Electromagnetic Formation Flight Progress Report: July 2002 Submitted to: Lt. Col. John Comtois Technical Scientific Officer National Reconnaissance Office Contract Number: NRO-000-02-C0387-CLIN0001 MIT WBS Element: 6893087 Submitted by: Prof. David W. Miller Space Systems Laboratory Massachusetts Institute of TechnologyDESCRIPTION OF THE EFFORT The Massachusetts Institute of Technology Space Systems Lab (SSL) and the Lockheed Martin Advanced Technology Center (ATC) are collaborating to explore the potential for an Electro-Magnetic Formation Flight (EMFF) system applicable to Earth-orbiting satellites flying in close formation. PROGRESS OVERVIEW At MIT, work on electro-magnetic formation flight (EMFF) has been pursued on two fronts: the MIT conceive, design, implement and operate (CDIO) class, and the MIT Space Systems Lab research group, as described in the April 2002 progress report. The CDIO class has completed its first semester performing trades on and preliminary design of a six-degree-of-freedom electromagnetic formation flight testbed, called “ElectroMagnetic Formation Flight of Rotating Clustered Entities,” or “EMFFORCE.” EMFFORCE will utilize electromagnets to control the size and attitude of a cluster of bodies. The MIT Space Systems Lab research staff is supporting the CDIO class with guidance in trades analysis and design. Recent work has focused on trade analyses for the design of the electromagnets that will be used in the EMFFORCE testbed. These electromagnets will act as actuators, controlling the relative degrees of freedom of the bodies that compose EMFFORCE. The following report summarizes recent progress on the trades performed in the design of the EMFFORCE actuation system.Preliminary Trade Analysis for the Electromagnet Design for the EMFFORCE Testbed 1 Actuation System Overview The actuation system includes the electromagnet and reaction wheel subsystems. The electromagnet is designed to provide the electromagnetic force and torque used to induce the spin-up of the vehicles in formation, as well as to reject disturbances in the system during steady-state spin. The reaction wheel assembly is designed to provide torque to counteract the torque induced by the electromagnet. Figure 1.2 shows two dipoles at rest in a perpendicular orientation at a separation distance 2m. The arrows represent the direction of the induced forces and torques on the dipoles. Figure 1.3 depicts the spin-up of two dipoles from an initially perpendicular orientation. As current is applied to the dipoles, they begin to move in the direction of the induced forces and torques. The reaction wheel is then used to apply torque to counteract the induced torque. This applied torque is then decreased until the dipoles are in steady-state rotation, aligned along the same axis. 2 Purpose of Electromagnet The amount of force induced by the electromagnet, made up of a solenoid coil and core material, is dependent upon the strength of its magnetic field. The solenoid coil produces a magnetic field when current is applied to it. When the solenoid is wrapped around a ferromagnetic material, this core has the effect of multiplying the strength of the magnetic field. Figure 1.3: Spin-up Figure 1.2: Perpendicular Dipoles S N SN 2 m3 Design Trade Analysis For the electromagnet, trade analyses were performed to select the core material, core configuration, and mass of the core, coil, and system for the EMFFORCE testbed. 3.1 Core Material One of the main focuses of the EMFFORCE project is to optimize the amount of force generated by the electromagnet while minimizing its mass. This force depends upon the strength of the induced magnetic field. Therefore, the core material will be selected based on magnetic properties that provide the maximum induced magnetic field at a minimum mass. When current is applied to a solenoid wrapped around a ferromagnetic core, a magnetic field, or B-field, is induced in the core. As the current is increased, the strength of the B-field increases until it reaches a strength that cannot be exceeded with increasing current. This is known as the saturation value, or Bsaturation, for a given material. Table 3.1 compares the Bsaturation values and densities of several ferromagnetic materials. Table 3.1: Properties of Magnetic Materials Material Composition Bsaturation [Tesla] Density [g/cm3] Iron 99.91% Fe 2.15 7.88 Steel 98.5% Fe 2.10 7.88 45 Permalloy 54.7% Fe, 45.0% Ni 1.60 8.17 78 Permalloy 21.2% Fe, 78.5% Ni 1.07 8.60 Supermalloy 15.7% Fe, 79.0% Ni, 5.0% Mo 0.80 8.77 Iron and steel have the highest values for Bsaturation of 2.15 and 2.10 Tesla, respectively. Both iron and steel have the same density, 7.88 g/cm3, which is the lowest among the magnetic materials considered. High Bsaturation values combined with lower densities are desirable properties to increase the strength of the induced magnetic field while keeping the mass as low as possible.EM Core MaterialInduced Field vs. Applied Field00.511.522.50 5000 10000 15000 20000 25000 30000 35000H [Amps/m]B [Tesla]AISI 1010 steel Remko soft pure iron Figure 3.1 is a plot of induced versus applied field for iron and steel. For higher values of the applied field, both iron and steel reach a Bsaturation point at approximately 2.1 Tesla. However, for lower values of the applied field, this plot shows that iron has higher induced field values than steel. This is important because it indicates that iron will induce a larger magnetic field, and therefore higher levels of force, than steel at low current levels. Another magnetic property that must be considered is the hysteresis effect. When a magnetic material is magnetized by an applied field, it will not completely demagnetize when the applied field is reduced to zero. In order to bring the induced field back to zero, a field must be applied in the opposite direction. The strength of this applied field is called the coercive force. The coercive force for iron and steel are 79.6 and 143.2 Amps/m, respectively. Therefore, hysteresis is less of an issue with iron since it requires less of an applied force in the opposite direction to drive the induced field back to zero. Recently, another possibility for the electromagnet core was discovered. Hiperco 50A, a magnetic alloy made up of 48.9% Fe and 49.0% Co, exhibits better magnetic properties than iron, which was