I've made small space stations before, mainly as tanker depots and places to drop unwanted kerbals, but I never went in for the recent fad of building large, complicated space stations just because they looked cool. Early on, I tried docking three Mark Twains to a tanker station (when I was trying to perfect the launcher for the Mark Twain), and ran into lots of lag... so it seemed that any big station project would suffer from lag problems.
But a space station seems like an important part of a space transportation system as a central transfer and refueling hub. A place where outgoing crew lifted by SSTO Crew Carriers can transfer to trans-moon or interplanetary crew transports, or returning transports can drop off crew to be sent down to Kerbin. Also a place where transports and tugs can get refueled before heading back out.
So, the station will need a tank farm to hold fuel and RCS propellant, and lots of docking ports for easy temporary attachment of vessels, and some containers for stashing transferring kerbals. The station will not have a large permanent crew, so no need for lots of living space or emergency escape capsules. There won't be large power needs, so I'll do without big solar panel arrays (although solar panels look cool and seem to scream, "Space Station," they would only get in the way of docking and increase lag). Also, I want to build the station with the Reusable Rocket boosters from last time. Below we see the station core being lofted on a Reusable Rocket (variant #4).
The side boosters are recoverable and drop free at around 18,000 meters (they actually despawn later and disappear from the game before reaching the ground, but more about them later). In the picture below, the station core has been placed in a 121 km circular orbit and the sustainer can bee seen maneuvering 200 meters behind it, preparing to return to KSC.
Four launches of the basic Reusable Rocket #2 were used to lift four Big Orange Tanks to the station to serve as fuel depot storage.
Below, the first B.O.T. has docked with the station. Each B.O.T. has three radial engines for orbital maneuvers. These are deactivated after docking with the station. The B.O.T. payload is controlled using an OKTO probe body "tug pod" that has an RCS tank and additional RCS thrusters. The pod is detached (see below) and deorbited using the RCS thrusters after the tank is in place because I didn't want to end up with a lot of ASAS units attached to the station when I was done (so they won't end up fighting and shaking the station around). Also, this leaves behind open docking ports.
Unfortunately, the amount of RCS fuel in the tug pods is barely adequate for the job of getting the B.O.T.s attached to the station, and I ran out of monopropellant in the tug pod of the second tank just before I got the tank docked. So, as that B.O.T. was very slowly drifting further and further into the distance, I attached the other two tanks, being very frugal in the use of RCS propellant (for example: using the radial motors instead of the RCS as much as possible in rendezvousing with the station; being patient as the B.O.T. moved around during docking...not just using more RCS fuel to make things go faster; and paying better attention to which way the RCS jets fire...instead of just experimentally firing to see which way I need to go, and wasting fuel when I guessed wrong and had to undo that last pulse). In the picture below, the third tug pod has decoupled and is doing its retro burn to deorbit. It doesn't take much RCS fuel to deorbit the pod (less than 10 units of fuel), but I didn't even have that much left after docking the last tank in place...but I was able to refuel from the station after docking, of course.
Here's a closeup of the tug pod on its way down. OKTO probe core, RCS tank, ASAS, RCS quads, battery, and four small solar panels (used instead of an RTG because the pod was destined for immediate deorbit and disintegration).
In the meantime, the sustainer cores of the Reusable Rockets were being deorbited and landed back at KSC. Since there was enough fuel for a rocket-landing (over 300 liters left in the sustainer after dropping its payload off into parking orbit), I let MechJeb do the landings, targeting the same spot each time. Note that it's important not to ask MechJeb to land these sustainers too gently... Trying to land at 0.5 m/s results in a lot of wobbling around just before touchdown, and could result in the sustainer core falling over. The landing legs can easily handle a landing speed of 2 m/s, which MechJeb sets down with no wobble. If I were doing the landings, I would just use the parachutes, of course. I'm not as cocky as MechJeb.
I also wanted to see how the KSC would look after the five Reusable Rocket launches it took to build the space station basic structure, so I made some extra flights that followed the boosters down instead of continuing to focus on the sustainers heading upwards. I had to add a Stayputnik probe core to the top of each booster to be sure they would all survive the exercise and not get removed as junk. Below you can see one of the twenty side-boosters littering the KSC (with others spread around off in the distance), mostly west of the VAB, after the five lanches. None of the boosters were damaged and all landed intact. One of the boosters did fall over later as I was switching between them taking pictures, but it was even undamaged by that.
Below are the five sustainer cores that MechJeb landed on rocket power. Nice grouping. The closest any of the sustainers or boosters came to landing on a structure was the side-booster that landed near the Spaceplane Hanger (off in the distance on the left here).
