I've been talking to a friend, and he said that it's impossible to travel at exactly the same speed as the speed of sound is. He argued that it's only possible to break through the sound barrier using enough acceleration, but it's impossible to maintain speed exactly equal to that of sound. Is it true? And if it's true, why?
[Physics] Is it possible to travel at precisely the speed of sound
acousticsfluid dynamicsspeed
Related Solutions
According to the MTU webpage Speed of Sound in Air, some things to consider:
if the ideal gas model is a good model for a real gas, then you can expect, for any specific gas, that there will be no pressure dependence for the speed of sound. This is because as you change the pressure of the gas, you will also change its density by the same factor.
However, the atmosphere is a mixture of chemicals with some varying, especially water vapour (hence, humidity), hence not an ideal gas - from the Wikipedia Speed of Sound page:
Sound speed in air varies slightly with pressure only because air is not quite an ideal gas.
This variation in speed of sound according to the MTU webpage is extremely minimal at most (see vertical scale below) for the range of atmospheric pressures at low humidity and a slight bit more pronounced at extreme humidity (see graph below):
Compositional changes (mainly water vapour) and especially air temperature has the greatest effect.
If the tips of the propeller blades are moving near the speed of sound a shock wave can form. Supersonic flow has very different character than subsonic. A propeller designed to operate at subsonic speeds will be inefficient at supersonic ones due to shock waves. In general, shock waves cause a loss of efficiency. You might have noticed that subsonic airplanes often have swept wings. This is because flow over the top of the wings accelerates and can reach supersonic speeds even when the aircraft is traveling well below the speed of sound. Sweeping the wings decreases the normal component of the velocity relative to the wing. Likewise, you will sometimes see helicopter blades with the tips swept back. To avoid supersonic flow at the inlet of turbine blades you will observe an increase in area from where air enters the engine to when it reaches the blades. As long as the flow entering the engine is subsonic, the increase in area will slow down the speed of the fluid and avoid supersonic flow over the blades.
Best Answer
It's true that at the speed of sound, you will have a huge amount of drag. The reason is that the air in front of you has to move out of the way, and if you are moving at the speed of sound, the pressure wave that pushes the air out of the way is moving at exactly the same speed as you. So in the continuum mechanics limit, you can't push the air out of the way, and you might as well be plowing into a brick wall.
But we don't live in a continuum mechanics universe, we live in a world made of atoms, and the atoms in a gas bounce off your airplane. At the speed of sound, you get a large finite push-back which is a barrier, and above this, you still have to do the work to push a mass of air out of the way equal to your plane's cross section with ballistic particles.
As you go faster, the amount of drag decreases, since the atomic collisions don't lead to a pile-up on the nose-cone. But if you look at wikipedia's plot here, the maximum drag at the supersonic transition is only a factor of 2 or 3 higher than the drag at higher supersonic speed, so it is possible to travel at Mach 1, it is just not very fuel efficient.