Tag: air tanker design

The American Institute of Aeronautics and Astronautics (AIAA) Foundation has announced the winners of a Team Aircraft Design Competition open to undergraduate AIAA branches or at-large Student Members.  The task was to design from the ground up a purpose-built large air tanker.

Specifications for the aircraft included 4,000 to 8,000 gallon retardant capacity, 2,000 to 3,000 nm ferry range, it would use existing available engines, and have a dash speed of 300 to 400 knots. Other criteria included a drop speed of 125 to 150 knots and takeoff from a Balanced Field Length of 5,000 to 8,000 ft. with an assumption of +35°F standard atmosphere at an altitude of 5,000 ft. above mean sea-level.

The design teams awarded first, second, and third places all chose 8,000-gallon gravity-powered retardant tanks. The estimated prices of the aircraft are based on a manufacturing run of dozens of each aircraft. The teams’ complete proposals are at the links below.

First Place

The “Fireflighter” designed by a team from Nanyang Technological University in Singapore took first place. (See image above). It is powered by four turboprop engines and has a dash speed of 410 knots. The students estimate it would cost $75 million.

Second Place

Team Njord, from the University of Illinois at Urbana-Champaign designed the “Valkyrie,” powered by two jet engines. Its dash speed is 300 knots and would use two removable RADS-XXL retardant tanks, enabling it to carry 20,000 pounds of cargo at the aircraft’s ferry range of 3,000 nm. It would sell for approximately $186 million.

Third Place

The Albatross team, also from the University of Illinois at Urbana-Champaign, designed the “Firehawk” powered by two jet engines. It would have a dash speed of 380 knots and sell for about $91 million.

In 2015 I wrote about the concept of purpose-built air tankers, and emphasized that they do not need to be built around empty space that in the past carried hundreds of passengers. Here is an excerpt from that article.

“I’d like to see the K-MAX engineering team design from scratch a fixed wing air tanker built around the following components, glue them together, and then configure them to be airworthy, capable of flying at least 350 mph, and able to take off from Ramona, California with a full load of retardant on a 90 degree day;

• 5,000 to 10,000-gallon tank,
• cockpit for two (no passengers; possibly a third seat for an inspector pilot or trainee),
• fuel, and
• engines.”

In 2015 the American Institute of Aeronautics and Astronautics (AIAA) Foundation held a Graduate Team Aircraft Design Competition open to undergraduate AIAA branches or at-large Student Members. They competed for prizes ranging from $500 for first place to $125 for third.

In October, 2016 the AIAA announced the three winners. The team that finished first was California Polytechnic State University, Pomona and their Ember project. On March 26 we wrote about the third place winning team. And on March 29, the second place entry.

Here we will describe the Cal Poly Ember Aviation entry in the competition.

Their key driving concept was a mindset they called Size Zero:

The idea behind this was to eliminate any and all wasted space within the aircraft. The goal for our team was to utilize every inch within the aircraft, hence the size zero name for this design concept, in which there was zero wasted space within the airframe…. Our team stayed focused on making sure that every component installed on the aircraft, earned its way on to the aircraft structure.

The task from the AIAA was to design from the ground up a purpose-built large air tanker. Specifications for the aircraft included a crew of two pilots, 5,000 gallon retardant capacity, 2,500 nm ferry range, dash speed of 300 knots, and powered by turbofan or turboprop engines. Other criteria was a drop speed below 150 kt, stall speed of 90 kt, and takeoff from a Balanced Field Length of 5,000 ft. with an assumption of +35°F standard atmosphere at an altitude of 5,000 ft. above mean sea-level. In addition, fatigue stresses should be strongly considered.

Below are excerpts from the team’s 94-page proposal. Some light editing was done in the interest of brevity:

Twin boom tail
Given our effort to eliminate wasted space, more specifically the aft fuselage that must be in place to support the vertical and horizontal tails, a different kind of tail must be designed. In the case of our aircraft, a twin-boom tail was deemed to be an ideal solution to this problem. A twin boom tail not only allows for the elevator to be placed up high in free-stream clean air, thus increasing the efficiency of the horizontal stabilizer, but also allows for a smaller overall vertical tail area, given that there are two vertical surfaces, as opposed to a single tail surface. Given the heavy payload, the larger the elevator, generally the better the aircraft takeoff performance is. By utilizing this twin-boom design, a significantly larger horizontal stabilizer can be designed, without creating a difficult structures problem for supporting a tail that large and heavy on a conventional “cigar tube” aircraft. Our aircraft also is unlikely to be ever be reconfigured for any duty other than fire fighting, thus eliminating the necessity for reconfigurable aft fuselage space.

