That almost addictive feeling...

[G'th]

When your space shuttle finally comes together as an operational vehicle. What a great feeling that is. Specially after toiling away at it for several days trying all kinds of different configurations.

[cubinator]

...when you finally get your Rubik's Cube to work without a bunch of parts attaching to each other through thin air and getting stuck...

The feeling of accomplishment when you get something really hard done is great, especially when it's a real engineering problem as opposed to just some challenge in your average FPS or other "just for fun" game.

I still haven't got the second version up, it's still in testing. 6-8 moves actually takes a *long* time when it keeps running into what I call the "quantum entanglement glitch", and I've been really into Xevious right now... I might finally finish the first Square-in-the-Middle pattern tomorrow and release the cube for everyone to use.

[autumnalequinox]

when you spend all day at your boring, boring job fantasizing about a new, interesting way to undertake a mission and just can't wait to get home and try it.

And then you get home and you have butchered your original idea by thinking about it allllllllll day.

[samstarman5]

[Madrias]

When you finally get your SSTO design working, and you use that design as your basic shuttle to do crew transfers to your space station.

[Overland]

Making a self steering trailer for land trains and seeing the wheels turn like they do in a diagram for trucks you found...

 

That itself found by reading history for  on a 1932 land train

 

 

Using a real world solution for a problem in KSP..and have it work

 

Walk the length of a land train connecting steering mechanisms..checking couplings...

Then seeing the future of KSP as a turn based logistical rostered simulator for land trains....

 

Beautiful it is

[Mjarf]

When I can finally do the actual manned launch for the actual mission with the interplanetary craft I've worked on for a week. It also makes me happy when the little lander rendezvous with the mothership and dock in the cargobay. I can almost hear the rest of the crew say (in Kerbalish) "Welcome back buddy, how was Duna?"

[purpleivan]

Completing a rendezvous burn of a big ship to find that you've ending up within 50m of your target.

Edited June 24, 2016 by purpleivan