Re: Skycrane

From: Tom (tom.cuddihy_at_gmail.com)
Date: 01/10/05 

Date: 9 Jan 2005 21:13:59 -0800

freddo411@gmail.com wrote:
> I presume you mean this mission:
> http://marsprogram.jpl.nasa.gov/missions/future/msl.html
>
> I too was surprize to hear about the proposed "skycrane" idea for
> landing. Frankly, I can think of a number of failure modes unique to
> this scheme; oscillations in the cord -- twisting, swinging,
bouncing.
> Helo lifts are notoriously prone to these.
>
> In fact, the MER rovers had a fairly tricky system involving the
> descent parachute / braking rockets that responded to possible
swinging
> oscillations. And this was before touchdown.

The MER rovers' landing system, described in detail here
http://www.cndyorks.gn.apc.org/yspace/articles/marsdoubts.htm
actually bears little resemblance to the skycrane idea. Leonard David
wrote a great article describing the MRL's proposed scheme here
http://www.space.com/businesstechnology/technology/mars_science_lab_040211.html

The changes added to the MER rovers all involved preventing excessive
lateral movement of the rover before it landed. Both added system were
intended to stop a sideways motion that had been insignificant for
Pathfinder's landing mode, but mattered for the larger MER rovers. The
TIRS was to prevent the backshell itself from tilting during the fall
so that retrorockets intended to be aimed down would not accidently
(because of backshell tilt) be pointing laterally, which instead of
slowing down the rover would speed it up (that would be bad). The
systems had to be added, and thus were considered design changes to
resolve unanticipated problems.

Lateral and transverse stability systems for the skycrane, on the other
hand, would be built in--unlike the MERs, this thing would be designed
to land at zero horizontal velocity. Presumably, you'd design it for
zero horizontal velocity before the payload even 'rappels' down the
line, hopefully getting rid of most of the swinging that way.
Anyway, most of the Skycrane type of landing would not see the same
type of problems as a helo lift. Most of the way down, it's just a
strait parachute type suspension using martian gravity. Oscillations of
the type resulting from the helicopter engines's natural frequency
would probably not be a concern for a rocket descent. All in all it
sounds like an excellent idea to me. The biggest advantage is that once
you land, there's nothing 'in the way'--like an enormous airbag or
landing platform--to give you extra complexity problems.

Tom

>
> I imagine that this must add up to lots of engineering complexity
which
> will be difficult to test and debug.
>
> I will wager that that a "skycrane" type landing will lead to added
> weight compared to a more conventional approach to
> 3 axis stabilized, rocket powered, descent with landing legs. In
fact,
> if they end up building and launching this, I'll wager it will crash.
>
> Anyone on this group think this is worth doing? I'd really like to
> hear the point of view in favor this this.
>
> Cheers
> --Fred