Below is the distribution of landed stages on the KSC property. The distances shown are from the launch pad. The makers labeled "4" are parts from the launcher that lifted the station core, and the markers labeled "2" are parts from the launchers of the Big Orange Tanks. I assume the side-boosters are all off to the west because of Kerbin's rotation toward the east. So, after the five launches to build the station, all launcher stages were recovered intact and ready for re-use on future flights, at least in my accounting of the costs.
Ah, but what of the little lost B.O.T. that didn't quite get attached to the station? To fetch that, I slapped together a Mini Tug to be lofted by the SSTO Crew Carrier. Also, in the meantime I had noticed that the space station had no provision for the docking of any craft with a Clamp-o-tron Junior docking port, so I also included a docking adapter module. It was a bit wobbly on boost, but Milski, Rodmy, and Dilger easily handled the flight. One of the things I have not been able to do is tell MechJeb how to fly the SSTO to orbit.
The extra mass of the Mini Tug and docking adapter resulted in the Crew Carrier having very little fuel once it reached the station...not enough to deorbit later. But that didn't matter because it could get the small amount of fuel it needed from the station! The Crew Carrier plus Mini Tug did not handle well (wobbly, lots of wasted RCS), so the Mini Tug was separated from the Crew Carrier after rendezvous with the station.
First order of business was to get the Crew Carrier docked to the station on any handy docking port. No problem. The docking ports on the station are mounted out away from the core modules to make things less crowded during docking, yet keeping the station core relatively compact. The middle set of docking ports are offset 45 degrees for more room, but I expect to use those ports less. There are also two axial ports on the top and bottom ends of the station. Second order of business was to use the Mini Tug to dock the Port Adapter to on of the docking ports. The Mini Tug handles like a charm by itself or with the docking port adapter attached. The Port Adapter has one large Clamp-o-tron (to dock to the station port), and five Clamp-a-tron Juniors in a cubic arrangement (the large Clamp-o-tron on one end of the Mini Tug is coupled to one of those Junior ports of the adapter during launch). Docked to two of the Junior ports are two "port converters" that are made of oppositely-facing Clamp-o-tron Senior and Junior ports (with an octagonal strut spacer in between). These adapters (if docked to by a ship that only has Senior docking ports, like the Mini Tug), would allow that ship to dock with another ship that has only a Junior docking port. For now, the converters are kept docked to a couple of the excess Junior ports where the Mini Tug could grab one if needed to go dock with a ship that has only Junior ports. But I expect to use Junior docking ports less in the future flights in this project.
Third order of business was to decouple the Mini Tug (leaving an outward-facing Junior docking port behind that would be the docking port of choice for any small ship with a Junior port to dock), and head over to the little lost B.O.T. and its empty tug pod. The B.O.T. had drifted less than 2 km away from the station in the time it took to do the rest of the station assembly, so the Mini Tug could easily reach it. The Mini Tug docked with the port on the end of the B.O.T., but rather than bring it back to the station, the Mini Tug refueled the B.O.T.'s tug pod, and then undocked, and the B.O.T. used its engines and RCS as intended to dock to the station...only being more frugal with its propellant this time. Then the Mini Tug also returned to the station.
Below we see that final B.O.T. attached to the station (its tug pod is still attached), and the Mini Tug has docked to one of the central ports. The Mini Tug has three RCS tanks, and ASAS balancing the OKTO probe body on the opposite side, and two RTGs laid alongside the central RCS tank to power the probe core. It has two sets of four RCS quads, and Clamp-o-tron Senior docking ports on each end. Such a fun little ship to fly around and dock with.
Milski and company moved the Crew Carrier to the North axial docking port of the station for better balance, since they'd be staying for a while to do the basic station system checks. There are two 4-kerbal habitat modules on the station, so they have plenty of room to hang out. After this construction phase, the space station has four nearly-full Big Orange Tanks and one "tuna can" tank for propellant, and two nearly-full large RCS tanks, and over a dozen free docking ports. Mini Tug valet parking is available. In general, reusable refueling ships will be used to keep the B.O.T.s and RCS tanks topped up for use by station traffic.
I left off any docking lights to reduce parts count and reduce lag, but if version 0.19 fixes the "lighting bug" and results in complete darkness when in the planet's shadow, I'll have to add some lights to facilitate any docking operations that can't wait until daylight. These lights will be added by docking another adapter (docked opposite to the Junior port docking adapter), with the lights angled to illuminate some docking ports on the North module. All of this assumes that this station will even survive the changeover to version 0.19. A couple more RTGs can be included in the adapter for power (to augment the two original RTGs located on the top of the South module).