Increasing orbital velocity without affecting size of orbit?

[jebster]

Is it possible in KSP to burn directly towards the body you are orbiting and increase your velocity?

[SargeRho]

The size of the orbit is directly determined by your velocity.

[Mr Shifty]

You could try it and see. If you burn directly toward the body, it will decrease your orbital altitude 90 degrees prograde from where you are and increase it 90 degrees retrograde. In other words, it makes your orbit more eccentric along an axis perpendicular to your burn axis.

[Yourself]

If you want to increase your orbital velocity but maintain the same semi-major axis, then, yes, you'll have to constantly thrust towards the body you're orbiting (after you've increased your horizontal velocity). This also doesn't mean you go full throttle towards the planet, you have to get the right amount of thrust to make up precisely for your increased velocity (in effect making up for gravity that isn't there).