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SpaceListener
Recent news about the progress of MSL Building Skycrane and MSL
centsworth_II
QUOTE (SpaceListener @ Nov 13 2008, 03:12 PM) *
Building Skycrane...

Where did you get that wording? I just see "descent stage" in the article.

I'm afraid it's a continuing obsession of mine to follow the descent stage vs. skycrane vs. skycrane maneuver phraseology saga. At the most recent MSL press briefing I noticed that one of the major officials in the project referred to "the descent stage, otherwise known as the sky crane."

As I understand it, the official stance is the descent stage is not to be called the skycrane. The landing maneuver is called the skycrane maneuver. Is it evil of me to be fascinated by the (to me) quixotic task of keeping the descent stage from being called "skycrane"?
mcaplinger
QUOTE (centsworth_II @ Nov 13 2008, 12:59 PM) *
Is it evil of me to be fascinated by the (to me) quixotic task of keeping the descent stage from being called "skycrane"?

Not evil, just a little obsessive/crazy. smile.gif

I think it's inevitable that it's going to be called the skycrane by many, even within the project, and the only reason to not do so is because "Skycrane" may be a registered trademark of Sikorsky. I note that the current licensed manufacturer of the S-64 calls it the "Aircrane".
antipode
What's a good, resilient, desert top predator on earth? MSL will be a desert hunter. The Fennec Fox is perhaps too diminutive. Here in Australia we have Dingoes but that's not going to work on the international stage! Coyote maybe? Maybe something avian and sharp eyed? 'Buzzard' perhaps!

ph34r.gif

Just kiddin - I think...

P
stevesliva
QUOTE (mcaplinger @ Nov 13 2008, 06:13 PM) *
and the only reason to not do so is because "Skycrane" may be a registered trademark of Sikorsky.


It is. USPTO lists it as an active trademark of United Technologies Corp, parent of Sikorsky.
SpaceListener
The only thing that bothers me is that after Skycrane drops MSL by tethers, it will fly away between 500 to 1,000 meters and then will fall off.

I must accept it since it has no posing legs but their thrusters.... It would be useful try to land smoothly or hover close to surface that their 8 thrusters would clean the surface and maybe make holes (only on sandy surface) like Phoenix did... so later MSL can snif them.
nprev
Re the whole "Skycrane" issue: Has anybody at JPL contacted United Technologies and asked them for permission to use the name? They might well be thrilled by the association, perhaps even fund some outreach efforts. Can't hurt to ask!
centsworth_II
QUOTE (SpaceListener @ Nov 13 2008, 07:01 PM) *
...after Skycrane drops MSL by tethers...

Obsessive it may be, but I will continue to make note* of misuse of the Skycrane moniker in these forums, in NASA news briefings, and in the press. I personally like the name for the descent stage and predict that the press will use it freely. I'm just curious to see if the misuse of the name will be actively discouraged, ignored, or accepted.

Yes, yes. I'm sick! tongue.gif

Edit:
*No I will not burden the forums with continued nitpicking at every opportunity. (Just a reminder every now and again.) laugh.gif Good - this should be your last nitpick - Moderator
mcaplinger
QUOTE (centsworth_II @ Nov 13 2008, 07:41 PM) *
Obsessive it may be, but I will continue to make note* of misuse of the Skycrane moniker...

I will note that US Patent D505105, "Skycrane landing system", held by a bunch of folks at JPL, uses this term explicitly. I'm not even sure who said it was a "misuse" to call it the skycrane, but I'd wonder what their motivation was. I could imagine some lawyers in the Caltech office of the general counsel decided it was trademark infringement or something, but IANAL.

I'm having a hard time seeing the ongoing relevance of this discussion, so we might be prepared for Doug to intervene.
djellison
QUOTE (centsworth_II @ Nov 14 2008, 03:41 AM) *
(Just a reminder every now and again.)


Here's a reminder. UMSF Rule 2.4

- 2.4 Don't rant. If you have a point to make...make it and move on. Do not litter your every post making the same point again and again. If you want somewhere to vent - get yourself a blog.

Official forum ruling...

... it doesn't matter what you call we as long as we know what people mean. Skycrane, descent stage, Rupert the Rocket, Piggyback Propellant Part, Dave the Decelerator. Having heard Richard Cook use both names within a single lecture, semantic debate over what it should or shouldn't be used is pointless. And we know what the media will call it - so anyone who continues to object, will have a LOT of objecting to do over the next 2 years. As 2.4 states, however, they can do it somewhere else. There's factual inaccuracy ( which this isn't ) and there's semantics ( which this is ).

