An output feedback structured model reference adaptive control law has been developed for spacecraft rendezvous and docking problems. The effect of bounded output errors on controller performance is studied in detail. Output errors can represent an aggregation of sensor calibration errors, systematic bias, or some stochastic disturbances present in any real sensor measurements or state estimates. The performance of the control laws for stable, bounded tracking of the relative position and attitude trajectories is evaluated, considering unmodeled external as well as parametric disturbances and realistic position and attitude measurement errors. Essential ideas and results from computer simulations are presented to illustrate the performance of the algorithm developed in the paper.

Adaptive Output Feedback Control for Spacecraft Rendezvous and Docking Under Measurement Uncertainty

This paper addresses autonomous aerial refueling between an unmanned tanker aircraft and an unmanned receiver aircraft using the probe-and-drogue method An important consideration is the ability to achieve successful docking in the presence of exogenous inputs such as atmospheric turbulence Practical probe and drogue autonomous aerial refueling requires a reliable sensor capable of providing accurate relative position measurements of su‐cient bandwidth, integrated with a robust relative navigation and control algorithm This paper develops a Reference Observer Based Tracking Controller that does not require a model of the drogue or presumed knowledge of its position, and integrates it with an existing vision based relative navigation sensor A trajectory generation module is used to translate the relative drogue position measured by the sensor into a smooth reference trajectory, and an output injection observer is used to estimate the states to be tracked by the receiver aircraft Accurate tracking is provided by a state feedback controller with good disturbance rejection properties A frequency domain stability analysis for the combined reference observer and controller shows that the system is robust to sensor noise, atmospheric turbulence, and high frequency unmodeled dynamics Feasibility and performance of the total system is demonstrated by simulated docking maneuvers of an unmanned receiver aircraft docking with the non-stationary drogue of an unmanned tanker, in the presence of atmospheric turbulence Performance characteristics of the vision based relative navigation sensor are also investigated, and the total system is compared to an earlier version Results presented in the paper indicate that the integrated sensor and controller enable precise aerial refueling, including consideration of realistic measurements errors, plant modeling errors, and disturbances

Trajectory Tracking Controller for Vision-Based Probe and Drogue Autonomous Aerial Refueling

https://people.bath.ac.uk/jldb20/pubs/Thomas.etal2014Advancesinair.pdf

Advances in air to air refuelling

This study describes an active flow control concept that uses counterflowing jets to significantly modify external flowfields and strongly disperse the shock waves of supersonic and hypersonic vehicles to reduce aerothermal loads and wave drag. The potential aerothermal and aerodynamic benefits of the concepts were investigated by conducting experiments on a 2.6%-scale Apollo capsule model in Mach 3.48 and 4.0 freestreams in a trisonic blowdown wind tunnel, as well as pretest computational fluid dynamics analyses of the flowfields, with and without counterflowing jets. The model employed three sonic and two supersonic (with design Mach numbers of 2.44 and 2.94) jet nozzles with exit diameters ranging from 0.25 to 0.5 in. The schlieren images were consistent with the pretest computational fluid dynamics predictions, showing a long penetration mode jet interaction at low jet flow rates of 0.05 and 0.1 Ib m /s, whereas a short penetration mode jet was revealed at higher flow rates. The long penetration mode jet appeared to be almost fully expanded and was unsteady, with the bow shock becoming so dispersed that it was no longer discernible. High-speed camera schlieren data revealed the bow shock to be dispersed into striations of compression waves, which suddenly coalesced to a weaker bow shock with a larger standoff distance as the flow rate reached a critical value. Heat transfer results showed a significant reduction in heat flux, even giving negative heat flux for some short penetration mode interactions, indicating that the flow wetting the model had a cooling effect, instead of heating, which could significantly impact thermal protection system requirements and design. The findings suggest that high-speed vehicle design and performance can benefit from the application ofcounterflowing jets as an active flow control.

/pdf/dynamics-of-shock-dispersion-and-interactions-in-supersonic-xdemtbdrgb.pdf

Dynamics of Shock Dispersion and Interactions in Supersonic Freestreams with Counterflowing Jets

A method is developed for modeling the aerodynamic coupling between aircraft Hying in close proximity. Velocities induced on a trailing aircraft by vortices from an aircraft upstream are written as a function of the relative separation and relative orientation between the two aircraft. The nonuniform vortex-induced wind and wind gradients acting on the trail aircraft are approximated as effective uniform wind and wind gradients. In a dynamic simulation, the effective wind can be used directly in the equations of motion, whereas the wind gradient can be used in the standard buildup equations for the aerodynamic moments. This removes necessity to explicitly compute the induced forces and moments. Various vortex models for estimating induced velocities and averaging schemes for computing effective wind components and gradients are assessed

Modeling of Aerodynamic Coupling Between Aircraft in Close Proximity

Results from a wind-tunnel test of two delta-wing aircraft in close proximity are presented and compared with predictions from a vortex lattice method. Large changes in lift, pitching moment, and rolling moment are found on the trail aircraft as it moves laterally relative to the lead aircraft. The magnitude of these changes is reduced as the trail aircraft moves vertically with respect to the lead aircraft. Lift-to-drag ratio of the trail aircraft is increased when the wing tips are slightly overlapped. Wake-induced lift is overpredicted slightly when the aircraft overlap in the spanwise direction. Wake-induced pitching and rolling moments are well predicted. A maximum induced drag reduction of 25% is measured on the trail aircraft, compared with a 40% predicted reduction. Three positional stability derivatives, change in lift and pitching moment with vertical position and change in rolling moment with lateral position, are studied. Predicted boundaries between stable and unstable regions were generally in good agreement with experimentally derived boundaries.