Additionally, the biggest disadvantage of a twin boom design is actually an advantage for this particular mission. Twin boom aircraft tend to have very small pod-like fuselages, but since our design is only ever required to carry a dense liquid payload the small fuselage greatly reduces wasted space.

Range
On one load of fuel it will be able to perform four sorties on a 200 nm radius (from base) per sortie with the capability of performing three retardant or water drops during each sortie. The aircraft shall be able to perform a ferry range of 2,500 nm.

Not pressurized
The aircraft will not be pressurized; however, we will have oxygen masks provided to the pilots in order for them to breathe oxygen with ease.

Engine
The engine finally selected for this design is the General Electric CF34-8C, which produces a peak static thrust of 23,600 lb, and an estimated SFC of 0.38 lbm/hr/lbf at cruise. This engine successfully met the thrust requirements, but also allowed the aircraft to meet the balanced field length requirement, without excess thrust and engine weight, that would have been a hindrance on an empty aircraft during a ferrying mission. Emphasis was placed on the GE CF34 line of engines during the selection process due to the overall simplicity of engine maintenance in comparison to engines manufactured by other countries that produced a similar thrust output, as well as the familiarity that many civilian aviation mechanics already have with the operation and maintenance of the CF34 engines.

To avoid an overly large vertical tail, the goal was to place the engines as close to each other as possible, thus limiting the yawing movement created by having an engine out. One of the best ways to prevent this yawing moment is to place the engines on the top surface of the wing, thus not being constrained by the width of the fuselage. By placing the engines on the top surface of the wing, our aircraft also gains the ability to land on unimproved runways and landing strips.

To protect the main wing spars and the fuel tank from an uncontained engine failure the engine nacelle will have a Kevlar shield around the entire engine, as well as a shield to protect the spars and the fuel tanks. To further prevent fire, the fuel tank will be made with a self-sealing bladder to prevent and limit the likelihood of fuel leaks if the tank becomes punctured.

Maximum takeoff weight
With the fuel (12,048 lbs) and retardant payload (45,000 lbs) weight added, the maximum takeoff weight is 89,076 lbs.

Cabin
The main cabin was designed for two pilots with exit doors on each side of the cockpit. There is also a barrier between the tank and the cabin, with a door on the back of the cockpit for easy access during maintenance.

Retardant tank
The aircraft has a retardant-tank-shaped fuselage to avoid having unnecessary empty space in the fuselage.

To mitigate corrosion of the 2024-T3 aluminum, the internal structure of the tank will be conversion coated with Alodine® 1201, after completion of assembly. Before each flight season, the internals of the tank must be sprayed with a paraffin based Tectyl type wax, to further mitigate corrosion, and to protect the surface conversion coating.

After each flight cycle/mission in which retardant is dropped, the payload tank will be flushed with water. If any retardant is allowed to solidify and gel, corrosion becomes an immediate risk. While flushing the tank after each flight day becomes wasteful, it is vital to maintaining a corrosion free airframe. In an attempt to prevent the wasting of water, the tank rinse water can be collected, and stored, to be mixed with more water and retardant concentrate for subsequent flights.

Auxiliary power unit
The APU will be in the aft fairing that is used to blend the payload tank into an aerodynamic shape. This not only utilizes the empty space created by the fairing, but also allows for a simple and effective exhaust vent at the tail of the fairing. Given the size constraints of the aft fairing, our selection was limited to auxiliary power units in use on small regional passenger aircraft. The APU that was chosen for this application was the UTC Aerospace APS-500R. It weighs approximately 120 lbs. dry, and includes an FAA TSO C77A certified full authority digital engine controller (FADEC) unit, and is more than capable of starting our selected GE CF34 series engine.

Situational Awareness
Our design features several cameras located throughout the airframe to improve situational awareness. One of these cameras will be a forward looking infrared (FLIR) camera. Infrared cameras would allow the pilots to see obstacles and terrain through dense smoke that would otherwise obscure their vision. Three more visible light cameras will be located in the tail and each wingtip. These cameras are for providing spatial awareness and to help the pilots gauge their clearance from obstacles when flying at low altitude and during ground operations at small airports.

