Assembling Space Stations Meant for Transit

[SgìobairOg]

So, similar vein to Boomerdog's Space Station post but little different info sought. I've been working on an orbiting fuel depot. Wanted to put it in high Kerbin orbit or Münar orbit to allow smaller ships to come up, refuel then head on to Minmus or one of the planets. I had a good amount of fuel in orbit, managed to dock everything together then tried to use the two engines I thought I had lined up together to push the assembly out to Mün. As soon as I applied thrust the depot just started flipping. I tried to move the side modules so their engines were pushing along the axis of the main tank but ran out of RCS fuel.

So it looks like pulling rather than pushing is the key to the erratic flight. But how can I go about precisely assembling the sections in orbit? Every station I see looks like all the parts are perfectly aligned and symmetrical but I don't know how I could do that without a whole lot of trial and error.

Right now I'm using mostly stock parts. I'm not opposed to mod packs if they're the only way to make this easier.

Eventual goal was to assemble a massive ship I could send out to the outer planets.

[Vonar]

Stations can be very unstable, I assemble most of my stations in orbit of the planet or moon that they will be located at. So for example a Munar station would have everything launched from Kerban and built in Munar orbit.

[Jean Deaux]

I've personally found it easier to wait to assemble/dock the station parts at their destination orbits, opting to send up refuel vessels to dock and resupply to extend the range after reaching Kerbal orbit and then pushing further along to their final destinations.

[HeadHunter67]

Also, if you can, design the station completely in the VAB, then break it into smaller sections for launch. It's not strictly necessary, but it gives you a better idea of what goes where and how, and the way everything will be when it's put together.

[Xiphos]

Your biggest challenge will be designing the tug to pull the station around. I have a tug that has not been fully tested as I am building the station to pull now. It's designed like so: (I'm at work or else I'd post pics) center is an orange tank with 4 structural wings(the rectangle ones) going out at 4 symmetry, at the rear. On the end of those, there are a 1440L tank with a skipper engine. On the front of the tug, there are 4 more 1440L tanks attached radially(Rotated 45 degrees from the engines, on hydraulic clamps so that they may be released incase of an emergency) These feed into the 2880L tank, and the engines draw from the 2880L tank. When fully fueled, it holds 14,400L of fuel. There is a large docking clamp on the end of the 2880L tank to attach anything, and a manned pod for torque on the front. (Although it might need more reaction wheels because when it's full of fuel it gets tough to rotate it) I also designed the launch vehicle to use the fuel from the tug to reach orbit, so you have to refuel it before use.

[PringleMan]

You are indeed correct in your thoughts about pulling rather than pushing. Think of this simple analogy. Pick up a pencil between your thumb and index finger. Let it hang between your fingers, and gently rock it back and forth. Notice how it easily returns to the rest position. Now, hold the pencil with it going up rather than down. While it can still be balanced, it only takes a tiny amount of force one way or the other to make it fall, and it does not easily return to the upright position (namely, you have to actively pick it back up).

This is because the most stable mass / potential energy orientation is with it hanging, and the force applied to the top, because in this example the bulk of mass is in the pencil body, not the point at which you are holding it (applying force). Likewise with rockets, your most massive section needs to be as low as possible to keep a steady and straight trajectory with minimal effort. So with your station, if you put a booster at the "bottom" of the mass, the natural tendency is going to be to topple end over end uncontrollably. Instead if you put a pulling booster at the top, with the station below it, it will exist already in a stable configuration and be much easier to control.

[Technical Ben]

There seems to be a limit on what length of rocket you can make. That and the power/weight or thrust/weight ratio. This ship is perfectly balanced, but wobbles and tumbles like mad. The only option I can think of is shortening it, and keeping the COG closer to the rockets?

I'll be trying shorter designs and will try and post the results.

Other than that, most of my stations are made at destination... out of spare bits of junk.

Although, the lower example in the picture flies perfectly. So I think weight plays a lot in control.

[Kulebron]

To handle a station better, make it as compact as possible and put RCS thrusters on struts, a bit outside the station. This way the station will have low inertia and RCS will make more moment.

Docking ports have low sideways strength, so an assembly like Mir station is hard to push and pull, because the modules that stick out will oscillate. But you can't make a long pencil (it has much rotation inertia), so in theory it's better to join two or more big orange tanks parallel to each other with two pairs of docking ports.

[Kulebron]

You are indeed correct in your thoughts about pulling rather than pushing. Think of this simple analogy. Pick up a pencil between your thumb and index finger. Let it hang between your fingers, and gently rock it back and forth. Notice how it easily returns to the rest position. Now, hold the pencil with it going up rather than down. While it can still be balanced, it only takes a tiny amount of force one way or the other to make it fall, and it does not easily return to the upright position (namely, you have to actively pick it back up).

