Let's put a stationary spacecraft some distance (A) from a planet. It will accelerate towards the planet in a straight line. Now lets imagine we have a tunnel through the planet (or maybe the planet is made from dark matter and interacts with our spacecraft only gravitationally). So I think the spacecraft will fly decelerating through the planet the same distance (A'), then stops and goes back and keeps repeating it.
Now lets forget it for a while and imagine our spacecraft in a high elliptical orbit around the same planet. And at apoapsis we burn retrograde, so our periapsis gets closer to the planet's center and we need to make a tunnel for it so it doesn't crash (or it is made of dark matter and just goes through the planet). Now at apoapsis we burn retrograde again and again until we completely stop and our ellipse will look like a straight line. So our spacecraft will go accelerating from apoapsis to the planet's center, then instantly turn around and start decelerating towards apoapsis again. Looks weird and for some reason not the way we had it in our first example.
So obviously there is a mistake somewhere, and I cannot find it. So I decided to ask for help here, on KSP forums, where I learned Orbital Mechanic in the first place.
