The protagonist, a Human, flew an 'S20' Spacecraft. The area of the solar system covered was the asteroid belt to Earth, i.e. the Inner Solar System. After sleeping on this I have dredged up the following. The S20 came from Mars to Earth to kidnap people. I think the Mars colony was splitting/rebelling from earth. Mars had a secret HQ embedded in an Asteroid. I may have read this in the 1980's. The S20 may have been para-military and the pilot was male.
Raiders from the Rings by Alan E. Nourse?
"A young adult novel in which the human race is divided into the Spacers and those who still remain on earth. There is a constant low-level war between the two groups, with the Spacers raiding earth for supplies and for women. The war turns hot in the book and three teenagers, one Spacer and two from Earth, must find a way to bring peace. "
However the spaceships are called S-80 not S20:
“They teach us to fly ships these days, not just pull levers,” Ben replied. “And that was one of the new S-80’s, too. Have you ever flown one? They make the old four-seaters look like cargo ships, handle so smoothly you hardly know you’re out there.”