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A Stagecoach to the Stars

Imagine the kind of spaceship we’ll need as we begin to expand the human presence into the nearby Solar System. We’d like something completely reusable, a vessel able to carry people in relative comfort everywhere from Mars to Venus, and perhaps as far out as the asteroid belt, where tempting Ceres awaits. Capable of refueling using in situ resources, these are ships not crafted for a single, specific mission but able to operate on demand without entering a planetary atmosphere. Brian McConnell, working with Centauri Dreams regular Alex Tolley, has been thinking about just such a ship for some time now. A software/electrical engineer, pilot and technology entrepreneur based in San Francisco, Brian here explains the concept he and Alex have come up with, one that Alex treated in a previous entry in these pages. The advantages of their ‘spacecoach’ are legion and Brian also offers a sound way to begin testing the concept. The author can be reached at bsmcconnell@gmail.com.

by Brian McConnell


“What if a spacecraft, like a cell, was made mostly of water?”

That’s what Alexander Tolley and I asked when we were working on our paper for the Journal of the British Interplanetary Society, “A Reference Design For A Simple, Durable and Refuelable Interplanetary Spacecraft” [1]. The paper explored the idea of a crewed spacecraft that used water as propellant in combination with solar electric propulsion. We dubbed them spacecoaches, as a nod to the stagecoaches of the Old West. Alex also gave the concept an excellent fictional treatment in Spaceward Ho!, also published here on Centauri Dreams. We are currently finishing a book about spacecoaches, to be published by Springer this fall. Subscribe to spacecoach.org for updates about the book and spacecoaches in general.

The HD 7924 Planetary System

The idea of crewed solar electric spacecraft is hardly new. In 1954, Ernst Stuhlinger proposed a “sun-ship” powered by solar steam turbines and cesium ion drives [2,3]. Since then solar electric propulsion has been used in a wide variety of uncrewed craft. Meanwhile, the convergence of several technologies will make crewed solar electric vehicles feasible in the near future.

The core idea behind the spacecoach architecture is the use of water, and potentially waste streams, as propellant in electric engines. Water, life support and consumables are critical elements in a long duration mission, and in a conventional ship, are dead weight that must be pushed around by propellant that cannot be used for other purposes. Water in a spacecoach, on the other hand, can be used for many things before it is reclaimed and sent to the engines, and it can be treated as working mass. This, combined with the increased propellant efficiency of electric engines, leads to a virtuous cycle that results in dramatic cost reductions compared to conventional ships while increasing mission capabilities. Cost reductions of one or two orders of magnitude, which would make travel to destinations throughout the inner solar system routine, are possible with this approach.

Water is, for example, an excellent radiation shielding material, comparable to lead on a per kilogram basis, except you can’t drink lead. It is an excellent thermal battery, and can simply be circulated in reservoirs wrapped around the ship to balance hot and cold zones (this same reservoir doubles as the radiation shield). When frozen into fibrous material to form pykrete, it forms a material as tough as concrete, which can potentially be used for debris shielding or for momentum wheels, and if positioned correctly, can double as a supplemental radiation shield. If mixed with dilute hydrogen peroxide, which is safely stored at low concentrations, oxygen can be generated by passing it through a catalyst, similar to a contact lens cleaner. Dilute H2O2 is also a potent disinfectant, and can also be used to process human waste, as is done in terrestrial wastewater treatment plants. Anything the crew eats or drinks can be counted as propellant, as the water can be reclaimed and used for propulsion. This greatly simplifies planning for long missions because the longer the mission is, the more propellant you have in the form of consumables. This will also provide excellent safety margins and enable crews to survive an Apollo 13 scenario in deep space.

A spaceship that is mostly water will be more like a cell than a conventional rocket plus capsule architecture. Space agriculture, or even aquaculture, becomes practical when water is abundant. Creature comforts that would be unthinkable in a conventional ship (hot baths anyone?) will be feasible in a spacecoach. Meanwhile, inflatable structures will eventually enable the construction of large, complex habitats that will be more like miniature O’Neill colonies than a conventional spaceship [4].

