Tuesday, September 8, 2015

The Utility of the Military SST

The Military Utility of the Supersonic Transport (SST)

In the late 1950s and the early 1960s, the supersonic transport (SST) seemed to be the future of civil aviation. As airliners had progressed from piston engine designs going barely 500 km/h to jets capable of nearly 900 km/h, increasing speeds even further was the logical next step. 

Ultimately technical challenges and economic forces (such as rising fuel prices) lead the SST to end up firmly on the margins of civil aviation. Only the Anglo-French Concorde entered service, and then in limited numbers. Occasionally there will be news of planned revivals of the SST, or of designs for supersonic business jets, but little concrete progress.

The fate of the civilian SST has been the subject of much discussion, both online and elsewhere. However, there has been much less speculation on the role of the SST in military service. One paper on the topic, which I recently discovered, is this one. It’s a bit old, and almost 70 pages long. Still, there’s worse ways to spend an hour.

It would seem at first that there would be little application for a military SST. Military transports are often required to carry very heavy and bulky equipment, and this is apparent from their design. These requirements are contradictory to those of supersonic flight, which demands a low frontal area and very streamlined design. Compare, for instance, the shape of the C-5 Galaxy to the Concorde. Additionally, military transports much frequently operate in harsh conditions, from short or unimproved runways. A design like the Concorde or L-2000 would be hard pressed to operate from the same runways a C-130 could.

The Boeing 2707. A military SST could look something like this. (Picture courtesy of Aerospace Projects Review).

On the other hand, there are certain advantages to possessing a military SST. The most obvious of these is rapid travel time. Compared to a C-5 or C-17, an SST could potentially cut hours off the travel time, depending on the route. This would be most obvious on transatlantic or transpacific flights (assuming the SST had enough range). While an SST would not be able to deploy an armored division, it could, for instance, deploy airborne forces or special operations teams great distances on short notice. Such a capability would be most useful in low intensity conflicts or sudden, unpredictable situations. For instance, if an opportunity came to eliminate a time sensitive target of high importance, an SST could deploy a special operations team from the US in less than half the time it might take a subsonic transport to. Alternatively, were an American embassy or other facility to come under attack from irregular forces, the SST could deploy force of airborne troops or other forces sufficient to hold out until heavier assets could arrive.

While these capabilities would be useful, they would impose certain constraints on the SST design. For one, at least a modicum of short takeoff capability would be needed. Not enough to take off from a 1,500 foot dirt runway, but at least good enough that you wouldn’t need a major international airport. This could be accomplished by various methods. Variable geometry is one option, though it would add significant weight and complexity, reducing payload, range, and reliability. Another option could be high lift devices, such as blown flaps, leading edge slats, or vortex generators. 

Were such an SST to be built, it would probably be in small numbers. Existing transports would be needed for previously mentioned roles (outsized cargo and STOL), so the SST would only replace a small portion of the fleet. Additionally, the cost of developing and building a bespoke SST airframe would be very high (if existing civilian SST designs were in service or development, it could be possible to use a military adaptation, reducing costs significant), which would also likely reduce the amount purchased. The increased fuel costs of an SST would make it even less attractive for conventional airlift missions. To me, it seems most likely that a military SST would be a “black” program, with very small numbers of highly capable airframes built in very small numbers, and used only for the most sensitive missions. Of course, the question of how to keep an operational fleet of very large supersonic aircraft traveling throughout the world secret is not an easy one. Still, it appears that the supersonic transport does have a viable, if very niche, military role.

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