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Well, Campbell gets lots and lots wrong, but the main thing for me is that he uses limitations of sea-going vessels or of aircraft for his spaceships.
The main one is his implicit assumption (eventually stated explicitly) that space ships save fuel by changing course gradually, so that their 'speed' relative to whatever situation they're in remains high. The most wasteful course, according to Campbell, is to directly reverse course.
This is sort-of true for waterships and aircraft. But not spaceships. They don't have anything for their wings or hull to press against, and so for spacecraft the most efficient was to move back the way they came (outside gravity wells) is to just flip end for end and accelerate directly in the opposite direction.
A curved course for a spacecraft is wasteful way to reverse course, as you're accelerating perpendicular to the way you want to go, twice!
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