[Rand]

"Rubato" da unmannedspaceflight un po' di risposte sul sistema Skycrane da un ingegnere che ci lavora (tutte le opinioni sono sue e non la posizione ufficiale di NASA/JPL/Caltech):

The skycrane maneuver still fascinates me, but I am confident it will be a mature and well-understood system by the time of EDL.

Apparently I exceeded the quote count limit for replies so ... I will respond to Oersted's questions by italicizing his questions ...

Despite our slip, I think the terminal descent & skycrane maneuver part of EDL (the guidance and control algorithms, the propulsion, the bridle and umbilical device (BUD), radar, the throttle valves and control on the descent stage) are all in pretty good shape at the moment (compared with the rover). EDL & skycrane was pretty much on track for '09. Which is not to say that we do not have a lot of work to go. So like you Oersted, I also feel that we will have a mature and well-understood system by the time of EDL. Ironically this EDL system (at least has the potential) of being easier to prove than MER and MPF. It is very similar to how we proved Phoenix's EDL.

Even more confident after the perfect Phoenix landing, where the engineers said they were sure all would go well after the radar had acquired the surface. After that the descent would just "go on rails", they said, or words to that effect.

I don't recall who said that, but there was some truth to that. On MPL and Phoenix, we used a radar altimeter/velocimeter that was not expressly designed for a high altitude / high speed near-vertical descent. In the years prior to landing Phoenix, we had some difficulty getting it to work the way we wanted it to work. But with tweaking we finally did.

It would be neat if the lander/skycrane itself could fly off and make a soft landing with its remaining fuel. There will now be to years extra for coding, so maybe a little proggie can be made for the skycrane computer that could try to effect that? Why would it be interesting to land softly? Maybe to scour some trenches that the rover could visit... Then again, a crashing lander should make a nice big hole on its own. smile.gif

hmmm.... you are not the first to suggest that we try for a soft descent stage landing, Oersted. While that would not be impossible to consider, it would be a lot of work and as you know (and MSL folks know all too well) time is money. However I will be more than happy to tease my friend Jeff about it (he's one of the main developers of the code that controls the "flyaway" mode of the descent stage).

From what I understood there are four main reasons for using the skycrane maneuver: 1) A parachute/airbag combo cannot deliver such a heavy system to the ground safely.

I saw some great answers above, but I will throw in my 2 cents.
I would say that parachute/unthrottled solid rocket/airbag combo cannot deliver such a heavy system to the ground safely.

As you might recall in one of the MER NOVA specials (where Dan Maas made a cool but terrifying animation of a high horizontal velocity landing that tore the airbags to shreds), the MER combo (even with the TIRS and DIMES add-on) resulted in uncomfortably high horizontal ("tangential") impact velocity and could also threaten the "normal" impact velocity airbag capability envelope. We found on MER that as the mass of the landed stuff increased, even with larger airbags, given available fabric strength we also needed to reduce the impact velocity. We found that we could not do that with a (unthrottlable) solid rocket propulsion system. If we were to swap the RAD and TIRS motors with a throttled liquid propulsion system, we COULD land with larger airbags (because the throttled system gives you a LOT more velocity control). However once you do that you now have the ability to control and reduce the touchdown velocity to the point that you really don't need airbags, nor a lander nor a righting systems (like the MPF/MER petals). In fact you can land on your wheels ...

But that pesky parachute is still hanging on trying to yank the prop system this way and that ... (it gave us fits on MER) .. what if we ditch the parachute like Phoenix does?

Viola .. you get MSL's EDL system.

You might recall picts of the old 2003 Mars Sample Return Lander that was being considered in the late 1990s (prior to the loss of MPL). It was basically an oversized Viking lander (same as Phoenix except that the descent engines were throttled rather than the pulse mode used by Phoenix). We were trying to use that lander design to land a large (MER-sized) rover on top of its upper equipment deck. The only trouble was that we needed a lot of heavy ramp hardware to get that rover down about a meter off of that deck down to the surface of Mars. (If your rover is still a meter above the surface of Mars, you really can't say that you have landed yet.)

2) The complexities of opening up a parachute can be avoided (deployment, squidding, shredding and other chaotic events).

Dang it. We can't seem to get away from needing a heatshield AND a parachute (or to be precise, some type of a supersonic decelerator). Until we invent something like a "supersonic tension cone" or "supersonic retro-propulsion" we are stuck with at least one parachute. I think the latter inventions are probably required in order to land really big things - like people.

