Using a skycrane for surface building

[MZ_per_X1]

I have used a skycrane to successfully land simple bases and rovers. I would like to build larger bases with ground assembly. It seems that skycranes could do the job, meaning pick up parts from the ground using docking ports and move the base parts to dock them to other parts. 

Let's say there's a skycrane parking on the ground. I lift it off the ground. How do I make it hover over to the object I'd like to pick up by docking. Main engine only? Rcs? Sas? I am a bit lost with this. I can fly a crane but not fine fly it, if it makes sense. 

[Laie]

There's just too many things to watch out for at once. It may be doable with two flightsticks, but if you're on WASD controls, by all means get some flight assistance mod.

[Cavscout74]

@Laie beat me to it, but I was about to say the same - it would be extremely difficult without some form of autopilot / auto-hover mod.   In the past I've tried to set up autonomous mining/refueling stations where a fuel tanker could land on a docking port on top of the surface mining facility, and never had luck with that. 

[peteletroll]

When the velocity is set to surface, radial out SAS keeps the skycrane vertical. This helps a lot...

[AHHans]

Well, assuming that you "only" use docking ports that point more or less straight up or down then you can use SAS to keep your craft from spinning out of control even in pure stock: if you select "control from here" from a downward facing docking port and put SAS in surface mode to "radial in", then SAS will docking port pointing straight down (choose "radial out" for an upward facing docking port). Then you can set the "main" engines to give just enough thrust to compensate for the gravity which makes the craft to nearly hover (nearly because you'll never the the thrust 100% right and because the weight of your craft changes with fuel burn). This in turn allows you to navigate around with RCS. This works better in lower gravity than in higher gravity: you'll need to hover quite a while (many minutes) on engine thrust (bring enough fuel!) and in lower gravity the acceleration is lower so you have more time to react.

Just today I docked a crew shuttle to the underside of my hanging Mun base, which I believe is kind of the same thing just in the other direction. So it is possible but it is fiddly work, so be prepared for frustration and several reloads. 

[Edit:] P.S. In case it wasn't clear: it can be done, but I don't recommend it. Except if you want to prove that you can do it.

 

Edited May 12, 2020 by AHHans

[Draalo]

7 hours ago, peteletroll said:

When the velocity is set to surface, radial out SAS keeps the skycrane vertical. This helps a lot...

I ve landed on a platform with dockingport multiple times on Minmus.

Just hover near the dockingport, set throttle to lets say TWR 0.95 so you start sinking. Use RCS and the keys I J K L to allign the retrograde marker with your target marker. Dont change throttle, use H to slow down your descent.

Good Luck, have fun

Edited May 13, 2020 by Draalo

[Curveball Anders]

Throttle Controlled Avionics might be the answer.

Been pondering to try for similar inventions but never got around to test it.

 

[Corax]

I may be stating the obvious, but unless you specifically want to tackle the challenge, using a ground crane instead could be an alternative.

Edited May 16, 2020 by Corax

[MZ_per_X1]

Thank you for your suggestions, everyone. 

I have been thinking and searching. I found an interesting video that shows the concept I'm thinking of. 

 

Looks finicky but I will try when I have mustered the patience. 

[Phormio]

I found this thread because I am working on the same thing. I am not a very good KSP pilot but I like to design things and let the computer fly them.

The best way I have found (there may be a better solution, and I would be surprised if this method this is original) is to use two mods in conjunction: mechjeb (MechJeb 2 --  2.9.2.0) and TCA (Throttle Controlled Avionics -- 3.7.1) both available thru CKAN. It took me about 40 minutes + infinite fuel to figure out how to make the two auto pilots play nice, so maybe this will save someone some trouble:

1. start with the sky crane and the module (whatever you are picking up) landed near one another on the surface, and as the sky crane controller, execute steps 2 - 12

2. disable TCA but leave MJ enabled, and ensure your primary lifting engines are online & fueled (but throttled to zero)

3. configure TCA like this (in the GUI, YELLOW means off; GREEN means ON):

a. VTOL Mode > OFF

b. VTOL Assist > OFF

c. Stabilizer > OFF

d. CPS > OFF! (this is collision avoidance)

e. AutoGear > OFF

f. AutoBrakes > OFF

g. AutoStage > OFF

i. AutoChute > OFF

j. Modules > OFF

h. Hor. Thrust > OFF

i. RCS Translation > ON

j. RCS Rotation > ON

k. Autotune engines' .... > ON

l. Hover > ON

m. Follow Terrain > ON

n. Auto Throttle > ON

o. Smart Engines > OFF 

4. Switch MechJeb Smart A.S.S. to OFF

5. click the downward-facing docking port on the sky crane and "control from here"

6. target the upward-facing docking port on the module to be lifted and "set as target"

7. enable reaction control system; make sure the RCS light is illuminated on the navball

8. enable TCA (enable button on the biggest TCA window . ..  TCA layout is a bit of a mess, but this is due to it being pretty involved; it makes me wish KSP supported multi monitor without having to do a rain dance). ***If the engines fire at this point, or if random unactivated engines activate themselves, you've misconfigured  TCA somewhere***

9. in the "hover" TCA toolbar, make your altitude 10 meters: this should cause the engines to fire and the sky crane to hover ten meters above the surface

10. open the MJ "docking autopilot" menu and enable autopilot: this should cause the sky crane to use RCS to position itself over the module and align the docking ports

11. wait until you see that the sky crane is motionless relative to the target; you should see the MJ autopilot starting to fight with the TCA autopilot as MJ tries to use RCS to push the docking ports together and TCA increases thrust to maintain the 10m altitude

12. as soon as this fight starts to happen, click the "-10" button on the TCA hover toolbar, making your desired altitude zero / RED : this should cause TCA to move downwards toward the target, and allow MJ to dock the sky crane to the module

If you want to make sure that MJ has enough authority to slow the final decent and docking maneuver to less than a meter per second, it is a good idea to put lots of RCS thrusters  pointing DOWNWARDS; this way mechanical jebediah will be able to cancel the remainder of the vertical speed that is leftover from what TCA thinks should be a typical landing velocity. Or just set TCA speed limit to .05.

I have not figured out the best way to precisely place my building blocks yet.. I only can pick them up and crudely place them down so far. But the picking up part I have down.

Maybe this saves someone 40 minutes. Cheers.