Ballistics and astronautics
At first glance it may seem that space flight (Fig. 13A, overleaf) is purely concerned with ballistics and is completely divorced from aerodynamics, but as we hinted in Chapter 1 at the speeds and heights of modern flight aeroplanes behave to some extent like missiles, and missiles, given the required thrust and speed, can actually become satellites. Moreover, both missiles and artificial satellites must pass through the earth’s atmosphere when they are launched, and there experience aerodynamic forces, and – what is far more important – the most hopeful means of returning them safely to earth again is to use aerodynamic forces to slow them up and control them.
In short, the subjects of aerodynamics, ballistics, and astronautics have merged into one subject, the mechanics of flight, and no apology is needed for the inclusion of this chapter in a book on that subject.
The upper atmosphere
The atmosphere with which we have been concerned in the flight of aeroplanes – i. e. the troposphere and the stratosphere – is sometimes called the lower atmosphere; the remainder is called the upper atmosphere (Fig. 13.1, overleaf).
In the lower atmosphere the temperature had dropped from an average of + 15°C (288 K) at sea-level to —57°C (217K) at the base of the stratosphere, and had then remained more or less constant. The pressure and density of the air had both dropped to a mere fraction of their values at sea-level, about 1 per cent in fact. One might almost be tempted to think that not much more could happen, but such an assumption would be very far from the truth.
There is a lot of atmosphere above 20 km – several hundred kilometres of it, we don’t exactly know, it merges so gradually into space that there is really no
Fig 13A Shuttle lift off (By courtesy of NASA)
exact limit to it – but a great deal happens in these hundreds of kilometres. The temperature, for instance, behaves in a very strange way; it may have been fairly easy to explain its drop in the troposphere, not quite so easy to explain why it should then remain constant in the stratosphere, but what about its next move? For from 217 К it proceeds to rise again – in what is called the mesosphere – to a new maximum which is nearly as high as at sea-level, perhaps 271K; then, after a pause, down it goes again to another minimum at the top of the mesosphere. Estimates vary of just how cold it is at this height (only 80 kilometres, by the way, only the distance from London to Brighton), but all agree that it is lower than in the stratosphere, lower, that is to say, than anywhere on earth, perhaps 181К ( —92°C). But its strange behaviour doesn’t stop at that and, once more after a pause at this level, as the name of the next region, the thermosphere, suggests, it proceeds to rise again, and this time it really excels itself rising steadily, inexorably to over 1200 К at 200 km, nearly 1500 К at 400 km, and still upward in the exosphere until it reaches over 1500 К at the outer fringes of the atmosphere.
Fig 13.1 The upper atmosphere
The figures given are based on the US Standard Atmosphere, 1962, which was prepared under the sponsorship of NASA, the USAF and the US Weather Bureau.
An interesting point about these temperature changes in the upper atmosphere is their effect upon the speed of sound which, as we learned in Chapter 11 , rises with the temperature, being proportional to the square root of the absolute temperature. The interest is not so much in the effects of this on shock waves, or on the flight of rockets, but rather in that one method of estimating the temperatures in the upper atmosphere is by measuring the speed of sound there.
While these strange and erratic changes of temperature have been taking place the density and the pressure of the air have fallen to values that are so low that they are almost meaningless if expressed in the ordinary units of mechanics; at a mere 100 km, for instance, the density is less than one-millionth of that at ground level.
It is believed that at these heights there may be great winds, of hundreds, perhaps even a thousand kilometres per hour. The air above about the 70 km level is ‘electrified’ or ionised, that is to say it contains sufficient free electrons to affect the propagation of radio waves. For this reason the portion of the atmosphere above this level is sometimes called the ionosphere, which really overlaps both the mesosphere and the thermosphere. Then there are the mysterious cosmic rays which come from outer space, and from which on the earth’s surface we are protected by the atmosphere, but beyond this we know very little about them except that they may be the most dangerous hazard of all since they affect living tissues. Then there are the much more readily understandable meteors, ‘shooting stars’ as we usually call them, but actually particles of stone or iron which have travelled through outer space and may enter the earth’s atmosphere at speeds of 100 kilometres per second, and which have masses of anything from a tiny fraction of a gram up to hundreds of kilograms. The larger ones are very rare, but some of these have actually survived the passage through the atmosphere without burning up, and have ‘landed’ on the earth causing craters of considerable size – these are called meteorites.
To prospective space travellers all this may sound rather alarming, but there are some redeeming features. The winds, for instance, wouldn’t even ‘stir the hair on one’s head’ for the simple reason that the air has practically no density, no substance. For the same reason the extreme temperatures are not ‘felt’ by a satellite or space-ship (what is felt is the temperature rise of the body itself, caused by the skin friction at the terrific speeds; it is this which burns up the meteors, it is this which has eventually caused the disintegration of many man – launched satellites on re-entering the atmosphere – but all this has little or nothing to do with the actual temperature of the atmosphere). Then, as regards the very low densities and pressures, no-one is going to venture outside the vehicle, or walk in space, or even put his head out of the window to see whether the wind stirs the hair on his head, unless he is wearing a space-suit, and we have long ago learned to pressurise vehicles because this is required even for the modest heights in the lower atmosphere. Moreover, the strong outer casing of the vehicle which is required for pressurising will in itself give protection at least from the small and common meteors, and to some extent even from the cosmic rays, the greatest unknown. So altogether the prospect is not as bad as it might at first seem to be.