Boost assist for uphill.

https://what-if.xkcd.com/21/

The GAU-8 Avenger fires up to sixty one-pound bullets a second. It produces almost five tons of recoil force, which is crazy considering that it’s mounted in a type of plane (the A-10 “Warthog”) whose two engines produce only four tons of thrust each. If you put two of them in one aircraft, and fired both guns forward while opening up the throttle, the guns would win and you’d accelerate backward.

To put it another way: If I mounted a GAU-8 on my car, put the car in neutral, and started firing backward from a standstill, I would be breaking the interstate speed limit in less than three seconds.