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Big lander on Mars - is this really possible?, Could "Mach 5 problem" ruin our dream to walk on Mars?
Guest_Zvezdichko_*
post Sep 15 2007, 04:51 PM
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I read a very interesting article about the problems when it comes to big landers on the Red Planet ( including Mars Sample Return and manned Mars landers ).

http://www.universetoday.com/2007/07/17/th...the-red-planet/

Some quotes:

QUOTE
The major conclusion that came from the session was that no one has yet figured out how to safely get large masses from speeds of entry and orbit down to the surface of Mars. "We call it the Supersonic Transition Problem," said Manning. "Unique to Mars, there is a velocity-altitude gap below Mach 5. The gap is between the delivery capability of large entry systems at Mars and the capability of super-and sub-sonic decelerator technologies to get below the speed of sound."
Plainly put, with our current capabilities, a large, heavy vehicle, streaking through Mars' thin, volatile atmosphere only has about ninety seconds to slow from Mach 5 to under Mach 1, change and re-orient itself from a being a spacecraft to a lander, deploy parachutes to slow down further, then use thrusters to translate to the landing site and finally, gently touch down.


QUOTE
Apollo and Soyuz capsules and the proposed Crew Exploration Vehicle (CEV) will all decelerate to less than Mach 1 at about twenty kilometers above the ground just by skimming through Earth's luxuriously thick atmosphere and using a heat shield.


then:

QUOTE
Parachutes can only be opened at speeds less than Mach 2, and a heavy spacecraft on Mars would never go that slow by using just a heat shield. "And there are no parachutes that you could use to slow this vehicle down,��? said Manning. "That's it. You can't land a CEV on Mars unless you don't mind it being a crater on the surface."


If what's said here is true, we don't have any chance to land in a way we know - using a standart heat shield, parachute or landing thrusters.
That "Hypercone" concept is interesting and surely could be integrated for a Mars Sample Return mission, but what about a human landing mission? Wouldn't this be a challenge even to be launched with the proposed rockets like Ares 5?
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tasp
post Sep 15 2007, 04:54 PM
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Hmmmmmm.

Sounds like a job for an Orion pusher plate and some nuclear propulsion modules . . . .


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Guest_Zvezdichko_*
post Sep 15 2007, 05:08 PM
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I have a crazy idea...

I suggest... a Vostok type solution smile.gif A person could slow down into a small lander, then heat shield deployment, rockets mounted on the backshell fire and... the astronaut skycranes to the surface...
Maybe not so crazy to be fulfilled.
The Mars ascend vehicle may be assembled on the surface.
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stevesliva
post Sep 15 2007, 05:31 PM
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I wonder if the solution would be to stay aloft longer... perhaps a lift-generating lander design....
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ngunn
post Sep 15 2007, 09:40 PM
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Sooner or later we will want to land heavy things on completely airless worlds. Perhaps it's time to think of Mars that way. Its very thin atmosphere provides interesting skyscapes when viewed from the surface but in fact it would be almost invisible without the suspended dust. It's a lot more like a vacuum than it is like 'air' as we know it.
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djellison
post Sep 15 2007, 09:45 PM
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BUT - it's still enough to shed 90%+ of the entry velocity. To try and land on something as massive as Mars without using the atmosphere would take the payload-to-entry ratio currently about 1/4 and make it a whole lot worse.

Doug
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ugordan
post Sep 15 2007, 09:48 PM
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Wasn't the gist of the problem with Mars and really heavy payloads is that you basically need retrorockets which ever way you decide to land, but the atmosphere gets in your way for this? Rocket exhaust into supersonic air coming back at you and the sorts.


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ngunn
post Sep 15 2007, 09:56 PM
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How about retro-rockets to bring the thing to a halt just above the atmosphere, then parachute from there?
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ugordan
post Sep 15 2007, 10:01 PM
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You can't open a parachute in near vacuum and by the time you hit sufficiently dense atmosphere you'd be going quite fast again. That's just wasting propellant. Just like launching rockets, except in reverse - gravity losses etc. Imagine cutting your engines at a point in ascent for an extended period of time. Gravity doesn't stop just because you have and the more you don't burn, the greater the fuel waste is. It just increases your required total delta-V to launch. Or land in this case.

Besides, you can't land with parachutes anyway. Their purpose is to reduce the speed to subsonic.


