In 2015 I wrote about the concept of purpose-built air tankers, and emphasized that they do not need to 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
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.
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.
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.
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.
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.
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.
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.
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.]