The Hague Family

Aircraft Nose Cones

Also known as "those pointy things at the front".

My working life was spent in the Defence Industry, specifically British Aircraft Corporation, that changed to British Aerospace, then BAe Systems, before my division was sold to Chelton, which became Cobham - so I had a few company name changes during my 34 years of sitting in the same building. My first job was to Production Engineer the Milan Anti-Tank Missile Launch Tube, followed by the Tornado Nose Cone (two variants), technical a radome, plus a few other radomes, before finishing with the before finishing with the Typghoon (Eurofighter) Nose Cone Radome.

So what precisely is a radome? It is a cover, made from radar transparent materials, that is fitted over a radar transmitter/receiver. They are fitted to aircraft, ships and missiles. They provide protection to the transmitter electronics from the weather and other elements (rain, hail, cloud, sand) and often require to be specially shaped for the application. A ship borne radome usually does not need to be resistant to sand (up to a point) and probably does not need to be aerodynamic, but an aircraft radome, and particularly one fitted as a nose cone on the aircraft, usually do need to be aerodynamic. Equally, aircraft, especially military jets, are festooned with radomes and radar sets. The Typhoon has radar transmitters covered by a radome on the nose, on the root leading edge of each wing, in pods on the end of each wing, in the top of the tail fin, just in front of the cockpit canopy, and just behind it, in blisters on the fuselage and so on. Each performs a different job: terrain following; weather; threat detection (has the plane been illuminated by a ground-based anti-aircraft missile system, or an in-flight missile homing device?); enemy target detection and acquisition; guidance to a missile fired by the plane; identification friend or foe (IFF); transponder; and more. Though you may think it is just a cover to the electronics, the cover must not degrade the transmitted and reflected microwave electromagnetic energy (the radar beam) that does the particular function of the antenna.

The best way to think of a radar beam is to think of it as a beam of light. Light is a very specific part of the electro-magnetic spectrum that out eyes are able to detect. Radar is a differnt, specific part of that same spectrum, that very fortunately, our eyes cannot detect. Just as well, since if we could, we would be blinded by the vast swarms of such radiation that envelop us throughout our daily lives. Mobile phones, Wi-Fi signals, television signals, satellite signals, and even the Cosmos are all flooding the surface of the Earth, and the individuals wandering over it, with electro-magnetic radiation that we cannot see, but can be detatected by the appropriate electronic equipment. Is this actually safe for humans? In my opinion, we have evolved over many millions of years, so natural radiation is probably not harmful.It is pretty low level anyway. Recent medical developments have created X-Ray machines, but their use on humans is carefully controlled, and the operators take refuge behind a screen when they take the picture. But as technology has increased, so man-made radiation has multiplied many, many times. And when you make a telephone call on a mobile, you are putting a radar transmitter in very close proximity to your brain. There is still a strong debate about just how dangerous, or not, this really is. And even when the mobile is in your pocket, it is continually "chatting" with the nearest antenna using electromagnetic radiation to do so. Aircraft radar stets, for example, transmit their radiation, or radar beams, at a much greater intensity than your mobile phone. I would not stand in front of an aircraft nose cone IF it's radar set is switched on. If you are young and are looking to one day start a family, I would advise you didn't either.

All this is by-the-by. This page is supposed to give you the low-down on the nose cone of an aircraft, which may be just a little more complicated than you might originally have supposed.

As I have said, a radar beam is very analogous to a beam of visible light. We use much the same terminology to describe things realated to them - a beam of light illuminates an object, a radar beam illuminates a "target" (not necessarily a threat, airport radar track commercial aircraft in the sky); visible light gets reflected by objects in its path (which allows us to see them), a radar beam gets reflected by the target allowing the reciever to "see" the object, and so on.

