I am told that even using the MRA4 development aircraft as 'flatbeds', rolling on the existing R1 mission kit (Tigershark for Comint and the Marlborough Comms Elint suite), would have resulted in a better all round Sigint solution than is offered by the Rivet Joint, and would have leveraged on the MRA4 support arrangements, making it value for money, too. The main R5 solution proposed was even better, since it packaged together all of the winning kit from 'Helix'.....
And we'd have had modern kit in modern, supportable airframes, rather than on an orphan fleet of antique KC-135As (the three airframes selected were built as KC-135As).