How to do a special kind of hover surface docking

[THX1138]

I need to do a strange kind of docking that I'm finding pretty tricky. I have something on the surface of the Mun that I want to land on top of so that I can dock onto it and lift it away. I would search for it on YouTube if I knew what to search for. Has anyone here done this before?

 

[nightingale]

I've seen @5thHorseman do it in his previous youtube series (the contractor one), but that was on Minmus (Mun will be much tougher).  It's doable, but can be really tricky.  My suggestion would be to get well above what you're docking to, get the horizontal velocity zero'd out and then do a pinpoint landing (easier said than done).  Otherwise, trying to get your vertical and horizontal velocities to line up at the same time would be a massive headache.

Also, Docking Port Alignment Indicator may help in lining things up right.

[Starhawk]

I've done this with the klaw on Vall.

Even that was very tricky.  Actually docking will be a significant challenge in Munar gravity.

You'll definitely want very fine control of your horizontal velocity.

Good luck.

Happy landings!

[Van Disaster]

DPAI is a very good suggestion; when I do this myself I use Mechjeb for SmartASS ( you can just use SAS for that to hold pointing up ) and the Translatron which is more useful: it holds your vertical velocity fixed to whatever you tell it to. I get nearby using a landing technique until I'm fairly close, hold vertical at zero, zero out as much horizontal as possible, point upwards and then use RCS to move over the landed part & line up. You could just land completely, sort your controls out & hop up a few metres, of course. If you don't want to use a throttle mod, thrust-limit the engines so you have barely enough thrust to lift off and you'll find it easier to hover.

Haven't done that sort of thing for a while, but back in 0.90 I had a system of dropping resource pods down to Munbases; occasionally I'd go down & retrieve a pod for reuse.

Spoiler

Spoiler

Make sure your tug craft has a top & bottom set of RCS thrusters, and try and make sure they're well balanced: you *really* don't want lateral RCS tilting the craft.

Edited July 7, 2016 by Van Disaster

[Hodari]

It's certainly possible, but won't be easy at all.  Your best bet will probably be:

1.  Have a few radially attached engines pointing downwards and make sure they're either far enough away from the center or weak enough that they won't damage the craft you're  landing on top of.

2.  Also have some RCS thrusters around the sides of your craft so you can use those for lateral movement during the final docking.

3.  Follow normal pinpoint landing process until you're directly over the target.  Keep your craft pointing directly upwards and adjust the thrust on your main engines to control your vertical speed.  Keep yourself just hovering 10-20 feet above the target.  Use your RCS to perform fine adjustments to your horizontal position until you are properly aligned and then slightly lower the main engine thrust to SLOWLY lower yourself down.

[soulsource]

I'm doing this with my mining station on Minmus. It's pretty easy in low gravity environment, but I guess it's much more difficult on high gravity objects. My strategy is as follows:

• Try to get on top of your target already at high altitude (navball - target indicator is your friend)
• Stop your horizontal velocity wrt the surface already high up
• use both hands on the keyboard. WASDQE for rotation, IJKLHN for translation (RCS is a must...)
• Main throttle should be used to keep your craft hovering
• Beware of engine exhaust. That stuff is hot. I use radial engines mounted far away from the docking port for that reason.

 

[Geschosskopf]

1 hour ago, THX1138 said:

Has anyone here done this before?

I have recently done this for the 1st time with the help of 2 instrumentation mods:

• RCS Build Aid so I have pure translation with no rotation
• Throttle Controlled Avionics to handle the vertical axis.

The basic procedure is to land near the target either manually or with an autopilot.  FWIW, the new version of TCA can do that for you, too.  Then you use TCA to hover at an altitude just above the target and RCS to move right over it.  Once over it, tell TCA to bring you down very slowly until you dock.

Obviously, you need enough TWR to hover, and this has to be mounted so as not to damage the lower vehicle.  Depending on how big the hover engines are, you might have to fly sufficiently high above the target so the flames don't hurt it when you translate over it.  Also, in TCA, these engines should be set to vertical only, not vertical and maneuver.  And finally, because hovering will consume some fuel, you need to have built the ship with RCS Build Aid so the CoM doesn't move as fuel burns.

[Empress Neptune]

This came up as a potential arrangement for my first ore mining system. I was going to leave the drills on the ground and have tanks go back and forth from the surface. I never built this system, but I did test a wheeled docking port on Kerbin. There was no need to line anything up while hovering. Just drive over the target and dock. Definitely less cool points though.

[Vanamonde]

Skip to around the 28 minute mark for an example.

[Aegolius13]

If you're open to mods, you could also try using the parts from Kerbal Assembly System (winch and electromagnet / harpoon / grappling hook).  For example, a kerbal can grab the magnet off the winch, walk it over, and attach it the other thing, so you don't need to make  precision dock.  You can then then tow it into orbit, and then dock normally if desired.  I tried this once with the electromagnet to lift a piece of debris off the Mun to fulfill a contract.  

But there are problems.  Many problems.  The towed item can come loose if you pull too hard.  It can throw your ship off course.  It can bang into your ship and damage stuff.  The electromagnet can run out of electricity.  The electromagnet can turn off if you try to reload.  Or it can do all those things at once.  (Sorry, working through some past trauma here).

 

[Leftotian]

Do you actually want to hover or do you just need to lift stuff? The easiest thing to do is to make long(so your docking port is just a bit higher than the target) legs with aircraft landing gear at the bottom, land anywhere near, drive on top of it(use rcs to push your craft around) and lower your gear to dock.