In the book, Alex and I 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 [5]. The ability to generate artificial gravity while providing ample radiation protection solves two of the thorniest problems in long duration spaceflight. Alex wrote an excellent fictional treatment of the concept for Centauri Dreams called Spaceward Ho! This is intended as a straw man design to kickstart design competitions. We 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 (similar to what Bigelow Aerospace did by flying its Genesis I and II habitats in low earth orbit).


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 (thin film PV material coincidentally is much more resistant to radiation than conventional silicon PV material) [6]. While thin film solar is not as efficient as silicon in terms of power per unit area, from a power density (watts/kilogram) standpoint, it offers multiple order of magnitude improvements, and will continue to improve for decades due to dematerialization in manufacturing processes.

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. One interesting discovery we made while doing our analysis is that ultra high specific impulse engines, such as VASIMR, are neither necessary nor desirable. Engines that operate at the low end of the electric propulsion envelope still yield excellent economics due to the synergies created by using water as propellant, while also being able to operate with less electrical power per unit of thrust, which reduces PV array size and mass.

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. [7]

Spacecoaches will not be mission specific ships. Even the first generation ships will be able to travel to many destinations within the inner solar system. They will be fully reusable, travelling from a high earth orbit or a Lagrange point to and from their destinations, without ever entering a planetary atmosphere. Spacecoaches will be able to travel to cislunar space, Mars, Venus, NEOs and maybe even Ceres and the Asteroid Belt. They can also be dispatched for asteroid interception and deflection missions on short notice. This is a huge departure from conventional spacecraft which are purpose built for a specific mission, usually Mars, that is planned decades in advance. Mars is certainly an interesting destination, but Ceres, with its abundant water resources and shallow gravity well, may turn out to be an even more interesting destination for human exploration and settlement.

The amount of water required for propellant on any given route will vary depending on the delta-v needed, and also the specific impulse of the engines on board, but water is easy to handle and store. Need to add an extra two kilometers per second to your delta-v budget? Just add water! (or replace the electric engines with slightly more efficient models). Because water is so easy to handle compared to conventional propellants, this will also simplify the construction and operation of orbiting fuel depots, which will be little more than orbiting water tanks.

Simplicity and upgradability is another key design element of the spacecoach. We assume that component technologies will continue to improve for decades. So instead of designing spacecoaches to fly only with today’s technology, they will be designed more like personal computers were in the 1980s. The original PCs were built around a common electrical and communication bus, the ISA bus, which allowed memory, CPUs and peripherals from many manufacturers to be combined. If you wanted to, you could buy the component parts from catalogs and build your own PC from scratch.

We envision something similar for the spacecoach, for the electrical system and engines in particular, which will have standard electrical and fluid interconnects, and uniform form factor requirements. The engines will also be mounted in a sealable compartment that can be pressurized so the crew can replace or upgrade engines without doing an EVA. This will not only make spacecoaches field upgradable, but will also reduce the need to design engines for extreme reliability. If a few units fail, crews would replace them in an operation not much different than replacing a rack mounted server. Upgrading engines will be the best way to improve performance and reduce costs, as a small increase in specific impulse can yield significant mass and cost reductions, especially for high delta-v routes like Ceres and the Asteroid Belt.

And what about cost?

Mention crewed missions to Mars, much less anywhere else, and people automatically assume you’re talking tens of billions of dollars as a starting point. We 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 [8]), 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/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).

If you are part of a team working on electric propulsion technology, here’s one way you can help make these a reality. Test your engine with water vapor, carbon dioxide and gasified waste (or a good analogue), and publish your results. The most important parameters ship designers will be interested in are specific impulse, efficiency (ideally the “wall plug” efficiency of the entire system so it can be modeled as a black box) and thrust/mass ratio. We already know several SEP technologies work reasonably well with water, but it will be great to examine all systems to see how well each works with water, compare performance across a variety of technologies, and identify opportunities for further improvement.

It is easy to be cynical about new spaceflight concepts, especially one that promises large cost reductions, but most of this can be validated on the ground and via uncrewed testbeds in a short time and at little expense. It is a paradigm shift, and that will take people some time to accept. The rocket + capsule design pattern served us well in the early years of spaceflight, so its hard to get away from that, but it’s time to move on to something that is more adaptable, something that’s more like a ship that can sail wherever her captain wants to go.

