Actually juzz- I think you have the Lagg-3 in mind not the La-5FN. The Lagg-3 was quickly put to ground attack/support roles after the Yak 9 and La-5 came out and was always outfitted with it's racks and rockets afterwards. Thats why it's performance figures are a little misleading- it was actually a damn fine plane that had the same airframe as the La-5 with a bit less power. But it was always tested with it's ordinance racks on making it look obselete after 1943.
The La-5FN in reality almost never had those bomb racks on her. The squadrons flying the La-5FN used her as a fighter only and would never have consented to the drag and high instability problems those bomb racks caused. The racks were usually just used in special occasions.
The Rockets however you are dead on about. The RS82 rockets came in 3 rocket frames and every Soviet fighter was capable of using them. All they did was screw the frame to the underside of the wing and hook up the ignition wire. The plumbing for the wire was standard in every Soviet fighter after 1942 even if some of those planes never have a recorded use of it.
The La-5FN probably (don't hold me too it!) never used the rockets though. Like I said they were considered a valuable fighter for a-a combat and would not be burdened with a-g ordinance. And both the La-7 and La-5FN served in a theatre window where air cover for ground attack planes was more important than adapting for more ground attack options.
However the Yak-9U we know for sure did have use of this option. When Korea started the NK forces used Yak-9U had planes with the rocket rails on. Unfortunatly I have yet to see a WWII picture of a Yak-9U with the rails attached- not even the 9T seem to have used them and these planes attacked ground targets more than any other Yak fighter.
As far as mandatory- I think only the provision for the ignition wire was mandatory. Certainly the inclusion of ground ordinance was not. By 1943 VVS was allready splitting into specialized design theories. Making fighters that could do ground attack like US fighters wasn't the path they followed.
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If your in range, so is the enemy.
[This message has been edited by Sorrow[S=A] (edited 02-10-2001).]