So lets think of a radar beam and the things that affect it in a similar way to a beam of light. If you think of a building, they most usually include a "window" made of glass, a material which is transparent to visible light. In the same way, a cover over a radar emitter must be transparent to the radar beam. If you shine a toirch (a visible light emitter) at a window, particularly at night when there is l;ess light coming in from outside), you will see a reflection of the torch face in the window. Therefore, some of that visible light FAILS to pass through the window glass and is reflected back to you. This means there is less illumination passing through the window to illuminate the object outside. This can be considered as a loss of power of the light beam. Even worse, the amount of the beam that reaches the target, and that is reflected back, is not 100%, there is always some scatter at the surface of the illuminated object, another power loss. And finally, the reflected light coming back to you has to pass through the window again, suffering a reflection again and a further loss of power, before it finally reaches your esyes, and you can see the obejct outside the building. Even this is a simplification, do you remeber your school Physics experiment passing a beam of light through a block of glass? The reflecti0on occurs at a change of density of the medium through which the beam of light is passing, so it not only reflects off the front surface of the pane of glass (air to glass), it also reflects off the back face of the pane (glass to air), and on both journeys - out ward and inward. So shining a torch at an object the other side of a pane of glass is not going to be super efficient in terms of allowing you to see, that is to collect the reflected light that has passed through the glass twice, in your eye. But we live with it, what else can we do except go outside to look at the object. The same things occur to the radar beam passing through the radome cover on its outward journey, illuminating the target and being scattered, some being reflected back to pass through the cover again before finally reaching the receiver, or detector. But the option of the detector moving outside the cover is denied the aircraft nose cone radar set.

You may start to suppose the whole idea of a radar beam in this situation to be very hit and miss. Well, there are things we can do to help the situation. The only thing is, that they compl;icate the design, and therefore the manufacture techniques, of the radome cover immeasurably, compared to the design of a pane of glass. Lets look at a few:

The Radome Materials. Thinking of the pane of glass, the materials must be transparent to visible light; be stiff enough to be fitted to a window frame without warping over time; be strong enough to withstand the impact of rain and hail, and these days, a stone thrown by a bad person. Reducing the reflections of visible light is employed in some specialist glass, but not in "normal" window glass. Quite frankly, it is not really necessary. Thinking of the radome cover, the materials must be transparent to radar beams; be stiff enough to not sag or warp especially if the aeroplane is performing a violent manouver such as may be needed in a military jet aircraft; be strong enough to withstand the impact of rain, hail and sand (which at speeds in excess of Mach 2 are siginicantly damaging to any structure) plus some other objects we will come to; be aerodynamically shaped to provide a minimum of air resistance (again, particularly in fast jets whose radomes are invariably pointed, compared to commercial airliners whose nose cones are more rounded, the reason why will come along soon); and last, but not least, and for once different to the beam of light, must not significantly reduce the power of the radar beam, outward and inward.

This latter point is very important. The more power a radar set emits, the hotter it gets inside the radome risking degradation, and the more dangerous the beam emitted is to humans in close proximity. But a powerful radar beam is better for "seeing" further away. A military jet whose radar can detect an enemy at 30 miles distance is far greater technically than one that can only detect the same threat at 20 miles distance, and much safer to the pilot. The further to see, the more power is needed after passage through the radome, so designing it to reduce those boundary reflections is a good thing. And get it optimal for the outweard journey, it is then optimal for the inward as well. But we have said that Physics dictates the electro-magnetic beam is reflected at a boundary between materials of differing density, right? So what can we do to address that?

The answer is a quirk of wave action that I cheerfully admit makes absoolutely no sense to me at all. Waves, and electro-magnetic radiation is a wave (though some physicists insist light acts like both a wave and a particle), interact with each other. The usual example is a couple of pebbles tossed in a calm lake, but i cannot see the effect very easily myslef. To try explain it, tow waves interacting sum their amplitudes to make a single "wave". If the two waves are synchronous (that is, are of the same wavelength, same amplitude, and their shapes are exactly overlaid on each other), the resulting single wave is the same wavelength, but the amplitude is doubled. That is, the energy of the two waves has produced a single wave of double the energy. OK, I can see that. But what I have trouble with is this: if the two waves are exactly 180° "out of phase", that is the peak of one wave is exactly overlaid on the trough of the other, and they are of the same wavelength so this out of phase is continuous, the two waves cancel each other out. The resulting "wave" is, well, of no amplitude, and therefore of no power. How two positives can add up to zero is what I cannot handle. Because even though the waves are out of phase, "power" was still needed to generate them. But with waves, they do. What a wonderful world God created for us!