Spacecoaches will form the basis for a real world Starfleet, a fleet which will grow as ships are built, and which will reach new destinations as component technologies continue to improve in the coming decades. They will open the inner solar system out to the Asteroid Belt to human exploration and settlement, and with some spacecoaches operating in cislunar space, humanity will also have a rapid response capability should we be surprised by the discovery of an Earth threatening object.

Visit spacecoach.org to learn more, and to subscribe for notices about the upcoming book, which examines the spacecoach reference design and potential missions in detail. If you are interested in obtaining an advance copy of the book, acting as a technical reviewer or inviting us to speak, please get in touch.


[1] “Reference Design for a Simple, Durable and Refuelable Interplanetary Spacecraft”, B. S. McConnell; A. M. Tolley (2010), JBIS, 63, 108-119

[2] Image credit: Frank Tinsley/American Bosch Arma Corporation, 1954

[3] “Possibilities of Electrical Space Ship Propulsion,” E. Stuhlinger, Bericht über den V Internationalen Astronautischen Kongreß, Frederich Hecht, editor, 1955, pp. 100-119; paper presented at the Fifth International Astronautical Congress in Innsbruck, Austria, 5-7 August 1954

[4] “A Shape Grammar for Space Architecture – I. Pressurized Membranes”, Val Stavrev* Aeromedia, Sofia, Bulgaria, 40th International Conference on Environmental Systems, http://www.spacearchitect.org/pubs/AIAA-2010-6071.pdf

[5] Image credit: Rüdiger Klaehn

[6] “Super radiation tolerance of CIGS solar cells demonstrated in space by MDS-1 satellite”, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, 18-18 May 2003, pp. 693 – 696 Vol.1

[7] Estimate based on BA330 mass per cubic meter of habitable space, per Bigelow Aerospace’s published specifications

[8] Per SpaceX published launch cost and delivery capacity for Falcon 9 Heavy, as of April 2015


Comments on this entry are closed.

  • CharlesJQuarra May 1, 2015, 14:10

    “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”

    in the meantime, the dummies at NASA are proposing pour billions over a decade to build a giant expensive and inefficient rocket launch system, whose only mission in the foreseeable future will be to haul a rock to cislunar space

  • Daniel Highmountain May 1, 2015, 14:54

    At what speed will these ships travel? How long will it take to go from say Venus to Earth or from Earth to Mars? Months or more?

  • Brian McConnell May 1, 2015, 15:26

    @Daniel, if the PV array is sized appropriately, travel times are comparable to a conventional ship on a Hohmann trajectory, and as more efficient engines are developed, it will be possible to reduce travel time. With ample radiation shielding and artificial gravity, spacecoaches make faster travel time nice to have, but not a requirement due to health issues.

  • Alex Tolley May 1, 2015, 15:29

    Daniel – we used minimum energy Hohmann transfers. The times are therefore comparable to other propulsion techniques using approximately the same trajectories. IOW, we are not going for high energy transfers, like the proposed 39 days to Mars using VASIMR.

    While electric engines do not preclude faster transfers, the trade offs in terms of energy (PV panel mass) and water (propulsion, consumables and shield mass) seem to work out quite nicely for the energy required for intermediate Isp numbers – 800-3000 seconds.

  • Roy Clymer May 1, 2015, 17:35

    The most exciting space news I’ve read since Zubrin’s “The Case for Mars” !!

  • Gerry May 1, 2015, 18:59

    Based on the information in the text, it looks as though each 10-20 kw thruster might yield around 1 Newton of thrust. If so, you would want 10-20 MW of electrical energy from sunlight to achieve 1,000 Newtons of thrust. So with perhaps 20% efficiency from advanced thin-films, you’d need to have a collecting area of, very roughly, 50-100,000 square meters. That would be a square about 250-350 meters on a side, which sounds reasonable and matches the illustration. That’s also a good rotational diameter for the generation artificial gravity without undue coriolis effects.

