How did they plan to assemble the Constellation transfer vehicles in orbit?

[Elthy]

While building modular interplanetar transferstages i noticed that all the tech required to dock several fueltanks greatly increased the dry mass of those extra tanks. I thought about different methodes to assemble them, e.g. with a tug or a roboterarm. I wonder what they planned to do in real life, the task doesnt seem that easy. A design would need to acomplish the following things:

The payload has to be placed next to the target. In KSP i use the upper stage for that with with either a direct launch (timed by mechjeb) or a hohmann transfer from a lower orbit. This isnt that easy in real life, both variants require a large change in velocity right next to the target which isnt done in real life. Instead a slower aproach is used with several burns to rendevous. This requires a restart capable engine and quite much deltav on the payload (on launches to the ISS they dont use the upper stage engines for this, but e.g. engines on a service module)

The docking. There are two possible variants: Use RCS or a roboter arm mounted on one of the docking parts or a third one (like the spaceshuttle during construction of the ISS). An arm is harder to controll than RCS (in KSP) but also lighter (you need quite much monopropellant to since those full tanks are very heavy) In real life you dont have to care about time and how hard something is to controll, a robot arm would be easier since its lighter.

The actual connection. If you just want to connect a payload to a combined tank/engine section you would just need to bolt it together somehow since data can be transmited wireless. With fueltransfer it gets way harder, im not sure how they do it on the ISS.

What were the designs for constellation?

[Nibb31]

What were the designs for constellation?

It didn't get into that level of detail. The DRM reference designs are conceptual only with no actual engineering behind them so they shouldn't really be taken at face value.

You don't necessarily need big engines to do the RV manoeuvers. Soyuz or Dragon use RCS only. So did Apollo or Gemini. The main engines were only for deorbit. All you need is an avionics bay, RCS tanks and thrusters.

Docking would have been autonomous. Berthing like on the ISS doesn't make much sense when you have two similar spacecraft of similar mass. You still need to precisely RV and station-keep with the target, so you might as well go straight in and dock.

Some sort of large docking ring would have had to be developed, because both CBM and NDS would be inappropriate.

As for fuel transfer, Progress and ATV have special valves in the docking collar. All you need is some sort of fluid connector integrated into the docking mechanism. It's not unsurmountable and it's already done for other fluids.