Radome designers were quick to sieze on this strange quirk, and use it for their own ends. When a radome beam encounters a denser material (the radome), some of the beam is passed through, some is reflected. The percentage of the beam that is reflected depends on the materials making up the radome; I'll come to that aspect later. The reflected beam is also shifted 180° in phase. The image shows a radar beam hitting a radome, and some being transmitted through and some is reflected. Look at the phase of the reflected beam. Now the part of the beam that enters the radome is NOT phase shifted, but the amounbt of the transmitted beam that is reflected by the second surace, where it encouneters a boundary of less density, is phase shifted. Naturally, this reflected beam gets partially reflected by the first surface, but since (hopefully!) the power of these reflections is getting much less, we can ignore that complication for now. So if 90% of a beam is transmitted through the first boundary, and 10% is reflected by the second boundary, only 1% gets reflected the second time, so we can effectively ignore the second reflection. Radar designers realised that since 90% of that 10% reflected energy from the "outside" boundary will pass through the "inside" boundary, if they can get the phases precisely 180° out-of-phase, that effect of the two waves cancelling each other out can be used. The net effect is that the two beams, one reflected from the first boundary, one reflected by the second boundary and passing through the first boundary, will not have the same amplitude (since the first reflection is 10% of the original power, and the beam emerging from the inside surface that was reflected by the outer surface is 90% of 10% of 90%, i.e. 8% of the original beam power) the beams do not cancel out. Buit it is close: 10% of the initial beam was reflected by the first inner boundary, and 10% of the 10% of the 90% = 8% was reflected by the second boundary and able to pass through the inner boundary. If the phases are exactly 180° apart, the net power of the wave reflected by the inner surface is just 2%! So 10% of someting, plus 10% of 10% of 90% of the same thing sums to 2%.

For a rigorous explanation of what is happening, check out Wiki or search the net for "phase change of electromagnetic radiation encountering a denser medium".

OK, that is all very interesating, but how did the radome designers make use of the principle? In order to make the emerging wave exactly 180° out of phase with the wave reflected from the first boundary, means the distance between the first and second boundaries must be exactly half the wavelength of the beam. Visible light has a wavelength of around 1 µm. To make a pane of glass utilise the physics of wave addition just described would require the thickness to be a multiple of that, and ideally 1 µm. Not very much, and mass producing panes of glass to tight thickness tolerances is hugely expensive, and totally unjustified! The wavelength of the radar beams used in military applications is almost universally graded as "TOP SECRET". If your enemy knew the frequency (which is a function of the wavelength) of your sophisticated radar equipment, he can transmit "noise" on that same frequency and you are, well, buggered actually. Although this is the most effective way of "jamming" an enemy radar, there are others: "Window" used by both sides during the Second World War, for example. Obviously, the radome designer needs to klnow the value, so he can design a radome with a thickness equal to half that wavelength.

The relationship between frequency and wavelength uses the speed of the wave, which for electromagnetic radiation is the speed of light: "c" = 299,792,458 metres per second. If the wavelength is ten micro metre (10 µm or 10 x 10-6 metres), then the wave performs one wave cycle over that distance. The time to cover that distance is ten micro metres divided by "c" which is around 3.34 X 10-14 seconds. Put another way, in one second the wave will make one divided by 3.34 X 10-14 cycles, which is the frequency of the wave, which comes out as 29,979,245,800,000 cycles (Hz) more usually expressed in THz (tera hertz). This is 29,979,245,800,000/1024/1024/1024/1024 = 27.27 THz. The Wikipedia page approximates this to 30 THz.

The relationship equations in simple English are:

F (frequency in cycles per second) = c (speed of light in metres per second) divided by W (wavelength in metres). This is the number of wave cycles performed by a wave travelling at the speed of light and whose single wave cycle is W metres long.

W (wavelength in metres) = c (speed of light in metres per second) divided by F (frequency in cycles per second). This is the length of a wave travelling at the speed of light and performing F wave cycles per second.

As with everything in our man-made world, absolutely NOTHING is perfect. I was once asked to Production Engineer a new radome, designed by a Microwave Engineer straight out of college. One of his specifications stated "Wall Thickness = 10.6 mm". I asked him what tolerance, and he said (with an astonished look on his face): "No, that's what it needs to be." I explained that with no tolerance, he had actually specified the thickness to be 10.6000000000... with zero recurring an infinite number of times. At some point, a single atom would make the thickness exceed that value. He was thus introduced to the concept that the real world is not perfect. Not only that, specifying a tolerance which was not practical for a human being to reproduce, either manually or in tooling, was an equal waste of time. However expert our laminators or resin transfer moulds were, no person, as well as no thing, is ever perfect.

