set apoaspsis and periapsis in systemInit

src/orbits.cpp

@@ -16,6 +16,8 @@ systemInit(grav_body gb, orbital_elements el)
 	s.e3d = ellipseInit3D(s.ep, ELLIPSE_VERT_COUNT);
 	s.epsilon = orbitGetSpecificEnergy(s.ep.a, gb.mu);
 	s.h = orbitGetAngularMomentum(s.ep.p, gb.mu);
+	s.r_apoapsis = s.ep.a - s.ep.c;
+	s.r_periapsis = 2 * s.ep.a - s.r_apoapsis;
 	s.time_step = DEFAULT_TIME_STEP;
 
 	return s;