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Skycrane, Innovative landing technology
centsworth_II
post Dec 14 2008, 07:47 PM
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QUOTE (Oersted @ Dec 14 2008, 12:56 PM) *
Did I get the reasons right?

I would say that a major reason would be to eliminate the need for a landing platform on which the rover would sit during landing and off of which it would need to drive to start exploring. Can you imagine how massive a landing platform for the MSL would need to be?
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dvandorn
post Dec 14 2008, 08:12 PM
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A landing platform for MSL would mass roughly the same as the skycrane descent stage. You bring the whole thing to zero velocity relative to the ground, so you're using about the same mass of propellants, etc. (in fact, the skycrane descent stage uses more propellants, since it has to hover and then fly a ways away to avoid dropping onto the rover). All you'd have to do to make the descent stage into a bottoms-down lander is add a solid top for the rover to sit on, and maybe some short, stubby legs (and maybe not, the "landing pallet" concept was just barely beat out by the skycrane maneuver for MSL, in which the descent stage simply plopped onto the surface, with MSL on top, without legs).

What the skycrane maneuver buys you is instant mobility. You don't have to have deployable ramps or any other means of driving the rover off of the top of its lander, the rover is plopped onto the surface on its six wheels, and can immediately drive away.

When you trade the extra fuel needed to hover and fly away from the rover for the solid deck and ramps of the pallet, you get roughly the same mass.

-the other Doug


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Doc
post Dec 14 2008, 08:38 PM
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Silly question time.
Every time I look at the EDL process I keep wondering how well can the skycrane maneuver away from the detached parachute and back shell? Other spacecraft were either in free fall (eg rovers) or in controlled free fall (phoenix) to get away from the parachute and back shell.


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djellison
post Dec 14 2008, 08:44 PM
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QUOTE (dvandorn @ Dec 14 2008, 08:12 PM) *
When you trade the extra fuel needed to hover and fly away from the rover for the solid deck and ramps of the pallet, you get roughly the same mass.


Really? Got the numbers for that?
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djellison
post Dec 14 2008, 08:46 PM
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QUOTE (Doc @ Dec 14 2008, 08:38 PM) *
how well can the skycrane maneuver away from the detached parachute and back shell?


I don't think it will be in freefall ( the animation shows the engines firing up before separation from the backshell ) - but perhaps with a low throttle setting on the engines to fall away from the backshell.
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nprev
post Dec 14 2008, 09:31 PM
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I keep thinking that it doesn't have to be a particularly difficult maneuver. If the descent stage maintains constant thrust after bridle release and uses the release event as a cue to tilt the platform just a few degrees in any direction, then it should gain altitude while simultaneously veering away from MSL.

The only potential problem would be if there was a cliff or some other topographical feature very near MSL (like a few meters) that the thing might hit, but it seems probable that the landing site will be chosen to minimize that risk.


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mcaplinger
post Dec 14 2008, 10:23 PM
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QUOTE (dvandorn @ Dec 14 2008, 12:12 PM) *
When you trade the extra fuel needed to hover and fly away from the rover for the solid deck and ramps of the pallet, you get roughly the same mass.

Maybe, maybe not. The propellant is at least a known mass quality given the mass of the descent stage, while the ramps could be pretty involved depending on the design constraints (rock size distribution, allowable tilt, etc.)

I haven't found a definitive trade study analysis of the skycrane, but http://trs-new.jpl.nasa.gov/dspace/handle/2014/40017 is a start.


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dvandorn
post Dec 14 2008, 10:33 PM
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QUOTE (djellison @ Dec 14 2008, 02:44 PM) *
Really? Got the numbers for that?

What I got is the fact that the same size/mass rover was designed for either a pallet landing or a skycrane landing. Ergo, the mass of the pallet descent stage vs. the skycrane descent stage would have to be roughly similar.

-the other Doug


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Guest_Enceladus75_*
post Dec 15 2008, 12:35 AM
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I think the skycrane element of the MSL is very impreressive - real 21st century technology in action (now, they just need to hurry up with the domestic robots they've been promising us we would see "by 2000" rolleyes.gif ).

Question: If it's successful (and I believe it will be) will skycranes be the standard mode for all or indeed most future non-manned Mars and other planetary landing missions?
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mcaplinger
post Dec 15 2008, 01:51 AM
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QUOTE (dvandorn @ Dec 14 2008, 02:33 PM) *
What I got is the fact that the same size/mass rover was designed for either a pallet landing or a skycrane landing.

That's not clear; the original rover design when they were still talking about pallets could have been quite different.


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mcaplinger
post Dec 15 2008, 01:53 AM
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QUOTE (Enceladus75 @ Dec 14 2008, 04:35 PM) *
Question: If it's successful... will skycranes be the standard mode for all or indeed most future non-manned Mars and other planetary landing missions?

If you read the tech reports so far, that's how it's being sold, since the payload design is decoupled from the descent stage design.

I'm neutral on the skycrane concept at the moment; I haven't seen definitive trade studies and some of the literature seems more like marketing than engineering to me. But we'll see.


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dvandorn
post Dec 15 2008, 05:18 AM
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QUOTE (mcaplinger @ Dec 14 2008, 07:51 PM) *
That's not clear; the original rover design when they were still talking about pallets could have been quite different.

OK -- I can accept that. I just thought I had remembered that the rover design was pretty well determined by late summer of 2005, when the skycrane maneuver testing was done that was to validate the concept and allow pallets and legged landers to finally be cast aside.

-the other Doug


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MarsEngineer
post Dec 15 2008, 08:03 AM
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Hi everybody.

It has been a long time since I posted (not long after Phoenix landed).

QUOTE (Oersted @ Dec 14 2008, 09:56 AM) *
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.

QUOTE (Oersted @ Dec 14 2008, 09:56 AM) *
Can't wait till 2011! - But will have to, it seems... smile.gif


Me too!


still cranking away ...

-Rob Manning

PS No one I work with really minds if people use the word "skycrane" for the name of the separated descent stage or the rover/descent stage maneuver. We have developed a habit of calling the awkward-looking jet pack that straddles the rover the "descent stage". But feel free to call it the "skycrane" if you prefer. I won't mind. biggrin.gif

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Phil Stooke
post Dec 15 2008, 02:54 PM
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Wouldn't it be nice if this great new EDL system could actually be used more than once...?

Phil


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mcaplinger
post Dec 15 2008, 03:55 PM
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It's clearly intended for that, but obviously the programmatic/budgetary considerations involved are above the pay grade of anyone here.

Removing full inline quoting isn't though - ADMIN


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