Surely, a much cheaper and readily achievable option would be to run a tram-rail service every 15 mins from Doncaster Station to the airfield boundary and then some form of relay to the terminal? Its not like the pre existing line is busy.......
There are track capacity issues at Doncaster station, which explains why the line has such a limited service.
The half-hourly X4 bus only takes 22 minutes, and I've never seen it with more than five people on it.
It is usually cheaper, quicker and more convenient for passengers at DSA to share a taxi, get a mate to drop them off or even park there - apparently very reasonable.