you should not be able to kill radar tower with 20mm < aeroplane guns in the first place.
It would more than a rocket too.
I seem to recall somewhere it was said that HTC coad took into account the differing types of rounds, some of them being inciniary, but as far as WE can SEE the weapon's lethality has components such as armored hard or soft. Objects have hardness. I could
imagine fuel powered generators, heaters, electrical shortages or wooden and fabric furnishings causing fires in the radar structure.
How about an hardened ammo bunker... 
We see wonderful art work in this game. I was very impressed when I looked at an old photo of Duxford Field and noticed the suprising resemblence of the tower there to the one in the game. Also, the HQ building has a great resemblence to one of the Zoo Flak Towers. With this in mind ammo bunkers as pictured in the game, I would agree would be impenetrable by simple plane rounds. I could
imagine that a lesser structure however may very well be vulnerable to the varying rounds carried in aircraft. Also, chain reaction explosions from nearby vehicles, debris, electrical shortages ......etc...... (I have a good imagination
)
Look familliar?
How about this?
