Nuts and bolts numbers talk on exactly what the Ship Upgrades in Void Destroyer 2 do.
What Your Slots Do
Every ship has a combination of Active Upgrade Slots, Primary Upgrade Slots, and Secondary Upgrade slots. You can buy more slots, and even convert slots from one to another.
Active Upgrade Slots are castable spells. They do something cool, then recharge. Activate them with SHIFT+1, SHIFT+2, etc. Press again to terminate them early, which can be very useful when accelerating uncontrollably (forward thrust, I'm looking at you.) The AI will never use Active abilities, even if they're your fleet ships.
Primary Upgrades are passive improvements to a ship. Faster firing guns, more acceleration, adding an aftermarket shield generator, turret, etc. Your primary upgrades persist even if the ship is made a Fleet Ship, and the AI makes excellent use of certain upgrades such as Engine Optimization for dodging.
Secondary Upgrade slots have not yet been implemented.
There are currently two standard weapon upgrades.
Allow you to shoot longer before reloading by 70%. If you had 20 round magazines before, you have 34 round magazines now. Does not increase reload times. This is an indirect increase to DPS and is recommended.
Re-Fire Rate Booster
Makes your guns shoot faster by 80%. If they shot 10/s before, they shoot 18/s now. This empties the magazine faster and needs more reloads, but the fact is that your DPS has been directly boosted. Recommended.
There are also non-standard upgrades available to only certain ships.
Available on the Hornet, this added turret significantly increases your damage dealing ability. It also shoots behind, below, and to the sides.
Available on the Pug, replaces your normal turrets with Railgun turrets. These weapons have almost zero travel time and knock enemy fighters around through space. Seems like a DPS upgrade, more testing is required.
(None at time of writing).
A value is added to the Armor Rating found on your ship. Can bring your armor up from 0. Understanding armor is important to surviving in VD2. Armor value is directly subtracted from all incoming damage, per bullet. An enemy machine gun is likely to deal about 4 damage per shot. If your armor value is 2, then you take half damage from that source.
A value is added to your ship's Shield Generator. Can bring your shield value up from 0. Shields take damage before the rest of the ship does, and regenerate over time for free.
Your shield generator rapidly refills itself for 10 seconds, even if you are taking fire. The recharge is so fast that it can outpace incoming damage assuming you aren't being slammed by multiple sources. Cools down in 30 seconds.
Front Energy Barrier
An impenetrable energy shield appears in the front 45 degree arc of your ship. You can shoot through it, but enemies can't. This is your trump card in 1 on 1 engagements and can make a light fighter easily triumph over a heavy fighter. On for 22 seconds, recharges in 15.
Doubles your armor value for 15 seconds. Depending on what your base armor is, the doubled value may be high enough to ignore incoming damage from some sources. Recharges in 25 seconds. If your armor value was 0, it doubles to 0.
Your engines, weapons and shields shut down for 20 seconds, during which time your hull rapidly repairs itself. This is a terrible thing to do in direct combat, but if you've just gunned your engines and gotten clear of incoming fire, this can save you. Recharges in 45.
Pull a distant object toward your ship. Range is 5k. Force imparted will depend on the size difference between the two ships. It is very important to terminate this ability early (push the activation key again) otherwise you'll find your tractor target ramming you with great speed.
Understanding engine upgrades requires a familiarity with how VD's flight model works with regard to velocity. Ship acceleration is given in terms of two figures: Cruise Speed and Time to Reach.
Cruise speed is the fastest a ship can go before severe diminishing returns come into effect, making it harder and harder to accelerate. Time To Reach is how long it takes to reach Cruise Speed. A ship with a Cruise Speed of 100, with a Time To Reach of 2 seconds, accelerates at 50 meters per second, per second. Once Cruise Speed is hit, acceleration slows down. All ships travel at their Cruise Speed on the overworld.
You can use the afterburner (default tab) for 215% as long as normal. The afterburner recharges as normal when not in use.
Only found on larger ships, this upgrade lets you pitch and yaw faster, making the ship feel more nimble.
Increases Cruise Speed. How much will vary based on ship class. A gunship gains 50% increase. If Cruise Speed was previously 60 in 4 seconds, it is now 90 in 4 seconds, a gain in both Overworld Speed and also in Acceleration. Remember that your fleet's overworld speed is the Cruise Speed of your slowest ship. Engine Optimization on your fleet's slowest ships can save you a lot of time.
Improves Time To Reach. Typically the reduction is to 85% of its former value. If cruise speed was previously 60 in 4 seconds, it is now 60 in 3.4 seconds. This is a gain in Acceleraiton, but no improvement is made in Overworld. Typically you should only buy this if you already have Engine Optimization and still crave more acceleration.
Supercharge your acceleration for 10 seconds. This ability can push you far past the normal limits of Cruise Speed with no diminishing returns. This ability is essential if you want to get your corvettes moving at 1000 m/s and catch those pesky fighters.
Gravity Drive Trigger
Jump forward 2-5 kilometers almost instantly, then kill all forward velocity. The exact distance travelled will depend on your ship's Cruise Speed. This is a great way to change places fast, and is also handy for killing too much foward velocity in a ship that otherwise can't stop quickly. If you just need to get from A to B quickly, typically Forward Thrust is the better buy.
(None at time of writing).
Instantly finish the cooldown on all your other Active abilities. This can feel almost like cheating as you pull a seemingly endless supply of Front Energy Barriers and Shield Recharges out of nowhere. Cools down in 60.
Prevent the enemy from targeting you, unless they're already targeting you. Doesn't fool turrets. Needs more testing.
Makes everything that was targeting you suddenly target something else. There's nothing preventing them from choosing to retarget you though, unless you're also running Sensor Shield, or happen to be surrounded by dozens of allies that your enemies would target instead.
Primary Upgrade Slot
Gives you another primary upgrade slot, no strings attached. Somewhat expensive.
Active Upgrade Slot
Gives you another active upgrade slot. Somewhat expensive.
Overhaul - PUS
Gives you yet another primary upgrade slot. Very expensive.
Overhaul - AUS
Gives you yet another active upgrade slot. Very expensive.
Convert Primary to Active
Adds an Active upgrade slot, at the cost of subtracting a Primary Slot. Fairly cheap.
Convert Active to Primary
Adds a Primary upgrade slot, at the cost of subtracting an Active Slot. Fairly cheap.
A clever, wealthy player can add as many as 3 primary slots to any ship by buying the slot, the overhaul, and converting an active slot. This is enough to seriously trick out your favorite craft. If you don't have enough Active slots to do the conversion, buy one, then convert it!