One of my pipe dreams in FGFS is cruising over Mach 5 using a Russian/Soviet aircraft, regardless of altitude. The fictional Firefox aircraft ticks these boxes, with immensely powerful turbo-ramjets and a highly sophisticated avionics suite. My ideas for FlightGear's implementation of the Firefox hypersonic jet is a wide range of liveries, including ones from the Soviet, Chinese, North Korean and allied air forces. Special liveries advertising the Firefox browser and Linux kernel are ideal for highlighting their comparative speed benefits over the competition. Another idea for the Firefox's autopilot is the use of a special variant of IT-AUTOFLIGHT which is specifically modified for speeds from zero to over Mach 5, and altitudes from sea level to it's ludicrously high service ceiling. Extra functions are implemented in the autopilot for in-flight refuelling, spin recovery, interception and formation flight.
The Firefox's engines are high-bypass turbo-ramjet engines with computer control and oversight, enabling the plane to overtake a speeding bullet like a Formula 1 car speeding past a snail. One of the engines' features is the unique intakes to compress the low pressure hypersonic flow to a ludicrously high-pressure subsonic flow before it enters the engine's combustion chamber at speed. The engine behaves like a turbojet at rest and during subsonic flight and as a ramjet when flying faster than sound, with extra turbojet thrust until Mach 2. When the plane is flying at Mach 2 or higher speeds, the engines are operating as ramjets for extra fuel efficiency and power, leaving the gas turbine section for supplementary bleed air, electricity and hydraulic power. My estimation for the Firefox's intakes' performance is the very high 98.5 percent pressure recovery ratio when flying at Mach 3, superior to the American SR-71 Blackbird's 90 percent. The engines are so big that the engines are started by air turbine starters, the prime choice for turbine engines when they're too big for electric starter motors. The air turbine starters depend on the bleed air system to function, so this jet is designed with an auxiliary power unit to satisfy those requirements.
The whitepaper about the Firefox aircraft is available at
http://www.redgiantcreative.com/dl/whitepaper.pdf.
Typical Firefox startup procedure [cold and dark to ready]: Turn batteries on, check warning lights for electrics, switch fuel pumps on, start APU, start one engine, switch APU off, start another engine using running engine's bleed air, turn alternators on, boot avionics up and it's ready to fly.