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.