i'm gonna go out on a limb here an assume that by "SDK" the OP could have meant "source code"
and to that, the answer is, as always - "at Martin's computer" :hmm:
even though every so often someone comes about asking for the sources, Orbiter is not an open-source project...
yet, don't mistake open-source for open-ended -
while not being open-source prevents you from making core changes to the simulator engine, (and i don't dare judge Martin's choice on that - remember, he IS smarter than most of us and the Probe combined
)
the open-end nature of it, (hence SDK != source-code) makes it a very welcoming platform for addons...
in fact, it's one of the best, most comprehensive SDK's i've ever seen around - and i've done my share of addon-making for other games/sims... the Orbiter SDK is a standards-setting piece of genius work
so ingenius it is, once my boss (i'm a game programmer by trade) complimented me on how my coding had an overall intuitive SDK-like feel... i then proceeded to tell him, i'm just well-inspired - he who deserves true merit has a doctor's degree and dwells in the UK (even tho his profile says "Jamaica")
so well, no source codes... we all take it that the doc knows what he's doing (and we mortals would probably have a hard time keeping up, from what i've seen in the sdk samples)
but that SDK is a masterpiece, and you most likely already have it... so start by reading the API_Guide.pdf file - its quite a nice read (and has served me as a general physics reference when coding other non-orbital games too)
