Terrorist robots dissected - anatomy of a scare
DIY cruise missiles - not as easy as you think
You can buy a small, petrol-powered robochopper which might cost under $60k, carries up to 50lb of payload and could fit into the back of a van or small lorry all ready to fly. Pull up somewhere quiet the other side of St James' Park, launch the bird and drive away.
It has controllers and software capable of automatically flying a 3D waypoint route, apparently good enough to stay inside an area no more than six metres wide and one metre up and down.
So the robochopper zooms along gaily without further input from you just above the rooftops and trees, crosses Horse Guards and comes in where you choose - roof, window, maybe doorway of Number 10, accurate to maybe +/- 3 metres. Fifty pounds of explosive device goes off. Job done.
Maybe not, though. You've definitely got your headlines but you're fairly unlikely to have bagged your prime minister, unless you could predict in advance - almost to the second - just when he'd be between the door and the armoured car (even his press secretary probably can't predict it that closely). Even then, a bit of wind, an alert close-protection man, and you're out of luck. He'll be into the car or the building before the explosion.
One can, of course, destroy or partly destroy buildings and armoured limos with less than 50lb of explosives - the Brighton hotel bomb was smaller than that - but the stuff has to be carefully placed and prepared, probably inside the target. If all you can manage is crashlanding the bomb within an area six metres across you need to be thinking in terms of hundreds of pounds warhead weight for these targets.
The new, US "miniaturised" GBU-39 smartbomb - ultra-accurate, designed to get a kill using the minimum necessary amount of bang - is in the 250lb class. Most of this is actually the steel penetration case, which would let it punch inside No 10 or the PM's limo before detonating its 50lb charge. But the robochopper can't hit hard or fast enough to get this kind of result, even if it could lift the weight.
Probably the best plan would be to make a 50lb large-diameter shaped charge of the sort now being used in roadside "superbombs" in Iraq (it isn't too hard) and hang that under your robochopper. Fly the chopper at the PM's vehicle. To have any serious chance of success you will need to manually pilot the weapon in; GPS will quite likely miss altogether even if the vehicle is obliging enough to remain stationary, and with an explosively formed slug you need to be fairly accurate.
A $50k-60k robocopter can be handled remotely using a live vid feed, but now you've got serious problems. Your drone is no longer electronically stealthy - it can be detected a damn sight more easily than a blackbird. It can also be meddled with, and it will be. Even ordinary military patrols these days deploy sensors and jammers which can warn of an elctronic threat in advance and perhaps cancel it out.