    Am I correct in assuming that you’d probably be looking at 8 months or so to Mars, and 15 months to Ceres? Being able to refuel for the return trip at the destination could cut those numbers down, though, as could operating at the high-end of the ISP spectrum at lower thrust. If you could manage around 25 km/s delta-vee each way, you might make a trip to Mars in less than 3 months, or Ceres in about seven, if I’m not totally mistaken…

  • Harold May 1, 2015, 19:11

    I don’t think humaned exploration of space will be possible, or at least be effective, unless a standard fuel is used and engines are designed to be swapped out on racks. Ships dedicated to zero atmosphere could potentially have very long operation lives, and advancements in engines will likely outpace advancements in life support systems or hull design.

  • Paul Gilster May 1, 2015, 19:52

    Roy Clymer writes:

    The most exciting space news I’ve read since Zubrin’s “The Case for Mars” !!

    Agreed! I love this concept and think it has spectacular possibilities.

  • Brian McConnell May 1, 2015, 20:04


    A good rule of thumb to estimate power requirements is to use the formula:

    P/T ~ 20 x Isp

    This gives a decent estimate of the power/thrust ratio for an engine that is 50% efficient (wall plug efficiency) as a function of specific impulse. So if the engine runs at 1,500s (electrodeless Lorentz force thrusts were shown to operate at roughly this level with water), that works out to 30kw/N. As you can see, power/thrust scales in proportion to Isp, which is why ultra high Isp engines like VASIMR are not attractive for SEP. We found the sweet spot for Isp is between 2,000 to 3,000s.

    OTOH, we don’t need huge amounts of thrust if the engines can be vectored while the ship is rotating (straightforward to do with a gimbaled rack and linear actuators), as they can be running for most of the journey. This enables the use of a small engine cluster and PV array (ship design competitions should surface the most interesting and adaptable optimizations for this).

  • James Stilwell May 1, 2015, 20:46

    A great post…Glad to hear these ideas are being taken seriously…A debris shield is critical as well as radiation shielding in the form of a water jacket…Habital inflatable rooms are waiting to be lifted into orbit…Perhaps there is water at the Moon’s south pole…and if so we have reason to begin your enterprise…A privateer will step forward once it is demonstrated that there’s water available outside our gravity well and, say, only six days away…keep the spirit going!

  • Harold Daughety May 2, 2015, 0:25

    This brings to mind the wagon trains going west, with the billowing canvas covered wagons – really, it sounds like a measured step back in order to make a leap forward. I would envision an inner rotating cylinder inside the water-shielded outer hull, with food production, water and waste processing and living quarters in the rotating cylinder and all else in the zero gravity annular space. I should be very possible to develop legumes that could provide all necessary amino acids and fats, and carbohydrates can come from many plant sources. I can imagine sweet potatoes growing in pots of growing media with the vines forming curtain walls around living spaces. I think there is something grand in all this!

  • Rüdiger Klaehn May 2, 2015, 3:47

    Regarding trip duration: with improvements in thin film solar cells, it will eventually be possible to have short trip times. But the requirement for absurdly short trip times in NASA missions is driven by the fact that these craft have cramped quarters, zero gravity, and insufficient shielding against galactic cosmic radiation and solar flares. So with every additional month in space, you have more bone loss and various other ill effects of living in zero gravity. And you also increase the chance of being hit by a solar flare.

    In a space coach you have more than adequate shielding against galactic cosmic radiation and solar flares. The sleeping quarters are surrounded by a very thick blanket of water and water-containing consumables. So a solar flare is basically a non-event. In addition, you have artificial gravity to prevent bone loss and other issues. And you have very comfortable living conditions (normal toilets and daily showers, frozen food instead of freeze-dried food). So what’s the problem if your trip takes a bit longer?

  • Mark Zambelli May 2, 2015, 8:11

    I can imagine the McConnell-Tolley fleet whizzing around all tied together nicely with the new Deep Space Atomic Clocks mentioned a few days ago. I hope this idea for these new spacecoaches does springboard to fruition after setting off an explosion of interest and technology competitions/research.

  • Alex Tolley May 2, 2015, 12:33

    @Harold – I like that design idea. This is the sort of thinking that we hope to elicit in design competitions.

