Dynamical investigation of a manned capsule/tether re-entry system

Abstract

The use of tethers in space has an exciting promise in future astronautical applications, with the possibility of providing more sophisticated functionality to satellites and spacecraft. Some of these applications include adjoining satellites, creating microgravity environments, generating power, and transferring momentum between spacecraft. The focus of this project is to investigate the possible reduction in convective heat flux and temperatures on a manned capsule as a result of re-entry with an attached momentum exchange tether, including how various tether parameters affect these results. Using a “bottom-up” approach by modeling the system as a series of lumped masses and rigid rods (links) in conjunction with Lagrange’s equations, software was developed using Mathematica® that is capable of generating the equations of motion for any arbitrary number of links. To solve the resulting equations of motion, a separate dynamic dumbbell station/capsule swing model was developed for both an inelastic and elastic tether to provide a realistic initial condition vector for the re-entry simulation. Mathematica® was also used to solve the equations of motion numerically, allowing for a simple and effective software workflow. For the analysis, the resulting motion of the capsule was studied in the case of varying tether length, tether diameter, and capsule mass. Accordingly, the resulting heat loads on the capsule were calculated using a simple 1-D multilayer heat transfer model based on the previously-analyzed dynamic cases. For certain cases, the presence of a tether can reduce the convective heat flux by almost 60% and the surface temperature by just over 20% when compared to an equivalent tether-less system. It is debatable as to whether or not a momentum exchange tether would serve as an effective hypersonic parachute for a manned re-entry mission. Such a conclusion depends on yet unexplored or undetermined parameters (e.g. the fatigue properties of the capsule’s heat shield or the actual weight of the capsule before re-entry). Nevertheless, if designing a mission to use a relatively light capsule and a long/thick tether, this feasibility study certainly suggests that further investigation into this subset of tether de-orbiting is warranted.