Doug
bgarlick
Sorry if this has been addressed [often] before, but why have the decent stage hover while the rover is lowered on tethers?
I can see that having the decent stage carry the rover all the way to the surface and then fly away would kick up lots of dust that could
damage/contaminate the rover, but wouldn't it be simpler to create some thin metal/composite encapsulation around the rover to protect it during
this stage instead of having the extra propellant to hover and the extra decent complexity?
Once the decent stage has flown away and the dust has settled the rover could then hatch/'pop out' of the light,protective shell and drive away.
djellison
QUOTE (bgarlick @ Nov 14 2008, 09:02 PM) *
t wouldn't it be simpler to create some thin metal/composite encapsulation around the rover


Simpler to somehow get a large lid off the rover after landing? I don't think so. It would be a lot heavier, it would mean the rover having to withstand the mass of the rover AND the descent stage on its suspension thus reducing margin on performance for landing.

The rope-rappel may seem crazy - but it's a lot less crazy and a lot safer for the rover than many other options.

There are videos out there somewhere of some rappel tests- but for the life of me I can't find them again.

Bingo : http://mars.jpl.nasa.gov/msl/spotlight/20081023.html
remcook
I can imagine a rover being in front of a rocket exhaust possibly causing a problem... No idea how much chance there is for that but it seems more so than from a distance. Just a random thought (as usual)...
centsworth_II
QUOTE (bgarlick @ Nov 14 2008, 04:02 PM) *
...why have the decent stage hover while the rover is lowered on tethers?

I don't think the lowering on a tether adds any time to the decent. If this video is accurate, the lowering takes place during the slow, steady portion of final decent. In fact, it is the extra weight of a protective enclosure that would require extra propellant. The goal is to maximize the weight of the science package and minimize the auxiliary weight needed to place that package on Mars.
Tom Tamlyn
>[W]hy have the decent stage hover while the rover is lowered on tethers?
>I can see that having the decent stage carry the rover all the way to the surface and then fly
>away would kick up lots of dust that could damage/contaminate the rover ...


I don't think that the principal justification for the skycrane technique is the dust cloud problem. See Rob Manning's earlier comments in post # 168 on why the engineers consider the skycrane technique easier to understand and thus more reliable than others under consideration.

TTT
monitorlizard
If you look at images and CG of Skycrane, you can see that the engines are angled away from vertical (outward, away from MSL). This means engine exhaust will not hit the rover. Any dust clouds formed would be away from the point of landing. Turbulence might bring a little dust back to the rover, but not much, I imagine.

I'm sure the length of the tethers was also chosen to minimize rover/debris interaction.
MahFL
Hopefully MSL will experience at least 6 seasons of duststorms. That will be a lot of dust.
Oersted
Well, the MSL will have all its optics and instruments folded away during landing, so it will be less prone to experience dust problems during landing than later on.

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

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

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. 2) The complexities of opening up a parachute can be avoided (deployment, squidding, shredding and other chaotic events). 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? 4) The lander/skycrane rockets will not impinge too much on the rover as it is lowered to the surface.

Did I get the reasons right? I mentioned them chronologically, I wonder what their order of importance is....

I have a few questions regarding the manuever 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.

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.

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?

Can't wait till 2011! - But will have to, it seems... smile.gif
ElkGroveDan
QUOTE (Oersted @ Dec 14 2008, 09:56 AM) *
2) The complexities of opening up a parachute can be avoided (deployment, squidding, shredding and other chaotic events).


MSL has chutes as part of EDL.
mcaplinger
QUOTE (Oersted @ Dec 14 2008, 09:56 AM) *
Did I get the reasons right?

The primary advantages are not having the mass of the airbags, which doesn't scale with landed mass, and not having the mass of any roll-off/airbag support structure (what was called the "lander" for MER.) As noted, MSL still has a parachute.

The simplification of the pendulum is a non-issue, and I would argue that MSL is no better, and maybe worse, for contamination potential compared with a "rover-enclosed" design like MER.

The main reason for not using airbags is that you can't have a rover with mass higher than MER with airbags, at least not without paying a huge mass penalty.
centsworth_II
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?
dvandorn
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
Doc
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.
djellison
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?
djellison
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.
nprev
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.
mcaplinger
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.
dvandorn
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
Enceladus75
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?
mcaplinger
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.
mcaplinger
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.
dvandorn
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
MarsEngineer
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

******
Comments and opinions expressed are the author's and not of JPL, Caltech nor NASA.
Phil Stooke
Wouldn't it be nice if this great new EDL system could actually be used more than once...?

Phil
mcaplinger
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
MarsEngineer
QUOTE (Phil Stooke @ Dec 15 2008, 06:54 AM) *
Wouldn't it be nice if this great new EDL system could actually be used more than once...?