  • Larry Kennedy May 2, 2015, 13:40

    @Harold Daughety Some number crunchers insist that space grown bulk food doesn’t add up for anything less than permanent colonies. But hey, even if they are right, all the more incentive for some homesteaders to set up on an asteroid somewhere.

  • Brian McConnell May 2, 2015, 14:55

    @Larry we’ve gotten mixed feedback on agriculture. Our view is that it’s probably not going to be the primary source of food, but nobody is going to complain about having fresh tomatoes, peppers, etc to supplement the frozen or dehydrated food brought on board. Plus, on a long trip, gardening will be good way to pass the time.

  • NS May 2, 2015, 15:52

    An article that highlights the importance of shielding for manned space flights:


  • Harold Daughety May 2, 2015, 17:29

    Once I had a sweet potato in a pot – 12 inch diameter red earthenware – and the vines covered about 12 feet of fence. Pretty to look at, some oxygen production, a bit of green old Earth, and a nice dinner twice a year.
    On H2O2, I have used 28% as a chemical reagent. Under refrigerator conditions it lasted about 3 months with a loss to under 25 percent. But one drop on bare skin would cause a painful, slow healing wound. It requires careful handling.

  • James Stilwell May 2, 2015, 17:36

    Suggestion: Plan a steam catapult at the site of water deposits on the Moon’s pole…Orbit several of the three-man rooms for the orbiting lunar catapult workshop. The workshop also functions as catcher of the water tugs coming up to orbit…You’ll want the water you’ll need already in orbit before making final preparations for your first trip out…and back…Bill it as a training exercise…a teaching exercise…DOD knows spacefaring knowledge doesn’t come cheap…Solar powered versions of the inflatables should safely store luke warm water in polar orbit…How many inflatables? Work up the cost sheets some day…Triple it and go for the money ASAP! DOD will like your plan and want it sitting on the shelf to commence when unexpected training needs arise…US SPACE COMMAND.COM likes options like these…

  • Adam May 2, 2015, 19:32

    Brian & Alex

    What about algae based food machines, like what Mark Edwards has advocated for years?


  • Brian McConnell May 2, 2015, 21:10

    @NS with water you get a 50x reduction in radiation per meter, so by wrapping the primary crew area in water reservoirs you’ll have good protection.

    @Harold, H2O2, if present would be drugstore strength (<10%) for that reason.

    @James, Deimos may be a more interesting site for isru. If the regolith has water ice content, its a much shallower gravity well.

  • Peter Popov May 3, 2015, 7:49

    Great concept. A quick question on the water as propellant in an electric thruster. After ionizing it, it will decompose into hydrogen and oxygen, both of which are highly reactive chemicals. How does this translate to engine wear and tear?

  • JTS May 3, 2015, 10:29

    Wouldn’t large quantities of water in liquid form be necessary for construction? Pykrete starts as a liquid. Can you provide any details on how large quantities of liquid water would be obtained in orbit? The mechanics of construction are hard to envision.

  • Harold Daughety May 3, 2015, 11:51

    Adam, I have tasted algae wafers – even with flavorings, they were at best blah and not at all appetizing. For survival food, that might suffice, but then, I would not want to survive that bad. There will already be enough in pioneering space coaches to drive everyone berserk. Maybe second generation space coaches will have stations posted on the major routes where travelers can buy a good meal, stock up on food and enjoy whatever entertainment is available – something like today’s mega truck stops.

  • Harold Daughety May 3, 2015, 11:57

    Peter Popov, preventing hydrogen embrittlement would require exotic alloys. HHowever, high pressure steam production is a well-developed technology and the design considerations are probably well known.

  • Alex Tolley May 3, 2015, 12:13

    @adam. I think what you are asking is why we don’t consider an ECLSS, perhaps utilizing algae. Let me address this.
    The reason is that the spacecoach consumables are propellant, rather than dead weight that must be minized to save fuel.

    2H2O => 2H2 + O2 [1] electrolysis
    CH2O + O2 => CO2 + H2O [2] respiration

    Eqn [1] shows that the water is a convenient storage for the bulk of the O2 consumption. Of teh shelf electrolysis will provide the O2 for the flight, eliminating cryostorage. This is a huge safety factor. The Apollo 13 O2 issue pretty much disappears for many missions.

