Best way to recover parts from MUN (or any moon) surface?

[JStevenson]

Hello,

I have been playing KSP now for a couple of years.  Just started a new career after my last one basically "Saturated" with science. 

Anyways, I am trying to accomplish one of those "Recover Part XYZ from the surface of MUN" missions.  I recall experiencing similar frustrations back 2 years ago trying to do this mission type. I recall I used something like a Skycrane to snag the part using a downward facing Claw.  But I do recall it was VERY hard to center the Skycrane or simply control it with sufficient fine control to actually accomplish the grab.  Even in free space I find the Claw somewhat unreliable and takes several "hits" sometimes before it actually grabs the part in space.

Anyways, so my question really is in two parts:

1.  Is a Skycrane the best thing to use to grab a part on the surface of the MUN (or Minmus, or whatever) and haul it back into space?

2.  Assume (2) is yes - how the heck to I build and then fly a Skycrane with sufficient control precision to actually so the delicate maneuvre required to position the Claw EXACT over and then drop on it to grab it?   Is there a trick to how to fly one of these things (i.e. seems like you need control ability like with a helicopter)?

I built what I thought was a good design, got to the MUN and landed it very close to the part.  Using short hops its now basically inside the radius of the 4 landing legs.  I Quicksaved here for the final maneuvre.  But after about a dozen repeats I have yet to find the right way to control the darned thing to actually snag the part (which turns out to be a Wheesley Turbofan engine - how the heck this got on MUN is a mystery!)

 

 

[bewing]

First, it is extremely important to understand the major limitation of a klaw. It will only attach if it impacts a target part almost exactly perpendicular to the target's surface. When you are just trying to attach at random, that means it'll take a dozen or two attempts.

So, you really have two choices for grabbing something laying on a surface. You go after it with a skycrane or a rover. (Theoretically, you can also scoop it up with an MK3 ramp, but I only know one guy crazy enough to actually do that.) Skycranes work best on very low gravity bodies. Meaning Minmus or smaller. A skycrane on the Mun needs to thrust so hard to stay airborne that it's very hard to control. And remember that you are going to need a dozen or two grabbing attempts. A rover is a little more convenient because it comes in from the side, and you can aim better to get a perpendicular attachment. However, there are plenty of slopes on the Mun that a rover cannot climb. So the part may be unretrievable with a rover.

To maneuver a skycrane, it's best to separate the lift and translate functions. You lock SAS to normal, then use RCS to translate over your target.

So, in the end, my opinion is to never ever ever take a "recover part from the surface" contract.

 

 

1 hour ago, JStevenson said:

(which turns out to be a Wheesley Turbofan engine - how the heck this got on MUN is a mystery!)

Not exactly a mystery, just heavily classified.

 

[JoeSchmuckatelli]

Would a lander with a bay work?  Use a rover with claw to capture, then deposit the engine into the bay and take off? 

[archiebald]

2 hours ago, bewing said:

First, it is extremely important to understand the major limitation of a klaw. It will only attach if it impacts a target part almost exactly perpendicular to the target's surface. When you are just trying to attach at random, that means it'll take a dozen or two attempts.

This is so true, it is worth a like.

If you don't mind using mods, then the best one to help out here is TCA (Throttle Controlled Avionics).  Big learning curve on this one, but it is very useful once you figure it out.  It has some of the functionality of MechJeb for launch and landing but also is great for VTOL and hover, auto levelling, goto waypoints, terrain following etc.  Also VERY good if you have multiple engines and an off center COM so you can create landers that are normally not possible - it will balance thrust on each engine individually in real-time to keep the vessel level.

With TCA running on a skycrane, lifting debris from the Mun is WAY easier, notwithstanding the limitations of the Klaw.  You let TCA hold you steady while you line up over the debris and cut engines, I've done many of these recoveries now - would not have considered them without TCA.

Make sure to get the updated version 3.6.2.2 which is good for KSP 1.9.1. (on page 89 of the thread)

 

Edited May 23, 2020 by archiebald

[bewing]

2 hours ago, JoeSchmuckatelli said:

Would a lander with a bay work?  Use a rover with claw to capture, then deposit the engine into the bay and take off? 

Not unless the bay has another klaw in it, in which case you don't need the bay. If a part is loose in a bay, then you can't timewarp or it will fall out.

[allista]

1 hour ago, bewing said:

Not unless the bay has another klaw in it, in which case you don't need the bay. If a part is loose in a bay, then you can't timewarp or it will fall out.