Comparison of Predicted and Measured Formation Flight Interference Effects

This paper describes the derivation of a new set of nonlinear, 6{DOF equations of motion of a receiver aircraft undergoing an aerial refueling, including the efiect of timevarying mass and inertia properties associated with the fuel transfer and the tanker’s vortex induced wind efiect. Since the Center of Mass (CM) of the receiver is time{varying during the fuel transfer, the equations are written in a reference frame whose origin is at the CM of the receiver before fuel transfer begins and stays flxed at that position even though the CM is moving during the refueling. Due to the fact that aerial refueling simulation and control deal with the position and orientation of the receiver relative to the tanker, the equations of motion are derived in terms of the translational and rotational position and velocity with respect to the tanker. Further, the derivation of the equations takes into account the momentum transfer into the receiver due to the fuel transfer. The receiver aircraft before fuel transfer is treated as a rigid body made up of ‘n’ particles. The dynamic efiects due to fuel transfer are modeled by considering the mass change to be conflned to a flnite number of lumped masses, which would normally represent the fuel tanks on the receiver aircraft. Once the refueling begins, by using the design parameters such as the shape, size and location of the individual fuel tanks and the rate of fuel ∞owing into each of them, the mass and location of the individual lumped masses are calculated and fed into the equations of motion as exogenous inputs. The new receiver equations of motion are implemented in an integrated simulation environment with a feedback controller for receiver station-keeping as well as the full set of nonlinear, 6{DOF equations of motion of the tanker aircraft and a feedback controller to ∞y the tanker on a U-turn maneuver.

/pdf/derivation-of-the-dynamics-equations-of-receiver-aircraft-in-49t7bju1xi.pdf

Derivation of the Dynamics Equations of Receiver Aircraft in Aerial Refueling

T HIS paper focuses on the development of an integrated simulation environment and control algorithms for a receiver aircraft in boom–receptacle refueling (BRR) operation while the tanker flies in a racetrack maneuver. A racetrack maneuver is the standard pattern flown by tanker aircraft, with straight legs and bank turns [1]. This paper applies the earlier work by the authors on mathematical modeling of relative motion [2,3] and aerodynamic coupling [4] to the simulation of aerial refueling, and it develops control laws for the motion of the receiver relative to the tanker that flies in racetrack maneuvers. An integrated simulation environment is developed to take into account tanker maneuvers, motion of the receiver relative to the tanker, and the aerodynamic coupling due to the trailingwake vortex of the tanker. The separate dynamicmodel of the tanker, including its own controller, allows the simulation of the standard racetrack maneuvers of the tanker in aerial refueling operations. The mathematical model of the receiver expressed in terms of the relative position and orientation with respect to the tanker’s body frame facilitates the formulation, in a single framework, of maneuver and stationkeeping of the receiver behind the tanker. For the racetrack maneuvers of the tanker, a linear quadratic regulator (LQR)-based multi-input/multi-output (MIMO) state feedback and integral control technique is developed to track commanded speed, altitude, and yaw rate. Similarly, for the relative motion of the receiver, an LQR-based MIMO state feedback and integral control technique is designed to track the commanded trajectory expressed in the body frame of the tanker. Both controllers schedule their corresponding feedback and integral gains based on the commanded speed and yaw rate of the tanker. The tanker aircraft model represents KC-135R, and the receiver aircraft model is a tailless fighter aircraft with innovative control effectors (ICE) and thrust-vectoring capability. Because the receiver has redundant control variables, various control allocation schemes are investigated for trajectory tracking and stationkeeping while the tanker flies in various racetrack maneuvers with different commanded turn rates.

Control and Simulation of Relative Motion for Aerial Refueling in Racetrack Maneuvers

: This report is a User's Manual for the 2011 Revision of the Missile Datcom computer program. It supersedes AFRL-RB-WP-TR-2009-3015. In missile preliminary design it is necessary to quickly and economically estimate the aerodynamics of a wide variety of missile configuration designs. Since the ultimate shape and aerodynamic performance are so dependent upon the subsystems utilized, such as payload size, propulsion system selection and launch mechanism, the designer must be capable of predicting a wide variety of configurations accurately. The fundamental purpose of Missile Datcom is to provide an aerodynamic design tool which has the predictive accuracy suitable for preliminary design, and the capability for the user to easily substitute methods to fit specific applications.

/pdf/missile-datcom-user-s-manual-2011-revision-1zr3fuh2dx.pdf

William Blake

Papers

Modeling of Aerodynamic Coupling Between Aircraft in Close Proximity

Comparison of Predicted and Measured Formation Flight Interference Effects

Derivation of the Dynamics Equations of Receiver Aircraft in Aerial Refueling

Control and Simulation of Relative Motion for Aerial Refueling in Racetrack Maneuvers

MISSILE DATCOM User's Manual - 2011 Revision