On a separate page (Nav Kit) I have outlined what the equipment was at the navigators station, on this page I will mention some of the items and their role in the Navigation Bombing System, (NBS). This section will grow as I find time to detail more of the units.

For the anoraks, take a look at this site

http://www.tatjavanvark.nl/tvve/dduck0.html

Offsets and how they were used. Handling magnetic variation Input of weapon ballistics to NBS The Calc 3 My thanks to G Croft for some of the forward throw formula. Forward throw info from Roy Brocklebank

The photos below were taken off the web, but I cannot remember the sites.  When I find them again I will give full acknowledgement.  My thanks anyway.

NBS was an integrated system which was used, as its name suggests, for both Navigation and Bombing.  Main elements were the H2S radar, Green Satin, GPI-6, Heading Reference System (HRS) and the various analogue units of the bombing system.  The main elements for the radar operator are shown in the following photos, these do not include the HRS and the GPI-6 which I intend to deal with on pages of their own.

Other pages have detailed how drift and groundspeed are obtained from Green Satin, and how the H2S PPI picture can be interpreted so this page will give some information on how offsets were used, how weapon ballistics information was input to the bombing computer and how magnetic deviation was handled by the navigation system.

The shot below is from the http://www.tatjavanvark.nl/tvve/dduck0.html siteand shows all the ‘black boxes’ of the NBS system on a test rig.  After leaving the course at Lindholme I like to think that I knew what they all were and what they all did.  Not any more.

The photo above shows some of the equipment at the nav Radar station; Antenna is the control unit for the radar antenna (CU 595); camera is the R88 camera which takes photos of the radar display during a bomb run; 626 is the control handle for ‘moving’ the PPI display; Radar is the 9” PPI display; Int (CU 585) and Ext are the internal and external Offsets.

Offsets.

The radar on the Vulcan was designed a long time ago, it was a derivative of the H2S system used at the back end of WW2, primarily on Lancaster bombers.  While an old design, the equipment as installed in the Vulcan was good, at least for the role assigned to the aircraft.  The bombing system which was integrated with the radar was pretty Heath Robinson by today’s standards, it was mechanical and analogue, but there were no electronic computers at that time that could fit into an aircraft. The radar could take advantage of the size of the Vulcan (and the other two 'V' bombers), and could house a 6 foot aerial in the nose. The aerial was oblong in shape, not circular, and could give high definition, a priority requirement for a radar needed for selecting a ground target amongst all the associated 'ground clutter'. The radar could rotate through a full 360 degrees, but could also be selected by the Nav Radar to sweep a small arc once a target had been identified. Sweeping in this way rather than having the aerial on constant rotation would allow more frequent 'painting' of the target, so allowing the Radar to further tune the definition of the picture and maintain radar markers over the target or fix point. The sweep could be selected around any mean point in the 360 degrees, and the width of the sweep could also be varied. The selection was down to the operator, but he would also need to take account of the fact that the narrower the sweep selected, the less of the surrounding area was visible on radar. (The aerial controls are on the CU 595 in the photo above.)  This could be an issue at low-level in a hilly or mountainous area where the radar is an added flight safety aid.  As well as selecting sweep the aerial could also be deflected downwards and again this could be used by the Radar to fine tune his picture and the definition of the target return. As you get closer to a target you need to start deflecting the aerial downwards to maintain the strength of return from the target or fix point. However, while the radar was good there could often be circumstances where the nature of the target was such that the radar return would either be negligible, infrequent or possibly too large.  If the target was an underground missile silo (for example) it might well be that there were no surface buildings (they could be remote) or the top of the silo was level with the ground giving little or no radar return.  In this situation it is necessary to find something in the vicinity of the target which does give a decent radar return, and use this as an aiming marker. These are called ‘offset’ aiming points.

This is a good shot taken from one of Ken Townsend’s photos. It is from XL360 at Midlands Air Museum, Coventry.  The shot shows the CU585, the two dials at the bottom are the internal offset dials; they are labeled N.S. (north/south) and E.W. (East/West) and they are wound on using the crank handles underneath the dials. Between the dials is the selector switch for internal/external offsets and for PC (primary check) mode.  The snakes nest of cables shows the ‘ergonomic’ design. On the left the ‘height corr’ crank always got fouled by the thick black cable next to it.