For this I'd also add the Hangar to the mix:

A Hangar can store your rover without the need to physically connect it to some part (klaw or docking port) of the mothership. Just drive inside, activate the Hangar, and the rover will vanish; then you can fly anywhere and spawn it again when needed.

[AHHans]

9 hours ago, JStevenson said:

1.  Is a Skycrane the best thing to use to grab a part on the surface of the MUN (or Minmus, or whatever) and haul it back into space?

Well, I use a rover with a claw up front. Well actually a plane/rover/rocket contraption, with a retractable nose-gear that will scrape the claw over the ground when the nose-gear is retracted, a main engine to push it foreward, and lifter engines to get it off the ground (and the nose upwards) when it's time to lift. Up to now I always managed to grab the target with a combination for pushing it around and raising/lowering the nose(-gear). Once the target part is grabbed it has enough thrust (and gimbal range) to get to space - with or without the help of a handy hill.

[Hotel26]

12 hours ago, JStevenson said:

fly a Skycrane with sufficient control precision to actually so the delicate maneuvre required to position

I routinely do top-docking with senior docking rings (which are my standard) and with claws, when necessary.

I long thought this was an impossible manuever but I have several craft (Spider and Sparrow) that can do it.

I do in fact refuel this way on the Mun (reconnaissance Ladybugs and Hummingbirds) as well as perform precision-placement of parts onto assemblies (e.g. Spider lowering ORBs^[2] from KP Luna Vista onto Snowflake) already resting on the surface.

So, from that experience I'd convey two points:

1. there's no shame in designing for use of horizontal RCS for positioning and I use MP or Vernors (LFOX) depending upon the load.  I tune the thrust percentages as required for the job.  Then set SAS to maintain a vertical control attitude (Radial Out Sfc mode).  Use attitude tilt to come down from above as close as possible to the "(fireman's) pole" above the target; then just maintain a moderate sink rate with vertical thrust while using the horizontal RCS to pinpoint the approach to the dock.  (Light & maneuverable craft (not skycranes) don't need this, as attitude-"jinking"^[1] will be sufficient for horizontal vectoring.)
2. this tip (from @Atkara) is not super-intuitive at first but it's worth studying as an option: once you have drift stabilized over the target, first be sure to punch out SAS Radial Out (Sfc) to SAS Hold (tap 'F' key); then select your bottom dock (or claw) on your Skycrane as control(!!!!); then re-select SAS for Radial In (Sfc mode).  The result of this is that SAS is still holding your vertical attitude but the navball will show the approach to the target in horizontal plan form in a prograde orientation that matches your controls (instead of reversed) and that unloads the pilot-workload just that much to be useful.

I sometimes land nearby first to jigger the thrust percentages on the main engines to give me finer control over vertical thrust and therefore sink rate.  (But Spider also features vertical RCS which allows additional "boost" to fine-tune sink rate just before touch-down.  This can be a critical feature.)  It's important to keep the sink rate moderately sufficient so that the horizontal situation doesn't get twitchy on the navball, but not so much that pilot reaction time is pressured too much.  Steady & gradual does it.  If you're a hot-shot, just add the vertical thrust tuning to your worklist and do it near final approach to obviate the need to land first, but take things one step at a time as you learn this procedure until you can pull it all together.

Here's the take-away.  It's completely possible, it works, and it is fun (but ever stressful!!).  Once you can do this, your KSP confidence quotient will SOAR.

I use top-dockers with and without a claw attachment.  If you land a docking ring on another, even if the alignment is off (due to ground slope), the magnetic capture will help you if you let it.  And if you land on a level ring but not exactly, you can still use the RCS thrusters horizontally to get aligned.  If you're heavy, you may still need to apply a little vert thrust to be light enough for that, but it's always possible.

[1] "jinking" is how I describe quick taps to the attitude-rotational controls, 'WASD', while leaving SAS Radial still engaged so that SAS immediately returns you to the vertical-reference attitude.

[2] a Spider transporting an ORB:

 

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Edited May 23, 2020 by Hotel26

[JStevenson]

Thanks guys!  These are all very good suggestions - especially the one about the TCA mod.  Sounds like what I have been doing is ABOUT right without this one.  And yes I have done at least a dozen attempts so far (maybe even 2 dozen!)

