Responding to Doug's comments:

>Whilst I have my doubts about skycrane - I dont see anything
>wrong, in principle, of using viking style deep-throttle motors to
>come to a hovering point ( nothing wrong or impossible with that at
>all ) - and then drop the lander down a cable to the ground.

Viking was a nice piece of engineering. I have two copies of the Viking program final report "Entry Data Analysis for Viking Landers 1 and 2" by Ingoldby, et al, NASA Contractor Report 159388 (TN-3770218). My at-home copy is falling apart because I've gone over it so many times. The Viking Landers almost augered-in because their radar altimeters were spoofed by the iron rich dust rising up from the Martian surface due to Viking's rocket plumes. Viking thought it was approaching the Martian surface at a much faster rate than it really was. Fortunately the lander rocket control law was robust enough that this "unknown" didn't kill the vehicle. The Viking aeroshell flew towards the Martian surface along a lifting trajectory. Unlike Pathfinder and MER, Viking's aeroshell was 3-axis controlled by a reaction control system (RCS). This was crucial because the designers were not aware of the Mach = 1.7 instability that occurs with a 70 deg. sphere-cone. Viking's RCS blew a lot of propellant keeping the vehicle from tumbling just before the aeroshell jettisoned. Again, Viking's engineers were sharp enough to build their control law to be sufficiently robust to deal with this unknown. This was a reocurring theme through out Viking's EDL (Entry Descent and Landing).

>It's easy to look at the MER EDL animations based on the UHF
>MGS relayed data and go "woah - you wanna do that with a
>hovering thing?" - but obviosuly - most of the swinging in that
>system is due to the parachute and three points of mass in a line.

The halter on MER and Pathfinder was deliberately sized to allow the parachute swing to quickly damp down in the thin Martian atmosphere and to keep the payload far away from the RAD's rocket plume.

>Skycrane will be one point of mass until seconds before
>touchdown and then only two points of mass. In principle, more
>stable than MER.

No, the payload will be well within the turbulent shear layer of the rocket's plume. Modelling this rocket plume in a computer simulation or through experiment for a swinging, descending vehicle in a poorly understood atmosphere at Martian gravity is probably beyond any near term technology.

>However - with all the experience and success out of JPL - I find it
>hard to see why you can be so 100% confident that it WILL
>WITHOUT DOUBT FAIL. Are you saying that you understand
>skycrane more than the people designing, simulating and testing it?

First of all, beware of the fallacy that if NASA management thinks something is true then it must be so. NASA management was confident tha Challenger's SRB O-rings were safe even though there was this annoying nebbish in a back room saying the O-rings were brittle and would fail. Likewise NASA managment was also confident that the RCC leading edge tiles on Columbia were secure even though annoying people were claiming that RCC was too brittle to be struck by large junks of ice. NASA managment shows a good understanding about meeting milestones and fulfilling project deliverables but is sometimes confused by the finer points of aerospace engineering.

>If MSL goes to mars with Sky Crane - it MIGHT auger in - in the
>same way Spirit and Opportunity might have augered in as well.
>But one things for sure - if MSL is going to mars - it's getting
>there via Skycrane and no other way.

No, MSL should go on the back burner until a better EDL technology is developed. For the time being, we can explore the Martian surface with the excellent and flight proven MER technology.

>So instead of dismissing the whole idea off hand - why not be
>constructive and explain, with facts, calculations and evidence,
>WHY, exactly, Skycrane will fail - and present this evidence to JPL
>to demonstrate that they're doing something wrong. If you're not
>prepared to do that - then why should we take your dismisal
>seriously?

I would respond that a significant fraction of the engineers at NASA Ames are telling the folks at JPL at every opportunity that MSL will auger in if Sky Crane is used (I did). Sky Crane is an emotive issue at NASA Ames and will cause some spacecraft engineers (not me) to go into spewing hysterics if the subject is brought up.

On a lighter note, let's do a little science experiment:

Take two yard sticks and tape their ends together so it forms a "V". Then attach a string to the point where the yard sticks are taped together. On the other end of the string, attach a small toy that can stand upright. Turn the yard stick "V" so it points upwards and the toy is dangling below on its string. Now balance the two ends of the yard sticks on the palms of your left and right hands and try to lower the toy upright onto a tall stool. It isn't easy. Now give a hair blower to one of your kids and ask him or her to randomly hit the toy with a stream of air as you try to place it on the stool. For extra credit, have your youngest kid climb up on your back and wiggle around while you're trying to land the toy on the stool. I claim that successfully landing a payload on Mars with Sky Crane is harder to do than this science experiment. JPL is betting almost a billion dollars and the future of the Mars Program that Sky Crane will actually work.

Gary