For training flights the Nav Radar would normally select his own offsets, for a Group Exercise or a war mission the offsets may have been provided by the mission planners.  When selecting offsets the Radar is looking for something that is relatively close to the target and is likely to give a decent radar return.  When making his selection the Radar could do this by studying a large scale map and using his experience to determine a feature that he believes will suit his purpose or he may use a training film from a previous bombing run. A camera (R88) is mounted above the display to take photos of the screen during the bombing run, this may be triggered manually by the operator or automatically by the bombing circuits, the film can then be used for later analysis of how well the operator used his equipment, and for selection of better offset aiming points.

Mounted above the radar screen is a camera that takes a film of the bomb run, these are then archived in the station Operations Building and these are available to other Nav Radars for flight planning purposes.  Using a film the Radar would study the approach to the target, frame by frame, looking for small but persistent radar returns that he feels would be suitable for use as an offset aiming point. When he is sure he has selected a return that would be persistent for long enough to act as an aiming point he then needs to interpret a large scale map and determine which map feature is producing the radar return that he wants to use.  I say that the return needs to be persistent for long enough to act as an aiming point, it is possible that the return can only be picked up for a certain number of miles and on a particular heading. The Radar needs to be aware of this and ensure that he is set up to have the offset selected as he comes within the range 'window', the aiming markers should then be close to the return for long enough for him to make the system corrections, getting the markers over the offset, and so feeding correct range and bearing to the target to allow an accurate weapon release. An example of such an offset appearing only for a short time could be as follows; the track to target could be up a valley giving a lot of radar 'cutoff' on each side of track as the high ground produces a radar shadow on the other side of the high ground, a few miles before the release point there could be another river valley coming in from the side with a small pumping station two miles up the river valley at right angles to track. Because of the high ground along track the radar can only paint down the river valley as the aircraft goes past the entrance to this valley, for the short time that the radar 'paints' down the river valley the pumping station gives a good radar return. As the valley entrance moves astern the pumping station moves into radar shadow and is no longer visible on the radar.

Having chosen the feature that he believes will give the required radar return the Nav Radar needs to accurately measure the displacement of the pumping station from the actual target. If you have used an Ordnance Survey map you will be aware that somewhere on the map there will be an indication of Magnetic North, True North and the variation. The Radar then needs to measure the number of yards the Offset point is from the target point, both North/South and East/West, these are the values that the Radar will set on the bombing system so that while he aims at the offset point the bombing system will in fact produce a heading steer towards the target and compute the weapon release point from the actual target.  The values of the offsets will be set on either the Internal or the External Offsets in the photo above.

In the photo above M.P. is the main panel displaying wind drums, Lat & Long, groundspeed and heading; the Rad Alt is the radio altimeter; O2 is the Oxygen panel for the Radar operator; Pwr is the main power control panel for the radar equipment.

Automatic Variation Setting Control (AVSC).

Anyone who has been walking using a map and compass will be aware that there is a difference between True North and Magnetic North, you may also be aware that the magnetic north pole is not a fixed point, it actually moves with time as the earth's magnetic field changes, and it is actually situated over northern Canada, rather than at the geographic North Pole. The difference between True and Magnetic North is Magnetic Variation, and is important.  Most compass systems are measuring relative to magnetic north whereas the map that the Nav Plotter is using to plot his aircraft position is drawn around True north. In simplistic terms if he is currently at a Longitude of 5 degrees west and he wants to fly due north on that longitude line by flying due north on a compass bearing, then if no allowance is made for the magnetic variation at his position, he will in fact fly some 5 to 7 degrees off course in the UK (magnetic variation is of that order in the UK). Given that if you fly 60 miles with a 1 degree heading error you will be 1 mile off track, then the magnetic variation of 5 to 7 degrees becomes a very substantial error.  This variation has to be taken account of in the heading system.  In the map below which is in the Goose Bay area the magnetic variation line can be seen and it is some 34^o West (see bottom right corner of map).