3) A skycrane maneuver presents the guidance computers with just a two-module pendular movement, not a three-module movement as in parachute-backshell-lander. Is that really a big issue, though, with present-day computational powers?

It turned out not to be an issue. Ironically, it was initial worry about the two-body & pendulum modes that gave a lot of people concern about the skycrane architecture when we first proposed it in early 2000. What we learned on MER (with its 3-body dynamics) is that damping the pendulum dynamics with a closed loop might not be so hard. Sure enough, when this architecture came up again in 2002 further analysis showed that damping the dynamics was a lot more straight forward than we initially thought. (Do an experiment ... suspend a yo-yo on the end of its string. Hang on tight and with your eyes closed have someone induce a swinging motion of the yo-yo .... with your eyes still closed see how fast you can move your hand right, left, front and back to try to stop the swing motion. You might be surprised at how quickly you can do it. You should be able to do in less than 4 seconds.)

The rover's computer can easily do the same thing as your hand. It uses its inertial measurement unit on the descent stage to feel the same forces your fingers feel - completely inertially ... no strain gauges or load cells required!

4) The lander/skycrane rockets will not impinge too much on the rover as it is lowered to the surface.

That is correct. Like MER and MPF, we needed to cant the rocket nozzles to prevent impingement on the rover. You might ask, why not leave the thrust UNDER the rover (like the pallet lander design option)? There are two answers, the first is that our fuel tanks take a lot of space and could not fit inside the pallet lander. The second is that, despite what you think you know about landing rockets "tail first" (with the thrusters close to the surface: like Buck Rogers, Apollo, Viking, DC-X and Phoenix), you might be surprised that there are a lot of interesting technical challenges with putting thrusters so close to the surface. (Did you know that the Apollo landers each sent a cloud of lunar dust particles into lunar orbit during each landing? The command module flew through that fine cloud.) While not insurmountable, it does present some interesting challenges. For example, to overcome ground effects of the thrusters, we had to land Phoenix about 5 times faster than the MSL rover will touch down. (Knowing we were going to landing Phoenix on a flat tundra-like surface made our EDL job a lot easier.) There are some advantages to keeping your thrusters a few meters above the ground.

I have a few questions regarding the maneuver though, maybe someone can clear them up.

Will the bridle deployment be part of the soft-landing effort? - I.e. will the speed of deployment be used to control landing speed together with the rocket firings? - It seems to me that bridle deployment will be completed before touchdown, though, but I just want to know if bridle deployment has ever been considered a means of controlling touchdown speed.

great question ... we considered using the bridle deployment as a touchdown aid for a long time before we decided that it was too complex. We time the start of the deployment phase so that the mobility system and the bridle deployment are all completed prior to landing. This made testing a lot easier too.

Will the bridle deployment be undertaking during descent or in a hover phase? I guess it is more complicated during descent due to wind forces, but waiting until a steady hover has been achieved will consume more fuel, of course.

We wait until the vehicle is descending vertically at a constant velocity (and at the correct estimated height) before the rover is released and is lowered on the bridle. It does take a bit more fuel but not a lot.

Will only radar be used during descent, or will there also be a photographic system to determine drift (as with MER) and possible nature of the landing site? Will the skycrane be equipped to actively try to avoid rocks and unsuitable terrain and translate horizontally in an Armstrong-esque fashion, if necessary?

Not on this mission. The combination of a small landing area (made possible by the Apollo-like Earth-entry closed loop guidance), the wonderful MRO imaging and 3-D reconstruction of the landing sites plus MSLs very slow and safe landing on wheels allows us to land safely with the rover's "eyes closed" with very high probability. (MSL Rover can land on surfaces steeper and rockier than any prior Mars lander.) MSL can land on slopes as steep as 20 deg (or more) and can land on 50 cm high rocks. Even MER and MPF could not do that it's first bounce.

COULD we add a camera and do terminal hazard assessment and avoidance in real time? Maybe. But that is a lot of work that we really do not need to do right now. Maybe someday if we were to do this again and we wanted to land on more challenging terrain (and to be sure, there is a lot of that kind of terrain on Mars) we could do terrain-relative navigation and use on-board maps to land on a-priori safe sites (my personal preference). Maybe.