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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 Ám. 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 Ám. 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 Ám 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: