My big fear: when the payload is hand grenades [1], i.e. a "multicopter bomber".
You can launch the multicopters from places where you're not seen. You can even do this in nighttime and have them land and wait undetected on some rooftop until you want them to continue their mission.
A handfull of such multicopters will make more terror than 9/11 ever could do because 9/11 was (almost) concentrated to a single place. 9/11 had passenger lists so it was easy to find out who took control of the planes. Multicopter bombers have no passenger lists, no sender address. You don't know how many more planes are in the air and when they will strike.
Redundancy and multi-target missions are even cheaper and more efficient with multicopters than in the 9/11 attacks. Multicopters are so cheap that the terrorists can launch a bunch of them and not care if half of them fail to get to their target. Multicopters bombers works for targeting e.g. Disney Parks and other places with a lot of easily frightened people out in the open.
With 9/11 it was easy to get away from the attacks (i.e. just leave that part of Manhatten). Multicopter bombers are like a swarm of supersized wasps. You don't know where to go, you don't know when they'll be back.
When you say "my big fear", I can't help but read "m irrational fear".
Do you _really_ fear "multicopter bombers" with a fleet "like a swarm of supersized wasps"?
To me, that's in the same class as people wo are afraid of flying, yet happily drive their car to the airport.
Realistically, terrorists don't skill up with brand new technologies to try carrying hand grenades around, they use cars/trucks/airplanes. If _I_ had a budget set to get explosives in amongst people, I'd be buying or renting cars/pickups/small trucks which can carry thousands of pounds of explosives around a city relatively inconspicuously. I wouldn't be training up teams of rc-pilots/hobby-UVA-geeks to be able to deliver a pound or three of explosive at a time.
a) buying a few hand grenades and premade multicopters? Hobbyists in Germany have already multicopters which are GPS pre-programmed so you don't need any pilots to control them during the mission.
b) creating thousands of pounds of explosives? Try read the detailed diary from the 2011 Norwegian terrorist to see how much time and trouble it took produce his bomb: way more time and effort than most would think.
The next 10 years will make multicopters much more accessible to non-geeks, but "thousands of pounds of explosives" won't be any easier than today. New tech gets old very quickly.
Why do you need multicopters? Terrorists already can (and some have) plant small timed bombs in crowded areas. I don't really see much difference other than that they can target more secure areas. This would be useful if they wanted to attack military targets, but since their usual goal is civilian casualties, they can by definition already get to places civilians congregate.
I have absolutely no idea where where I can purchase a live grenade. However, living in a rural area, I'd have no trouble finding people who know how to make explosives from common (and cheap) farm materials. Hell, I was making blackpowder when I was 11 years old.
Now add to that the fact that in the US I can buy a used car or pickup truck cheaper than I can buy a cheap GPS-equipped model helicopter and I definitely know which option I'd be more worried about.
Hobbyists in Germany have already multicopters which are GPS pre-programmed so you don't need any pilots to control them during the mission.
Hobbyists everywhere can have this - Arducopter (DIYDrones.com) and other projects have had this for years. Arduino + compass + GPS + accelerometer + altimeter + optional XBee radio (~2km+ range on some models) + autopilot software... preprogrammed routes, live telemetry and course changes via Xbee and ground control software.
This has been happening since 2000, over Afghanistan, Pakistan, Iraq, Yemen, Libya, and Somalia.
With guided rocket propelled "grenades" and the terrorists are the CIA and pals in USA government.
You should put your talent to good use, I bet you have it in you to write a first rate thriller.
(I'm not being viscicious - plenty of people have imagination, plenty of other people can rationalise about what could work in practice, drawing these together is a skill).
You can launch the multicopters from places where you're not seen. You can even do this in nighttime and have them land and wait undetected on some rooftop until you want them to continue their mission.
A handfull of such multicopters will make more terror than 9/11 ever could do because 9/11 was (almost) concentrated to a single place. 9/11 had passenger lists so it was easy to find out who took control of the planes. Multicopter bombers have no passenger lists, no sender address. You don't know how many more planes are in the air and when they will strike.
Redundancy and multi-target missions are even cheaper and more efficient with multicopters than in the 9/11 attacks. Multicopters are so cheap that the terrorists can launch a bunch of them and not care if half of them fail to get to their target. Multicopters bombers works for targeting e.g. Disney Parks and other places with a lot of easily frightened people out in the open.
With 9/11 it was easy to get away from the attacks (i.e. just leave that part of Manhatten). Multicopter bombers are like a swarm of supersized wasps. You don't know where to go, you don't know when they'll be back.
1: Homemade "grenades": http://en.wikipedia.org/wiki/David_Copeland