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The Chinese have accomplished quite a bit with the service module left in space following the return of their latest lunar mission. I was wondering if something similar could be done using the 'trunk' section of a SpaceX Dragon cargo capsule. The DragonLab write-up says the solar arrays can supply up to 4KW, with a 1 year on-orbit lifetime. If it was left at the ISS it could become the basis for a good-sized free-flying satellite.

Since this is such an obvious idea, I'm wondering if I have overlooked something …

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The current trunk design has no thrusters and no flight control system. It's basically an empty shell that needs a Dragon to control it.

Dragon trunk interior

The trunk doesn't have a means to attach it to the ISS. You'd need a parking spot on the ISS and an attachment mechanism on both the ISS and trunk. Doable, but not present in the current version.
The trunk is not built for on-orbit disassembly. If you want to work on it in orbit, you'd want to optimize the design to make it easy to remove e.g. the solar arrays, with big, easily-accessible bolts instead of tiny, hidden ones. Add some hand/footholds, etc.

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    $\begingroup$ Would it not be possible to strip it for parts while docked to the ISS? For instance, use the solar panels for some space assembled satellite? $\endgroup$ – neelsg Feb 16 '15 at 9:43
  • $\begingroup$ I think the idea of the question is to put propulsion (for station keeping only) and other systems into it as active payload. As a service module with generous power supply, it might service its trunk payload as it services the Dragon, and live on as an independent satellite after leaving its primary ISS mission. $\endgroup$ – LocalFluff Feb 16 '15 at 9:56
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The Dragon trunk is mostly a simple structure used to provide an interface between the second stage and the Dragon. It carries the solar panels, and allows for carrying bulky unpressurized cargo. It has no orbital maneuvering system, docking module, or anything else of use.

On the other hand the Chinese Shenzou design is heavily influenced by the Soviet/Russian design of the Soyuz. In that model, the spacecraft has three modules, a propulsion module that is unpressurized at its base. The middle module is the reentry module, and the top module is the orbital module.

A pedestrian benefit of this design is that there are two rooms, while in orbit, with the washroom in one for some modicum of privacy.

When a Soyuz or Shenzou is done with its mission it discards both the Orbital and Propulsion modules and only the re-entry module returns. The Chinese advancement over Soyuz was to leave that Orbital module behind for science missions. A clever use.

It seems unlikely that they could 'stack' them up, since the interface between the modules does not seem to be the kind that would allow reconnecting modules in orbit after use.

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