Fairing diameter is one of the more absolute restrictions that shows up in mission planning.

Obviously there's a drag impact at liftoff, but I'd be very interested to see a quantitative analysis as to why a ten-metre-diameter fairing is not a practical object. Has anyone got a pointer to one?

I don't, but it's an interesting question. Suspect that in addition to drag it also has a lot to do with the aerodynamics of the entire stack. If the fairing diameter significantly exceeds that of the booster there's gonna be a lot of chaotic airflow right underneath the fairing during transit through the lower atmosphere which might cause a lot of bad things like vibration, oscillation outside the authority of the stability control system, etc.

Think that the biggest fairing I've heard of was 6m for current boosters, but 5m seems more like the usual max.

There's also more than just drag....you can also move the center of pressure too far forward. In essence, like throwing a dart backwards...it'll want to flip on you.

Drag is a problem though - a first order approximation....a 10m fairing presents 75sqm of x-section drag...whereas a 5m fairing only 20sqm. This URL shows some numbers.....these wouldn't necessarily quadruple ( you would be going slower I'm the thicker atmosphere ) but a different and slower flight profile would also inflict higher gravity losses.

http://gravityloss.wordpress.com/2008/01/1...or-scalability/

NSF is probably a better resource for this type of question:

http://forum.nasaspaceflight.com/index.php?topic=29319.0

Yeah, NSF ain't bad for things like this.

These guys would know:
http://www.ata-e.com/asymmetric-payload-fairing

I think the practical limit is that the fairing itself just gets too massive. But I'm not a rocket scientist.

So let's ask a rocket scientist Any volonteers?

If you read the NSF thread I linked to above, you'll find that the general rule of thumb is that the fairing diameter should be no more than 1.5x the diameter of the core stage. ULA advertises custom fairings up to 7.2 meters for Atlas V, but I'm not sure anyone has taken them up on this. Obviously larger fairings or asymmetrical fairings are possible up to the size allowed by the pad infrastructure, if cost is no object.

In the real world of robotic mission design, any proposal that doesn't use a standard fairing available from the LV vendor with flight heritage is likely to be DOA.