How long is falcon 9 transonic for?
It is suggested that 45 seconds after start the speed is 340 m/s at 8 000 m altitude, i.e. the rocket is transonic for the first time. 150 seconds after start the speed is 1 700 m/s at 80 000 m altitude, where separation occurs. The first stage – mass say 105 tons - now moves on by its own momentum for 135 seconds up to 200 000 m altitude being slowed down by
boost back burns and gravity.
285 seconds after start the velocity is 0 m/s at 200 000 m altitude but far out to sea and the first stage flies and drops back to landing zone 1 assisted by more
boost back burns. Maybe between 280 and 290 seconds after start the speed is subsonic for a short while.
The return bit takes 300 seconds, so 585 seconds (9 minutes and 45 seconds) after start the empty first stage - mass now 50 tons - has landed again. There is an
entry burn at around 130 000 m altitude during the return but, I assume the speed is then transonic >1 100 m/s. The
landing burn may take 30 seconds starting at say 17 000 m altitude and the rocket is maybe transonic for at least 20 seconds and subsonic for only the last 10 seconds. All maneouvres/burns are automatic and controlled by computers.
The whole return and recovery is a fairy tale or fraud in my opinion. Reason is
fuel and
fuel consumption. 55 tons of fuel is not sufficient for the proposed
boost back, entry and
landing burns. You need four times more fuel for the return and recovery but then you will not get off the ground. Also the hypersonic grid fins steering the rocket during the return are also a joke. Most of the trip is in very thin air or almost vacuum.