A website, spacecoach.org, expands on a 2010 paper which describes a design for a reusable interplanetary spacecraft made mostly out of water or pykrete (ice frozen with fiber material).
The proposed design “burns” water in microwave electrothermal engines, a type of electric propulsion system that has been tested with water as propellant, and proven to be several times more fuel efficient than conventional chemical rockets. The ability to use water, as well as waste streams, as propellant radically alters the economics of deep space missions, reducing the cost of a mission by potentially one hundred fold, making deep space missions comparable in cost to current manned missions to low earth orbit.
The ships made mostly of water, powered by microwave engines, will be capable of reaching destinations throughout the solar system, at just 1/10th to 1/100th the cost of conventional chemical rockets.
The system described in the paper is based entirely on existing technologies that have already been flight tested or are well under development, and is feasible with present day technology and Earth launch platforms to low orbit.
These ships, in addition to being cheaper to build, will be fully reusable, and will be mostly organic structures that will be far more comfortable than conventional capsule designs, and more like a scaled down version of Gerard K O'Neil's proposed space colonies than a metal ship.
They have coined the term spacecoach to describe these ships, a reference to the prairie schooners of the Old West.
They present a reference design that combines inflatable structures and thin film PV arrays to form a kite-like structure that both has a large PV array area, and can be rotated to provide artificial gravity in the outer areas. The ability to generate artificial gravity while providing ample radiation protection solves two of the thorniest problems in long duration spaceflight.
They envision a series of design competitions for water compatible electric propulsion technologies, large scale solar arrays, and overall ship designs. Much of the reference design can be validated in ground based competitions and experiments, followed by uncrewed test vehicles.
Spacecoaches are possible not because of any one insight or breakthrough, but because of the convergence of improvements in component technologies, specifically thin film photovoltaics, electric propulsion, and inflatable structures. The combination of the three, particularly when you add water for propulsion, leads to one or two order of magnitude improvements in mission economics.
Thin film solar photovoltaics, which enable the construction of large area PV sails, will enable ships to generate hundreds of kilowatts to several megawatts of electrical power.
SEP (solar electric propulsion) is a well understood, flight ready technology. Engines that function with water or gasified waste will be well suited to the spacecoach architecture. We simply need to test existing SEP technologies with water and waste streams to pin down performance and efficiency numbers, which can be done via an X-Prize style engineering competition. Scaling them to propel a large (40 tonne) ship will be done by clustering them in arrays, so there will be no need to build a single high power engine when an array of many 10-20 kilowatt units will do just fine, while also adding redundancy
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