THE ACFT WAS BUILT AS A ONE-TIME ONLY ACFT FOR RECORD BREAKING DISTANCE FLYING, PRESUMABLY FOR AN AROUND-THE-WORLD FLIGHT WITHOUT REFUELING. IMMEDIATELY AFTER TAKEOFF, THE PLT RADIOED THE CHASE PLANE THAT HE HAD SOME MINOR PROBLEM & WAS GOING TO LAND. AFTER TURNING FINAL THE PLT REPORTED "SOMETHING BROKE, I'M GOING IN..." THE ACFT CRASHED AT A SLIGHT NOSE DOWN ATTITUDE 1/2 MI FROM THE END OF THE RWY. CONTINUITY OF FLIGHT CONTROL WAS ESTABLISHED & NO EVIDENCE OF PREIMPACT FLIGHT CONTROL WAS EVIDENT. THERE WERE NO DRAWINGS OR DATA AVAILABLE FOR THE DESIGN OF THE ACFT. THE FUSELAGE & EMPENNAGE REPORTEDLY HAD NOT BEEN STATIC TESTED. A BREAK AT THE REAR OF THE COCKPIT APPEARED TO BE IN AN AREA OF POOR DESIGN. THE COMPOSITE STRUCTURE BEHIND THE COCKPIT RAILS LOOKED QUESTIONABLE IN ITS CROSS-SECTIONAL AREA TO HANDLE THE BENDING LOADS IN THIS AREA. AN IN FLIGHT SEPARATION OF THE FUSELAGE AT THE REAR OF THE COCKPUT BY AS MUCH AS 1 INCH WOULD HAVE PLACED THE STABILIZER IN A 3 DEG NOSE UP ANGLE RENDERING THE ELEVATOR INSUFFICIENT TO HOLD THE NOSE UP.