Aeroshell as descent stage and skycrane

[Stephen W]

What is the point of having different stages for different parts in the mission? Wouldn't it be easier to add retrorockets to the aeroshell and cut the parachute? The descent stage already weighs almost twice that of the aeroshell so it shouldn't really affect the TWR or fuel needed.

Furthermore, this has already been used three times (albeit without cutting the parachute) on the two MERs, Spirit and Opportunity, and also Pathfinder, so the technology is mostly there.

The benifits could also include more space in the vehicle and/or more fuel for landing.

[DreamyPickle]

I'm confused, are you suggesting to drop the parachutes entirely? A large amount of speed is dissipated by those parachutes so a lot more fuel would be required for landing.

If you just to avoid separating the aeroshell from the skycrane: what are you saving, just the staging mechanism?

[Stephen W]

I just don't see why they can't integrate the skycrane into the aeroshell. Fewer points of failure and theoretically more simple. Just like the MERs but probably needing to separate the parachute just prior to powered descent, just like they would have done anyway.

[Crispy]

The descent stage already weighs almost twice that of the aeroshell so it shouldn't really affect the TWR or fuel needed.

If that's true, then by discarding the aeroshell, you reduce your mass by 33%
That's a significant improvement to TWR.

The parachute has to be detached, so you may as well get rid of as much extraneous mass as you can at the same time.

Just think of the aeroshell as a really big parachute attachment fixture.

[Jim]

What is the point of having different stages for different parts in the mission? Wouldn't it be easier to add retrorockets to the aeroshell and cut the parachute? The descent stage already weighs almost twice that of the aeroshell so it shouldn't really affect the TWR or fuel needed.

Furthermore, this has already been used three times (albeit without cutting the parachute) on the two MERs, Spirit and Opportunity, and also Pathfinder, so the technology is mostly there.

The benifits could also include more space in the vehicle and/or more fuel for landing.

The rover and the descent stage are not volume limited, they are mass limited.  The parachute and aeroshell can only decelerate a given mass. 

Backshell with thrusters would be too much extra mass to decelerate and maneuver.  The most of the descent stage structure would still be needed on the back shell to hold the propellant tanks and thrusters, the rover attach system and  the bridle system. The descent stage structure (without propellant) is only slightly more than the backshell. 

It is like staging a rocket but in reverse.  Same reason we have two stage rockets vs single stage.  Backshell and parachute are the first stage and the descent stage is the second stage.  The descent stage by itself with the backshell is lighter than the backshell with descent stage functions because the latter would require much more propellant to decelerate and hover the heavier backshell with descent stage functions.

BS + DS < BS/DS + Prop

Also, there isn't really room to put more propellant in the backshell.  It would still need to occupy the same area to maintain vehicle balance and give the rover the same volume.

[Jim]

I just don't see why they can't integrate the skycrane into the aeroshell. Fewer points of failure and theoretically more simple. Just like the MERs but probably needing to separate the parachute just prior to powered descent, just like they would have done anyway.

MER just had solid motors, much easier to attach than all the systems of the MSL/M2020 descent stage.