Atmospheric Flight on Titan

In the far future, it is possible that mankind will settle bodies other than Earth. One question which is not commonly thought of is the question of atmospheric flight on places other than Earth. Obviously, it would be impossible to fly an aircraft on bodies without an atmosphere, such as the moon. However, on others, such as Venus, Jupiter, or Mars, it could be possible. One especially good candidate is Titan. Despite having less than 1% the mass of Earth, Titan retains a massive atmosphere. Combined with that moon's low gravity, this means that flying should be easy there.

One of the first questions that must be solved when designing an aircraft for Titan is the propulsion system. Obviously, a simple chemical rocket could be used. One attractive propellant combination would be a methane/LOX engine. Water ice is believed to exist in quantity below the surface of Titan, and from this LOX can be made. Though the majority of the atmosphere of Titan is nitrogen, methane is also present, and the quantity increases at lower altitudes (up to about 4-5%). The surface hydrocarbon deposits also contain massive amounts of methane, as well as other hydrocarbons such as ethane and propane, which could be used as fuel or refined into methane.

However, chemical rockets are inefficient, so what other options are there? One exotic option, suitable for a large aircraft, would be a nuclear turbine, similar to the J87. Operation would be very similar to on Earth: the incoming Titanian atmosphere would be heated by the nuclear reactor, then expelled at high speed, producing thrust. This concept would have the advantage of virtually infinite range (though this would be of somewhat less consequence on a small body such as Titan). However, the mass of the reactor and shielding would severely cut into payload, making this method only suitable for a very large aircraft.

Aircraft on Earth carry their fuel with them, and harvest oxidizer from the atmosphere. What if we reversed this? What if our hypothetical Titanian aircraft instead harvested fuel from the environment, while carrying its own oxidizer?

We will use liquid oxygen (LOX) as our oxidizer, and methane as our fuel. The atmosphere of Titan has a pressure of about 146 kPa at the surface, and about 4% of this is methane. This means that the partial pressure of methane at the surface is roughly 5-6 kPa. This is close to the partial pressure of oxygen in Earth's atmosphere at an altitude of 10,000m. Given that operation of conventional jet engines is possible and that altitude is possible on Earth, it is reasonable to assume that there will be enough methane for our engine to operate on Titan, at least at low altitudes.

Analysis of the properties of methane indicates that it will be liquid at Titan's surface. On the other hand, nitrogen exists as a gas in Titan's atmosphere. This is useful; through careful design of the inlet, it would be possible to separate the methane fuel from the useless nitrogen. With the methane obtained, and liquid oxygen carried on board, our engine would function similar to a liquid air cycle engine. (More info on this type of engine can be found here). Though the specific impulse of the engine itself would be approximately the same as a conventional methane/liquid oxygen engine, the elimination of the need to carry fuel would dramatically increase payload and efficiency.

Alternatively, rather than separating methane out from the nitrogen, the Titanian atmosphere could  be inducted in a gaseous state. This would allow a Brayton cycle engine to be used, as is done in a conventional jet turbine (LOX would again be used as the oxidizer). Though I have not yet done the math, it seems to me as though this would offer increased efficiency over the previous option (the Liquid Methane Cycle Engine?), though less thrust. Which option is better for our far-future Titanian aircraft would depend on the specific application.

Air Superiority F-106 - Pulling Out All the Stops

Months ago, I wrote a piece on the feasibility of the F-106 Delta Dart as an air superiority fighter. I ultimately came to the conclusion that the F-106 could, with slight modifications, serve as a decent multirole aircraft. But what if we made more than slight modifications?

Suppose that, at sometime around 1980 a country using the F-106 wanted to modify them such that they could be used in the air superiority role for at least another decade. How would they go about doing this?

Those of you who are more reasonable might ask why the country in question doesn't just buy a bunch of upgraded F-4s or MiG-23s (or even some F-15s or 16s) and use them instead. Well, they could, but that would be a quite boring end to this post.