The above cautionary tale is the first in many things which mean the microwave engineer does not get a finished product that is precisely as the antenna emitting frequency would dictate it needs to be. Look at this list:

• As stated above, the thickness achievable in hand lamintaing GRP with resin on or in a form is pretty broad. It depends on how skilled the laminator is, and what variations there are in his materials:
• The thickness of the glass fabric can vary, which means the thickness of the finished product made by laminating x number of glass layers which would nominally get as close as possible to the desired overall thickness will also vary. [I did have a Structural Design enginer who once told me the build had to be 2.4 layers thick. Think about it - he hadn't.] Also bear in mind that military radomes are usually low quantity. Boeing sell hundreds of 737, we sell relatively few Typhoons. So when you go to a manufacturer of glass fabric for a military radome, you tell him he must use a specific, very high grade, of raw glass filament that is transparent to radar waves (which he has to source and stock probably solely for your requirement, so in awkward small quantities). For a glass filament based fabric, it must be "sized" in order for the resin to adhere to it. The supplier of the raw filament needs to apply this size (a chemical which adheres to the fibre and permits the particular resin to adhere to the size) to a controlled thickness and then "cooked" to fix it, the chemical make up of which is dependent on the resin and glass itself. The resin is also usually highly specific to be radar transparent, so the size in chemical form has to be purchased by the filament maker in small specialist quantities, and stored which he doesn't like either. He then has to apply it and fix it to a small (in his terms) quantity of fibre before he can deliver it to the glass fabric weaver. The weaver has to set up his loom for a small (in his terms) production run, which he doesn't want to do - setting up looms is not creating saleable product, well, not directly. Every time any of these suppliers do something which occurs intermittently, and with small quantiities involved, there gets to be a variation in the product. So the composition and thickness of the raw drawn filament, and the composition and thickness of the size on it, and then the weave density of the fabric, and the thickness of the fabric, are all more variable in high-tech military applications than they are in low-tech commercial applications, where the quantities are much larger and more economic. Needless to say, one of the ways all these suppliers got back at us was to to charge the absolute Earth for our "specials". In many instances, this was a blatant attempt to get us to go elsewhere and save them all the hassle of these awkward orders, but more often than not, there wasn't any elsewhere we could go to.
• The resin suffers from the same variables as the glass. The raw materials are often exotic to be radar transparent, and we order small quantities. A yearly order from us of one kilogramme for example, compares to the resin manufacturer's standard order for a commercial resin of a number of tonnes. The stills have to be set up, the exotic raw materials bought in small quantities, and the recipe weighed out again in small quantities. Imagine making 100 tonnes of a resin, 30% of which is component "A". You weigh out 30 tonnes of the component, or more precisely, the maximum quantity your still can handle, easily a tonne or more. So you would make 30 batches. The variation in component A is the accuracy of your one tonne scales or whatever, and in percentage terms on the whole batch, is pretty insignificant. Then you get an order for 1 kg of one of our resins, 30% of which is component "B". You need to weigh out 300 grams of the stuff. Granted, you wouldn't use a scale of 1 tonne capcity (with divisions of probably around 200 grams at best) but even then, a specialist scale of 1 kg capcity is likely to be accurate to 10 grams or so. 10 grams is 5% of the required quantity, so already there will always be a 5% variation in the content of the resin mix. And it applies to all the componets in the mix. All are exotic, and therefore are already variable, and the quantities used in the small resin mixes are variable, so the resin we get out of stores to make our radomes is also variable. On top of that even, the resin is liquid and stays that way until we, the manufacturer want to use it. We then have to weigh out a comparatively small quantity of the raw resin, plus the appropriate quantities of hardener, cayalyst and accelerator, in even smaller ratios (often 100:4:3:2). Obviously, we use the most accurate scales we can find for the capacity (maximum weight) we want to weigh, bit its just another variation. Incidentally, it was not possible to buy the whole component order quantity of resin, or fabric, to help out our suppliers a little, since both materials have a "shelf-life" beyond which you cannot use them. Or if you did, the component would most likely fail specification and be scrapped. So if the shelf-life was a typical six months, we only bought what we calculated we would use in the six months. If we underestimated, production was stopped until we could persuade all the suppliers to do another awkward batch for us; if we over estimated, we threw away incredibly valuable stock we had paid for. Stock control was real FUN in our business, especially before computers when it was all manually hand written on paper (remember that?)
