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Curiosity
Why didn't they just use bigger parachutes? Use the supersonic 'chute
to slow the vehicle down and then two or more large 'chutes for final descent. |
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Curiosity
Alan Erskine wrote:
Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. Weight of final payload. The same reason Apollo did the strange lunar orbit rendezvous. Certian bizzare solutions weigh less but are higher risk. Pour enough extra money into the solution to make it more robust. The atmosphere of Mars is too thin to get the speed down far enough. One strategy or another needs to be used at the very end of the approach. |
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Curiosity
On Mon, 06 Aug 2012 14:50:06 +1000, Alan Erskine
wrote: Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. Thin atmosphere, little wind. The parachute could come down and envelope the lander after touchdown. Brian |
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Curiosity
On 6/08/2012 2:50 PM, Alan Erskine wrote:
Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. The ground-level density of the Martian atmosphere is about 1/50 that of air on Earth. So a parachute (if it could be made no more massive itself) would have to be 50 times greater in area than an Earth parachute to achieve the same descent velocity. It's hardly practical. Also, as others have pointed out, you have to get rid of the parachute before landing to avoid having the craft end up underneath it. Sylvia. |
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Curiosity
On 7/08/2012 11:01 AM, Sylvia Else wrote:
On 6/08/2012 2:50 PM, Alan Erskine wrote: Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. The ground-level density of the Martian atmosphere is about 1/50 that of air on Earth. So a parachute (if it could be made no more massive itself) would have to be 50 times greater in area than an Earth parachute to achieve the same descent velocity. It's hardly practical. Also, as others have pointed out, you have to get rid of the parachute before landing to avoid having the craft end up underneath it. Sylvia. Then another method. Instead of the skycrane, why not have a lander like the two previous rovers, but cover Curiosity with a shelter. Once on the surface, the shelter could be opened up at the front like an aircraft hangar (there are deployable shelters that do this) and the rover just drives down a ramp. |
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Curiosity
On 7/08/2012 11:36 AM, Alan Erskine wrote:
On 7/08/2012 11:01 AM, Sylvia Else wrote: On 6/08/2012 2:50 PM, Alan Erskine wrote: Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. The ground-level density of the Martian atmosphere is about 1/50 that of air on Earth. So a parachute (if it could be made no more massive itself) would have to be 50 times greater in area than an Earth parachute to achieve the same descent velocity. It's hardly practical. Also, as others have pointed out, you have to get rid of the parachute before landing to avoid having the craft end up underneath it. Sylvia. Then another method. Instead of the skycrane, why not have a lander like the two previous rovers, but cover Curiosity with a shelter. Once on the surface, the shelter could be opened up at the front like an aircraft hangar (there are deployable shelters that do this) and the rover just drives down a ramp. The landers of the two previous rovers were wrapped in inflated balls, and bounced to a stop. This however meant that they could land any way up, and had to be able to deal with that. Curiosity was said to be just too big for that sort of approach. The approach also makes it more difficult (impossible!) to ensure that the lander doesn't end up stuck on top of or beside an obstacle that will prevent the rover from departing. Curiosity's exact landing site appears to be decided by the onboard computer based on radar imagery. A shelter is still goving to have to be landed using rockets, and the closer your rocket motors get to the ground, the more you have to deal with turbulence as the rocket exhaust interacts with the uneven ground (and possibly digs holes in it). Once the shelter is down, you have to arrange for the door to open. If it opens downwards, then it may get stuck on an obstacle. If it opens upwards (Curiosity probably doesn't need a ramp), then gravity cannot be used to open it, and you need a motor (two for redundancy) and power to operate it (also redundant). It all adds to the mass. I dare say NASA looked at these sorts of alternatives. The skycrane addresses the problem of the rockets getting too close to the ground, and is actually probably quite a simple mechanism, using pyrotechnics to release Curiosity, and gravity to extend the cables, which are subsequently also released by pyrotechnics. Sylvia. |
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Curiosity
On Tue, 07 Aug 2012 11:36:36 +1000, Alan Erskine
wrote: Then another method. Instead of the skycrane, why not have a lander like the two previous rovers, but cover Curiosity with a shelter. Once on the surface, the shelter could be opened up at the front like an aircraft hangar (there are deployable shelters that do this) and the rover just drives down a ramp. You could do that with SLS, but I think that beast will be too wide for any exsting launch vehicle. Brian |
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Curiosity
Sylvia Else wrote:
On 6/08/2012 2:50 PM, Alan Erskine wrote: Why didn't they just use bigger parachutes? Use the supersonic 'chute to slow the vehicle down and then two or more large 'chutes for final descent. The ground-level density of the Martian atmosphere is about 1/50 that of air on Earth. So a parachute (if it could be made no more massive itself) would have to be 50 times greater in area than an Earth parachute to achieve the same descent velocity. It's hardly practical. Does the difference in gravitational pull between Earth and Mars come into play as well? rick jones -- Don't anthropomorphize computers. They hate that. - Anonymous these opinions are mine, all mine; HP might not want them anyway... feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH... |
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Curiosity
On 8/08/2012 12:11 AM, Fred J. McCall wrote:
Curiosity is a lot bigger than the previous landers. Without rockets to slow it down down low, you get a smoking hole because you simply can't put a big enough parachute on it to slow it down enough. Then you put the 'skycrane' _under_ the rover; not above it. Cut out the 'middle-man' of those cables and make the whole thing a lot simpler. Reduce the complexity and increase the reliability. |
#10
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Curiosity
Alan Erskine writes:
On 8/08/2012 12:11 AM, Fred J. McCall wrote: Curiosity is a lot bigger than the previous landers. Without rockets to slow it down down low, you get a smoking hole because you simply can't put a big enough parachute on it to slow it down enough. Then you put the 'skycrane' _under_ the rover; not above it. Cut out the 'middle-man' of those cables and make the whole thing a lot simpler. Reduce the complexity and increase the reliability. You'll need landing legs then, have to make sure that the rover can leave the platform even with a random rock blocking the ramp (which means having two or three ramps), you need to make sure that the debris thrown around by the rocket exhaust hitting the ground from a short distance doesn't damage anything... Looks more like actually adding lots of complexity just to remove some simple cables. The rover will have to end up with its wheels on the ground sooner or later anyway. Landing it this way to begin with and throwing all the then useless lander hardware off to the side is just a rational thing to do. Jochem -- "A designer knows he has arrived at perfection not when there is no longer anything to add, but when there is no longer anything to take away." - Antoine de Saint-Exupery |
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