How to stabilize docked craft ?

[Khazar]

Hi.

I'm trying to build craft that would be some sort of mothership carrying fuel and 4 small, detachable crafts that could do various tasks like surface landings while main craft could stay in orbit (in career mode I have many contracts around the mun, that I would like do in single mission).

I thought that it would be a good idea to use "recon craft's" engines for the propulsion of mothership, so i't wouldn't need separate engines (or reduce their number more likely) and save weight this way (layout 1). Docking ports cant' however hold those crafts in place and they start to shake and swing until the hit something and blow up. Is there a way to hold them that would still allow to undocking and docking ?

So far I could only leave Kerbin's atmosphere in one piece If I had separate, fixed main engines and didn't use recon crafts engines when getting into orbit (layout 2).

If I would use strut connectors, decouplers or other parts to stabilize docked crafts, would I be able to undock and then dock ?

http://www./view/uu2h3e4sqln81hn/layout.JPG

Edited March 31, 2015 by Khazar

[RainDreamer]

Struts won't reattach after undocking. If you want a stock only option, you better build a proper frame/enclosure to hold the smaller crafts in place, or place them in a way that minimize wobbling, like along the side of the mothership instead of sticking out perpendicularly.

[Khazar]

I have found answer in other topic that have showed up just a few minutes after I started mine. Struts are disconnected during undocking (didn't know that) so I can use them. I believe that after leaving Kerbin atmosfere I will not need struts anymore. In space I will not have to do so much quick maneuvering as during ascending to orbit.

[AlextheBodacious]

Time warp a little.

[Khazar]

Time warp a little.

hmm you mean during burn or anytime ? I don't think I would need warp time during burns, so it shouldn't be a problem unless it would tear my craft apart even if its not using engines. That would be a problem :/

[impyre]

AlextheBodacious was making reference to a technique used to stop vehicles from wobbling just after docking in space. Just after docking, vessels can wobble (especially if each has its own SAS module and stability assist is enabled). Disabling stability assist and timewarping (*not* physics warp) will help stop this before it destroys the vessels.

He didn't realize that you were talking about during launch/burns.

[Tex_NL]

Get Kerbal Attachment System (KAS). Among other things it allows your Kerbals to connect struts during EVA.

[pincushionman]

Multidock. In theory that should help distribute the forces better.

…but I've also heard that causes its own problems too, so take care.

[michaelsteele3]

Duct tape.

Edited March 23, 2015 by michaelsteele3
duct tape

[GreenWolf]

There's a few different ways to approach the problem of building motherships. Many of them can be used together to increase stability.

The first, and the most obvious, is to use the largest docking ports available. The 2.5 meter docking ports are stronger than the 1.25 meter ports. Depending on where you are in the tech tree, this might not be an option. You could also try triple docking the 1.25 meter ports, but this takes some skill and luck to pull off consistently.

The second method is to reduce the number of docked craft. It's generally more stable to have two big craft instead of 4 small craft.

You could also try doing a puller configuration instead of a pusher configuration. Stick your engines in the front, and string out the modules behind it, like a train. It will still wobble while trying to turn, but it's more stable while performing burns.

If you insist on doing a pusher, build a "core module". This should contain the engines and most of the fuel. As you add more modules, try and keep the center of mass of the whole craft inside the core.

For what you're doing (multiple Mün missions with a modular ship), a mothership is almost overkill. However, what I would suggest is instead build one single lander with around 2000 m/s of delta-v in a single stage (pretty easy to do). This will act as your drive unit and your lander. Attach some fuel tanks in front or behind it, and you have your mothership. When you get to the Mun, fill up the tanks on the lander, then detach it from the rest of of the craft and land. Once you're done with the mission, takeoff and rendezvous with the fuel tanks in orbit to refuel. You can keep doing this until you run out of fuel in the orbital tanks.

[Sovnheim]

This seems like a good idea - The problem is that if one of your reckon crafts is destroyed then you have an unbalanced mothership with reduced power.

