Space Coaches – Reusable Refuelable Interplanetary Spacecraft made mostly of water at one hundredth the cost of conventional chemical rockets

A website,, 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

Inflatable/expandable structures are just now beginning to be recognized as a flight ready technology, with Bigelow Aerospace’s BEAM unit due to fly on the ISS later this year. Bigelow already has two uncrewed inflatable habitats in low earth orbit. The basic idea with inflatable structures is to replace a rigid metal hull with a flexible high strength Kevlar type material and utilize pressurization to inflate and deploy the structure. This also enables a large habitable space to be compacted into a standard cargo fairing, thus requiring a minimal number of surface launches for initial delivery to orbit. We expect this technology to improve, both in terms of mass per unit of habitable space (currently about 60 kg per m3), and in terms of the types of shapes that can be created.

ISP Prize, engine competition for better thrust with low power requirements

The Isp Prize will be open to electrothermal engine designers. The goal of this competition is to identify candidate engine designs that meet the following criteria:

* Modular design with standardized connectors and power, so the engines are easily replaced.
* User serviceable design to enable crew to repair or upgrade units in flight without leaving the spacecraft on an EVA.
* Runs at a specific impulse of 800 seconds or greater, preferably 1,500 to 3,000s (the range in which Hall Effect thrusters operate).
* Uses water vapor and optionally carbon/nitrogen rich exhaust gases to generate thrust.
* Can be scaled up by arranging engines in large clusters.

Most electrodeless electric propulsion systems should work to some degree with water (an engine design competition, the Isp Prize, will be conducted to determine which systems work best with water). At the moment, ELF thrusters look especially promising, having already been tested with water and carbon dioxide with good results.

Ion drives, VASIMR, and helicon drives all operate at very high specific impulse, meaning they use very little propellant. However they also require large amounts of power to generate useful thrust, and often assume the use of nuclear power plants to provide enough power for manned missions.

MET and ELF engines, as well as other technologies such as Hall Effect thrusters, operate at a lower specific impulse, but still much better than chemical rockets. Because of this, they require less power to generate a unit of thrust compared to more advanced electric motors, and can be powered by existing solar photovoltaic arrays.

Current research has demonstrated that MET engines should run at a specific impulse of about 900 seconds using water vapor as propellant, while ELF thrusters have been demonstrated with water at a specific impulse of 1,700 seconds. You should use specific impulse in the range of 800 to 2,000 seconds in calculating how much water will be required to achieve the trajectory required for various missions. They think it should be possible to increase performance further, which will make high delta-v missions such as a crewed trip to Ceres feasible.

Spacecoach Design Competition.

In conjunction with the engine design competition, they are planning an integrated architectural and systems design competition where teams will develop detailed designs for spacecoaches and proposed missions. This competition will be open to multidisciplinary teams, and will also include cash prizes for the winning entries, which will be thoroughly vetted by spaceflight experts at leading corporations and research institutions. This competition will begin some time after the Isp Prize, and will use data from that competition to provide ship designers with data they can use to refine their models.

Water Focus and Costs

They modeled approximate round-trip mission costs to destinations throughout the inner solar system, using a 40,000 kilogram (40 tonne) dry hull and SpaceX’s published launch costs to get materials, including water, into low earth orbit ($1,700/kg via Falcon 9 Heavy), with electric propulsion (Isp between 1,500 to 3,000s) from there (electrode-less Lorentz force thrusters using water operate in this range). Among the missions we modeled were EML-2 (Earth Moon Lagrange point 2) to and from cislunar space, Martian moons, NEO interception, Venus orbit and Ceres. Even with engines operating at the low end of the electric propulsion performance envelope, our models predicted per mission costs in the hundreds of millions of dollars, a one or two order of magnitude reduction compared to conventional missions, some of which, such as a crewed mission to Ceres, simply are not possible via chemical propulsion.

Such large cost reductions are possible due to a combination of the fuel efficiency of electric engines, and the synergies created by using water as propellant. On one hand electric engines require far less propellant for a given delta-v. On the other, virtually everything the crew consumes or uses for life support can eventually be sent to the engines. As a result the only dead weight on the ship is the hull and whatever non-consumable materials and equipment are brought on board, which will also allow spacecoaches to carry larger crews. Reusability will also enable operators to amortize development and construction costs across many missions.

Spacecoaches are also well suited for in situ resource utilization. Should we reach low gravity destinations with accessible water (Ceres is an especially interesting location), it will eventually be possible to refuel spacecoaches at these destinations, or even ship water inbound to cislunar depots. We assume for now that spacecoaches are fully supplied from Earth, but exploring ISRU destinations and capabilities will be a high priority early on. Partially reusable launch vehicles offer another way to reduce costs. Water will be an ideal payload for a heavily re-used Falcon 9R booster. Unlike most payloads, it has essentially zero replacement cost, so the launch operator can fly the reusable boosters until they fail, and can learn about potential failure modes and fixes in the process (all while delivering more water to orbit).


Pykrete is a frozen composite material made of approximately 14 percent sawdust or some other form of wood pulp (such as paper) and 86 percent ice by weight (6 to 1 by weight). Its use was proposed during World War II by Geoffrey Pyke to the British Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (because of low thermal conductivity), and its vastly improved strength and toughness over ice; it is closer in form to concrete.

Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process. However, it can be repaired and maintained using seawater. The mixture can be moulded into any shape and frozen, and it will be extremely tough and durable, as long as it is kept at or below freezing.

Block of Pykrete

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