Your concept is flawed. I think you are unconsciously applying "airfight/ship logic".
If a device intercept a target path THEN MATCH ORBIT, what you obtain is a device that stay at THE SAME DISTANCE of the enemy while they admire the firework (because they'll have the same orbital parameter)
The explosion would create debris yes, but those debris will only be as fast as the explosion with no added speed (and matching orbit is the most slowest you can get in relation of a target).
It IS a valid solution if you want to **** up the orbit in the long term, and if you enemy is too fragile to survive any impact. But if you want to annihilate as efficiently as possible something that could survive it, you'll need to do it differently.
I think you mixed up space-physics with the air/sea medium where FRICTION will make a mine stop in front of a vehicle that actively accelerate toward it.
But there's no "dive" in orbit, or rather you are constantly "diving" aka "letting gravity shape your trajectory"
What you want is to intercept the target itself. (as in get at the same position at the same time)
Then you have the choice :
1) Match orbit at close range, bringing your spaceship at rest with one another. "same speed"
2) Not match orbit, we fly by each other then our inertia pull us away.
3) Collide with it.
- My ship launched a drone
(In a very bad, horribly inefficient and sub optimal way here)
- We will follow the drone as it "dive" on the target
(note : it make no difference if the planet surface is "below you" and the enemy "above", space don't care that way)
- My drone is on a FLYBY trajectory, it will pass by the enemy, then (if it doesn't match speed) it's inertia will pull it away.
When the drone get at range for its railgun :
There will be 3 phases (assuming it doesn't fire its thruster)
- Approach : relative velocity will add up to the projectiles' speed, (more kinetic energy = more damage) (same goes for the enemy)
- The closest approach : It only last an instant, and would add no speed to your projectile, you would only be the closest from your enemy. (same goes for the enemy)
- Escape : relative velocity will subtract from the projectiles' speed, (meaning less energy) (same goes for the enemy)
(because this game do allow you to test and even design your own vehicles and weapons)
If my drone was a Kinetic-missiles (meant to collide with the target) then the approach would give less time for the enemy to intercept it.
If my drone was a Explosive-missile I would have it explode at the closest approach
However since my drone is throwing unpropelled projectile it mean I only had a shorter window of opportunity to fire with a buff, before it become a debuff.
If my drone dropped a "bomb" it would have to do so on a collision trajectory and then burn to avoid colliding with the target itself. The bomb would only inherit the relative speed, making it slower than a missile and easier to intercept.
LAST AND MOST IMPORTANT POINT :
If I want to meet my enemy again, I would have to maneuver far away on the orbit to keep it economical, or waste TOO MUCH fuel doing it during escape (I tried, and my drone ran out)
So for/against a warship that can only win/loose during a longer fight it would have been better to match velocity, then get close enough (fast enough) to overwhelm the enemy with drone.
Ideally your warship/drone would matched velocity at their best weapon's range. Problem is : the enemy get a vote too, if I do that the enemy would shoot at me while I'm busy maneuvering.
So as said, the closest you get of a "dive" is an intercept course where the source of gravity is behind your enemy. Which bring no tactical advantage over an intercept from a lower orbit.
ps: there's a lot more subtlety and different lateral thinking and winning approach I didn't go into. But it show you the BALLISTIC logic of a real space battle.
edit 1: reworded to be clearer