Is it possible to output 3D models of my rocket?

[SunRuikang]

I want to 3D-print my rocket. So I need the function “output 'obj'models”. Is it possible?

[Gargamel]

Shapeways used to do 3d prints of rockets you built, so yes there is a mechanism to do this, but No, I don't know the details. 

[Mephisto81]

I recall some articles from some time ago.
Here you go:

https://www.matterhackers.com/articles/print-your-ships-from-kerbal-space-program

 

Basically, you build your vessel in KSP. Then, you use Blender to merge the different parts into a single vessel. In Blender, you adjust your vessel and generate a 3d printable file.
Have not done it by myself. Articles is also quite old. But it should give you a start for your own research.

Here is a link to Blender. Might be outdated.

Mephisto
( Updates after updates...)

[Sharpy]

yup, there was a program that converted .craft to .blend files. People would use that for posing awesome scenes impossible in the game engine, but one could also convert the 3D file to STL or such.