Assembling a Rocket in orbit.

[OUScooby]

I'm fairly new to the game and I've yet to explore the outer most reaches of the system, and while I've reached Duna, Eve and Dres, I've only successfully returned from the Mun and Minmus. Given the size of the rockets I've constructed to reach the Mun and back, and with most of that rocket being used to simply get into orbit of Kerbin onto a path to the Mun, I was wondering if it might simplify construction and require a far smaller sized rocket, design the rocket in individual stages, launch those stages into Kerbin orbit and then assemble the parts with the docking ports. The only thing I can't see working is how would you control such a rocket once it's all docketed together? Is there a way to control multiple docked ships as one craft and not as different parts each requiring independent control?

[MRab2]

Short answer is yes. My current mission involves a mothership carrying 8 smaller vessels. Just shut down the engines you don't want to fire during your burns.

[Tristavius]

When two ships join through docking they automatically become one vessel. As MRab2 said, you may need to manually shut down some engines, RCS ports and so on for the craft being carried.

A 'mothership' carrying one or more smaller crafts (landers, rovers, satellites etc) is a common approach to interplanetary travel. The mothership can contain most of the fuel and the engines for the transfer, along with power generation and Kerbal storage. The best best is to launch it into low Kerbin orbit, launch and dock your smaller crafts then refuel them all. Your landers can be made with the minimum delta v required to land and take off from your target world and can be disposed of if you wish once they've accomplished their mission (better not to drag any dead weight back to Kerbin at the end).

For example I recently went to collect a whole load of stranded Kerbals on Duna. I had a large carrier ship with a lander, a manned rover and 2 satellites in tow. The rover was depployed to go and round up the Kerbals and bring them to the lander, which made several trips to ferry them all back to the mothership (refueling from the mothership each time). The rover was left abandoned when the last man left and the lander was also ditched in orbit once they'd all returned. The satellites were launched to Duna and Ike polar orbits. The mothership then returned them all to a Kerbin space station where they all boarded a space plane to return them to KSC. The mothership was again refueled and was ready for it's next cargo and mission.

[Klajan]

Yes, docking large crafts in orbit is one of the best aproches to interplanetary travel. I am currently building a interplanetary spacecraft, capable of traveling to every planet in the Kerbol System. I have to shoot 7 different part into orbit and dock them, but this way I have enough fuel and thrust to easily get to laythe and back. If my memory is correct, it should consist out of ~700 parts and has a total weight of around 300-400t. My "Mothership" consists out of fueltanks and a bit of RCS fuel and other things, then I have an enginearray with 9 Nuclear engines docked to the back and in the front I have and large Habitant module. I am currently building the landers and and transportvehicles for laythe and will soon be ready to depart.

[Kegereneku]

On the next picture there is 4 independents parts.

- a fairly small Duna Lander-Ascent

- a tank with 3 nuclear engines

- a fuel-tank alone with a Mechjeb

- an orbital-truck with 4 nuclear engines (that I don't mean to take to Duna)

I have also built this mothership long before, each tank+engine can be separated and replaced.

Later I intend to build a mothership with fixed orange tank but swappable engine.

[capi3101]

Docked ships act as one craft. You will need to shut down any engines you don't want to use during your burns manually and make sure your control point is in the direction you want to travel; if you start your interplanetary transfer burn and notice your speed going down, it's because you've got a control point that's pointed in the opposite direction you want to go. Pick another one ("Control from here"), reorient your ship and try again. Couldn't tell you how many times I've done that one.

The RCS thrusters will act as one while your craft are all docked with one another to turn the whole shebang in one go. Keep plenty of fuel in each RCS tank and you should be good to go. Incidentally, RCS is a good way to make subtle but significant changes to your transfer burns; it can make all the difference between aerobraking in Eve's orbit and landing on it by mistake...

Parts connected by single docking ports will have a tendency to wobble under thrust; bear that in mind if your design gets to be long. Most folks get around this by sticking more than one docking port on their parts; that can create its own headaches.

Good luck.

[Sunsparc]

Best way I've seen to do it is to build a custom dock with 4 ports on it, which increases stability. Otherwise, the ship will wobble like nobody's business while burning.

[capi3101]

Best way I've seen to do it is to build a custom dock with 4 ports on it, which increases stability. Otherwise, the ship will wobble like nobody's business while burning.

Yep. That's what I use.

[Rhomphaia]

Alternately have the mother-ship tow the payload behind it rather than pushing it. Wobble is minimized and saves on weight, also easier to dock.

[capi3101]

Alternately have the mother-ship tow the payload behind it rather than pushing it. Wobble is minimized and saves on weight, also easier to dock.

Never thought of that...and it makes sense: something being pulled is inherently more controllable than something being pushed. Turning might still be an issue; then again it might not.

Something new to try.

[RoboRay]

Never thought of that...and it makes sense: something being pulled is inherently more controllable than something being pushed. Turning might still be an issue; then again it might not.

Stopping presents some challenges, as well, if you're towing with cables (KAS). Even if the towed payload misses the towing vessel, the shock on the cables when they reach full extension could snap them, if you start thrusting before the slack is all out. If you're pulling with a rigid connection, it doesn't make any difference stability-wise.

But there are mods that work much better than using multiple docking ports. I think Docking Struts is still broken in KSP 0.19x...

...but Quantum Struts work great for adding strut connections between docked craft in orbit.

Edited May 14, 2013 by RoboRay

[Temstar]

Never thought of that...and it makes sense: something being pulled is inherently more controllable than something being pushed.

No it's not:

http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

In addition KSP doesn't know to steer rockets with CoT above the CoM you have to reverse the control input, so it's unable to steer pendulum rockets at all.

[Fuzzy Dunlop]

Pendulum rocket fallacy only applies to rigid rockets, and if our rockets were rigid we wouldn't have an issue.

[Rhomphaia]

Not talking about pendulum rockets or towing cables though, just having the payload docked to the center rear of the transport, where the transports engines are mounted on nacelles around it. far more stable than docking it to a single port at the nose of the transport and without the weight penalty and frustration of docking four ports at once