Flight at hypersonic speeds
In concluding this subject we must not leave an impression that once we have conquered the problems of supersonic flight, we have finished. Far from it – no sooner do we learn to get through one region of speeds than another, with new problems to solve, opens up before us.
Rather strangely, too, there does not seem to be much argument about where the change takes place in this case – above a Mach Number of 5 we talk of hypersonic speeds instead of supersonic speeds. It doesn’t pay to try to see the sense of this rather extraordinary use of the English language, or perhaps we should say of Tatin and Greek, which makes hypersonic superior as regards speed to supersonic; nor shall we get very far if we try to discover just why the Mach Number of 5 is so significant; actually it is a number of different factors that decides the issue, and that is far beyond the scope of this book.
To us hypersonic flight is simply supersonic flight – only more so. Here we are deep into kinetic heating effects with all the associated problems; at a Mach Number of 5 at about 61 km the temperature given by the formula is over 1000°C and by Mach 15 it has risen to over 10 000°C. The actual temperatures are found to be rather lower, but not so much lower as to give any real comfort!
Mach Tines are inclined at very acute angles; Fig. 12.27 shows the shock waves and Mach Tines over a double-wedge section at a Mach Number of 10, and Fig. 12.28 is sketched from a photograph of a bullet moving at the same Mach Number. These suggest an arrow type of aircraft as being most suitable (Fig. 12.29).
One feature of hypersonic flow is a thickening of the boundary layer and an increased importance of the nature of the flow within the boundary layer.
Aerofoil shape seems to matter even less than in supersonic flight; lift and drag coefficients tend towards a constant value as the Mach Number increases.
An interesting aspect of this part of the subject is the wide variety of experimental methods used to investigate it – arc-heated jets, gun tunnels, shock tubes, shock tunnels, hot shot tunnels, models moved by rockets or guns, and ballistic ranges; names that are all to some extent descriptive of the methods
Fig 12.27 Hypersonic shock and expansion waves
Fig 12.28 From a photograph of a bullet moving at Mach 10
Fig 12.29 Arrow-head: shape of the future?
employed, each of which would need a chapter to itself. Mach Numbers of 15 can be achieved in still air on ballistic ranges, and even more if the projectile is fired against the airflow in a wind tunnel.
But even hypersonic flow is not the end; at Mach Numbers of 8 or 9 something entirely new begins to happen – molecules, first of oxygen, then at even higher speeds of nitrogen, dissociate, or split into atoms and ions, thus changing the very nature of the air and its physical properties (another phenomenon that is experienced by space-ships on re-entry into the atmosphere, and which affects radio communication with them). At this stage there are possibilities of the control of the flow by electro-magnetic devices.
And so it goes on. Nothing, so far, suggests an end. Figure 12.30 shows how speed records have gone up – and up – and up.