Well, listen, it is complicated
As you probably noticed, your Ursa bomber fully packed with Helioses is as fast[slow
] as another Ursa without any bombs.
I think you should give all weapons(mostly secondaries) an $Weight value which defines how heavy the missile is in kilos, and how much speed does it gives your bomber when it is fired.
Like this:
Helios
$Weight: 2.0
Your bomber obtains +2 m/s when a Helios is fired.
The $Max velocity can stand for the velocity without any secondaries. The $Weight is decreased from the $Max velocity when the mission begins, and the $Weight of the fired missile is added to maximal velocity.
eeehhh .... understandable?