Curiosity

On 8/08/2012 5:45 AM, Jochem Huhmann wrote:
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.

It didn't hurt Viking 1 or 2.

Alan Erskine writes:

On 8/08/2012 5:45 AM, Jochem Huhmann wrote:
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.

It didn't hurt Viking 1 or 2.

The Vikings had no 900kg rovers rolling off them. And nobody would have
bothered with the skycrane concept if all you'd wanted to do is landing
a platform with a few experiments.

But in the case of Curiosity the rover *is* the probe, so minimizing the
landing equipment is imperative (in this case it even was the rover's
avionics that controlled entry, descent and landing). There's little
sense in wasting mass for landing legs and ramps and debris protection,
if this is just for landing a dumb platform to drive a rover off of. If
all you want is to have the rover on the ground actually landing it
right onto its wheels makes lots of sense.


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

Alan Erskine wrote:
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.

When every gram counts you pull tricks to increase the payload. You'll
notice that Curiosity is a lot bigger than any of the provious probes.
The reason is more than newer rockets at leaunch time.

Compare with the lunar orbit rendezvous that Apollo used to get to the
Moon and back. A stunt to save payload weight then paint it green with
enough extra money you're pretty sure it will work correctly the first
time.

In article ,
Rick Jones wrote:

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

Of course.

On Aug 7, 3:10*pm, Alan Erskine wrote:

* Reduce the complexity and increase the reliability.

No, that increases mass and complexity and decreases the reliability.

The rover would be on top of a lander, which means the cg is higher,
which means the lander legs would have to have a wider span, which
means more complicated mechanism to stow them in the same size heat
shield. That doesn't include all the mechanisms for stowing the ramps
that would also be complicated, since they would have be designed to
allow the rover to roll off either direction and account for rocks
being under or in front of the ramp.

No, the reasons for the skycrane landing method are that is decreases
mass and complexity and increases the reliability.