What is the max load the SCA can carry? Can it ferry with a fully loaded payload bay in the orbiter ( +/- 50k lbs ).
It isn't weight limited but range limited. The heavier the orbiter, the shorter the range.
True, but 'up to a point, minister'.
I'm working from other transport aircraft but I strongly suspect that the SCA is no difference.
The baseline is the Payload – Range Chart, which gives you the relationship between payload and range for a reasonably optimal flight profile.
On that chart there will be a region where you have filled the tanks and you can go no further than that fuel will take you, no mater how much payload you offload.
There is _usually_ a region where you trade fuel and payload, leading to less payload = more range, as Jim says. The relationship can be complex but to a first approximation you have a fixed maximum weight determined by the strength of the wings or the maximum weight the undercarriage can take.
Then there is _usually_ a region where you can add no more payload because the undercarriage can't take the impact of landing above that weight. In this region you can't gain anything by flying a shorter distance with more payload and taking less fuel because for maximum payload you will plan to land with as little fuel as you can get away with so the limit is determined by the payload.
That's the basics. But other limits also come into it. A key one is how much reserve fuel you have to carry to ensure you can land somewhere safe should you have to divert en-route. Sometimes a less direct route can mean more payload because you fly nearer airfields you can land at and so you need less reserve fuel. You also need more fuel if you are flying into a head wind.
Then the airfields can add limits. On landing your weight might be limited by three things – the maximum impact the runway can take from a landing aircraft, the maximum weight the taxiways can take and, sometimes, the length of the runway. The runway length is a limit because the distance to stop the aircraft varies with weight and you need enough space to stop, then some allowance for errors and failures. Most of the time the length of the landing strip isn't an issue because you need a lot more on take-off
On take-off the taxiway limits apply and the runway has to have enough space for you to accelerate to flying speed – which gets larger the heavier you are. Then you need to allow extra space to stop in should you abort the take-off
On top of all this is the balance issue. An aircrafts centre of gravity is a precious thing and unless you can distribute that heavy weight properly it isn't going to be flown.
Finally there is the payload support problem. In SCA terms you have to be user that none of the pylons are overstressed, on take-off or during manoeuvres In more typical transport aircraft you have to be sure that the tie-down chains aren't overloaded and the point loading under the tyres of a vehicle isn't to high.
I'm not actually involved with real flying but I model transport aircraft lift capability as part of my job. The considerations above are why I get a real flight planning cell to give me maximum payloads for various aircraft and trips rather than trying it for myself and why I laugh when someone asks for the 'maximum payload' of an aircraft – there is a maximum but its rarely possible to use it.
I've seen people take the 'maximum payload' of an aircraft and design a load to that limit. Then they act all crestfallen when you point out that it can't get to where its needed because the distance it can be carried is less than the distance they need to fly over water.
Rick