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.

Could the F-106 have been turned into a multirole aircraft?

The F-106 Delta Dart, the 'Ultimate Interceptor' was one of the highest performing military aircraft of the 1950s, and might have been the best bomber killer at the time it was introduced (only the English Electric Lightning was comparable). However, compared to designs such as the F-4 Phantom, it was highly specialized, with armament and systems highly optimized for the bomber interception role. Other aircraft, such as the Phantom, would be used for other tasks during the 1960s and 1970s.

However, what if (for whatever reason), the USAF had not decided to procure the F-4? Could the F-106 have been turned into a reasonably effective air superiority fighter and/or attack aircraft? Let's find out.

The first question is the basic suitability of the airframe. According to the excellent Design for Air Combat manual, the following characteristics are required of a fighter aircraft;

Air-combat fighter

This type approaches its target either under ground or airborne direction, using its own radar, or by chance, at low to medium altitudes, and armed with a gun and highly manoeuvrable short/medium-range missiles. While the highest supersonic speeds are not necessary, the best possible instantaneous turning performance is essential. Airfield performance is important, since the fighter will be operating from first-line-of-defence bases. Its aerodynamic design will be governed by the need for good lift/drag at high g, high usable lift, low drag at all speeds, and high control power. It will require high thrust/weight with and without afterburning, and low combat fuel consumption.

Compare these to the requirements for an interceptor;

Interceptor fighter

Usually directed towards a non-manoeuvring target by ground or airborne control radar. It requires high speed and longitudinal acceleration, together with good high-altitude performance and long range. Its weapon load is characterised by long-range all-altitude missiles and associated radar. Since neither turning performance nor capability in low-speed combat are important, the primary aerodynamic design is largely governed by low profile and wave drag, and high cruise lift/drag. Low fuel consumption is also significant.

 

The F-106 has good supersonic speed (capable of Mach 2.3), and low drag. The thrust to weight ratio is also fairly good, roughly comparable to the F-4 at MTOW. What about the instantaneous turn performance? That's primarily a function of wing loading. Here's how the Phantom and Delta Dart compare;

F-106 wing loading at MTOW: ~309 kg/m^2
F-4 wing loading at MTOW: ~569 kg/m^2

As you can see, the F-106 has significantly lower wingloading that the Phantom. This means that its instantaneous turn performance should be far superior. This is supported by anecodotal accounts. According to the F-106 data sheet , the aircraft's structure could sustain a loading of 7.0 g at combat weight, not quite as good as something like the F-16, but not too shabby.

It's not all perfect, though. While the F-106's large delta wing will give it good instantaneous turn performance, it does mean that it's going to bleed energy quite quickly in a sustained turning engagement (as I recall, the MiG-21 had similar issues). This would put it a disadvantage in a low speed engagement against an aircraft such as the MiG-17. It should be noted that the low aspect ratio of the F-106's wing would help it do better in a turn fight at supersonic speeds.

The main issue with using the bog-standard F-106 as an air superiority fighter is its weaponry. The performance of the AIM-4 Falcon against maneuvering targets could charitably be described as shit. During the Vietnam conflict, it performed so poorly  that the US Air Force hurriedly adapted the US Navy's AIM-9 Sidewinder. Clearly, if we want to turn the F-106 into a multirole aircraft, we're going to have to drop the Falcon. The most logical replacement is the AIM-9. However, there is a problem. The AIM-9 is significantly longer than the AIM-4. Here's a diagram of the F-106, showing the internal weapons bay with AIM-4s;

Based on this diagram, and me doing some eyeballing, it looks like you might not able to fit two AIM-9s in the weapons bay lengthwise. Fortunately, the AIM-9's fin span is narrower than the AIM-4s, so you could still fit in, at least 2 horizontally, possibly 3. I would also note that the AIM-4 is over half the length of the radar guided AIM-7, so you should be able to fit at least two inside.

Of course, you can't just stick AIM-9s onto the launch rails and expect them to work. The avionics are going to have to be adapted somehow. This could potentially add weight and complexity. However, the F-106, in the interceptor role, was fitted with electronics needed to work with the Semi-Automatic Ground Environment (SAGE), a highly advanced computer system capable of autonomously flying the F-106 to intercept Soviet bombers. A variant used primarily as an air superiority aircraft would not need these electronics, and they could be removed. (As an added bonus, the necessary adaptations to carry the nuclear-tipped AIR-2 Genie rocket could also be removed.) As a result, I believe conversion of the F-106 to carry Sidewinders should be at least theoretically possible.

