X-313 "SpeedDemon" - The Kerbal Griller

But Kraken damned hot while flying, the cabin's internal temperature reaches easily 400°K (~126°C or ~260°F) with less than 5 minutes of flight at Mach 3.7. But it comfortably flies around Kerbin without sweating (the poor pilot, on the other hand, ends the endeavour soaked in sweat and pretty well done).

She's pretty stable on flight, tough.