Phil



I completely agree Phil. To an extent that is not obvious from the EDL animations, there is a lot more commonality than you would think. But obviously we could do a lot better. While the MSL EDL architecture may not make sense for very small landers, I beleive that it is general purpose enough to be able to be used for a wide range of future landers and rovers. The historical diversity in the design of Mars landing systems is not as inexplicable as it looks. Someday one of us will have to write an engineering history. (I have been on the hook for a paper on this topic for a year or two now.)

-Rob
Oersted
Thanks for your replies everybody, with a special thanks of course going to MarsEngineer for his long, detailed and highly informative reply!

QUOTE (MarsEngineer @ Dec 15 2008, 09:03 AM) *
I will respond to Oersted's questions by italicizing his questions ...

[...]

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).


...If that descent stage should gently alight on the surface of Mars, I'll give out a whoop almost as big as that for MSL itself smile.gif

QUOTE (MarsEngineer @ Dec 15 2008, 09:03 AM) *
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.


Oops, ascribe that brainfart of mine to a late-night posting: of course MSL has a parachute!

When I hear about supersonic tension cones, I often think back on old quasi-sci-fi projects such as General Electric's MOOSE (Manned Orbital Operations Safety Equipment): http://boggsspace.com/strange_but_true.asp

- So what we need for future Mars EDL systems is a big properly shaped Mylar bag, inside that a deployable carbon-fibre erector set and lots of quickly-hardening ablative foam, and voilá, there's your supersized supersonic tension cone! Well, that'll be the day...

From considerations like these as well as the amazing chrysalis MER pop-out deployment, I am thinking that JPL would do well to hire some Japanese origami experts... Space exploration seems to be unfolding ever more smile.gif - That is, unless we go for supersonic retro propulsion, in which case we should be looking more for some punchy German Huntsville kind-of-guys who know how to weld together a kick-*** rocket engine.

QUOTE (MarsEngineer @ Dec 15 2008, 09:03 AM) *
(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.)


No, I most certainly didn't, that's amazing! - I did know that the Surveyor 3 lander was practically sanded by moon dust from the Apollo 12 landing, but never heard about a cloud of dust reaching up to the CM.

QUOTE (MarsEngineer @ Dec 15 2008, 09:03 AM) *
I have a few questions regarding the maneuver though, maybe someone can clear them up.

[...]

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.


Just one last question: - Does this mean that the MSL rocker-bogie suspension system will be up and running at the moment of touchdown? Outstretched wheels ready to absorb the impact as best possible? Wheel position instantaneously changing to accommodate a possible big rock?

For future, heavier landers, it could, I guess, be worthwhile to beef up the rocker-bogie suspension for that one-off event. The stresses of landing and roving are so very different, it would be less than optimal to have to over-design the whole rocker-bogie for just that one single forceful bump. - Could it maybe be done with cheap and light one-time compression "sticks" under the main body or inserted into the rocker-bogie structure, to fall away after the initial jolt? Not replacing but supplementing the wheels. I am thinking of small versions of the ingenious Apollo LM legs that were designed to handle just one single compression by collapsing metal foil.

- If you say the weight is better spent on a perfecting soft-landing procedures I won't argue though!

Thanks again for all the replies, I am glad this thread got going again!
centsworth_II
QUOTE (Oersted @ Dec 18 2008, 06:14 PM) *
Just one last question: - Does this mean that the MSL rocker-bogie suspension system will be up and running at the moment of touchdown?

It sounds like it from this:
"During powered descent, the vehicle uses radar and its propulsion system to control position and velocity. At approximately 20m above the surface (as measured by the radar), the rover is lowered on a tether from the propulsion system (known as the descent stage) in a 'sky crane' configuration and placed directly on the martian surface with its mobility system (i.e., wheels and suspension) fully deployed." http://marsoweb.nas.nasa.gov/landingsites/...uide_v4.5.1.pdf
imipak
The video of the tether / bridle / rappel tests that Doug posted earlier in the thread mentions tests of landing on a simulated rock. However now I come to watch it again, and going on the appearance of the test mass, those are tests of the bridle mechanism alone, rather than the rover wheels and suspension system. I can't see how the force of the impact on the wheels and bogies would be any greater for landing on an uneven, rocky surface, than for landing on a flat one, though?
hendric
If you are landing on a smooth surface, all three wheels on one side encounter the ground at essentially the same time. If one of your wheels hits a 50cm boulder on a 20 degree slope, that wheel will be moving upwards, pushing the rest of the wheels down, making them hit the ground "harder". Plus, the bogey linkages will be in a different configuration, putting stress in different places etc. Take a toy car and hold it horizontal, then tap it against a table. Now put something under one side and do it again, and you'll see that the opposite side is coming down faster than the rest of the car, and taking on more stress than before during impact.
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