• So the number of variables which effect just how precisely the final component, the radome, conforms to the specification is often questionable. When making the Tornado ADV nose cone radome, for example, to achieve the 186 final quantity required thew build of 232 radomes, the others being scrapped due to manufacturing defects, about 65% of those being variations in the materials (as our best guess), the rest tooling malfunctions and the occasional very rare, operator error. The cost of the materials was well into the tens of thousands of pounds, and the labour to build it not much less, so the cost to us as a company of a scrapped nose cone radome was dreaful. The only way we could survive was to pass that cost onto our customer by way of including a scrap allowance in the unit price, which eventually fell to the MOD, and finally the tax-payer. You. Even then, the company Board, and especially the money men, were not at all pleased to hear that we produced another scrap unit.
• OK, but the Microwave Enginner was not the only person defining the materials to be used, the tolerances to be applied to their manufacture by our suppliers, and so on. Once the Microwave Engineer had designed his "best" radome within all these limits, along came the Structural Engineer. The Microwave Engineer was only concerned with one thing: that the degradation to the radar beam by its passage through the radome wall on the outward and inward journeys was only minimally degraded, and then in a consistent fashion for every radome so fabricated, that he could measure and allow for in the set up of the radar antenna underneath the radome. The Structural Engineer cares nothing for this. He has to design a radome nose cone that fits onto the aircraft with a reliable opening mechanism to allow radar engineers access to the works underneath, and stays on the aircraft despite whatever violent manouvers the plane makes. It also has to be strong enough to withstand the impact of the various things that do impact aircraft in flight - and also on the ground. [Diversion: two examples of ground damage. (1) We were once asked to visit an American airbase (I'll leave it nameless). Its F14s had been carefully lined up side-by-side on the taxiway for a high profile visit political. Afterwrads, a bored aircraft fitter decided to walk along the line and at each aircraft, jump up and swing on the metal pitot tube protruding from the nose cone, cracking the resin in the nose where the pitot entered, and bending them. All of them. The yanks wanted to know if we could do a rapid repair to them all as the squadron was suddenly heavily under-manned, despite using all the spares they had! We couldn't, I don't know what happened eventually, either to the damaged radomes, or to the un-named aircraft fitter. (2) In the days when Concorde was flying, British Airways had a huge maintenance hangar at Heathrow for them. One day,a Concorde was backed into the hangar, and the fitter closed the big hangar double doors as it was a bit chilly with them open. Unfortunately, the plane was not far enough in, and the doors closed firmly on the nose cone each side with a resounding "crunch". In this case, there were no spares, and a Concorde AOG "Aircraft On Ground" was a very expensive commodity earning nothing for BA. As we had originally manufactured ALL the Concorde nose cone radomes (BA and Air France) we eventually were able to repair the thing, sadly just in time for the awful Paris crash.] Enough of these diversions. You can bet your bottom dollar that the Microwave Design would be totally unacceptable Structurally. And, naturally, vice versa. So these two experts threw slightly modified compromised versions of their own designs at each other for analysis, until a mutually acceptable solution was found. Usually, with quite audible mutterings about how much they had each had to deviate from their ideal design to suit the other.
• Sort of tied in with the Structural Engineer was the Aerodynamicist. The former had to make sure the thing stayed on the aircraft under all conditions and withstood everything concievable thrown at it, but the Aerodynamicist wanted it to be a wonderful stream-lined extension at the front of the fuselage. At last, the POINTY THING! But if you remember the Microwave Design needed the radar beam to impinge on the inner surface as near normal to that surface as possible, otherwise the reflected waves may be correctly out of phase but not overlap each other, so the reduction in power loss was, itself, not as effective. To the Microwave Engineer, the "best" shape for a radome is hemi-spherical. He can get away with it on ship borne radar sets, for example the Type 42 Naval Destroyers with the "14 Foot Radomes" front and aft we made to house the Marconi Missile System, but designing such a shape for the front of a military jet aircraft, he is on a loser. Military jets have to fly fast (have an aerodynamic nose), as well have a working radar (more bulbous nose), and again the design is generally a compromise. Those aircraft that do not need to fly supersonically for example, such as commercial aircraft like the Airbus A300, or military transport aircraft such as the Hercules, have much more bulbous nose cones than military fighter jets such as the F14 and Typhoon.