“dla,

You’d better be one genius hobbyist to take down a Predator (100mph cruising speed at 25000ft ceiling) with your hobbyist rocket!”
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…or a Reaper at twice that altitude.
Then again, you’d need a hell of a booster rocket on a stinger to take down either!

The original Stinger had a max altitude of ~ 15,000ft. and a cost of ~ $76,000.
Not that many block 2′s with a max altitude of ~ 26,000ft are out there on the market, at much higher cost.

Additionally, the terrorist would need a handy dandy portable radar setup to find either the Predator or the Reaper prior to arming the Stinger.
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UAV’s are incredibly cheap compared to manned platforms, esp manned systems capable of 24 hr lingering time:
The fuel for them and the tankers alone render UAV costs insignificant.