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MSL "Heat shield woes"
Stu
post Feb 17 2008, 04:20 PM
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"MSL Heat Shield Woes" - Aviation Week

Anyone else see this?


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centsworth_II
post Feb 17 2008, 04:31 PM
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From the article:
"The relative orbital positions of the Earth and Mars allow for a shorter
trip time in 2011, although the spacecraft will hit Mars' atmosphere at a
higher velocity, which has implications for the TPS..."


Maybe this is naive, but can't they control the velocity with which
MSL hits Mars by reducing the boost it gets at the beginning of its
journey and lengthening the trip time?
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djellison
post Feb 17 2008, 05:39 PM
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Do that, and you don't reach Mars at all. The journey time ( and arrival speed ) are dictated by the trajectory which is dictated by the orbital mechanics.

Doug
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centsworth_II
post Feb 17 2008, 06:13 PM
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I bet Dawn wouldn't have any trouble. smile.gif
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nprev
post Feb 18 2008, 06:59 AM
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Not to be cynical, but 'pray for launch delay' is a very common mantra in some circumstances for many programs (mostly those confined to Earth orbit)... wink.gif


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mchan
post Feb 18 2008, 08:01 AM
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The delay for MSL would be 2 years and very costly. Doubt there are many prayers for a delay in this particular case.

The 2011 launch date is the faster trajectory, so launching in 2009 would avoid the problem.


This post has been edited by mchan: Feb 18 2008, 08:10 AM
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AndyG
post Feb 18 2008, 11:24 AM
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Is the article correct?

QUOTE
NASA had planned to use a super lightweight ablative (SLA) heat shield for MSL, similar to what is used on the space shuttle's external tank


I thought the SLA for ablative re-entry use was nothing like that used on the tank? Am I wrong?

Andy
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rlorenz
post Feb 18 2008, 01:43 PM
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QUOTE (djellison @ Feb 17 2008, 12:39 PM) *
Do that, and you don't reach Mars at all. The journey time ( and arrival speed ) are dictated by the trajectory which is dictated by the orbital mechanics.


Yes - sort of.

The launch date, launch speed, arrival date, arrival speed and arrival location are all coupled. Not all
points in this five-dimensional space are populated for a given launch vehicle, and some points are
not populated at all.

For a given launch vehicle, clearly you can only attain some utter maximum speed, and below that
the launcher has a mass/speed performance envelope. The launch speed dictates the relationship
between launch and arrival dates, and arrival speeds.

Specify some other parameters (like launch speed) and you introduce a coupling between arrival
date and/or speed and location. It might be possible to tune the trajectory to a lower entry speed,
but not if you force the landing site to be in a particular place.

Thus it might be that you could 'take the slow road' to Mars, but maybe not to get exactly where
you want to.

I think Doug meant all of the above, but was just saying 'orbital mechanics' as shorthand... ;-)
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djellison
post Feb 18 2008, 02:22 PM
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Yeah...I meant every word of that...all of it.

Easy solution - read the first 20 pages of


Planetary Landers and Entry Probes

For more R.L. et.al goodness on such matters.

That's what I've been doing over the weekend. I'll need to read it twice for it to go in, but at least I know what b-plane means now.

Doug
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ElkGroveDan
post Feb 18 2008, 04:36 PM
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Ouch $115...but you can get a used one on Amazon for $105 rolleyes.gif.

Why can't you rocket scientists write in popular paperback? Also if you throw in a murder mystery and a love scene between discussions of thruster vectors, we can convince our other-halves that it's a worthwhile purchase.


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Greg Hullender
post Feb 18 2008, 05:00 PM
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Part of what I love about orbital mechanics problems is that they're so rich. In this case, although there's certainly a unique minimum-energy transfer orbit (which also has minimum arrival velocity) at any given launch window, there's a vast abundance of higher energy and higher arrival-speed orbits. That's true even if you insist on the transfer orbit being smoothly tangent to the orbit of Mars (as opposed to simply smashing straight into it from the side.) Also, since Mars has a highly elliptical orbit, even minimum-energy orbits will vary from one launch window to the next, with lower energy (and thus lower arrival velocity) when Mars is closer to the sun at arrival time.

Moral: You can get to Mars faster than a Hohmann orbit, but you'll be going faster when you get there.

--Greg
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edstrick
post Feb 19 2008, 08:38 AM
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Mariners 6 and 7 <1969> had unusually short trip times to Mars, something like 5 months. They were launched as second generation Mariners on Atlas Centaur rockets instead of the Atlas Agena that launched the early Mariners and Rangers. Launch vehicle had more or less the same capability as the one that launched Mariner 9 to orbit Mars on the next opportunity, BIG fuel tanks and all. Mariner 69 were flybys and a little ?more? than half the weight of Mariner 9, so the launches had energy to spare. They were kicked into fast flybys, that also had a mission irrelevant gravity assist that kicked the aphelion out to the edge of the inner belt. Had the record as the furthest solar powered spacecraft from the sun till NEAR, Stardust, or perhaps Rosetta got further out at aphelion.

You would NOT have wanted to retro into Mars orbit from those encounters. The "Hyperbolic excess velocity" -- very useful term in these discussions, the speed the spacecraft approaches and receeds from the planet at effectively infinity (ignoring the sun) -- was much larger than for an orbit insertion bound mission.
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Guest_Sunspot_*
post Feb 19 2008, 09:58 PM
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Guests






Is MSL really going to be ready to launch on time? unsure.gif unsure.gif
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djellison
post Feb 19 2008, 10:03 PM
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To be honest, compared to the trouble MER had, MSL's got it easy.

Doug
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elakdawalla
post Feb 20 2008, 05:07 PM
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Alan just said at MEPAG that, just three months after the Herculean efforts to solve the last budget overrun (which was $62M), MSL has come to HQ to request another $165 to $200 million. sad.gif

--Emily


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