Staying High—Engineering a Mid-Air Refueling System
Sanjeev Pal posted on June 30, 2016 |
Air-to-air refueling diagram. (Image courtesy of Mentor Graphics.)

Air-to-air refueling diagram. (Image courtesy of Mentor Graphics.)

Air-to-air refueling is essential for military aircraft that have to fly continuously over very long distances when landings and takeoffs are not an option. This can happen if planes are flying where there are no air bases or over hostile territory.

Long-range bombers may have missions half way around the world. Fighter jets may have to stay in the air longer than their relatively small fuel tanks will allow, or may find they have exhausted their fuel unexpectedly, such as with supersonic flight or evasive maneuvers. To help in these situations, large tanker aircraft are deployed that carry sufficient fuel to refill several smaller aircraft in a single mission.

But as you might imagine, injecting very volatile jet fuel from one aircraft to another while both are moving at high speed and altitude is fraught with risk.

Design Issues

While ground fueling is the norm, midair refueling will add a host of challenges for the aerospace engineer.

“Military aircraft fuel systems can be very complex because they have to be able to handle many different operational scenarios such as ground fueling as well as air-to-air refueling,” said Michael Croegaert, who serves as senior military and aerospace industry manager at Mentor Graphics. “The aircraft must maintain a balance all while undergoing dramatic altitude changes and possibly an engine or component failure. This all must be done while minimizing weight and utilizing irregularly shaped spaces for fuel storage.”

Engineers need to simulate these operational conditions in order to ensure their designs are safe and functional in these environments. This is no small feat.


To make both air-to-air refueling and on-land fueling safer, a process called inerting is used. An inert gas is pumped into the ullage (the nonfuel volume of a tank). This reduces the concentration of oxygen and volatile vapors.

Simulations can help engineers ensure the fuel ullage vapor is inert as it is added to the tanks. The simulations help determine the amount of nitrogen that needs to be produced by the on-board inert gas generation system to help drive out the oxygen in the tank that would support the combustion of the fuel vapor.

“Engineers calculate this amount by simulating fuel flow rate in and out of a tank and determining the amount of ullage space that is in the fuel tank at any point in time,” explained Croegaert.“The amount of air that is entering or leaving the tank through the vents can also be determined with this simulation.”

System-level simulation tools like Mentor Graphics’ Flowmaster allow for these designs to be tested for various scenarios in a short time relative to testing.

“Flowmaster provides a large component library, including tailored aerospace components,which allows engineers to quickly construct and simulate the entire fuel system,” explained Croegaert.“A single model can be used to run all of the different test scenarios that must be considered.”

Sizing and Validating Fueling Design withSteady-State 1D Simulations

Figure 1 - A wing mounted aerial refueling pod (image from Cobham website)
Figure 1 - A wing mounted aerial refueling pod (image from Cobham website)
Model of an aircraft refueling system. (Image courtesy of Mentor Graphics.)
Model of an aircraft refueling system. (Image courtesy of Mentor Graphics.)

Ideal Conditions or Worst Case?

To determine an optimum specification and size of fuel system components, engineers will often turn to 1D steady-state simulations.

“It is much simpler to determine sizing and flow balancing requirements in a steady state than in transient conditions,” instructed Croegaert. “A good example of this is line sizing. There is an optimum flow rate and pressure for refueling the aircraft to achieve this along with balancing the inflow from either side of the aircraft to the other.Steady-state analyses can be run to find the optimum line size and flow restrictor sizes for achieving the optimum performance.”

However, a steady state is not always the case. A fuel system needs to operate in many scenarios. Engineers must consider extreme situations and ensure their designs will handle them. This is where parametric simulations can help.

“Simulation helps engineers to quickly determine the areas of concern in a model that maybe impacted by high pressure spikes or overly fast or overly slow valve actuation,” said Croegaert. “Conducting a parametric study allows the user to run a series of analyses while changing specific variables such as piping size, orifice diameter or valve control logic to find the optimal design that meets all of the design criteria for the specific system.”

Transient 1D CFD Simulations Ensure Air-to-Air Is Safe and Balanced

Pressure vs. time transient solution of an AAR receiver tank. (Image courtesy of Mentor Graphics.)

Pressure vs. time transient solution of an AAR receiver tank. (Image courtesy of Mentor Graphics.)

Transient simulation of fuel systems can help engineers ensure the aircraft will remain balanced during air-to-air refueling. Fuel tanks can be in either wing. Filling one fuel tank too fast can cause the aircraft to roll and disconnect from the tanker aircraft.

“To understand the aircraft balance, several dynamic factors must be considered, including control valve operation, aircraft pitch and roll and fuel flow rates,” explained Croegaert.“All of these factors must be considered when running transient simulations to figure out if the refueling control logic can properly adjust the system to maintain balance.”

The potential for an unbalanced fill can be reduced with control systems that monitor the fuel level and flow rates in each tank. This control system will then constantly adjust valves to minimize the imbalance.

“Flowmaster has a catalog of control components such as PID controllers, gauges and user-programmable controllers that allow engineers to virtually recreate the actual control system to come up with an optimum design,” explained Croegaert. “Alternatively, Flowmaster also allows the user to connect to Simulink directly and run a co-simulation where the control system is modeled in Simulink alongside the fuel system simulation in Flowmaster.”

Fluid Hammer, Cavitation and Erosion

As the receiving aircraft disconnects from the tanker during an air-to-air fueling, the check valves close rapidly. This can cause fluid hammer on both aircraft. “The effect on piping causes excessive pressure surges, damaging the entire system,” said Croegaert.

Simulation can help determine the maximum pressure tolerance and pressure spike limits of the system. Flowmaster provides simulations for fluid hammer and other rapid transient events in the fuel system that might cause damage. Engineers can evaluate system responses for the full range of temperatures and pressures experienced during a mid-air refueling.

A pressure drop below the vapor pressure of a liquid will make for bubbles that will attach themselves to surfaces and implode. If this is done often enough, such as with fuel impellers, the result will be pitting of the surface, a process known as cavitation. Erosion is sure to follow. Also, the bubbles can coalesce, adding compressibility to the otherwise incompressible fluid.

“Pumps over spinning, rapid valve actuation and shifting orifice sizes can lead to cavitation in fuel systems,” said Croegaert. “While many of these can be determined during steady-state analysis, some of them show up only over time. This makes it useful to run both steady-state and transient-state simulations.”

Combining the 3D simulation of the different complex components with 1D piping system analysis will allow engineers to quickly test many scenarios and reduce costs and development time. Sensors and simulation data have made modern refueling operations safer. However, by continuously learning from sensor data and simulations, refueling systems will become more reliable, safer and cost-effective. The best part?Engineers can design these optimized systems with less time spent on physical testing in the field.

To learn more about the simulation of aircraft refueling, read this white paper.

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