Long-range bombers may have missions halfway 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 during supersonic flight or evasive maneuvers. In these situations, large tanker aircraft are deployed that carry sufficient fuel to refill several smaller aircraft in a single mission (Figure 1). The task of injecting volatile jet fuel from one aircraft to another while both are moving at high speed and altitude is fraught with risk.

Figure 1. Designing a safe and effective air-to-air refueling is a challenge for aerospace engineers.

Design Issues

Military aircraft fuel systems are complex because they have to handle many different operational scenarios, including ground fueling as well as air-to-air refueling. Design challenges for aerospace engineers include making sure the aircraft can maintain balance while undergoing dramatic altitude changes and possibly an engine or component failure, minimizing weight, and using irregularly shaped spaces for fuel storage. They need to simulate these operational conditions to ensure their designs are safe and functional in these environments.


Engineers use a process called inerting to make both air-to-air refueling and on-land fueling safer. An inert gas is pumped into the ullage (the nonfuel volume of a tank) to reduce the concentration of oxygen and volatile vapors. Simulations help engineers ensure the fuel ullage vapor is inert while it is added to the tanks. The simulations calculate the amount of nitrogen that needs to be produced by the onboard 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. 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 such as Mentor Graphics Flowmaster allow for these designs to be evaluated for various scenarios in a short time relative to testing (Figure 2). The software includes a large component library with tailored aerospace components that allows engineers to quickly construct and simulate the entire fuel system. A single model can be used to run all of the different test scenarios that must be considered.

Sizing and Validating Fueling Design with Steady-State 1D Simulations

Figure 2. 3D model of a wing-mounted aerial refueling pod and 1D model of the aircraft refueling system.

Engineers often use 1D steady-state simulations to determine an optimum specification and size of fuel system components because determining sizing and flow balancing requirements is simpler in a steady state than in transient conditions. A good example of this is line sizing. There is an optimum flow rate and pressure for refueling the aircraft that can be achieved along with balancing the inflow from either side of the aircraft to the other. Steady-state analyses find the optimum line size and flow restrictor sizes for achieving the optimum performance.

However, a steady state is not always the case in real-life situations. The fuel system needs to operate in many scenarios. Engineers must consider extreme situations and ensure their designs will handle them. Parametric simulations can help engineers quickly determine the areas of concern in a model that might be affected by high pressure spikes or overly fast or overly slow valve actuation. Conducting a parametric study allows an engineer 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

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. Several dynamic factors must be considered to understand the aircraft balance, including control valve operation, aircraft pitch and roll, and fuel flow rates. 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.

Figure 3. Pressure versus time for locations upstream and downstream of the reception coupling.

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 constantly adjusts valves to minimize the imbalance. The simulation software has a catalog of control components such as PID controllers, gauges and programmable controllers that allow engineers to virtually recreate the actual control system to come up with an optimum design. Alternatively, the engineer can connect to Simulink directly and run a co-simulation where the control system is modeled in Simulink alongside the fuel-system simulation.

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. Simulation assists in determining the maximum pressure tolerance and pressure spike limits of the system (Figure 3). The thermal simulation software provides scenarios 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 event.

A pressure drop below the vapor pressure of a liquid will create bubbles that attach themselves to surfaces and implode. If this occurs often enough, such as with fuel impellers, the result will be surface pitting, 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. While many of these can be determined during steady-state analysis, some of them show up only over time. This is why running both steady-state and transient- state simulations is useful.

Combining the 3D simulation of the different complex components with 1D piping system analysis allows 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. And with the latest simulation tools, engineers can design these optimized systems with less time spent on physical testing in the field.

This article was written by Sanjeev Pal, Industry Analyst, Neovion Group (Charlotte, NC), and Michael Croegart, senior product marketing manager, Mentor Graphics (Chicago, IL). For more information, Click Here .