Hmm so many places to fit this, here seems best I guess.
As mentioned recently in "The Fleet" section I came up against a couple issues with T20 vehicle and craft Structural Integrity. What I'd like is a home-brew rule that doesn't unbalance the game while allowing a little more detail without too much work.
Right now SI is based solely on the vessel's size which is not a bad starting point, much like the level 0 armor factor included in the basic chassis/hull.
First, one problem I have with the current SI tables may be a minor nit to pick but possibly an errata issue too.
1) p253 the "Vehicle Structural Integrity" table needs to have a "Base SI" column entry of "0.1" for the "Fine" "Size" row to work with the formula.
2) p261 the "Hull Structural Integrity" table needs the entries in the SI column to be changed from "per additional X ton" to "per X ton" to maintain the proper progression.
Second, and the grittier fun, pumping up the SI.
"Ramming speed Mr. Stryker!"
"But sir, we're in a Yugo!?"
1) It seems to me that Armor should bump up the SI but that would be a bigger wrinkle (changing published designs) so I'm willing to accept that it is just added onto whatever structure is there to begin with.
2) Vessels with higher accelerations should have more SI to withstand said acceleration but again this too is not factored so I'll also accept that the stated SI is rather the free SI left after the minimum for the drives is accounted for, perhaps as part of the drives themselves.
3) Which brings us to adding SI, best treated like adding Armor and not done on every ship. In fact using the Armor Factoring system may be the easiest. I propose the following system. Let me know if it seems reasonable and if I've explained it clearly.
Structural Integrity Upgrades
The base Structural Integrity (SI) of a vessel, be it a vehicle, small craft, spaceship or starship, is included in the original chassis or hull construction. In some cases though a builder may desire a more robust frame for certain applications.
To add SI units first determine the vessel's Armor Factoring (p259) or Base Armor Volume (p235). Using this number and the appropriate Armor section (p268 or p236) select the desired SI upgrade factor just as you would for adding Armor factors with the same double for the first factor and TL limit. This will give you the cost and volume required for the SI upgrade.
eg1-pt1) A TL5 helicopter (see p300) with a basic SI of 55 (see errata) in its civilian form is built to military spec to better survive combat with a greater SI, in this case adding the maximum allowed for TL5. The helicopter's Base Armor Volume (used to calculate the SI upgrade) is 50vl. 5 levels of upgrade (based on TL) will be 6 (2+1+1+1+1) times 4 (TL 4-9) times 50vl = 1200vl. The cost will be cr3,000 + (cr9 times 1200) = cr13,800 before discounts.
Next go to the appropriate SI table (p261 or p253) and look across from the vessel's size to the find the Additional SI column and multiply the added factor by the SI upgrade factor above to find the bonus to the vessel's basic SI.
eg1-pt2) We now know that the TL5 mil spec helicopter will have an SI upgrade factor of 5. Looking on the table (p253) the 5000vl helicopter has an Additional SI factor of 2.5. So 5 times 2.5 = 12.5 SI (round fractions down) = 12 SI added to the basic 55 SI for a robust 67 SI total for the military version of the helicopter, comparable to other vehicles twice its size.