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Simulation of the Mars Science Laboratory Sky Crane
An anonymous reader points us to Gizmodo for a fascinating video of NASA's Sky Crane. "When I read that the UFO-looking Mars Science Laboratory's aeroshell would use a floating crane — called Sky Crane by NASA — to softly land the rover on Mars, I couldn't believe it. Now, watching this hyper-realistic NASA simulation, I still can't believe how the whole thing works. I don't know about you, but the whole operation mesmerizes me to no end."
Trebuchet's don't scale very well. AFAIR, neither did the beach ball. This thing is lots bigger and heavier.
Faster! Faster! Faster would be better!
It has to do with mass, the MSL rover, at 900 kg, is much too heavy to land using the airbag methods that landed the 180 kg Spirit and Opportunity rovers. To give a sense of scale, the MSL rover is the size of a minivan, while Spirit and Opportunity are the size of small riding mowers. See:
http://en.wikipedia.org/wiki/Image:Mars_Science_Laboratory_empty_chassis.jpg
I think the idea is that as you get more massive that doesn't work as well anymore. The weight of the airbags becomes untenable... I haven't looked at the math in few years, so unfortunately I can't be more specific.
Mars is one of the hardest places to land because its atmosphere is so wispy; on Earth some simple parachutes and a well-shaped capsule do the trick. On Venus the atmosphere is so thick anything you drop in will happily land softly as long as it doesn't melt. The moon and other such places you really only have the landing rocket option, which can be heavy but not particularly complicated.
On Mars though, the atmosphere is too thin to allow the capsule to slow it down to subsonic speeds on its own, meaning supersonic chutes are necessary if you want to use the atmosphere to slow you down. If you want to land with a rocket, you run into issues of trying to light an engine with supersonic flow going into the nozzle; trying to light it and flip around I imagine introduces some pretty wretched dynamic and structural problems. That tends to mean a series of parachutes including custom Mach 2 or Mach 3 chutes that would never be needed on Earth, or in this case using an aeroshell as well. Even then, you're still going too fast, so you need to slow down more. As suggested before, the airbags have worked in the past but don't scale well with higher mass vehicles. Thus you really need some kind of rocket (that ignite at subsonic speed); I'm not sure why a sky crane works better than some other system with rockets, I'd imagine its the easiest method of separation and also allows you to use less fuel since the crane itself doesn't have to slow down to a safe speed (i.e. release it down and reel it back up to reduce landing speed.)
Also, they had this option out there three years ago when I worked on a Mars mission for a class, so it's been out there a while and is probably as well developed as a non-tested system can be.
So what? This thing is much larger than anything landed anywhere before, with the exception of the manned LEM.
The Apollo capsule had an atmosphere much thicker (as in height) and much thicker (as in density) than the Mars landers have available. The Apollo LEM could use rocket braking because of the Moon's low gravity.
Mars is a stone cold bitch to land on because the atmosphere is too thin to completely rely on parachutes, and Mars' gravity is too high to rely completely on rockets.
Not exactly. It is radioisotope power but it is not nuclear power in the sense that there is a reaction going on. The simple decay gives off heat. As I recall it's not that much, either. Something on the order of 110 watts. Still, it's much better than relying on solar panels. Here's a nice page of fact sheets for the mission.
http://www.rootstrikers.org/
The problem with soft-landing heavy objects on Mars is that there's not enough atmosphere for aerobraking and parachutes to do the job, so the approaches used for Earth re-entry won't work. There's too much gravity for landing on rockets. as with lunar landers, without most of the payload being landing fuel. The problem gets harder as the mass goes up. This was realized only about five years ago, to the embarrassment of some within NASA. So there are now various complicated hybrid schemes, like this.
The scheme with the cables does not look promising. Unlike Luna, Mars has winds and weather. This looks like one of the student lander designs from NASA's high school curriculum.
One bad feature of this design is that the actual landing forces have to be taken by the rover's suspension. Previous designs had the rover inside the landing module, not underneath it. That approach uses crushable components (air bags, crushable blocks, collapsible legs, etc.) to cushion the landing. With this "flying crane" approach, the autopilot has to do a really, really smooth landing or the rover will be broken.