Too big, too heavy and too short ranged.

I don't necessarily mean the actual phalanx system, it would probably be torn right off! However, I'm not sure that there is no solution for creating a computer controlled multiple gun system. Primarily, I'm not sure because I do not know a lot of the crucial numbers. When pilots are pulling hard G's in order to get behind their opponent, how far away are they from each other normally? Would the barrels have to be too long to make the idea possible? Don't know, just asking, that's why I like this forum.

B-52's carried radar-controlled tail guns but the concept is obsolete. It takes a large plane to carry the wieght and the range is hopelessly short in a missile battle.

Yep, it would only be useful if it had missile like range, in which case it would be more efficient to just point the nose of plane in the direction of the enemy and fire because the turning circle of a plane is relatively small compared to the distances involved.

Judging by the internal space that a proposed (2015?) laser system would fit into an F-35 (in place of the lift fan, and powered off the massive engine), is there the possibility then that a future large offensive aircraft (if another will ever be built) could have such a system installed? As there will be much info gathered from the ABL project (as well as THEL data already gained, and perhaps soon the "fits on a pallet in a Chinook" Army Tactical Laser), it seems to me that the tail end of a B-52 or B-1 is perfectly sized to fit a comparable system as the F-35 would mount. It's also fair assumption a shipborne laser CIWS will be developed from it or alongside it. So why not exploit it: a large, orbitting (atmosphere) bomber, armed with 2 or 3 gimballed laser defenses, would be nearly immune to everything except large-scale saturation attacks: if current fire control radars can get 20mm cannon fire to bring down a missile, certainly the tech 15 years from now could do it even better, without having to compensate for any bullet drop.

Would be a great weapon for FB-22 if it is built and it is under serious consideration, the FB-22 in its's revised heavy weapons carried in wing stealth pods configuration. As for range, 10kilometers minimum should be adequate to have reaction time to kill AAMs. The Russian R-72 monster was said to be able to lock onto an AAM at 0.005 square meters and be capable of a 180 degree turn after launch.

Would computer directed anti-missile guns work on a modern plane, yes. The caliber would have to be 12.7mm to 20mm to be effective and likely in a drum-fed gatling arrangement like the phalanx in order to be compact and brass-emissions free. The turrets would not have to be all that big, contrary to opinions shed, as the .50 cal systems on the robots in Iraq are under 200 pounds apiece. Here's the deal, even if you destroy missiles coming your way the fact is you are still vulnerable to ground-fired anti-aircraft shells and direct-fire from aircraft cannon. So in order for this to work you'd need a secondary missile system to handle the other threats. A plane the size of the A380 could probably be fortified enough to handle as many planes as a naval frigate could handle.

At some future point, I could actually see the logic behind a large airborne weapons platform: it could guide/direct/control a fleet of in-flight refuelable UCAVs. it could be armed with its own very long range ASMs. it could be armed with its own very long range AAMs. it could have all the tech (radar capabilities) of an AWACS and JointSTARS all rolled into one. it could be armed with defensive hard-kill laser systems. it could be direct-linked to an "SWACS", Spaceborne Warning And Control System, which would provide massive look-down coverage, targetting data, and guidance uplinking to allow the very long range ASMs and AAMs to adjust their flight to achieve their strikes:. Theoretically, such a system could launch a 300-500km air-breathing AAM. This would be an ideal "stay inside a safe zone and shoot down anything within a 500 mile diameter airspace" type of platform. Of course, the real issue would be, how many airfields would be able to accomodate these aerial behemoths that could easily grow to sizes exceeding the 747 or 380 (like the proposed Pelican transport.) Should we instead develop massive always-airborne blimps and drigibles that barely need to touch down (only to resupply/repair/recrew), and could maintain patrol altitudes above the jet stream (powering their laser weapons and electronics from the fuel cells and higher-efficiency solar arrays that should be available a couple decades from now)? Hell, even a modern 737 could be equipped with a very effective long-range strike package: a modern equivalent of the Tomcat's AWG-9 (300+km range should be achievable in such a massive radar, app 40 inches in diameter... or pursue that latest capabilities of Wedgetail-type modifications by turning them into attack-capable systems), a modern Phoenix equivalent (an air-breather than can do high Mach numbers out to the same 300+km range), and even the next generation of long range ASMs. I would think that the current fleet (and projected future types) of US spysats should be quite capable of providing adequate up-to-date long range (against ground targets) targetting data in many scenarios. Such a platform (aerial frigate) could actually compliment UCAVs, which themselves could provide forward targetting assistance/relaying to the missiles fired by the aerial frigate a couple hundred miles distant. There used to be a proposal to mod a 747 and outfit it with dozens of cruise missiles: a decade from now, surely we could also outfit Dreamliners with scores of Advanced Cruise Missiles, JASSM-ERs, and ALRAAMs, backed up by a very capable all-aspect, air-sea-land attack radar that can track and designate targets by the several hundreds. Again, UVACs and UAVs could also provide forward sensor enhancement, easily allowing command elements a real-time, several-thousand-sqare-miles view of everything within their battle environment. This could become the ideal "server" for the networked battlefield of the future, even providing its own offensive and defensive abilities. The fact that we could utilize many commercial airframes, with minimal structural modification, should be a plus and save a considerable amount of funds, the only real expense being the weapons and electronics packages.

The problem I see with a big aerial arsenal ship type thing is that no matter how large you can make the missiles on the plane someone will be able to make a longer ranged larger missile that is launched from the ground.

When you are at a higher elevation that your enemy the latter tires to reach your position. Your elevation gives the better sight and grasp on the situation in comparison to the ground-centric force. Likewise, a much smaller weapon is needed to know down the larger ground-launched weapons. My biggest fear for such a flying destroyer/cruiser plane would be direct fire from kinetic kill weapons. It would be asking an awful lot to both carry a super defensive package and an offense package to dominate as well.