Nightfighting in World War Two
By Lee Brimmicombe-Wood Part 3. Radars in the Sky
Ground control of interception was only part of the solution to effective nightfighting. The Luftwaffe's Dunaja
'dark fighting' system permitted a controller to guide a nightfighter close to a bomber with the aid of radar, but still ground radars gave imprecise information. If the controller was just a few hundred feet astray, the target would be lost in the darkness.
It was clear to all involved in night fighting that fighters had to carry their own radar sets. This way the controller merely needed to get the fighter close enough for its own Airborne Intercept (AI) radar to pick up the target. Then the pilot would complete the final stage of the interception alone. PHOTO: The crew of a Royal Air Force Blenheim night fighter climb aboard for a night patrol. Like many nightfighters in World War II the Blenheim was an adapted light bomber. High performance and agility were not a requirement for night fighting, with the result that twin-engined bombers and fighters that were vulnerable by day, found their metier at night.
AI radar was one of the great technical challenges of the first year of the war. A radar needed to be small enough to fit in an aircraft and powerful enough to find targets a few miles away. Transmission and receiving aerials had to be sufficiently small that they did not adversely affect aircraft performance. The system needed to be operated by the pilot or observer. These requirements, particularly for miniaturization and power, did not easily go hand in hand and it took a long time for the technology to emerge.
For the RAF the first practical AI system was the AI Mk IV. After various experiments with wavelength, they settled on 1.5m as having sufficient range and sensitivity. In turn this meant that reception aerial dipoles mounted on the wings could be kept to half a wavelength, or 75cm, without hurting aircraft performance too badly. The system had a range of around 2-3 miles.
Metre-wave radar aerials broadcast a large balloon-shaped lobe of radio pulses in front of the aircraft. This was sufficiently imprecise that a comparison of returns between reception dipoles on the port and starboard wings was necessary to give accuracy in azimuth.
Because of the inefficiency of the aerial dipoles, the radar leaked energy into weaker 'side lobes'. These would pick up signals from the ground and feed it into the radar. This meant that the practical range of the radar was limited to its height above the ground. The lower the aircraft flew, the more the ground return overwhelmed the scope. This was to make metre-wave AI radar useless to the RAF when trying to engage low-flying raiders such as minelayers, and was to drive the Allies towards developing radars with shorter wavelengths. ILLUSTRATION: A diagram of a 1.5m radar, showing an idealized main lobe and side lobes. If the aircraft flies any lower, the side lobes will begin to register a ground return, which will fill the radar scope at long range. The lower it flies, the more the ground return fills the scope.
The other big problem that these radars faced was that of minimum range. They could not see targets below a certain distance, and this was a product of the Pulse Width, or the length of time the pulse took to be broadcast by the radar. Difficulties with circuit design plagued the efforts of early radar engineers to produce short pulse widths. If the pulse width was too long, then a target at close range would return an echo before the pulse had finished broadcasting, so making the signal disappear from the radar.
Engineers never entirely got over this problem, meaning that minimum range became a tactical hurdle for the nightfighter. The aircraft had to try and pick up a target bomber before it dropped within minimum range. For the AI Mk IV radar the minimum range was 400 feet, and with a maximum visible distance in the region of 2,000 feet, this meant there was a critical narrow band within which the target must be visually detected.
If the problems of ground return and minimum range was not enough of a challenge for the nightfighter, there was the layout of the radar scopes. For AI Mk IV there were two scopes, showing the radar returns in elevation and azimuth. It took some skill to interpret the information, and as a result the natural division of responsibility in nightfighting fell between the observer, who could constantly monitor the scopes and call out information, and the pilot, who could keep his eyes out of the cockpit, scanning the darkness for the silhouettes of the enemy bombers. This saved the pilot from ruining his night vision by staring at a scope, but put great reliance on tight teamwork between himself and the radar operator. PHOTO: One of the AI Mk IV radar's two scopes. This indicates whether the target is to port or starboard of the aircraft. Pulses appear on the vertical timebase, indicating range. The magnitude of the pulse either side of the timebase indicates direction to port or starboard. Where the aircraft was near the ground a 'christmas tree' pulse, representing the ground return, would begin to march from the top of the scope down the timebase.
This division of labour meant that the archetypal nightfighter would be a twin-engined aircraft, something with space for two crew, enough power from the engine alternators to run the radar, great endurance, plenty of firepower, and the space to take all the electronic kit. Though there were exceptions to this--the American Navy and Marine Corps would invest in single-seat fighters--this set the pattern for the years ahead. Initial RAF deployment of AI used the Blenheim light bomber as a platform, but this was too slow to take on fast German bombers, so soon the more powerful Beaufighter was adopted as the mainstay British nightfighter.
While AI Mk IV was being perfected in late 1940, the Germans were developing their own system by repurposing a radio altimeter into an AI radar. This was to become the FuG 202 Lichtenstein BC
, which would be operationally trialled in mid-1941 and become the standard Luftwaffe AI equipment for the next few years. This operated on a shorter wavelength than the British AI, around 50cm, which meant a fractionally shorter range than AI Mk IV. It had a useful minimum range of 200m (~650 feet). Four nose aerials functioned as transmitters and receivers and the radar operator had no less than three display scopes, one each for range, azimuth and elevation. PHOTO: The Me110 became the mainstay of the Luftwaffe's nightfighter arm, but lacked the speed to battle the RAF's heavy bombers. Its replacement, the Me210, proved an abject failure, and so the Zerstörer soldiered on long beyond its sell-by date. This Lichtenstein-equipped Me110G-4 was the ultimate version of the type, with nitrous boost for its engines at high altitude.
The Germans would fit Lichtenstein
aboard their own two-seat platform, the Me110 Zerstörer
. The heavy 'destroyer fighter' concept had been found wanting in the day fighting of the Battle of Britain, but at night it would come into its own. The only problem was that the extra weight and drag of nightfighting equipment, such as radar aerials and flame dampers to hide the exhaust flares, took 10% or more off the aircraft's top speed. The Me110 could get by when chasing older, slower British bombers but would soon find itself outpaced when a new generation of four-engined bombers appeared.
Now that AI radar had been perfected sufficiently to be useful, the next challenge was to incorporate it into a tactical system. Next:
Integrating AI and ground control. Himmelbett