Why Couldn't a Double-decker Concorde Be Made?

The passenger market for the services of Concorde was minuscule. Concorde consumed horrific amounts of fuel to travel at supersonic speed and that fuel consumption was reflected in the ticket price that was many times that of a trans-Atlantic flight aboard a Boeing 747To reduce horrific fuel consumption, it was important to minimize the cross-sectional area of Concorde. A double decker version of Concorde would have greatly increased cross sectional area and greatly increased fuel consumption. The aircraft might have required an additional 2-engines, larger wings and larger fuel tanks. If the airline could have filled the seats, ticket prices might have been marginally lower.A double decker Concorde built during the late-1960s to early 1980s era would have been a major market risk, especially in view of the drastic increases in oil prices that occurred during early 1970s and again during early 1980s. Why could not a double-decker Concorde be made?

1. When a jet is flying at supersonic speed, how come an observer doesn't hear the sound waves from when the jet was travelling at sub sonic speeds before the jet passes?

Because the jet is going faster than the speed of sound... so they have passed the sound waves they made when they were going below the speed of sound

2. If the flow rate of water down my sink is limited by the conservation of mass, why doesnu2019t the water flow velocity increase to sonic or supersonic speed?

It is possible this is the most astonishing question I have read on Quora (so far...there's time in 2019). The mass of water will stop flowing out the instant the sink is empty, not a gram more nor less (conserving mass), and its rate is limited by its viscosity, the orifice through which it flows and the force applied.If you had an amazing sink with a "sudden open" drain GTE the diameter of the sink, the water would still not flow out at the speed of sound, even if "pushed" by 14. 7 psi at the top and pulled by a vacuum at the bottom.Screwing around with this notion, if the force at the top were a piston that accelerated the water through a pipe the size of the sink, and the sink held 10L of water, with a drain of 1000 cm^2 opening, the 10L would need to be driven to a velocity of 2.9 km/s, emptying the 10 ml depth. F=ma, m=10kg and a=dv/dt, with dv=2.9/s and dt to be imagined. So if a=9.8 m/s^2 (1G force) 2.9km/s takes about 300 s with no resistance (let it be so). OTH, if you hope to have it happen in just 1 sec (dt), the force will be 300 times larger so 2900 m/s^2 = a, and to move 10kg to that speed requires 2900 m/s^2 * 10 kg = 29,000 kg-m/s^2, 2.9x10^4 Nt29,000 newtons is the force of 6 elephants on the piston to empty your sink. It's not happening imo, and in any case has nothing to do with conservation of mass. If is dangerous to heat water or any liquid in a sealed container in a microwave oven. The discussion was for illustrative purpose only and the reader is advised not to attempt such an experiment without consulting a PE/safety engineer first.If the flow rate of water down my sink is limited by the conservation of mass, why doesn't the water flow velocity increase to sonic or supersonic speed?.

3. Why does a conical structure of vapor appear around a fighter jet when it breaks a supersonic speed threshold?

It is called vapour cones. It is about pressure dropping rapidly in humidity

4. Is it possible that high-speed UFOs spotted by the military utilize electromagnetic energy and mercury to accelerate at supersonic speed?

We must also do a comparative analysis of the similarity of these "TIC TAC" shaped UFOs with the "FOO FIGHTERS" that were ever present during World War II in the air during the United States and allied flights in the war zones of Europe. We did discover that there was a huge batch of elemental mercury present in caves in Germany and/or Poland during World War II and theorists believe they may be connected to the foo-fighters as a form of an ionic thruster propellant.The action of these "tic tacs" are somewhat similar with the foo-fighters of World War II and are at least 70 years technologically advanced. Is it possible that high-speed UFOs spotted by the military utilize electromagnetic energy and mercury to accelerate at supersonic speed?Is it possible the tic-tac-shaped high-speed UFO spotted off the west coast by the USS Nimitz utilized electromagnetic energy to accelerate at such a high rate of speed?