Because of the contorted shape of the Earth's magnetic field the variation from True North varies with position on the globe and it is not realistic for the Nav Plotter to be constantly applying the local variation to the heading system all the way through a sortie, it needs to be done automatically.  In the Vulcan this was done by another analogue wonder, a three dimensional cam, or model of the variation in the northern hemisphere is part of the heading system.  This cam is about 4 inches in diameter, made out of hard wax or plastic and has a small roller that travels across its 3D surface, see the diagrams below. At the start of a sortie the cam is set to the local variation and then during the sortie the cam will slowly rotate so that the point under the roller will represent the variation at the geographic position of the aircraft.  The system has limitations, one of them is that it cannot operate from 90^oN to 90^oS, the roller would slip as it approached the rotational centre of the cam.  At frequent intervals during the sortie the Plotter will check the variation read out on the cam with that at his fix position on the map, if there is a small error, he will correct it, the big fear is that the cam could 'run away'. If it did then it would start to feed completely erroneous variation settings to the Heading system, and that would have pretty dire consequences. Heading and variation checks throughout a sortie are vital flight safety checks for the Navigation team to carry out.

Diagram 1 below is the AVSC from the side.

The diagram above shows a side view of the cam, with the stylus roller measuring variation East or West by its vertical displacement as the cam rotates, and the diagram shows that it can only give automatic readout between Latitudes of 80^oN and 60^oS. The Latitude is determined by the stylus displacement from the centre of the cam towards the outer rim.  The diagram below shows that a drive from the North/South numerators in the NBS system will rotate the sylus arm about its vertical support, so moving the stylus from the centre of the cam towards the outer rim. At the same time the NBS East/West numerators cause the cam to rotate so causing the stylus to rise or fall vertically over the surface of the three dimensional cam model. A classic analogue approach to the problem. 

Diagram 2 below is the AVSC from above.

Weapon Ballistics film.

This analogue approach to problem solving was also extended to the input of the weapon ballistic characteristics to the bombing computer. For a bomb release system to be able to compute the release point for a weapon at any height or speed, the system needs to know the ballistics of the weapon, and how it will fall given a release height and release speed. All bombs will have different ballistic properties, and these will also change again if the weapon is released in a retarded mode rather than free-fall.  In the retarded mode the weapon will either deploy a system of 'paddles' as airbrakes, or possible a small parachute, the intent of both systems is to delay the fall of the weapon to allow the delivery aircraft more time to escape the blast effects of the weapon.  Such a retard system will obviously have a marked effect on the ballistic properties of the weapon, its forward throw, and so its release point.

At the flight planning stage of the sortie the Nav Radar will have decided the weapon that he will be simulating on his bomb run, and the release mode, he then inserts a cartridge into the bomb computer system which fits this weapon specification. The cartridge contains a 35 mm film, similar to that used in a domestic camera.  The film has a number of black and white squares on it that pass in front of a light source in the bomb system, the film is read when the Radar selects 'bomb' on his equipment at the start of the bomb run.  Given the height and speed at the time that the film is read, the light and dark pattern is interpreted by the system and converted into a distance prior to target where the weapon should be released. It is important that during the bomb run the speed and height should not change appreciably as this could cause the film to start a fresh run through the system, and this can take some 20 seconds. Going back to the film used in a domestic camera, you may have noticed that the cartridge containing the film has a pattern of black and white squares on it, these match up with a set of electrical contacts in your camera and allow the camera to automatically read the 'speed' of the film, the ASA number, from the cartridge.  This is very similar to the process going on in the bomb computer when it reads the 35 mm film.