My advice :

1/ For your reckon crafts, make them as light as possible. I have had similar problems which I solved with very light reckon probes & manned crafts powered with monopropellant. They can perform rendez-vous easily to refuel. So far it has worked well for me, especially for missions at close range on small bodies (Mun & Minmus).

2/ Add a big badass engine on your mothership that is able to push your craft as far as you want. You'll have shorter and safer burns. A Rockomax Skipper (Mainsail might be overpowered for missions to the mun)

3/ Add some radial docking ports for refueling and return, but send them attached with decouplers. This will make your ship far more stable for the journey to the Mun. Remember that your ship will be lighter on the way back, therefore easier to maneuver.

Last but not least, consider that it is much easier to return from the mun than to go there. A small manned craft (1-kerbal) with a medium sized monopropellant tank is enough to return home. You may want to leave the heavy mothership behind - especially if its gas tanks are almost empty.

Hope that helps

[Khazar]

Thank you for your answers.

I have no access to large docking ports yet, also I have reached mods "capacity" and KSP started to crash quite often at this point. I guess I'll have to remove some modes I have installed to save memory for the new ones.

I haven't tried multidocking so far but it seems to be quite good idea.

The second method is to reduce the number of docked craft. It's generally more stable to have two big craft instead of 4 small craft.

My carreer situation is that I have to take crew reports at more than 20 locations, more like 30 I guess, as well as plant a flag, take some science data from surface ad place a basic station in the orbit.

To get those reports done I will have to change orbit many times or spent much time waiting to get near every location.

So I thought that the best idea would be to take 4 small crafts that would take care of crew reports, 1 lander to place the flag and gather the data. All that would be detachable. Then there would be a main command module with science lab and other stuff required for this "space station contract". I hope that I could just undock it on specific orbit and then dock it back instead of leaving it for good. After I accomplishing as many contract as I can, I would like to assembly every craft and go back to Kerbin. Refuel, exchange crew (to get exp) and science samples, and wait in orbit until next big mission.

If you insist on doing a pusher, build a "core module". This should contain the engines and most of the fuel. As you add more modules, try and keep the center of mass of the whole craft inside the core.

This is what I did before, it worked well enough, but this way is much less efficient. The recon craft's engines couldn't be used to help during take off. As I had to completely redesign tanker craft that was supposed to bring fuel for the mission, but was unable to maneuver well enough to dock, I decided to redesign both crafts so this time I could use recon crafts thrust.

For what you're doing (multiple Mün missions with a modular ship), a mothership is almost overkill.

2/ Add a big badass engine on your mothership that is able to push your craft as far as you want. You'll have shorter and safer burns. A Rockomax Skipper (Mainsail might be overpowered for missions to the mun)

Hope that helps

I used 4 main sail engines to power this ship so I guess it must be overkill for sure

This seems like a good idea - The problem is that if one of your reckon crafts is destroyed then you have an unbalanced mothership with reduced power. (...)

Last but not least, consider that it is much easier to return from the mun than to go there.

Having 4 recon crafts each quite powerful (considering they contribute during kerbin take off) I should be ok until I loose all of them. Besides docks where recon crafts are placed I also have one dock in the front and one in the back. If I loose 1 ship, I can move the one from opposite side to the rear so I could use all three of them, If i loose 2 I am symmetric, and having just one I just place it in the back and it should be ok.

[TheXRuler]

I saw this and thought I'd draw up something quickly. I used this to build my station, and it's also the only way I have managed to get a 2.5m docking hub.

If you launch this thing atop a rocket with the decoupler and dock it with the central docking port you can dock your recon craft where the dark blue triangles are you can use them as drive engines. Stronger engines increase wobble, also they have lower ISP which means you need more fuel for the same dV.

Of course using this design would require quite a lot of rendevouz and docking, which can be quite hard. But then again it is almost essential for interplanetary missions and station building, so getting some practice is always good.

