Big lander on Mars - is this really possible?, Could "Mach 5 problem" ruin our dream to walk on Mars? |
Big lander on Mars - is this really possible?, Could "Mach 5 problem" ruin our dream to walk on Mars? |
Guest_Zvezdichko_* |
Sep 15 2007, 04:51 PM
Post
#1
|
Guests |
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? |
|
|
Sep 15 2007, 04:54 PM
Post
#2
|
|
Member Group: Members Posts: 903 Joined: 30-January 05 Member No.: 162 |
Hmmmmmm.
Sounds like a job for an Orion pusher plate and some nuclear propulsion modules . . . . |
|
|
Guest_Zvezdichko_* |
Sep 15 2007, 05:08 PM
Post
#3
|
Guests |
I have a crazy idea...
I suggest... a Vostok type solution 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. |
|
|
Sep 15 2007, 05:31 PM
Post
#4
|
|
Senior Member Group: Members Posts: 1582 Joined: 14-October 05 From: Vermont Member No.: 530 |
I wonder if the solution would be to stay aloft longer... perhaps a lift-generating lander design....
|
|
|
Sep 15 2007, 09:40 PM
Post
#5
|
|
Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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.
|
|
|
Sep 15 2007, 09:45 PM
Post
#6
|
|
Founder Group: Chairman Posts: 14432 Joined: 8-February 04 Member No.: 1 |
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 |
|
|
Sep 15 2007, 09:48 PM
Post
#7
|
|
Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
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.
-------------------- |
|
|
Sep 15 2007, 09:56 PM
Post
#8
|
|
Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
How about retro-rockets to bring the thing to a halt just above the atmosphere, then parachute from there?
|
|
|
Sep 15 2007, 10:01 PM
Post
#9
|
|
Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
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. -------------------- |
|
|
Sep 15 2007, 10:12 PM
Post
#10
|
|
Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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 . .
|
|
|
Sep 15 2007, 10:20 PM
Post
#11
|
|
Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
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. -------------------- |
|
|
Sep 15 2007, 11:55 PM
Post
#12
|
|
Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
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?... )
-------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
|
|
|
Sep 16 2007, 02:16 AM
Post
#13
|
|
Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
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 |
|
|
Sep 16 2007, 04:43 AM
Post
#14
|
|
Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
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. |
|
|
Sep 16 2007, 05:34 AM
Post
#15
|
|
Senior Member Group: Members Posts: 3419 Joined: 9-February 04 From: Minneapolis, MN, USA Member No.: 15 |
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 -------------------- “The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
|
|
|
Lo-Fi Version | Time is now: 26th April 2024 - 07:47 AM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |