The only time the motors are needed is to make minor adjustments to the course to keep the craft on target. The point is that friction does not slow them down. Although friction is just about zero in space there is still the effect of gravity so the path taken by the spacecraft in the solar system will be curved and not straight. Think about whirling an object round on a piece of string. The answer is obvious.
Strings are very good at pulling forces but they are no use for pushing.
Go back to our object on the end of the piece of string. If it is moving in a circle then it is clear that it is constantly moving but that equally it is constantly changing direction. According to Newton's first law a body will continue moving in a straight line unless acted upon by a force. The object must therefore be acted upon by a force because it is not moving in a straight line. At first that sounds a bit ridiculous.