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  #1  
Old August 6th 12, 05:50 AM posted to sci.space.policy
Alan Erskine[_3_]
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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.
  #2  
Old August 6th 12, 08:08 PM posted to sci.space.policy
Doug Freyburger
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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.
  #3  
Old August 6th 12, 11:40 PM posted to sci.space.policy
Brian Thorn[_2_]
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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
  #4  
Old August 7th 12, 02:01 AM posted to sci.space.policy
Sylvia Else[_2_]
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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.




  #5  
Old August 7th 12, 02:36 AM posted to sci.space.policy
Alan Erskine[_3_]
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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.

  #6  
Old August 7th 12, 03:52 AM posted to sci.space.policy
Sylvia Else[_2_]
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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.
  #7  
Old August 7th 12, 04:32 AM posted to sci.space.policy
Brian Thorn[_2_]
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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
  #8  
Old August 7th 12, 06:12 PM posted to sci.space.policy
Rick Jones
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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
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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...
  #9  
Old August 7th 12, 08:10 PM posted to sci.space.policy
Alan Erskine[_3_]
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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  
Old August 7th 12, 08:45 PM posted to sci.space.policy
Jochem Huhmann
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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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