The engine always points straight up, but your fingers don't, so this example works only in atmosphere, where if you incline a "pencil", aerodynamic force starts pushing it sideways more and more because of velocity. The engine is "neutral", it still pushes it straightly. In space, there's no such force as drag, and as soon as your ship is symmetric, you can put an engine in the back safely. To some degree an off-centered rocket is flyable, as SAS and pivoted engines cope with it.

[Captain Sierra]

Just use radial symmetry. Then your stuff works. If you have stuff mounted on radial docking ports, just have two of them.

[M5000]

I would think it's easier to assemble your station Kerbin-side and push it to the destination, rather than taking it to the destination and assembling there.

Reasons:

-Less long-distance piloting, therefore less room for errors.

-No need to run parallel missions so that everything departs from one transfer window in one vehicle.

-IF you were using parallel missions with separate modules, you would either have to wait for a transfer window and hope you get all the modules sent on their merry way at once (Eg, you better be ready) and if you miss one or want to send another module, you have to wait for another transfer window. Alternatively, if you send them each on their own window it takes forever.

-Less cost (if you play that way) because rather than having to design a rocket that can bring modules up from Kerbin and then also bring them interplanetary, you can design it to bring the modules to LKO and just tack on a single tug at the end.

-Less debris in the destination orbit. (Station building is likely to leave some debris in the form of decoupled struts, fairings, stack separators, or possibly entire transfer stages. Leave the debris back home so it's easier to manage later.)

-Allows for, possibly, larger payloads per-launch. The extra mass it would take to bring up not only the module to be added on, but the transfer stage, could allow for larger modules in the station. If you build your station in as large of parts as possible, it is likely to be more structurally sound, since you are using less docking/berthing connections.

So, those are my arguments FOR building your station in LKO and then simply tacking a tug onto the FRONT of the station and pulling it along to its destination.. So here are some design tips that I've seen as valid...

-Try using these mod parts. They are Common Berthing Mechanisms that you can use in place of docking ports. They come in the four most common standard sizes, from .625m to 3.75m, and they are incredibly rigid. There is also an active and a passive (male/female, of sorts) for each size. However, this should not matter, and is really just for aesthetics. Also, they should link up with the stock docking ports, but certainly you will want to test this before departing if you are relying on them for a pivotal part of your mission.

-Try designing your non-core modules to all be the same size. Perhaps a module initially turns out to be smaller than some of the other ones...Add a reserve of fuel or maybe some solar panels or structural bits to help even out the size and mass. This will allow you to play with your station like Legos.

-A generally good design is to use 1.25m CBM (the mod parts up there) ports, placed around a central "core" or "shaft" of a station, which can be 2.5 or 3.75m in diameter, depending on application, using said above CBMs to link up the parts. 4x symmetry for the modules you will attach radially is a good idea, remember to make these the same size.

-Fuel tanks make a great central core structure, and, they, you know, hold fuel.

-For your propulsion, it is advisable to PULL your station, rather than to push it. When you push a structure of considerable mass, you impart an acceleration upon it, and it becomes similar to if that structure were sitting on the surface of a planet. The gravity of this "invisible planet" is entirely dependent on your TWR, but the effects can be devastating. To fix this problem, create a propulsion module that can dock on to the front of your station, and point the engines backward, held out on some kind of outrigger, possibly, like a gigantic space train. One more thing that is also very important if you go this route: ENGINE GIMBALS DO NOT WORK PROPERLY WHEN PLACED AHEAD OF THE CENTER OF MASS. So, assign an action group to disable or toggle these gimbals, so you can turn them off when needed. Steer using reaction wheels instead, which should be integrated into your design. When building the modules, try putting a small reaction wheel on each module so that the forces can be distributed more evenly.

-Plan your RCS thruster placement. Use them efficiently and you won't need to bring very much RCS.

-Use struts if possible.

-Turn on fine controls

-Symmetry is key.

-Plan your station before you build it. Try drawing a sloppy schematic in paint or on paper even, let your imagination run and you'll find that more often than not, when planned properly, your station will come together beautifully.

-And above everything: BE GENTLE. This may be a near or multi-megaton mass station, but it is as delicate as a baby. Be precise. Be gentle. Tread lightly. Baby steps.

So, that noted.. Here's my first large station design I ever made. Actually, it's more of an interplanetary ship, designed for a grand tour mission. It's also an example of what NOT to do:

I was so confident that it would work. Left it unpaused in orbit and went to lunch and came back to this: