@Rawr, this is certainly not AC. In AC, when you reach a x2 speed, you keep your waiting time. In this game the time you have waited time is reduced by half as well. :X
The game is better off with not spending points in the combo upgrade as you get very little from collecting them but if you shoot one you lost almost all of your combos.
Upgrading the Combo will increase your minimum combo and your maximum combo, meaning you wont lose all your combo when you shoot a green powerup, and also your potential to score higher will increase, due to higher cap on combo.
Thanks for playing! :)
Advanced research are available at 80 of each kind of troops (very expensive), 100 of each troops (not so expensive). Also specific alien research are available when you destroy certain number of aliens (1000 carriers, 10000 aliens, xx aliens with a magnet on his head, xx speedsters). Speedsters are those very fast moving aliens, they give 1 RP when destroyed. Getting 25(not sure) parachute boxes opens a new research too (also too expensive right now).
I know you're saying that some of the upgrades improve RpS a lot, but I can't see which upgrades are for future troops, and which are for existing troops, and which affect both current and future stuff so it's hard to know which ones to buy now.
Upgrading the Combo will increase your minimum combo and your maximum combo, meaning you wont lose all your combo when you shoot a green powerup, and also your potential to score higher will increase, due to higher cap on combo. Thanks for playing! :)