Atmosphere Harvesting

Introduction

Serge Demetriades proposed a propulsive fluid accumulator (PROFAC) in 1959. The idea is to have a device that skims the extreme upper atmosphere to collect gas which is then liquefied and stored for latter use in a space tug. In the original concept, the oxygen component of the atmosphere was stored, while the nitrogen was used as a reaction mass to make up for the collector drag. A nuclear power plant provided to energy to expel nitrogen at high-velocity using some form of electric propulsion. Since propulsive mass makes up a large fraction of the payloads coming up from earth, such a device has the potential greatly increasing the amount of useful payload into space.

An immediate objection to the concept are the potential problems associated with a nuclear reactor in an inherently short term orbit. There have been variations on the idea that eliminate the reactor. One recent proposal by Paul Klinkman and his associates at WPI is to operate at a higher altitude and to use a electrodynamic tether as the propulsion device. The studies on this page follow this later line of reasoning. A cartoon of the overall system is shown below. I've never seen the idea of the filled tanks climbing the cable before but it's an option. Conventional docking could also be used to collect the product.

An electrodynamic tether uses current through a long cable to generate a drive force through the interaction with the earth's magnetic field, much like an electric motor. The tether also allows you to have the power generation equipment at a high altitude, thus reducing drag on large acreage solar panels or other energy collection devices.

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Studies

Sizing Considerations for Electrodynamic-Tether-Driven Atmospheric Harvestor

First published; 10/12/2011. Last update: 10/14/2011

Note that there were some numerical errors in the original 10/12 version. These have been corrected

This study walks through a sizing study to determine the mass of a harvestor needed to gather 1000kg of gas per day. The focus is on sizing the electrodynamic tether system and associated power array. The study uses the tether equilibrium model to determine the tether path and the compatible top and bottom node masses. There is no attempt to acutally determine the mass of equipment needed to gather and liquify the atmospheric gases. We simply assume a minimum mass of 5000kg for the equipment and size the remaining power and propulsion components to match.

The altitude of the collector is set by the maximum operating temperature of the tether. Two different approaches are investigated; a pure aluminum alloy tether and a mix of steel and aluminum. The pure aluminum tether operating at a higher altitude has the lower system mass. The sketch below shows some of the derived values for the system. The study determines various factors for a range of cable lengths.

The full notebook gives a function that steps through the design process, including an iteration to find a consistent set of masses and electrical system values. The notebook makes use of a revised version of the cable equilibrium solution (see study below), and a new atmosphere model.

Computable Document Format HTML Pages PDF Version Notebook Version Dependencies
cable_equilibrium_with_drag.m
atmosphereModel.m
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Equilibrium of an Electrodynamic Cable Propelling an Atmosphere Harvester

First published; 9/27/2011. Last update: 10/11/2011

A current passing through an orbital cable will interact with the earth's magnetic field through the electromotive relation F = J X B, where J is the current B is the magnetic field vector. One application of this propulsive force is to drag a mass collector through the upper atmosphere. The cable constantly makes up the momentum loss that comes from collecting gas. A possible system would involve a collector at the bottom of a long cable, and a solar power station at the upper end of the cable. The current loop would be completed with electron collection at one end, and an electron gun at the other. An example of a similar application for electrodynamic tether propulsion is to reboost the International Space Station. Some additional system details of a tether propulsion system are described in a related patent. The present study gives a solution approach to determine the steady-state conditions and cable trajectory for a system that drags a collector through the atmosphere. Functions are provided that allow one to determine some of the cable system parameters needed to meet a mass collection goal.

This particular notebook is heavily mathematical, and I recommend it only to readers that need to perform similar computations. The notebook includes the derivation of the governing differential equations and boundary conditions. Portions of the derivations are "live" Mathematica symbolic computing. Functions built from the results are also included. The functions use the built-in Mathematica numerical differential equation solver. Later, I hope to use the results to build up some practical designs.

The October 11 update adds the drag of the cable to the solution function. The original derivation had the drag terms, but the numerical function left them out because the atmospheric density model was not available for high altitudes. The atmosphere model has been updated, and the function was changed to use the results.

Computable Document Format HTML Pages PDF Version Notebook Version
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