Markus Schlotterer

Testbed for on-orbit servicing and formation flying dynamics emulation

The testbed for on-orbit servicing and formation flying dynamics emulation is a facility to emulate the force and momentum-free dynamics of multi-spacecraft missions on ground. The facility consists of a large granite table and several air cushion vehicles that float on this table. The granite table has a size of 4m x 2.5m and a thickness of 0.6m with a very high evenness. The air-cushion vehicles consist of two parts: a lower translation platform and an upper attitude platform. Both platforms are connected by a spherical air bearing. The translation platform carries the flat air bearings which enable the vehicle to float on the table, high pressure air tanks to support the flat and the spherical air bearings and a vertical linear actuator. With this actuator it is possible to adjust the altitude of the spherical air bearing and thereby the altitude of the attitude platform. All components needed for a control loop to actuate the air cushion vehicles are mounted on the attitude platform. These are 12 proportional cold gas thrusters with high pressure air tanks, 3 reaction wheels, an onboard computer with real-time operating system and Wireless LAN for communications and software upload, an inertial measurement unit (IMU) and a sensor for an infrared tracking system. In addition the attitude platform carries a power system with batteries to support all components with electrical energy and a balancing system to move the center of mass in the center of the spherical air bearing. Due to the linear air bearing the system can emulate 2 translational degrees of freedom. The third translational degree of freedom (parallel to gravity vector) can be actuated by the vertical linear actuator. As the attitude platform can tilt and rotate, 3 rotational degrees of freedom are emulated. So there is a total of 5+1 degrees of freedom that can be emulated by the testbed. One purpose of this testbed is the testing of control algorithms for multi-spacecraft missions. It can be used for precise formation control as well as path-planning and formation acquisition. In addition to the usage of real hardware as in Hardware-in-the-Loop simulations, the force and momentum-free dynamics are not simulated but are real (emulation). This is also a foundation for the second task of the testbed: the emulation of contact dynamics for Rendezvous and Docking missions. Models of docking adapters can be mounted on the attitude platform of the air-cushion vehicles and docking maneuvers with real contact dynamics can be tested on ground. A further purpose of the testbed is the qualification of dedicated relative navigation sensors and inter-spacecraft communication systems. These components can also be mounted on the attitude platform and tested in an agile environment. This paper describes the design and configuration of the testbed for on-orbit servicing and formation flying dynamics emulation. Results showing the quality of the force and momentum-free environment are presented. First control, navigation and thruster actuation algorithms have been developed and their design and the result of their usage within the testbed are also shown. Finally possible scenarios for the usage of this testbed are presented and an outlook on further developments and improvements is given.

Hybrid Navigation System for the SHEFEX-2 Mission

In October 2005 the first experiment of the DLR hypersonic SHarp Edge Flight EXperiment Program was successfully launched at the Andoya Rocket Range in northern Norway with the purpose to investigate possible new shapes for future launcher or re-entry vehicles applying a shape with facetted surfaces and sharp edges. For 2010 the execution of the SHEFEX-2 experiment is planned. It shall focus on hypersonic flight control using steerable canard fins while new thermal-protection system concepts will also be a subject of investigation. The accurate control of the vehicle using the canards requires a high accuracy in knowledge of the angle of attack and the side slip angle. Both angles can only be derived from the flight path and an accurate inertial attitude measurement. The first can be achieved by using GPS measurements. The second can not be provided by an Inertial Navigation System due to the fact that drifts due to launch vibrations are exceeding the requirement. Therefore a star tracker is foreseen to update the attitude information shortly before re-entry. Since the case of SHEFEX-2 describes a re-entry scenario which is applicable to other re-entry missions the need arises to develop an integrated navigation system which can provide the navigation solution with needed accuracy. This navigation system shall combine the measurements from inertial measurement unit (IMU), GPS receiver and star tracker. A further extension to include other sensors shall be foreseen. The paper will describe the concept of the integrated navigation. A special focus is also laid on the analysis how the star tracker can be integrated into the SHEFEX-2 probe.

Nonlinear Control for Proximity Operations Based on Differential Algebra

A new algorithm based on differential algebra is proposed to obtain a high-order Taylor expansion of the state-dependent Riccati equation solution. The main advantage of this approach is that the suboptimal solution of a class of nonlinear optimal control problems, characterized by a quadratic cost function and an input-affine plant model, is obtained by a mere evaluation of a polynomial expression, reducing the computational effort due to a well-known algorithm for the state-dependent Riccati equation solution. A relative position tracking and attitude synchronization problem involving docking maneuvering operations between two Earth satellites is investigated. Particularly, two possible docking scenarios are simulated by using a specific platform designed by DLR, German Aerospace Center, Institute of Space Systems to emulate the satellite motion on ground. The experiments show the effectiveness of the proposed differential-algebra-based algorithm and the potential computational benefit when it runs on r...

GTOC 9: Results from the German Aerospace Center (Team DLR)

This paper discusses the methods used by the team from the German Aerospace Center (DLR) for solving the 9th Global Trajectory Optimization Competition (GTOC) problem. The GTOC is an event taking place every year lasting roughly one month during which the best aerospace engineers and mathematicians world wide challenge themselves to solve a nearlyimpossible problem of trajectory design.