> They do exactly that. But Mars' atmosphere is so thin that terminal velocity of the capsule is still supersonic.
Also, that kind of blunt body is stable hypersonically, but unstable supersonically. As soon as you get to a mach number where a parachute can deploy stably (about mach 1.8 and below) you deploy it to stabilise the thing, and of course the drag is much greater than entry body alone.
>> Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck?
The point of the sky crane is not just to avoid kicking up dust, this notion seems to have become the received wisdom in the media though. The reasons include:
-Having rovers drive off from on top of things is difficult, it puts the CoG of the lander right up and if you have to drive off a sloped ramp having landed on a slope, the ramp can be really very sloped.
-Rocket forces nearer the ground can be quite an interesting control problem, in terms of the forces generated when you're near the ground and where all the exhaust goes.
-Having rocket fuel tanks landing on spikey rocks has the potential to really ruin your day.
-Knowing when to turn the rocket engines off (i.e. successfully detecting landing) is actually quite a hard problem. It's a very noisy problem, and getting it wrong was what destroyed Mars Polar Lander (i.e. it thought it landed when it was still up in the air, and so fell out of the sky). The skycrane has the rocket motors on the whole time and goes off an crashes.
Lots of this is explained in the video I linked to somewhere else in this family of comments.
Also, that kind of blunt body is stable hypersonically, but unstable supersonically. As soon as you get to a mach number where a parachute can deploy stably (about mach 1.8 and below) you deploy it to stabilise the thing, and of course the drag is much greater than entry body alone.
>> Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck?
The point of the sky crane is not just to avoid kicking up dust, this notion seems to have become the received wisdom in the media though. The reasons include:
-Having rovers drive off from on top of things is difficult, it puts the CoG of the lander right up and if you have to drive off a sloped ramp having landed on a slope, the ramp can be really very sloped.
-Rocket forces nearer the ground can be quite an interesting control problem, in terms of the forces generated when you're near the ground and where all the exhaust goes.
-Having rocket fuel tanks landing on spikey rocks has the potential to really ruin your day.
-Knowing when to turn the rocket engines off (i.e. successfully detecting landing) is actually quite a hard problem. It's a very noisy problem, and getting it wrong was what destroyed Mars Polar Lander (i.e. it thought it landed when it was still up in the air, and so fell out of the sky). The skycrane has the rocket motors on the whole time and goes off an crashes.
Lots of this is explained in the video I linked to somewhere else in this family of comments.