I suppose because they are catching them when they are at ~0m/s in the atmosphere so they're worth -8km/s worth of momentum to begin with. So you would have to

I suppose because they are catching them when they are at ~0m/s in the atmosphere so they're worth -8km/s worth of momentum to begin with. So you would have to fire them out a >8km/s to get a net +ve change in momentum.