Sunday, September 27, 2015

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.

    Wednesday, September 9, 2015

    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.

    Tuesday, September 8, 2015

    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.