I realized that what I need is the equivalent to the "altitude hold" feature in Orbiter.  Raise the Skycrane just enough about the, say 3-4m, then hold this altitude and with SAS in Normal Out.  I scaled back the max thrust of each of the 4 corner thrusters to 10% to give even more precise control.  I then use RCS thrusters to position in XY to be centered over the part.   I even switched to DOCKING mode to get the linear mode, and using for/aft RCS to trim the vertical position.  I *have* managed to do this several times now, and dropped on the Wheasley several times nice and centered and yet it didn't latch on to it.  In most cases it either skipped/bounced away or in some extreme cases exploded.  The lack of perpendicular surfaces is the problem.  The engine is basically sitting with flat front end down, so is essentially a cone sticking up.  Not much perpendicular surface there facing up!

After I posted my question, I independently also tried the "Rover with Claw" idea.  I built a special version of my "Constructor Mk1" rover but this time with a bunch of side-mounted tiny rockets to provide enough vertical lift that SHOULD allow the thing to fly off MUN once it has the part (assume the added weight of the Wheasley isn't too bad).  Until I get the better "Mk3 or Mk4" wheels I find the earlier constructor Rovers would best with a bit of Ballast to them, so I always build them using a horizontal fuel tank with a small liquid engine at the back.  The engine has helped me move heavy MUN base parts around and allow the Rover to get out of some bad jams (like being stuck down a steep slope)

Anyways, I spent an hour building it, launching it and getting to the surface of MUN right next to the failed Skycrane/Wheasley combo.  And now having still the problem of not latching on.  I'm simply pushing the darn thing around the surface.  Again because I think its too low and basically a cone.   The "Claw" is basically snagging it with the bottom two claw arms only, which I suspect is the problem. Once I do latch on, the location is at the bottom of a deep crater so plenty of nice hills to launch up on! 

Talk about alot of fuss over a stupid lost Wheasley engine, eh?  Oh well, at least I gave the Tourist who came along in the Skycrane alot of entertainment!   I always try to multi-task on missions and do as many contract elements as possible with each launch.  In this case, I took a silly tourist along who wanted to "Orbit and Land of MUN".   He must have tons of neat stories now to tell his fellow Kerbal friends.  Especially all the cussing coming from Valentina ;-)!

Here is a photo of the Skycrane VERY close to the Wheasley.  So near, yet so far.....

https://1drv.ms/u/s!Ap9Ol_DyLKPiicotBWUEGysvGxAurw

 

[swjr-swis]

16 hours ago, bewing said:

So, in the end, my opinion is to never ever ever take a "recover part from the surface" contract.

Quoted as Best Advice in this thread.

(Not to belittle @Hotel26's excellent description of how it can actually be done, but considering the tools we get in the -stock- game, the outright silly choice of marooned parts, and the almost-guaranteed abysmal return of investment, I quickly learned to completely pass on the 'recover debris from X' type of missions.)

[Klapaucius]

I don't play career, so not sure how big the parts are, but can't you land a rover (or wheeled lifter) that can straddle the part?  Then you have all the time in the world to position yourself.

I was thinking about Marine Travel Lifts. Obviously, this is not exact, but you could take the basic design and instead of the slings, dangle a claw from a crossbeam on top.

 

https://marinetravellifts.com/35-ton-marine-travel-lift-for-sale/

[magnemoe]

Excelent idea of an rover with an claw at the back. you need engines pointing the same direction of the claw, land on the claw and then tilt rover to it end on the wheels, now the claw module should also include probe core, parachutes and heat shield, decopler then the rover who is fuel tanks with wheels, you might want landing legs to raise the nose for takeoff. Test this on kerbin against the same type of capsule you will encounter. 

My way of doing this was an rover with an crane who drove over the rover but above is much simpler. 
 

[Empiro]

I've only done one such mission, and tried both the sky-claw and rover design. The sky-claw is really challenging because you're essentially attempting a docking maneuver with very limited time before running out of fuel, and you'll need to be constantly adjusting the thrust to maintain altitude.

At the end, I went with a rover that was essentially a rocket with a claw in the front. To land, it had wheels and radial engines that lands it horizontally. I also had a horizontal probe core on the side to help with the horizontal landing. For most of the way, it uses the efficient engine in the back, and when it's just about to land (maybe 100m off the ground), I deactivate it, switch to the radial engines, and control from the horizontal probe. After landing, I drove over to the target and clawed it. To get back into space, I turned off some of the radial engines (because the CoM shifts after you grab your target), and just used the engines to pitch the nose up so that I could use the rear main engine to boost into space.

All in all, it was fun and challenging, but so time consuming that I haven't done such a mission since.