Cost
The total research, test, development and evaluation costs were calculated at $3.6 to $4.5 billion in 2022 dollars.

The flyaway cost would be $36 to $45 million per unit with a breakeven point of 76 to 79 units, assuming a 12 percent profit margin.

The team computed the direct operational cost per mission at $22,970, including fuel and retardant. [Note from Bill: Those two items account for 90 percent of the cost, according to the team’s calculations, however current Forest Service contracts specify that the government will pay those costs, not the contractor. Subtracting those costs brings the figure down to approximately $2,297 per mission. The team estimated fuel needed for the standard mission profile to be 382 gallons of JP-8.]

In 2015 the American Institute of Aeronautics and Astronautics (AIAA) Foundation held a Graduate Team Aircraft Design Competition open to undergraduate AIAA branches or at-large Student Members. They competed for prizes ranging from $500 for first place to $125 for third.

In October, 2016 the AIAA announced the three winners. The team that finished second represented Sharif University in Tehran, Iran, for their Anahita aircraft. On March 26 we wrote about the third place winning team. In a later article we will cover the team that took first place, but here we will describe the Sharif University entry in the competition.

The task from the AIAA was to design from the ground up a purpose-built large air tanker. Specifications for the aircraft included a crew of two pilots, 5,000 gallon retardant capacity, 2,500 nm ferry range, dash speed of 300 knots, and powered by turbofan or turboprop engines. Other criteria was a drop speed below 150 kt, stall speed of 90 kt, and takeoff from a Balanced Field Length of 5,000 ft. with an assumption of +35°F standard atmosphere at an altitude of 5,000 ft. above mean sea-level. In addition, fatigue stresses should be strongly considered.

Not being fluent in Persian, I looked up “Anahita”, the name the team gave their project, and found that it refers to an Iranian goddess associated with fertility, healing and wisdom.

Curiously, there was a fairly lengthy section in the team’s proposal discussing the feasibility of converting a 737 into an air tanker. The proposal was written in 2016, and in May, 2017 Coulson announced that they had purchased six 737-300s to convert them into 4,000-gallon “Fireliner” air tankers. Britt Coulson said they saw an opportunity when Southwest Airlines made a decision to replace their 737-300’s with the new 737-Max.

The students designed an aircraft with a high wing, “H” tail, and two turbofan engines under the wings. It would carry enough fuel for two sorties of three drops each.

The retardant tank would be cylindrical,  8.5 feet long with a diameter of 2.5 feet. My calculations determined it would only hold 312 gallons, far short of the 5,000 gallon requirement.

The students specified that the tank would be pressurized, “…so the drop operation will be more precise and there will be no splashing.” But there was no description of how that would be accomplished — with an onboard air compressor, a ground-based compressor, or another method.

The proposal mentioned that the aircraft could also carry cargo, and in another section, that the tank could be removed in minutes. There were no other details about cargo; presumably the tank would be removed to make it  multi-role capable. The document described the cargo door in a 737 but there was no mention of one in their purpose-built air tanker design.

The aircraft will have two pilots, and since it must be able to drop on its own without the need for a lead plane, the designers determined that an observer would be on board to monitor the fire. Extra windows cannot be installed in the cockpit because it would greatly increase structural fatigue. So for the observer to have an effective view of the fire area, five cameras will be embedded in the skin providing imagery to a Virtual Reality (VR) helmet worn by the observer, similar to the helmet worn by F-35 pilots. Two cameras would be in the nose, one in the middle, and two in the rear.

In order to determine the location of the drop, two infrared cameras would map the fire and, the students wrote, “…with the help of the geological data of the terrain and wind speed and direction, the fire behavior can be predicted by a computer and the optimal location of drop can be realized.”

Cost
They found an air tanker study that recommended an optimal number of 28 federal large air tankers. Based on production of 28, the unit cost would be approximately $279M. If 120 units were manufactured for worldwide use, the cost per unit would drop to $126M.

From the Conclusion section of the proposal
“It was realized that the 160-day contracts do not result in economical solutions to the LCC of Anahita, therefore, additional capability of performing alternate missions was considered. The 160 contracts per year were predicted to reach 208 by 2100, this indicates that the firefighting operation hours will increase in the future. Increase in the fleet was recommended in order to lower the unit cost, this was justified based on the increasing number of fires and 37 international agreements on forest fires between US and other countries, it was then indicated money for every player is guaranteed by Export Credit Loan, so foreign contractors could also be involved.”

