>Everything, I've read agrees with this. However, I've just received an
>email that suggests a small rock could be entrained by the wind. Once
>entrained it would be traveling at a very high velocity and would pack
>quite a punch. The author did say that it would be difficult to get a
>large particle entrained and that the object would probably remain near
>the surface (10-50 cm). Can anyone comment on this?
I'm sure you can find more detail on this in a book on winds on earth,
but I'm going to make a flying leap of a guess on how this might work.
Let's assume that you can get rocks blown up into the air by any wind
which has force equal to or greater than their mass. This is probably
going to overestimate maximum rock size, but who knows. It's probably
within a factor of 10, so it's just fine ;-)
If Cd of a rock is 1.0 (simple assumption), density = 0.01 of earths
(earths at 0C and 1 atm is 1.3 kg/m^3 or abt 11.8 N/M^3),
and Vmax=400kph ~=110 m/s then...
dynamic pressure = 1/2 Cd S (density) (V)^2
= 1/2 (1.0) (S) (0.01 * 1.3) (110)^2
~= 78 kg/m^2 ~= 677 N/m^2
So we just need to estimate how big a rock has a surface area to weight
ratio such that 78 kg/m^2 >= mass ...
If a rock is a sphere, then vol = 4/3 pi r^3 and surface area (exposed)
is pi * r^2. If rock has density of 4.0 and with the mars gravity of 0.35
weight = 0.35 * 4.0 * 4/3 * pi * r^3 ~= 5.85 r^3
(converting from tons/m^3 into kg/m^3 gives 5850 kg * r^3)
surface area = 3.14 r^2
drag ~= 78kg * surf area in m^2
= 245 r^2
5850 r^3 ~= 245 r^2
r^3 ~= 0.042 r^2
divide both sides by r^2
r ~= 0.042 meter about 4.2cm
[At this point, someone will step forwards from the audience and shoot me
for using kg instead of newtons for force. Fortunately, I wear a bulletproof
vest and am an engineer, not a physicist, so I am doubly protected ;-)]
So we have a completely wild guess that the biggest rock that could be
lifted up in any way might be about 3" in diameter. Let's look and see
what the results might be if the largest rock lifted by wind can be
weighing 1/4 of its drag... it looks like the equations above scale
linearly, so we're looking at a 1 cm radius rock. If it's 1/10 of its
drag, then it's about 4 mm radius (about 1cm diameter). If it's 1/25
then we're looking at about 3mm diameter, or 1/10".
I'm going to make a guess that a rock which weighs as much as its drag
could roll around on the ground under a wind storm like that. One which
is a quarter of its drag is probably going to bounce along the ground
at some small fraction of local wind speed, never getting very high up.
One which has its weight 1/10 to 1/25 of its drag will probably be moving
up in the air, so we're looking at small pebbles to sand grains moving
at wind speed, maybe. Assuming my guesses are right.
Doubtful that it would be big enough to crack anything but would be
abrasive as hell...
>>But it's hard to give better advice about an
>>accident without knowing what you want. Does
>>your plot demand a death?
>The plot does demand a death. The loss of this character introduces
>several complications that the remaining crew will need to resolve on
>their own. Preferably, the death should be quick since there is an
>astronaut nearby who could rescue him. The other astronaut would have to
>suit up first.
Well, if you just need to kill someone, let's see... falls off short cliff
or into crater which was obscured by the sandstorm, breaking neck.
Trips and falls onto large rock which smashes some critical piece of
suit, either dies instantly or before help can arrive. Is caught outside
wearing low-insulation suit (optimized for "hot weather" on mars) by
accident as storm starts, and freezes to death before they get back inside.
Or more likely gets chilled which contributes to bad judgement and one
of the other possible fatality methods, as you stated it had to happen
fairly quickly... if he gets cold too quick because of the dust storm
his judgement could go fairly fast. Lots of people hurt themselves
being stubborn in cold weather.
>>If your plot needs a death or a quick rescue,
>>I'd suggest falling and cracking open his helmet.
>I like the idea of falling. Since he is a dust storm, stumbling over a
>rock is a real possibility. Could such a fall short circuit his suit? If
>so, what life support systems would be affected?
We're trying to de-emphasize electrical systems on mars suit work we're
doing, but there would still be some need. If nothing else fans to blow
the air around and through the carbon dioxide scrubbers.
Falling down and losing power because the batteries got smooshed,
and then dying because the CO2 scrubbers weren't working, would take
a little while (get Frank to work out the numbers for you. ;-)
>Cracking open his helmet would be ideal, since it would bring about a
>quick death. However, my research shows that the visor is made of Lexan,
>which is describe as an "unbreakable, clear polycarbonate." Now, I could
>not find Lexan in either the encyclopedia or the dictionary, but the brief
>description I did find leads me to believe that it is pretty tough stuff.
>Is it plausible for a visor made of Lexan to break from a fall?
It's not unbreakable. It's very strong, though. You could probably
hit it with a hammer and not break it open entirely, and even a little
crack wouldn't be instantly fatal, so that's not a good explanation.
More likely would be something like a air line getting pulled out or
tangled up and yanked out, or some fitting bashed off. It will take
a bit longer, but not that long, for all the air to leak out of a 1/2"
pipe you can't reach to plug on the back of your suit....
[Dramatic tension.. the radio calls as the guy gets cold, starts to
stumble part-blind back to base, trips over edge of crater, picks himself
up and realizes he's losing pressure from an air line he can't reach on
the back of the suit, but not so quickly he can't frantically try for
a couple of minutes to patch it, then try and run back to base before it
loses pressure entirely... you get the idea ;-) ]
-george william herbert