Keith Willshaw wrote:

"Ian" wrote in message
...



Maybe a bit simplistic but couldn't they equal the transfer rates by just
increasing the bore of the drogue feed pipe? It all ends up in the same
fuel
system (on the refuelling aircraft) so it can't be a pressure constraint?



That means increasing the bore of a longer length of pipe
than on a flying boom. I suspect there are weight restraints
at work here.

And size (hose takes up space, after all), and drag. More importantly, there are
limits on the size at the other end. While large a/c can carry around a large
diameter fixed probe with little effect on performance, fighters and attack a/c don't
have that option. So, for instance, on their Victor tankers the RAF used Mk. 20 wing
pods with smaller diameter hose/drogues (and lower flow rates) to refuel
fighter/attack a/c, and a centerline Mk. 17 HDU of greater diameter hose/drogue to
refuel bombers/transports (which have larger diameter probes than fighter/attack
aircraft, so can accept higher transfer rates, although still not as high as a boom).
A fighter just can't be carrying around such a massive piece of hardware all the time,
especially if it's fixed external (there wouldn't be enough room to house it
internally on a fighter, and no one seems to have tried a bomber-sized extendible
probe).

A boom receptacle, OTOH, is theoretically only limited in diameter by the fuel flow
rates that the a/c's internal fuel piping is designed to deal with; since the flow
rate in A/B is quite high, and since none of this is adding weight/drag outside the
airframe, transfer rates can be much higher with little/no weight penalty on the
receiver. The tanker is paying the weight/drag penalty of the boom, but it's a lot
larger and can afford it.

Guy