Swedish Strike Saturday: the AJS-37 Viggen

The AJS-37 Viggen is a modernized classic: a 1990s update of the 1971 AJ-37 Viggen.

Why is it a classic, though? You may be forgiven for not knowing. In fact, I did not know until I saw that Leatherneck Simulations1 are making a DCS AJS-37. So, on this first Swedish Strike Saturday, let’s take a look at why the Viggen is such an icon, and why you ought to be excited for it.

In doing so, we first have to take a trip back in history, back to Sweden circa 1961. The enemy du jour is the Soviet Bear. Although the Saab 35 Draken matches up well against Soviet fighters of the day, the Saab 32 Lansen, a late first-generation jet which handles the attack role, is looking a little long in the tooth. It’s time to make something better.

Much better. The Swedes had a history of pioneering aircraft designs out of Saab, and the Viggen was no exception.

It was the first canard aircraft to enter front-line service, and featured the first afterburning turbofan in a strike fighter. By date of start of development, the Viggen’s computer was the first integrated-circuit computer designed for use on an aircraft. For a time in the early 1960s, while development work was under way on the computer, Saab was the world’s largest buyer of integrated circuits. It was the first single-seat third-generation jet strike fighter to enter development, and the second to enter service2.

As one of the two first digital attack aircraft to enter service, it is, then, an object of some historical interest. Similarly, its computer is one of the first in the aviation world, and that makes it interesting to me (a computers guy). The CK37 (CK for Central Kalkylator) flight computer does just about everything data-related in the aircraft: it runs both of the cockpit displays (the HUD and the Central Indicator—think radar screen, but with navigational information, too), does navigational calculations, and handles weapon aiming.

Saab built the prototype, using individual transistors, in the 1960. It was table-sized, featured about 5,000 transistors, and ran at about 100,000 cycles per second. Total weight was about 450 pounds. Obviously, it wasn’t altogether suitable for aerial usage. Redesign efforts in 1961 used the newly-available ‘integrated circuit’.

Enter Fairchild, who beat Texas Instruments (!) for the contract. Their integrated circuits featured a whopping two transistors per square millimeter, ten times the density of discrete components. Some few years later, in 1964, Saab’s computing division delivered the final CK37 prototypes. This final version could run about 200,000 instructions per second, with about 28 kilobytes of magnetic core memory, with core density of about one core per millimeter3. It weighed about 150 pounds, comprised five computer units, and drew about 550 watts of power.

And, going by everything I’ve seen, it made for a tremendously effective aircraft. On seven hardpoints, the original Viggen could carry a combination of weapons: 135mm rockets, 120kg bombs, the RB-05A MCLOS missile, and the RB-04 anti-ship missile. Between the radar and the advanced (for its day) navigation system, the Viggen could fly in ugly weather, dropping unguided bombs precisely on any target it could see by radar. Although its air search capabilities were rudimentary, the radar could still cue Sidewinder seekers; on those grounds, it was not altogether ineffective as a fighter.

It did so without a navigator; the autopilot and navigation systems are sufficient to permit the pilot alone to fly and fight. By all accounts, the Viggen gave excellent service from its introduction date in 1971 to its retirement thirty-odd years later. Along the way, it gained the RB-75 missile4, and a variant called the JA-37. A fighter first and striker second, the JA-37 gained a better computer, a lookdown-shootdown radar, and support for the Skyflash5 missile. Much later, both the JA and the AJ Viggens saw some upgrades. The JA-37 became the JA-37D, with a glass cockpit and the ability to sling AMRAAMs6. The AJ-37 became the AJS-37, and that’s the plane we’re interested in today.

Development of the JAS-39 Gripen7, the follow-on to the Viggen and the Draken, began in 1979. It didn’t fly until 1988, and it didn’t enter service until 1997. In the interim, Swedish military planners began to get a little nervous about the state of their ground attack force. Though the Viggen was a solid workhorse, its armaments were outmoded, and its navigation system was fiddly.

Some of the Gripen’s weaponry was already available in the early 1990s, though, including the BK-90 submunitions dispenser8 and the RBS-15 anti-ship missile. The S-modification allows the Viggen to launch both, giving it access to modern smart weapons. At the same time, Saab’s designers added a data cartridge, greatly simplifying pre-mission preparation. The extra data capacity in the cartridge also allowed for a terrain contour matching function. The data cartridge contains information about the elevation contours expected during the mission and their locations; in flight, the computer correlates the expected contours to the actual, observed contours from the radar altimeter. This allows the computer to update the INS with true positions, correcting to some degree for drift during flight.

With those upgrades, the AJS-37 soldiered on until 2005, flying alongside the Gripen for eight years, at which point it was finally retired. An airplane of many firsts, it was also a notable last: the last of the great 1970s low-altitude strike fighters to fly its original mission profile. The Tornado, the F-111, and all the Viggen’s other contemporaries were upgraded to fly more modern, middle-altitude missions. The Viggen never lost its focus as a low-altitude interdictor.

Is the Viggen a good interdictor in its original threat environment? Do the upgrades make it better? Is it suitable for the modern world? How good is the Leatherneck recreation? This paragraph is where I had hoped to tell you that we would soon be finding out. Unfortunately, it’ll be a little longer than I had hoped; Leatherneck’s Viggen releases on January 27, and it isn’t looking like the Soapbox is big enough for a preview key. No matter—that just gives me more time to prep for the articles down the road. In February, you can expect two or three of them, touching on the answers to the questions posed at the start of this paragraph.

Stay tuned!


  1. Makers of the DCS MiG-21
  2. The A-7 Corsair came first, entering service in the late 1960s to the Viggen’s 1971 
  3. The CK37 divides its memory into 8192 words of 28 bits in length, with 1536 words as working space and the remainder write-protected data. 
  4. The AGM-65A Maverick; the Swedes have a thing about keeping American names for missiles. 
  5. Or RB-71. 
  6. Surprisingly, they call this one the AIM-120. 
  7. The Gripen is a longtime favorite of mine. 
  8. The Swedes are anti-cluster-bomb, so a weapon which drops explosive bomblets is called a ‘submunitions dispenser’. 

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  1. Pingback: Fishbreath Flies: DCS AJS 37 Viggen Review - The Soapbox

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