the real trick with 3d is to convert your object in your mind into simple building blocks, the hatch cover on the side is just an elliptic cylinder on its side intersecting with the original cylinder, the flanges- very short cylinders- the bolts hexagonal blocks, then merge all the parts together into one solid lump, if I want a smooth transition between parts I fillet the intersection to give me a radiused 'weld'.
I tend to work in the original units at 1:1, then I can scale the whole thing when finished to any scale I want (scale a copy and you still have the original to scale differently).