You can download a copy of their proposal.

The designers:

In 2015 the American Institute of Aeronautics and Astronautics (AIAA) Foundation held a Graduate Team Aircraft Design Competition open to undergraduate AIAA branches or at-large Student Members. They competed for prizes ranging from $500 for first place to $125 for third. The task was to design from the ground up a purpose-built large air tanker. Specifications for the aircraft included a crew of two pilots, 5,000 gallon retardant capacity, 2,500 nm ferry range, dash speed of 300 knots, and powered by turbofan or turboprop engines. Other criteria was a drop speed below 150 kt, stall speed of 90 kt, and takeoff from a Balanced Field Length of 5,000 ft. with an assumption of +35°F standard atmosphere at an altitude of 5,000 ft. above mean sea-level. In addition, fatigue stresses should be strongly considered.

In October, 2016 the AIAA announced the three winners. Today we are writing about the third place contestant — in subsequent days we will cover the second and first place winners. Yes, even though we covered the initial announcement of the competition, we are a little late to the party to write about how it turned out. (No sense in rushing into these things.)

Receiving the award for third place was a 10-person group from California State Polytechnic University, Pomona, who called themselves the AeroTactic Company Team. You can download their very, very technical 65-page proposal.

The team’s FF-1 Rainbird aircraft design features an integrated retardant tank with a gravity-fed dispersal system. It is powered by two Rolls Royce Tay turbofan engines underneath the wings. Winglets increased the Rainbird’s lift to drag ratio by 5%, consequently increasing its fuel efficiency.

Their rational for choosing to have it piloted rather than unmanned, was,  “A piloted aircraft is more flexible to changing scenarios compared to a UAV. In addition, an unpiloted aircraft would cost more to maintain and to build due to the additional sensors, hardware, and software required.”

The high density payload combined with the low stall speed requirement drove the design of the aircraft to contain large wings and a small fuselage.

The two engines are attached to the wing’s leading edge, protruding past the wing in order to balance the aircraft for all loading configurations. It is capable of carrying enough fuel for four sorties and can reload the retardant within ten minutes.

The 5,000-gallon tank is located at the center of gravity of the aircraft, limiting CG shift while releasing its load. Their goal was for the fuselage to be as small as possible while still being long enough to maintain aircraft stability, hence the larger length of the tank relative to its other dimensions. The tank can be refilled from three fill ports simultaneously.

The group determined that for the aircraft to reach a fire 200 nm away in the least amount of time, 54 minutes, it should cruise at 20,000 feet at 245 kt.

The cockpit, but not the entire aircraft, would be pressurized and air conditioned. (Not all three of the student entries have pressurization.)

The aircraft will have two infrared cameras, priced at $4,200 each.

Cost
The graduate students assumed 200 units would be produced over the next 80 years with combined 20-year life spans covering 100 years. They estimated the research and development would cost $1.89 billion, including building and testing three aircraft. Total cost of production would be $89 billion, inflated to 2022 dollars (when delivery would begin). The flyaway cost for each unit was estimated at $45.3 million in 2022 dollars, including 10% profit. At 131 units, the program would break even in costs and begin to generate profit.

Below is an excerpt from the proposal:

“The cost comparison between the [students’] FF-1 and the C-130 and DC 10 was obtained through the analysis of depreciation, the costs associated with retrofitting an existing aircraft into an air tanker, and the limited life of the aircraft in the year 2022. The cost of retrofitting an air tanker was approximated to be $23.71 million, not to mention the installation of a retardant delivery system which is another $6.38 million. The cost for the both the C-130 and DC-10 were obtained from Military Aircraft. Table 17.5-1 exhibits the costs of the FF-1 configurations and the retrofitted competing aircraft.”

The table below shows how the design met the required specifications.

The American Institute of Aeronautics and Astronautics is conducting a competition for the design of a large air tanker open to undergraduate AIAA branches or at-large Student Members. They will be competing for prizes ranging from $500 for first place to $125 for third.

The purpose-built air tanker in this design competition will have a crew of two pilots, 5,000 gallon retardant capacity, 2,500 nm ferry range, dash speed of 300 knots, and will be powered by turbofan or turboprop engines.

The winners will be announced in August of 2016. It will be interesting to see what they come up with.

Thanks and a tip of the hat go out to bean.