I'm going to use the development of the Atlas Cheetah as a model for our F-106 improvement program. The Cheetah was developed by apartheid-era South Africa from their existing Mirage III airframes, and involved a quite extensive rework of the aircraft, including a complete rebuild of the airframe, fitting of an upgraded engine (in the Cheetah C model), as well as new electronics and weaponry. The Mirage III dates from approximately the same time period as the F-106, and is broadly comparable in performance, so such a project should be (theoretically) feasible.

Calling our new aircraft the F-106M, we can start by looking at what we don't need. The fairly extensive electronics fit allowing the F-106A to interface with the SemiAutomatic Ground Environment (SAGE) is redundant, so it can go. That should save us a couple hundred kilos, and free up some space. Equipment needed for the carriage of nuclear weapons (such as the AIR-2) can also get removed. In its place an autocannon such as the M61 can be permanently fitted.

Though the F-106 was originally designed as an interceptor, it actually has decent turning ability, at least at high speeds. However, at low speeds, the aircraft tends to lose energy. This tends to be an issue inherent with dynamically stable pure deltas, such as the F-106. One option to improve lift at low speeds and high angles of attack would be to fit canards, such as on the AJ37. This would require extensive modification to the forward fuselage and air intakes of the aircraft. Another option would be to fit leading edge extensions to the wings, as are found on the F-18. These leading edge extensions generate vortices, which increase lift at high angles of attack. Additionally, they would shift the center of lift forward, reducing the static margin of the aircraft and improving performance. (Here's a brief explanation of why instability is good in an aircraft.) Another benefit would be reduction of takeoff distance, i

The second major change is in the powerplant. The J75 is not a bad engine by any means, but by 1980 it's getting quite long in the tooth. If possible, we should re-engine the F-106M with a newer powerplant. One of the best candidates is the Pratt & Whitney F100. Though the F100 is about a meter longer than the J75, it is only 9 cm larger in diameter. Given that the structure of our improved F-106M will be undergoing a substantial refurbishment, increasing the diameter of the engine bay and lengthening the fuselage slightly should not be an insurmountable obstacle. In fact, the largest challenge would probably be redesigning the intakes to accommodate the F100. Obviously, some sort of variable geometry intake should be retained, to take advantage of the F-106's exceptional speed (variable geometry intakes are virtually essential for speeds about Mach 2).

In exchange for all this trouble, the F100 gives us significantly improved fuel consumption compared to the J75. Additionally, the F100 has a thrust to weight ratio about double that of the J75, so it ends up weighing about half as much. With the weight of the F-106M reduced by up to 1,000 kilograms compared to the base model, range will be improved, as will the maneuverability of the aircraft (thanks to reduced wing loading).

Most of the other changes are internal. An improved radar would be essential, as the F-106s aging 1950s era fire control systems were designed to target large, slow targets at extreme range in conjunction with ground control, rather than air superiority operations. One candidate would be the AN/APG-63, which was fitted to early model F-15s. Whether this or another radar is used, it is likely that a redesign of the nose section would be required (as was planned for the F-106C, which would have been fitted with the radar from the aborted XF-108 program). Avionics for ground attack would also be essential, in order to give the F-106M true multirole capability. These could be fitted in place of the deleted SAGE equipment found on the F-106A. Not only would this include fitting a modern bomb sight, but compatibility with guided munitions such as the GBU-12. In order to allow the F-106M to utilize laser guided munitions, it could be possible to give the aircraft a built-in laser designator. However, a more likely option would be the fitting of a targeting pod to one of the underwing hardpoints on appropriate missions.

Additionally, compatibility with modern weapons, such as all aspect variants of the AIM-9 and AIM-7. A helmet mounted sight (as developed by the South Africans in the 1970s), when coupled with an all-aspect IR missile would result in a massive increase in dog fighting capability. This would keep the F-106M viable in a close in fight, despite its inferiority in this arena against designs such as the MiG-29 or F-16.

Numerous other, smaller changes could be made, such as replacing the old ejection seats with an improved model, and fitting a new RWR. While completely rebuilding an F-106 into a 4th generation fighter was never likely to happen, it's interesting to look at whether such an undertaking might have been possible at all.

This Blog Isn't Dead

After a long hiatus, I've decided to get back into this. Hopefully I'll stick with it.

In the meantime, I've rearranged some of the sidebars a bit. Go ahead and check out some of the links I've added.

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.