Hull Repair Drone
Automatically repairs 3-5 damage to your hull. Drone part is consumed once it finishes.
For a baseline, 51 datapoints were collected. On average, the Repair Drone fixed 3.75 ± 0.80 hull points (standard deviation of sample), all in the stated range of 3 to 5.
For 37 of the datapoints, current hull value was also collected in an attempt to see if it repairs more when the hull is more damaged. There was actually an extremely small tendency for it to repair more when the hull was in better shape (mx + b: 0.0079*hull_points + 3.702). However, a difference of +0.0079 hull points repaired versus current number of hull points is so small that it's probably just chance variation, and there is no relation to current hull state.
Repairing with Drone versus Stores
The standard "exchange rate" is that drones cost 8 scrap at stores. If repair drones fix 3.75 hull points on average, then the 8 scrap to buy a drone has effectively cost 2.13 scrap per hull point (8/3.75).
Hull repair costs 2 scrap at stores in sectors 1-3, 3 scrap in sectors 4-6, and 4 scrap in 7-8. This makes it hard to compare, since the price changes. For the sake of argument, let's say it costs 3.5 scrap to repair each hull point at stores. Most players are unlikely to buy a Repair Drone early in the game; change this number if your play style is different.
If repairs made with the drone cost 2.13 scrap per hull point versus 3.5 scrap per hull point in stores, then drones save you 1.36 scrap per use. This means the schematic's cost of 85 scrap would be realized after repairing about 62 hull points (85/1.36). In other words, over two complete restorations of your 30-point ship, using it ~16 times (62/3.75). Sixteen is quite a few drone parts to use.
Repairs made with this drone cost a bit less than Store repairs in the second half of the game, but if you buy it around sector 4 or 5, typical players may not even break even versus Store repairs by the end of the game. Meanwhile, you might have put those 85 scrap into other things that prevent damage or get you more scrap.
So it's probably not a bargain, for most players. But you're still welcome to consider it insurance. Especially if it was a drop, and you'd only get half back by cashing it in. If you already have it and can spare the drones, it is a cheaper way to repair your hull in sectors 4 and higher.
A handful of situations make the repair drone extremely effective. If you have a drone recovery arm, you can allow the drone to repair your hull exactly twice and then jump away, collecting the drone part before the drone disappears. This can be done once per sector for 2 free hull points. If you have excess drone parts, finding a repair drone can provide some very resource efficient repairs. Between rounds against the flagship, a hull repair drone can provide emergency repairs where they would otherwise be unavailable.
Like other drones, this one also dies if hit (by you or them).