Edited March 24, 2015 by TheXRuler
screwed up the imgur links

[Flixxbeatz]

This is why I don't do single-port docking (always either 3 or 4).

[Khazar]

hehe KSS "overkill", a fine name for a ship

I decided to launch a mothership first, and then recon crafts separately. It was impossible to keep under 256 parts limit otherwise (I want those recon crafts to be well equipped). This is how it looked before launch :

hehe KSS "overkill", a fine name for a ship I decided to launch a mothership first, and then recon crafts separately. It was impossible to keep under 256 parts limit otherwise (I want those recon crafts to be well equipped). This is how it looked before launch http://www./view/m2t6vkk1qenm2hz/005.bmp

and a recon craft :

http://www./view/ob6lada5y20jq47/004.bmp

Allready in orbit just after docking first recon craft in.

http://www./view/c2cu3iy5d375d7w/001.bmp

And here most recent view. 3 recon crafts docked,

http://www./view/cdo7ob7u7g7a1y6/002.bmp

I just have to bring last recon craft and some extra fuel and I'm good to go for the mun

Recon crafts have crew of 2 (pilot and scientist), they carry quite a lot of scientific instruments and they are supposed to be capable of surface landings in low gravity environments.

[stibbons]

Duct tape.

It's NASA Approved!

[Mapoko]

I had similar problem with my design earlier this week and ended up having the realization - docking ports. The only sure fire to do it. With KAS you have to reattach your struts every time you get back. I am still using KAS for other various things.

[Khazar]

This KAS mod is really great. Not only it allows to strut everything I want improving crafts stability greatly but it also allows to pup fuel without of docking. That helps a lot. No more docking with heavy, not well balanced tankers. Also i can remove and reuse some stuff from decommissioned crafts.

Now everything is going well, just made couple surface landings. One of them was on hillside at night This game is awesome.

http://www./view/4qjoj8g16gin1ao/MUN%20001.bmp

http://www./view/2vbmkdd8he88w9g/MUN%20002.bmp

http://www./view/v9rmd5bn6n84u78/mun%20003.bmp

I have one question. My main craft is at equatorial orbit, and one of recon crafts is at polar one. What would be the most efficient way to get rendezvous ? Burning normal/antinormal at point where orbits cross is the only way ? Are there any trick's to save some fuel ?

[Harry Rhodan]

bmp

You should really try to use jpg and not uncompressed bitmaps.

[TheXRuler]

Well, if you don't mind fiddling and waiting a little you could use a munar gravity assist to increase your inclination. Then burn at apoapsis to get the maximum efficiency for the rest of the inclination change and aerobrake back to a circular orbit. Changing inclination in a low orbit is incredibly expensive and should be avoided if at all possible. If you there is no way around it I recommend raising your apoapsis quite a lot (a few hundred if not thousand km) and making the burn at apoapsis. However if you are orbiting an airless body you will also have to burn retrograde at periapsis to circularize again. I usually try to plan my missions so that I do not need to do an inclination change while in low orbit or if that is not possible try getting a gravity assist to make the change for me. For precise gravity assists I find the [thread=47863]precise node[/thread] mod to be absolutely essential.

Also you could use a free image hosting website like imgur and wrap your URL's in the appropriate tags like so:

 [Spoiler="Whatever you want the spoiler to be called"][IMG]your image URL[/IMG][-/Spoiler] 

Do not include the "-" in the second spoiler brackets, I only put that in to prevent the forums from converting the code. Of course the spoiler is not in fact neccesarry, it is just good manners to not clutter up the forums with large images, since not everyone has unlimited internet.

Edited April 7, 2015 by TheXRuler

[Khazar]

Thanks for the info, I was off course trying to change inclination at low alt so I'll probably have some savings now.

As for images I was using bmp format because I thought that bmp images kep better quality and saving them as .jpg lowers the quality a lot. I'll use .jpg next time.

[TheXRuler]

No problem, we're all here to help and learn after all