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J35 Appreciation Station

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I wrote a little piece on the rb 04E based on the SFI's. Figured people might be interested.

Robot 04E (with "robot", abbreviated "rb", being military Swedish for "missile") was the AJ 37 Viggen's signature weapon: a radar-guided, sea-skimming anti-ship missile, developed from the rb 04C which had originally entered service in the 1961 on the A 32 Lansen. The E version entered service in 1975, with 315 missiles produced. Let's have a look at how it works.


Rb 04C or D on a A 32 Lansen.


Missiles on the assembly line at the air force's Central Aircraft Workshops in Arboga.

First, some background on the doctrine and use case that shaped the design of the missile. The Swedish armed forces expected the Warsaw Pact to attempt to secure a beachhead on the Swedish coast with a D-Day style invasion: a massive fleet of hundreds of ships with surface combatant screens protecting a core of various landing craft. The AJ 37's raison d'être was to attack a fleet like this. The rb 04E was mainly intended to be used against the screening combat ships, since if their AA was silenced the Viggens would be able to go to town on the vulnerable landing craft with less expensive weapons like bombs, autocannons and unguided rockets. In order to achieve saturation of the defenses and a reasonable chance to actually sink mutually supporting surface combatants, the plan was to deploy at least four but preferably six or more full squadrons in each attack wave (one squadron in the air was two flights of four aircraft, so six squadrons would be 48 aircraft). Since the plan involved launching up to close to a hundred missiles at the same time (or slightly less - some aircraft would be carrying countermeasures instead of missiles), getting the missiles to spread themselves out between different targets and not collide with each other or lock on each other was a very real concern, which will be apparent when we get into discussing the seeker.

Onwards to the technical details!


The missile's about four and a half meters long (14 ft 9 in), weighs around 625 kg total (1378 lbs), has a shaped charge warhead that weighs about 200 kg (441 lbs) and is powered by a solid rocket motor that produces a nominal thrust of 195 kp (1.9 kN, 430 lbf) for a nominal burn time of 65.5 seconds (can vary between 60 and 75 seconds depending on propellant temperature). The control surfaces are pneumatically actuated. The seeker is a frequency hopping monopulse radar with a parabolic receiver antenna located under the radome in the front of the missile (the text "TRYCK EJ HÄR" on the radome means "do not press here"). The antenna sweeps horizontally only, 28 degrees to each side. The missile cruises at an altitude of 10 meters above sea level, which it maintains by the use of a radar altimeter.

The AJ 37 can carry two rb 04E's on the inner underwing pylons. When pre-flighting the missile, the mechanic had a panel with five switches and a knob available to him for programming the missile - there really isn't much the pilot can configure from the cockpit. The panel looks like this:


The switches are intentionally only labeled with numbers for opsec reasons - the seeker electronics were highly classified and conscripts were not allowed to know much about how it worked. Switch 1 ("balkläge") is the missile's position on the aircraft; V (vänster, left), C (center) or H (höger, right). The centerline pylon © was initially planned as a possible launch position on the AJ 37 but the electronics to actually launch the missile from there were never implemented. The rest of the switches we'll cover when we get to the functionality they affect.

The missiles can be launched one by one or both together - in the latter case there's an automatic delay of about two seconds between the two, to avoid collisions. Targeting is simple: the pilot simply points the entire aircraft at the desired target, guided by the head-down radar screen, on which either a PPI or a B-scope is presented together with a wind-compensated aiming line (wind speed is taken from the aircraft computer, where it is either doppler calculated by the radar altimeter system or taken from the weather forecast as input during pre-flight procedures). The presentation looks like this:


B-scope and PPI, respectively.

The number 60 shown in the bottom right means that the range of the display is set to 60 km. The two short, curved lines on the PPI represent the ranges 12 and 24 km respectively, while the line marked "raktframlinje" is the wind-compensated aiming line. Originally, the 12 and 24 km lines represented minimum and maximum firing ranges for the missile, but at some point the procedure was improved to calculate the engagement envelope dynamically based on air pressure, temperature and speed of the launching aircraft (later manuals recommend a max launch range of about 20 km). The pilot can select if the missile's seeker should be in single ("ENKEL") or group ("GRUPP") targeting mode. In single target mode, the missile will simply lock on the first detected target. In group mode, the target selection process is more involved and we'll get back to it in a little bit. The missile can be launched at altitudes between 50 and 425 meters above sea level and airspeeds between Mach 0.7 and 0.92. The aircraft's radar does not need to be radiating to launch the missile, since the targeting is done just by pointing the aircraft the right way. In fact, the missile can be launched completely "blind" - this was particularly desirable on the Lansen, which did not have a radar in every aircraft. The flight lead could do the radar thing and the rest of the flight just launched when he did - a tactic that was also technically usable on the AJ 37. Once launched, the missile is completely autonomous and can no longer be controlled in any way by the launching aircraft.