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ngunn
post Sep 15 2007, 10:12 PM
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I'll admit I haven't a clue what I'm talking about on this - just a bit of late night fun. But I'm intrigued by the idea that a thin atmosphere may be worse for landings than none at all. Why not descend on rockets at sub-sonic speed, with parachute added when the atmospheric density warrants it? Please, I only want to learn here . . smile.gif
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ugordan
post Sep 15 2007, 10:20 PM
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As far as I understand, heatshields will not slow you enough to enable retrorockets before reaching ground. The problem is when you increase mass, the radius of the package increases only by the 1/3 power of mass (so surface area follows a 2/3 power relationship), there's a point when the surface area unit has so much mass behind it that the thing just slices through the atmosphere like a big asteroid.

This particular article was already linked to somewhere some time ago, perhaps there's some more info there.


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nprev
post Sep 15 2007, 11:55 PM
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The answer just might be a hypersonic glider after all...at least as an interim EDL stage. After tearing halfway (or more) around Mars to shed velocity & get subsonic, the lander could separate from the glider and do a powered descent from a reasonable altitude...unless the landing site is Meridiani or a similar flatland, in which case they could presumably skid the whole thing to a stop al a Werner Von Braun's original proposal. (How ironic would that be?... smile.gif )


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Mongo
post Sep 16 2007, 02:16 AM
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The problem sounds like the Martian atmosphere is too thin for terrestrial-style heat shields to work, because the mass to surface area ratio gets too high with larger packages. So forget about terrestrial-style heat shields, and go with martian-style heat shields: much larger diameter, but lightweight, shields supported by either a torus-shaped balloon around the package, or perhaps a cantilever system. The aerodynamic and thermal stresses should not be too high -- after all, the big problem is that the martian atmosphere is too thin for smaller heat shields to slow the package down in time.

Here is what I mean:



As for having only 90 seconds, I would think that if you aimed for a grazing trajectory (with a non-atmospheric-effects trajectory which would just miss Mars), the available deceleration time could be as much longer than 90 seconds as you need.

Bill
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edstrick
post Sep 16 2007, 04:43 AM
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The problem is not as big as it appears, but it's expensive to cure.

We have zero experience desigining rocket engines that fire THROUGH a heatshield INTO the oncoming atmosphere. In arm-waving-speculation (not theory), you can dribble gas... maybe water?... through a rocket nozzle that opens through a heatshield during the high-speed parts of entry, and fire the engine into the mach 5 or so ram-pressure.

It's gotta be a fairly big engine for a big lander. We CAN do that experiment on earth-atmosphere entry test vehicles. Get entry speed and angle right and you should be able to do a pretty good simulation of martian entry, other than the atmosphere being O2/N2, not CO2. We've had absolutely no reason to do that in the past.

A sufficiently complete test program to do it at the scale needed to prove the technology will cost a few "full-up" flights of a fly "boilerplate" vehicle. Who cares if it crashes after reaching Mach 1 intact, but it's gotta fly the entry as-if for-real-on-Mars.
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dvandorn
post Sep 16 2007, 05:34 AM
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The aerodynamic pressures going Mach 5 in the Martian atmosphere, at say 10km above the surface, are quite a bit lower than what you would find on Earth, and while there would be considerable blow-back, it wouldn't be as bad as what you might think.

The biggest problem with the approach to Mars is that you encounter maximum aerodynamic deceleration much closer to the surface than you do on an entry approach to Earth. The time frame between decelerating to a speed at which parachutes and rockets are effective and reaching zero altitude is far, far smaller than that on Earth. But, as has been pointed out, you need to take advantage of that aerodynamic deceleration, or else you need to send four times as much mass as you otherwise would in deceleration propellant.

I think nprev is right, we're going to need something that will maintain altitude through lift as it bleeds off speed, so that we can continue to use aerodynamic deceleration to slow the craft. Of course, that means huge wings (to generate enough lift int he thin air) that have to be strong, since they have to withstand Mach 5 (and above) winds for a decent percentage of an hour, not just four or five minutes. Even in the Martian atmosphere, I have a hard time imagining a deployable wing system that could deploy cleanly and then withstand the heating and shear forces they would encounter in the long, slow decelerating glide.

So, it's a tough nut to crack. We may have to develop entirely new materials and technologies to allow rocket-assisted aerodynamic entries, and/or we may have to assemble gliding vehicles in Martian orbit. In any event, I bet that even if a Mars program aimed at landing humans there by 1980 had been funded, it would have failed when faced with this issue...

-the other Doug


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