The shot above shows the film which delivers weapon ballistics information to the NBS system.  This was taken from the site http://www.tatjavanvark.nl/tvve/dduck0.html

I have included the photo below because it is the only one I have seen which shows the Calc 3. This is the large ‘dustbin’ in the bottom left of the photo, the aircraft is the Valiant, but in all three aircraft the Calc 3 was positioned behind the Radar’s seat.  The Calc 3 was one of the large analogue units in the NBS system, this one contained the ‘height chain’ which managed by a system of steel belts etc to produce an analogue input to the bombing system for plan range and forward throw. It was all done by triangles and analogue voltages, and other weird and wonderful things that I have now forgotten, maybe another Nav Radar can remind me. One of the ‘wonders’ was the Square-rooting Pin Wheel, this was a circular plate covered in little metal studs which by an analogue system was able to square root numbers. It was known to all as the ‘Rootin’ Tootin’ pin wheel’.

Roy Brocklebank has sent me a correction to what I have said regarding plan range calculation; this is an extract from the email.  “I just noticed you attributed the Calc 3 to calculation of plan range. That function was done by the triangle solver in the Calc 5A in the nose wheel bay.

This was linked through the Ind 301/CU626 and the marker system. Now I get a bit hazy here as my NBS Master Block Schematic is on loan to the Royal Institute of Navigation in Kensington Gore. Essentially as the range marker was drive out so a range carriage would be driven out in the Calc 5 in relation to slant range. A slug on the carriage had a steel band attached. The other end of the steel band was attached to a similar slug on the height carriage. That slug was moved up or down in relation to the aircraft height. The triangle thus described had height for the vertical, plan range for the horizontal and slant range for the hypotenuse.

In turbulence, especially at low level, the slugs could bounce thus giving a fluctuation on the range marker on the Ind 301. The odd blob of frozen grease was also known to affect the free running of the carriages.”

I have also added a few comments from Gerry Frew who worked on NBS at Scampton (‘79-82) “That and the article on your site regarding the NBS (and in particular, Roy Brocklebank's remarks re. the Calc 5) reminded me of one of the more bizarre aspects of maintenance. It was our responsibility to change u/s Calc 5s but on taking on the job for the first time, one could see the obvious flaw in the logic. You could undo the connectors and the nuts that held it in its mounting tray, but it would only slide forward a couple of inches before any further progress was impeded by the starboard nosewheel door hydraulic jack. This had to be disconnected and swung out of the way in order to get the Calc 5 out and its replacement in - not the sort of thing us fairies expected to have to do. However, try and find a handy rigger to take on such a mindless task. The solution was that NBS technicians were given special authority to disconnect and re-connect the jack, (but only on the starboard side!). We did have to get a rigger NCO to countersign the work, but that was generally an academic exercise. I still have scarred knuckles from trying to insert the split pin into the castellated nut which anchored the jack to the roof of the nosewheel bay. I also remain convinced to this day that Vulcan riggers were selected not by ability but by the number of double-joints they had.”  I take no responsibility for Gerry’s comments on riggers!

The 35mm film cassette for the particular bomb type to be dropped was inserted into the top of the Calc 3, there was also a crank handle and dial to allow the Radar to input a height reading to the Calc 3, but I can no longer remember if this was aircraft height or height of the target. Again, can anyone out there remind me? Take a look below at the info from Roy Brocklebank.

I was sent the following info via the guest book on the site, for which I’m grateful.  You never know when you might need the forward throw formula!  Thanks.

What a wonderful site! From joining the RAF in 1961 until being Commissioned in 1974 I was an Air Radar Fitter specializing in NBS. From D38 Mechanics Course at Yatesbury to Fitters Course ARF314 at Yatesbury / Feltwell. I worked on the flight lines of 7 Sqn (Valiants at Wittering), 35 Sqdn (Vulcans reformed at Conningsby) and in the NBS Bays of Cottesmore, Conningsby & Finningly. With reference to the Calc.3 comments; The 'Film Cassette' slotted into the top of the Calc.3 and the light path through the film activated the toggles of the 'Selector Switch' to set up the Forward Throw details. For some strange reason we were expected to remember the equation for forward throw (at least for exams), I don't remember it all now, and probably didn't back in the 60's but I do know it ended in ......"H Tan Lambda Cos Delta".

I will now be a frequent visitor to your site. Best Regards Flt.Lt. G Croft RAF Rtd.

