Saab Draken: Swedish Stealth fighter?

Draken_05.jpgOK, I’m asking for trouble with such a hyperbolic title, but hear me out. Yes, I know it wasn’t really a stealth fighter but I suspect its radar cross section (how large it appears to a radar) was remarkably small — and almost definitely the smallest of its generation. It was possibly even the stealthiest fighter until the F-16 come on the scene twenty years later.

Let’s start with a little bit of background detail. The Saab Draken was a Swedish fighter developed in the 1950s to counter Soviet bombers and their fighter escorts. Sweden’s neutrality meant it was better for them to develop their own military aircraft, and this they did with aplomb, creating a series of fighters specialising in low-maintenance (as there was a large conscripted force) and high deployability as in a war the air force would operate from hidden underground bases and sections of motorways acting as a guerilla force. 

The Draken was an immensely clever design, and remained in frontline service until 2005 (and in a training capacity in the US until 2009). It was powered by a licence-built Avon engine. Whereas the British Lightning had two Avons, the smaller Draken had only one, despite this, the Draken could reach Mach 2, had triple the range of the British fighter and had a similar (and later significantly better) armament (four Sidewinders versus two archaic British weapons).

Today fighters are designed to have the minimal radar cross section as stealth is a good safety measure against radar, the furthest seeing method of aircraft detection. In the 1950s speed was king, and immense compromises were made to reach high mach speeds. Some design features suitable for high speed flight are compatible with stealth, and occasionally a low radar cross section is arrived at by accident as a happy byproduct of aerodynamics and other considerations. Looking at the Draken its hard not to wonder what the radar cross section of this sleek design would have been.

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Highly swept leading edges

In some ways radar energy bounces off at a flat surface in the same way as a billiard ball would, so predicting what it will do is possible and can be tested.  Stealth aircraft hide from radar in several ways,  one being the the use of aircraft’s shape to divert returning radar ‘spikes’ away from the hostile radar. The leading edge of the Draken’s inner wing had an 80° sweep angle for high-speed performance, an extreme angle that would deflect radar energy away from the transmitting aircraft. The outer wing, swept at 60° for better performance at low speeds, was of an even greater angle than the Raptor (42 degrees) and not far off the F-117’s 67.5 degrees. The vertical tail is also highly swept, though a single straight up tail is doubly-bad offering a large signature from the side and creating the avoided at all cost 90 degree angles (with the wing) that provide a painfully loud radar return. The wings’ smoothness are not interrupted by many angle changes, ‘dog teeth’ or wingfences.


The compressor face of the engine, essentially a massive block of metal perpendicular to the flightpath (radars looking directly from the front at an aircraft will have the greatest notice of the aircraft’s arrival, hence stealth’s preoccupation with the aircraft’s frontal cross section) is a key contributor to an aircraft’s radar signature — that of the Draken is completely hidden away within the fuselage.

Materials

As far as we know the Draken did not incorporate any radar absorbent materials (RAM) or radar absorbent structures (RAS), it also of conventional materials (largely aluminium). The radar plate in the nose and the cockpit would be highly reflective, though the general canopy shape may less reflective than others of the time. 

Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes. I asked his opinion on the Draken’s signature and  how it would compare with its contemporary, the F-5 and the later F-16 (an aircraft known to have a small signature).

“I’m not an expert in the radar signature world, and to those that are, a single-figure rcs is not appropriate, But I did write (ages ago) a little algorithm that I used to provide an estimate to drive an optimising program. The estimation tool was explicitly not for LO aircraft. As one of your other commentators pointed out, the type and frequency of the illuminating radar is important, as are the details of the geometry. Testing and analysis is the only way to go – but for the three aircraft you mention, Draken, F-5E and F-16, one can make the following comments:

Draken, head-on, has relatively small intakes, relatively highly swept wing, relatively tidy boundary layer diverter, somewhat blended wing and fuselage, and should have a lower rcs than most of its contemporaries. From the quartering front, the fuselage and wing are relatively blended, which would help. Directly side on, the fin is going to give you a spike, as are the external pylons. And all of the aircraft will have significantly higher returns when carrying missiles. Interestingly, later aircraft have a IRST.

F-5E is a trim little aircraft. Compared to the Draken, although it too has smallish intakes, the boundary layer diverter arrangement is notably messy. From the quartering front, one can observe the flat fuselage side making a right-angle to the plane of the wing, and the fin and tailplane, and tailplane and rear fuselage, doing the same. So from the side, not only will there be a spike from the fin, but the wing-fuselage and fin-tail-fuselage geometries form corner reflectors, increasing radar returns.

F-16 has much higher power-to-weight than F-5 or Draken, and as a result has a large intake. Head-on this is likely to increase signature, but the inlet may (will in later/US models) be treated so as to counter this. The boundary layer diverter is large, and will contribute to the rcs. From quartering front, the leading edge strake and the blending of the wing and body will help, as will the gold flashing of the canopy, which prevents radar energy from entering the cockpit and reduces returns from that area. Side-on, the large fin and the under-fuselage strakes will contribute. Of the three aircraft, only the F-16 is likely to have benefited from RCS reduction treatments, almost certainly in the intake duct, around the radar and possibly some applique coatings.”

Without knowledge of the radar in question and the materials used in the Draken it is hard to make a precise estimation but it seems likely that it was stealthier than the Lightning, F-4 and even the F-104 (suffering as it did a circular fuselage, a T-tail and tiptanks). For its generation it may have indeed had a marginal advantage in radar conspicuity.

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