Physics, Performance, and Applications of Plasma Aerocapture

Abstract

The feasibility of magnetoshells for drag-modulated plasma aerocapture (DMPA) is investigated using analytical modeling and a novel demonstration experiment. DMPA proposes to generate drag through interaction of the atmosphere with a magnetized plasma. Instead of a rigid aeroshell, it uses a "magnetoshell" consisting of a magnetic field that confines the plasma, attaining very large effective drag area and enabling less risky flight profiles. Although there are clear advantages to magnetoshells over aeroshells, the fundamental physics of their operation is not well characterized. An understanding of these physics is needed to determine how their performance scales with spacecraft and mission design parameters and ultimately inform appropriate mission applications. An analytical model is developed to describe the interaction of a dipole magnetic plasma with a hypersonic rarefied neutral flow, analogous to a magnetoshell in aerocapture flight. Single particle trajectory analysis reveals a characteristic ion-trapping region enclosed by a particular magnetic flux surface that drives mass and energy capture from the flow by the plasma. The capture and deflection of particles is found to produce strong forces on the magnetic field depending on the extent of the ion-trapping region and the flow reactivity with the plasma. A global model is developed and averaged over the toroidal ion-trapping volume to describe the net mass and energy exchange between the neutral stream and plasma. Simulations over a large range of magnet and freestream parameters reveal three distinct physical regimes that have significant bearing on the magnitude of plasma/flow interaction. The transitions between these regimes exhibit characteristics of resistive critical ionization, whereby the relative kinetic energy between plasma and neutral gas collisionally heats electrons, driving rapid and complete ionization of the gas. Two regime transitions are observed with sudden exponential increases in plasma density occurring at velocity thresholds that depend on several energy loss mechanisms. The higher-velocity transition is a classical presentation of critical ionization where flow neutrals are ionized directly by plasma electrons. The other is a unique case in which charge exchange between ions and flow neutrals supplies both the particles and energy required to initiate critical ionization. This transition is distinct from any critical ionization effect reported in literature and indicates the existence of a lower critical velocity governed by collisional and diffusive effects as opposed to ionization energy losses only. The critical ionization thresholds increase the force on the magnet by up to two orders of magnitude compared to aerodynamic drag on an equivalently sized flow impediment. The application of DMPA to outer planet orbiter missions is analyzed in terms of performance results and spacecraft design scaling. A design framework for generating preliminary DMPA mission architectures is presented and a sample design is compared to an existing Neptune orbiter concept that uses an ADEPT drag skirt. Compared to ADEPT, the DMPA architecture is shown to deliver 70% higher orbiter mass and experience 30% lower stagnation heating. It achieves a ballistic coefficient of 4 kg/m^2 with a ballistic coefficient ratio of 67, both results indicating a high degree of control authority via the continuous drag modulation scheme. The robustness of DMPA trajectories to perturbations in the atmospheric density and entry flight path angle is analyzed with a Monte Carlo simulation. It is found that DMPA achieves orbit within 1% of the target apoapsis on average while 93% of all cases fall within 12.5% of the target. Compared to a variable-area aeroshell, DMPA achieves lower variance in apoapsis error, a factor of ten reduction in dynamic pressure, and a factor of three reduction in stagnation heating and total heat load. These improvements may be mission-enabling by drastically easing thermal protection system requirements to sub-kW/cm^2 heat rates compatible with heritage heat shield materials.Recommendations for using the design framework to optimize arbitrary DMPA architectures are discussed while the challenges and associated technology advancements in guidance/navigation, power, and space tethers enabling DMPA are characterized. Key physical assumptions about energy and momentum exchange during DMPA are tested here by a novel experiment that characterizes the interaction between a hypersonic free-molecular flow and a magnetized plasma. A neutral beam source accelerates ions from a helicon plasma to a conductive plate, neutralizes them, and reflects them as a collimated flow. A hollow cathode plasma with an applied axial magnetic field of 1 kG acts as the target for the flow. It is found that interaction with the flow causes an increase in both density and temperature of the target plasma by up to a factor of two. The voltage required to operate the hollow cathode at a fixed current is reduced by up to 5% while the neutral beam is operating, suggesting power deposition by the flow. A 0-D power balance model is invoked to show that flow kinetic energy is absorbed by the plasma at a rate of up to 50% of the hollow cathode power. This power correlates strongly with the increase in electron temperature, suggesting that flow kinetic energy is converted to electron thermal energy. Deflection of the flow by the plasma is not resolved due to extraneous forces on the measurement device and uncertainties in plasma properties. Using the results found here, it is shown that the experiment can feasibly scale to demonstrate significantly higher energy and momentum transfer as required for a plasma aerocapture proof-of-concept.