TEXT: PROPULSION SYSTEMS (Optional Rule) 
   
  If the starship that you are constructing has two different reliability ratings, instead 
  of giving the worst reliability for both (as stated in the Narrators Guide p.139), you 
  could have the propulsion system with the worst reliability rating take damage 
  first, then the system with the better reliability rating takes damage next, and so 
  on with, all effects cumulative. 
   
  Example: Let’s say you are building a heavy cruiser, and you have allocated to 
  all your systems (Hull Data, Tactical Data, and Operations Data, etc.), but not 
  your Propulsion Data. You elect to purchase an impulse system with a rating of a 
  C, and a warp system with a rating of a D. Now in combat your ship is hit and on 
  the Table 7.10 (NG p. 115) the roll comes up as Propulsion and 2 to Initiative 
  (impulse system has the worst rating). In the next round of combat the roll from 
  Table 7.10 comes up again, as Propulsion, and this time your warp system takes 
  the damage (in this case, the warp system with a rating of a D) giving you 2 to 
  Helm maneuvers. Now, later in combat, the roll for Propulsion comes up, and this 
  time your impulse system takes damage again and the result is 2 to Initiative. 
  Your ship now has a cumulative penalty of 4 to Initiative and a 2 to Helm 
  maneuvers. 
  
SOURCE: textdata/thevault/Star Trek RPG/Decipher/Homebrew Rules/CODA-Propulsion-Systems-Optional-Rule.pdf