Why I can't set true anomaly to more than 90 degrees in parabolic and hyperbolic orbits?
Unlike circular and elliptical orbits, parabolic and hyperbolic trajectories are not bounded paths. According to orbit equation, in parabolic trajectories (e=1), as true anomaly approaches 180o, the denominator approaches zero, so that the radius of spacecraft tends towards infinity. Also, in hyperbolic trajectories (e>1), the radial distance approaches infinity as the true anomaly approaches cos-1(-1/e) (true anomaly of the asymptote). Since it is impossible to simulate the spacecraft motion in infinity, the value of 90o is considered as the upper limit for true anomaly in parabolic and hyperbolic trajectories.
Why should I specify perigee radius instead of semimajor axis in parabolic orbits?
Parabolic trajectories are the limit of a sequence of ellipses where one focus is kept fixed as the other is allowed to move arbitrarily far away in one direction. The apoapis is considered to be at infinity, Therefore semimajor axis also tends to be infinity. Since both eccentricity and semimajor axis of all parabolic trajectories are the same (e=1, a=infinity), some other information like perigee radius is needed for simulation.