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Science and Technology "Somewhere, something incredible is waiting to be known." - Carl Sagan.

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Old March 9 2013, 12:03 AM   #76
sojourner
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Re: manned Mission to Mars discussion

I think it would have to hit one of the caps of co2 ice on the poles to do that.
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Old March 9 2013, 04:09 AM   #77
JanewayRulz!
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Re: manned Mission to Mars discussion

gturner wrote: View Post
Oh, PICA heat shielding on the Stardust sample mission worked fine at a re-entry velocity of 12.6 km/sec and a heating rate of 1,200 W/cm^2. The Dragon's PICA-X heatshield is already 3 inches think and only about a half-inch burns off during re-entry from orbit, so they have a huge margin. Re-entry from Mars might get into the 3,500 W/cm^2 range, and PICA hasn't been experimentally tested at those heating rates, but it's not predicted to be a serious issue. They would have to conduct such testing, though, and might have to switch to a different material. If need be, they can go up to 10 inches thick with the PICA shield.

Probably the main issue is that they'll want to very accurately model the aero-thermodynamics so they don't waste mass on an unnecessarily thick shield. Getting a heat-shield to survive a re-entry is obviously possible since meteors make even more extreme entries with an accidental shield. Their existence made Robert Goddard suggest an ablator back in the 1920's.

Besides, heat shield design is fun.
Hey, ya gotta find your fun where-ever you can.

W/cm square... watts?
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Old March 9 2013, 04:48 PM   #78
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Re: manned Mission to Mars discussion

Fairly useless except as a technology demonstrator, but it at least does give something for appearance to human beings, and we need that sort of thing. Eventually we'll mark any landing as a milestone towards colonization, but I doubt the first landing will reveal anything the landers haven't already shown us.

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Old March 10 2013, 01:03 AM   #79
gturner
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Re: manned Mission to Mars discussion

JanewayRulz! wrote: View Post
Hey, ya gotta find your fun where-ever you can.

W/cm square... watts?
Yep! So if your heat shield has to survive a peak heating rate of 1,300 Watts/cm^2, it's like aiming a 1,300 Watt hair dryer on each square centimeter of the heat shield surface.

What they generally do for testing is to hit the heat shield material with a plasma arc, sort of like what you'd find in a welding shop. I'm not sure how good a job a propane torch would do, but it would be a similar testing concept.

The other parameter is the total heat soak, which is in Joules (a Joule is one Watt for one second) per square centimeter. Your shield might survive a brief period at 1,300 Watts/cm^2 but erode away before re-entry was completed, so you have to check the total energy the shield can take. For example, one minute (60 seconds) at 1,000 W/cm^2 would be 60,000 Joules/cm^2. In older units common to NASA's early days, they'd have expressed this in BTU's/square inch.

So to test a heat shield material, you take a block of it and heat it with a torch, starting the burner on low, then cranking it up to eleven, then slowly backing off to simmer. As the heat soaks into the shield material the shield could start to melt internally or it might crack, both of which would be very bad.

For some applications you might just use a heatsink material, like a thick titanium skin which will heat up as you re-enter. The test there would be to hit it with the simulating heating it would experience and make sure its temperature never exceeds your design limits. For the Shuttle, the idea was that the tiles should never let the aluminum skin of the shuttle get too hot.

There are lots of links online that show pictures from testing the Apollo heat shield, where you can see the ablation and burning in various test blocks they tried out.
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Old March 13 2013, 03:07 PM   #80
JanewayRulz!
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Re: manned Mission to Mars discussion

^^Ahhh, now I see why you find heat shields fun. You get to play with welding torches! ;-)

Thanks for the tutorial.
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Old March 16 2013, 06:32 PM   #81
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Re: manned Mission to Mars discussion

Now I think it might be better if the plan were pushed forward a bit, but for a simpler unmanned probe.

This would be a bit simpler, and the probe would arrive right as the comet does.

Now you say this wouldn't require Falcon Heavy, but hear me out.

New Horizons will be able to get to Pluto quickly because the ration of probe to the rocket fuel under it was exaggerated even more--a top end Atlas with a Centaur and a solid upper stage both.

So if we have a fuel fat upper stage and a very small probe, would that be enough to slingshot around Mars' backside and then catch up with the comet, to fly along with it? I still don't think the trajectories would allow it--but that might be something to bring up elsewhere to get the question answered.

Usually, to catch up with something, you have to do a very long matching trajectory as Rosetta is doing, but with Mars there with the comet, could you do a slingshot so as to have a probe ride with that comet--or at least do a flyby?
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Old March 18 2013, 05:33 PM   #82
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Re: manned Mission to Mars discussion

publiusr wrote: View Post
Usually, to catch up with something, you have to do a very long matching trajectory as Rosetta is doing, but with Mars there with the comet, could you do a slingshot so as to have a probe ride with that comet--or at least do a flyby?
Under most circumstances, no. Comets on highly eliptical orbits have relatively high orbital velocities as they spiral inwards; boosting outwards to Mars puts you in a LOWER velocity with respect to the sun and takes much longer to do. IOW, it's a bit like jumping on a skateboard and trying to coast uphill to make a jump.

If you want to intercept something moving that fast, the better target is Venus, whose gravity is three times higher than Mars and is also lower in the sun's gravity well where potential energies are higher.
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