In order to give the F-106 true multirole capability, it must be able to use air to ground ordnance. The F-106, to my knowledge, never dropped bombs in real life. However, it did have underwing hardpoints (used to carry 370 gallon drop tanks). Photographic evidence indicates that they were at least theoretically capable of carrying air to ground ordnance;

http://www.f-106deltadart.com/photo_gallery/var/resizes/48th-FIS/48fis_korea_500lb-2.jpg?m=1391303353

 

I'm not sure whether the F-106 ever conducted test flights with this loadout, but I'm going to assume it did (why would you stick bombs on an aircraft in a forward deployed location if it can't even fly with them). Assuming those are Mk 82s on that rack, this gives the F-106 a bombload of 1500 kilograms (6x250kg). Not anywhere close to the Phantom, but nothing terrible. (In a scenario where the USAF doesn't get the F-4, the F-105 is going to be even more important in the strike role). Additionally, the underwing hardpoints could be used to carry additional air to air armaments. Likely options would be 2 AIM-7s, and 2 or 4 AIM-9s (four could be carried if a double rack such as was used on the F-8 was feasible). Four AIM-9s and two AIM-7s would be a respectable air-to-air loadout, not as much as the F-4 could carry, but then again, the F-106 is a smaller aircraft.

Based on this, I think it's clear that even without extreme modifications, the F-106 could be made into a capable multirole aircraft. It wouldn't be perfect, by any means, but perfect solutions rarely exist. A multirole F-106 could have been developed for relatively little cost and effort, and still been highly capable.

Not that better results might not be obtainable with more radical modifications...

The J-31 and the State of Chinese Jet Turbine Development

Recently, the PRC public displayed the J-31.

Not a bad looking aircraft

Based on what I've heard, it appears that the J-31 is intended to fill a somewhat similar role to the American F-35; a multipurpose medium weight fighter. Much noise has been made about the similarities between the J-31 and F-35. While I would be highly surprised if the PRC hadn't obtained classified information about the F-35's design, I doubt it's an exact copy, as some claim. Rather, I think it's a case of convergent evolution; any aircraft designed with low RCS in mind will have certain features, as a friend of mine explains here.

Especially sharp eyed observers will note that while the F-35 only has one engine, the J-31 has twice as many. I'm not sure whether this is a deliberate design choice, or because China has yet to produce an engine comparable to the F135 (which powers the F-35). However, I suspect it's the latter.

Historically, Chinese jet turbine design has been behind the US and other western countries. The primary evidence of this is their usage of Russian engines in many of their military aircraft. For instance, the J-10 is powered by the AL-31, the same engine that powers the Su-27. So far as I know, the J-20 prototype also uses a version of the AL-31 (although the production aircraft are supposed to be powered by a domestic engine). I highly doubt that the Chinese would use a foreign engine on such sensitive military projects if they were confident in their abilities to produce a good enough engine domestically.

Also consider the Y-20, one of the PLAAF's recently developed transport aircraft.

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmu5_8i3Onk5cPrPJlbcoaVrgphEGNfscVQf_tMZCy7TC71JXLhmr45Lqtx1XrkloiNXsGc1f_ISCeOJCDvkDNjz_Alub6RZypxX3yzeKro_EuF27-_GUq_sdkfujWxvi1mZ1sizQ6F8jt/s1600/y20.jpg

Despite having a modern airframe design, the prototype still uses same Soloviev D-30s as the Il-76. While the Soloviev isn't a bad engine, it's a bit dated (anecdotally, it's also quite loud). Not what I would put on my brand new transport aircraft. While the production version is planned to use an indigenous high-bypass turbofan, this is still an indication that the Chinese are slightly lagging in engine design.

Of course, design of high performance turbines is hard (citation needed). Just because the PRC isn't quite on the same level as the US or Europe doesn't mean that their engineers are idiots.

Planet Formation Around HL Tau

Phil Plait, of Bad Astronomy fame, has written an article about probable planet formation around the young star HL Tau. Link. As can be seen from the picture, there are several gaps in the protoplanetary disk, indicating that planet formation has already begun in earnest. Plait guesses there are at least 3 planets, although some in the corners of the internet that I hang out in suspect that some of the gaps could be due to resonances (in a manner similar to the Kirkwood gaps), and there are actually fewer planets.

In any case, while there have been numerous pictures of dust disks around young stars before (Beta Pictoris is one of the earliest I can remember), this is one of the best ones that I can remember. Pretty fortunate that it was oriented roughly face-on toward Earth.

Not Nominal

A short while ago, an Antares rocket launched from Wallops Island exploded shortly after takeoff. The launch was supposed to be an ISS resupply mission, and had been scheduled to launch yesterday, before being canceled as a boat intruded into the downrange safety area.

At this point, we don't really know what caused this. It will probably be months before we do. Still, I'm going to engage in a bit of idle speculation.

From the video, it looks like the rocket broke apart very suddenly and spectacularly, and didn't deviate much from its course beforehand. Since it detonated extremely close to the pad, it's almost certain that this wasn't an activation of the range safety system, which would be used to destroy the rocket if it had suffered a failure in the guidance system.

Compare the Antares explosion to a recent failed Proton launch:

This article states that the crash was caused by a shutdown of one of the booster engines. The Antares rocket has two engines, compared to the Proton's 6, so in the case of an unplanned shutdown, it would have fallen back to the pad almost vertically. That it exploded in midair suggests that something caused the propellant tanks to rupture and ignite. In my opinion, the most likely thing to cause this would be some sort of overpressure event in the engine. Numerous faults in the engine could cause such an event, and I don't know enough to even make a guess at what it could be.


