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Unmanned Spaceflight.com _ MSL _ Skycrane maneuver question

Posted by: kwan3217 Dec 31 2011, 08:17 PM

From the papers I have read about the landing sequence, I see the combined descent stage/lander descending to about 18.6m at 0.75m/s down when the rover is separated. The rover then descends at about 1m/s to the end of the 7.5m bridle. After that, the descent stage continues to drop at 0.75m/s until the rover contacts the ground. Once the rover is supported by the ground, a lot less thrust is needed by the descent stage to keep a constant speed. It takes about 1 second for the system to realize this, and another second while it cuts loose before it starts to ascend.

So while the rover lowers to the end of its bridle, what is the descent stage doing? Is it still descending at 0.75m/s, meaning the rover is descending at 1.75m/s relative to the ground? Also, when the rover is in contact with the ground, is the descent stage still descending at 0.75m/s? Does this bring it 1.5m closer to the rover before it detaches and flies off?

I'm trying to put together a timeline for one of my animations.

Posted by: MahFL Dec 31 2011, 09:03 PM

The pdf says the DS halts vertical motion, one assumes a slight throttle increase is needed, then the DS hovers for 187 milliseconds then DS flies away, after the umbilical is cut.

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/41708/1/08-0255.pdf

Posted by: MarsEngineer Jan 6 2012, 12:06 AM

You are both correct.

For the second or so that the rover/descent stage is "discovering" that the rover mass has been/is being off-loaded, indeed the descent stage is still moving down even though the rover is on the ground. The rover discovers the off-loading via noticing about a 50% decrease in the throttle setting that naturally occurs as the closed-loop velocity controller maintains the 0.75 m/s descent rate of the descent stage.

The electronic bridle is maintained somewhat taut by an umbilical management device on the descent stage. Likewise the triple bridle that support(ed) the rover is kept from becoming excessively slack by a combination of the bridle umbilical device and three bridle retraction mechanisms on the rover deck. After one second, the descent stage is told (by the rover) to stop its 0.75 m/s descent rate (which takes another half second) prior to being rapidly configured for release and flyaway. So yes, the descent stage does continue to descend for a meter or so after actual initial touchdown before coming to a halt and then flying away.

Although I am expected to know everything about MSL, I reserve the right to be wrong (occasionally).

-Rob Manning