I have been sent the Forward Throw formula by Jimmie Robb, or at least Jimmie believes this to be correct;

H will be height, but I cannot remember ‘g’, nor a value for Tau, not which angle is ‘Lamda’.  Can anyone add anything to the conundrum? Jimmie believes there may be a function missing in this formula. See below for an in-detail update on this.

As the bottom of the photo indicates, it is one taken by Damien Burke. It is also interesting to see that the seat in the Valiant look somewhat less substantial than those we had in the Vulcan. The Calc3 is the large dustbin in the bottom left of the photo, this contains the height chain.

I would like to thank Roy Brocklebank for the following information.  There is indeed an omission in the forward throw formula.

The formula should be FT=Vg (Sqr 2H/g +/- Hdot/g - Tau +Time Advance) + ½ stick length – H Tan Lambda Cos Delta.

Vg is the ground speed and all the functions inside the brackets are time functions. The product of Vg + t gives the basic forward throw for an ideal bomb, or more accurately the bomb firing button activation point. The stick length and the trig function, or trail distance, are distance functions which are then applied to the basic forward throw.

G is of course acceleration due to gravity and is taken as 32 feet/sec/sec.

H dot /g is the climb/dive correction hence it may be positive or negative.

Tau is a fixed value which represents the drag factor on the bomb. For an ideal bomb this would be 1. For a 1000lb I calculated the value of drag to be about 0.95 but I don’t know how the actual correction in the formula was calculated – the ballistic film did that). Tau is negative as bomb drag reduces forward throw.

Time advance was, in the Vulcan, 0.55 seconds which was calculated as the time it would take for the electrical firing pulse to reach the EMRU and release the bomb.

Delta is the drift angle.

Lambda is the trail angle. This is the angle between the vertical below the Full Range Point – the distance travelled by an ideal bomb in still air – and the predicted impact point of the real bomb in still air. The distance on the ground was thus represented by the function of aircraft height. As the bomb would actually be affected by the wind it would suffer from cross-trail H Tan Lambda Sin Delta and fell behind the aircraft it followed that it was in the air for a longer period and would thus suffer increased lag too hence the along track component of cross trail – H Tan Lambda Cos Delta.cross trail effects.

You mention the 35mm film scan. On first selecting BOMB the Calc 3 would initiate a double scan of the film. The 35 mm film, about 1 metre long, would be run through and the PE cell would be activated if there was a clear window as the film moved. As the film was run through a carriage within the Calc 3 would move and make or break a number of switches. This would take about 4.5 seconds when the film would be re-wound and a second scan commenced. These were known as the A and B bank scans. When both A bank and B bank scans had ended the forward throw calculated would be displayed on the Forward Throw meter.

If the aircraft changed height or airspeed from one speed or height band to another a second B bank scan lasting 4.5 seconds would be made. If the aircraft was descending or climbing close to its release point and a B bank scan was initiated there was a possibility of an inaccurate bomb door or bomb release pulse being generated. For that reason, at 10.55 seconds, the Calc 3 scan was frozen. 2.55 seconds later the bomb door opening pulse would activate followed at bomb release time by the bomb firing pulse.

The Calc 3 had a Primary Check function for ground use which could be used to increase the Calc 3 height if the height carriage automatic drive had failed. This function worked at 50 feet per second or 3000 feet per minute. This was not the climb limit for the Calc 3. During a 2H pop up attack the aircraft would climb from low level at about 10000 feet per minute to a release height of 10500 feet. It was found that the Calc 3, with a lower limit of 7000 feet, could pick up as the aircraft climbed through 7200 feet and calculate an accurate release in the climb.

Static tests in the Electronic Bay showed that the Calc 3 Height Carriage could actually climb at 50000 feet per minute.

The ballistic film did not produce a simple complete bomb fall profile but was divided into zones. I do not recall any actual figures but it would have been something like TAS up to 270 kts, 271-370, 371-490 etc. Similarly the height bands would have been something like 60000-45000, 45000-30000, 30000-17200 (Calc 3A for the early Mk 2s before they were refitted with the Mk 1 aircraft calc 3s.)