When the launch signal is given, the missile activates its internal batteries, releases its gyro from being slaved to the aircraft's attitude gyros, unlocks and pressurizes the aileron actuators, and when the batteries have reached full power (after about 0.6 seconds), it separates from the aircraft. 0.7 seconds after separation, the elevators and rudders are pressurized and the missile immediately starts diving at an angle of about 7 degrees. About 1.1 seconds after separation, the missile starts yawing either 2.5 or 7.5 degrees to either the left or the right - which direction and by how much is determined by the position of the knob (marked 6, "kurstillskott") on the switch panel on the missile. After 8 seconds, the missile returns to the launch course. The reason for this is to separate the missiles horizontally.

When the missile's radar altimeter detects that the missile has had an altitude under 120 meters above sea level for more than 100 milliseconds, the automatic 7 degree dive stops and the missile instead follows a descent profile that takes around 10 seconds to reach its cruise altitude of 10 meters. Missiles launched from the right pylon ignite their rocket engine when descending below 130 meters, while missiles launched from the left pylon ignite it upon reaching the cruise altitude, to further separate them in time and in altitude.


When the cruise altitude is reached, the seeker starts scanning for targets; the scan area (and lock envelope) is shown above. When a possible target is detected, the seeker activates a function called "three-view logic", which means that the ranging function continues seeking forward about 80 meters. Then, the antenna sweep is reversed and the ranging seeks about 250 meters backwards, then the sweep is reversed again and the ranging seeks about 300 meters forwards. If the seeker gets a return again during the first or second reversed sweep, the target is considered valid. If no return is received during the first or second reversed sweeps, the target search continues. On the other hand, if the seeker gets another return immediately after the first indication, caused by the size of the target, the three-view logic function is blocked and the seeker accepts the target immediately.

When the seeker has locked on a target, the range to the target is monitored. The range should be decreasing, since the missile is approaching it. If the closing speed is too low, for example because the seeker has locked on another missile flying in the same direction, the missile releases the lock and starts a new search. The seeker will not lock on targets that are located such that the missile cannot be maneuvered to hit them, either.

In group targeting mode, the seeker will assume that the target ships are traveling in columns, and can be programmed to lock on a target in the first, second or third row as seen from the attacking aircraft, using the target selection switch (marked 5, "målval") on the switch panel on the missile. In order for the missile to lock in group mode, two or more targets have to be detected in the same range sweep, and they have to be a maximum of 2700 meters from each other (this number looks arbitrary, but it's just about one and a half nautical miles). In order to allow for at least some flexibility in the line up, the seeker performs a fictional widening of the antenna lobe by copying detected targets and considering them for the next range sweep as well. This is all perhaps best explained with a picture:


The "angle jump" function, which can be enabled on the switch panel using the switch marked 4 ("vinkelhopp") makes the missile skip the first possible target it sees and lock on the next one instead, if one is found before the antenna sweep reaches the end position and turns around.

The missile also has an additional targeting mode, called "active + passive", which can be selected on the switch panel (switch marked 3, "följemod"). When this is selected, the missile is basically home-on-jam - if it detects it is being jammed, it will lock on the jammer after one full horizontal sweep. While locked on the jammer (passive targeting mode), the antenna is kept pointed at the signal source and the missile tracks the bearing to it. The range search stays active during the passive target tracking and if a target is detected in the jammer's direction, the missile will lock on that. If the jammer stops transmitting, the missile will keep going "blind" for two seconds; after that it resumes active targeting.


Rb 04E seeker unit.

The seeker keeps the missile pointed straight at the target until it is less than 4000 meters away, at which point the missile starts accounting for the target's speed and leads it. The seeker keeps tracking the target until it has closed to 250 meters, then the missile flies blind the last distance. If the warhead does not detonate when the target is passed, the missile re-starts targeting and simply locks on the first thing it sees (disregarding the single/group target selection and any previous considerations).

At 250 meters from the target, the missile arms its fuzes. The missile is not intended to actually hit the target - the warhead is a shaped charge that is focused downwards, so it is supposed to be detonated above the target. There are three different proximity fuzes - one magnetic, one temperature-sensitive and one based on the radar altimeter, which detects a sudden altitude change when passing over the target. There are two proximity fuze modes, selected with the switch marked 2 ("zonrör") on the switch panel - in mode 1, only the radar altimeter is active, while in mode 2, any two fuzes both giving the detonation signal is required. Additionally, there is also a contact fuze in the nose of the missile, which detonates it after a small delay if it should hit the target directly.


In summary, I find the group mode to of questionable utility since it requires the targets to line up almost perfectly, but I guess they did what they could to try to get the missile to be able to work against large ship formations. In the single target mode though the missile seems to be a pretty nasty piece of business for 1975, especially considering the radio silent mass usage doctrine and the fact that very few aircraft needed to actually radiate to enable a launch. The main weakness was probably that there were so few missiles purchased - about two missiles per AJ 37, total.

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