"What's the deal with taking off from Brussels?"
That is the M5 300 seat passenger aircraft project. LAPCAT and LAPCAT II. By Lapcat II their only competitor was the M8 German Kerosene SCRamjet, which seemed to have a lower heating load (if you could make a SCRamjet 10s of metres wide work).
It was funded under an EU framework agreement. Brussels paid the bill so they get to choose the reference flight path IIRC it was Brussels Canberra.