The Antares rocket uses a single AJ-26 engine in the first stage. This engine is better known as the NK-33, a development of the NK-15 engine used on the Soviet N1 lunar rocket. These engines were all built during the 1960s and 1970s, and subsequently refurbished by Aerojet for use in the Antares rocket. I'm certain that Aerojet has very robust quality control and inspection procedures, but it's not impossible that some microscopic defect could have slipped by. Additionally, until it was surpassed by a variant of SpaceX's Merlin engine, the NK-33 was the kerosene/liquid oxygen fueled engine with the highest thrust to weight ratio in the world; roughly 137:1 (the RD-275M had a slightly better thrust to weight ratio, but used UDMH/N2O4 propellants). That an engine with such performance was developed in the 1960s is a testament to the skill of Soviet engineers. It also suggests to me that the engine was very 'hot', basically squeezing everything possible out of the design with little room for error (I've also heard this about the RS-25). While this would allow higher performance, it (in theory) means that the engine could be more prone to failure. I'm not going to go so far as to say that this is definitively the cause of the launch failure, but it's my guess at this moment.

Ravaged Skies

 Found this quite good track (Ravaged Skies, by Mitch Murder) while browsing SA recently. Good music, and this video gets bonus points for using footage from 'The Right Stuff', an excellent movie which everyone and their children should watch. The Original Music Video Here, which is slightly different.

A minor historical note, while 'The Right Stuff' depicts Yeager's flight (and subsequent accident) in the NF-104 as being an unauthorized joyride, it in fact took place as part of a regularly scheduled test mission. However, according to this summary, the accident was very much Yeager's fault.

Soviet/Russian Spaceplane Concepts

During the Cold War, both the US and Soviet Union developed numerous concepts for reusable spaceplanes. Many of these concepts, such as the X-20, MiG-105, STS, and Buran, are well known. While flipping through my copy of Unflown Wings, I happened across a couple of other, lesser known spaceplane concepts developed in the Soviet Union and Russia during the past few decades. To my knowledge, none of these ever flew, but there's some interesting designs that were drawn up.

(credit for all pictures goes to Yefim Gordon and Sergey Komissarov)

 The first of these is the Tu-136, also known as Zvezda, which was developed during the late 1960s. As is apparent from the picture, it appears to be roughly analogous to the X-20, although there are some notable design differences. Like the X-20 it would have been vertically launched on top of an expendable launch vehicle, most likely an existing Soviet booster. Based on the weight given for the vehicle (7-9 tons), it is likely that an early version of the Proton rocket would have been the launch vehicle for the Tu-136. Like the X-20, the Tu-136 would have been intended for military usage in LEO, although the exact nature of these missions is unknown.
 Interestingly, it appears that this design was intended to be able to operate in both manned and unmanned modes.

 

The next design was the Tu-2000. This was not a single design, but rather a series of several conceptual spaceplane designs.  Unlike the Tu-136, they would have been launched horizontally, either from a runway or a purpose built (and truly mammoth) carrier aircraft. Given that the Tu-2000 was to function as an SSTO spaceplane, massive amounts of LH2/LOX fuel would have been needed. Some of the concepts would have had takeoff weights of almost 300 tons. Total orbital payload was projected to be in the 5-10 ton range.

In Unflown Wings, it is mentioned that the Tu-2000 would have featured up to six turbojet engines (in addition to liquid fuel rocket engines), in order to improve in-atmosphere performance. Interestingly, it is also mentioned that use of a nuclear rocket engine was considered, due to its higher specific impulse compared to chemical engines. This is probably the first time I have heard of a nuclear engine being proposed for such an application.

The MiG AKS (AKS roughly translates as 'aerospace system') was a proposal for a reusable spaceplane dating to the 1980s/1990s. As can be seen from the picture, it would have been launched from a high speed 'mothership' aircraft, likely travelling at hypersonic speeds. The mothership was planned to operate on conventional jet fuel at low speeds, and LH2 at hypersonic speeds. While this would have complicated logistics greatly, it was believed that the benefits of an air launch (reduced delta-v, more flexibile launch site) would have been worth the disadvantages.
There is also brief mention in Unflown Wings of a MiG designed spaceplane named 'Oryol'. This was intended to be ground launched, with one proposal having it launched by an electromagnetic mass driver. However, it was seen as less technically feasible than the AKS, and was not developed as far.

 One interesting design from the 1990s is the S-XXI, developed by the Myasishchev design bureau. Unlike most other Soviet/Russian concepts, which were intended to be fully capable of reaching orbit, and used for military missions, the S-XXI would have been suborbital, and used for space tourism. Launched from the back of a modified M-55 carrier aircraft, the S-XXI would have reached an altitude of just over 100 km on a ballistic trajectory, allowing the passengers to briefly experience weightlessness. Though it was intended to have flown in 2005, it was ultimately cancelled due to lack of funding.