    Eqn [2] is te heart of the recycling issue. The H2O is not deadweight and can be used as propellant. What about the CO2?
    Obviously we would scrub it, and with lots of solar energy and excess capacity, the Apollo 13 LM CO2 scrubbing issue that was so critical disappears. Without doing anything else, the spaceoach could dump the CO2 periodically to reduice this deadweight.

    However, since teh electric engines do not need to burn fuels and oxidizer, it may well be possible to add CO2 to the propellant stream to a add thrust. This is something that needs to be investigated.

    4H2 + CO2 => CH4 + 2 H2O [3] Sabatier reaction

    Eqn [3] is a chemical alternative, converting teh CO2 to methan and water (propellant) . The CH4 can be used for the maneuvering chemical engines, or again possibly used directly in teh electric engines as propellant.

    In preactice, food is not dry, byut contains a high fraction of water. In a conventional vehicle, the water coontent is useless, and recycling will just accumulate this water. Therefore teh solution is to minimize its mass by dehydrating or freeze drying, and using recycled water to rehydrate it. Nasa spends a lot of time making these food packets palatable. In contrast, the spacecoach treats the wet food as a water storage option, allowing the recovered water to be used as propellant. In extermis, it doesn’t much matter whether you keep your soup in dry granules and add water, or as bulk soup, ready to heat.

    The other waste stream is fecal matter. Again, water recovery is important, so teh spaceoach would emphasize means to reduce this to dry forms and recover teh water. This could be by direct application of energy, or possibly by composting that reduces the bulk via respiration.

    Finally, let me address the issue of ECLSS via biological recycling. We cannot do that today. So waiting for a reliable solution that we don’t need just slows the rate of progress. We cannot eat most algae directly, so we need other larger organisms to do so, e.g. fish. This poses problems of both maintenance and the extra deadweight of indigestible parts. A fish fillet in the freezer is fully digestible compared to say a freshly caught anchovey or carp from the algal tanks. Since one doesn’t want teh deadweight of various food processing equipment on teh ship, it seems to me that it is far preferable to have teh most delicious pre-pared foods availabe to eat on demand, than opting for some almost hair-shirt recycling diet more suitable for those committed to a farming lifestyle.

    Bottom line is that ECLSS is not needed to allow even Mars missions, and can be introduced when the technology becomes available for really deep space missions, simply because the bulk of the consumables mass is H2O and O2, and the H2O can be recycled easily and used as propellant. The O2 can be sources from the H2O, and teh H2 used either in the electric engines (demonstrated) or for other reaction purposes.

    I hope that clarifies our thinking.

  • Alex Tolley May 3, 2015, 12:14

    @Peter Popov Reactive H+, O-. You are correct that this could be an issue. This is why we emphasize the design of engines that can be easily maintaintained, e.g. replacing eroded liners. It is similar to carrying a barrel of grease on a prairie schooner, and key, hard to make components like axle bearings in case repair is needed on the journey. Design for maintenance, rather than perfect operation over teh lifetime of the unit.

  • Brian McConnell May 3, 2015, 12:56

    @Peter water seems to work fine in electrodeless engines (such as ELF thrusters) as the propellant does not come in contact with an electrode. This is a good point, and one of the reasons the design pattern has the engines housed in a chamber that can be sealed and pressurized, so the crew can service the engines, replace ablated parts, etc without doing an EVA.

  • Alex Tolley May 3, 2015, 12:59

    @JTS The first thing that should be obvious is that water is a lot easier to handle than cryo-fuels. All spacecraft to date have launched with all their fuel already in teh vehicle. The initial spacecoach launches will be similar, with the inflatable hulls deflated and the water pumped into the hull containers after reaching orbit.

    It gets more interersting when we consider refuelling in space. Water can be launched quite safely by any type of launcher – dumb boosters, epaceplanes, even space elevators. The containers just need to ensure that they stay pressurized to prevent the water was boiling off. Those containers would be docked with te spacecoach and teh water simply pumpled into the spacecoach hull. A leak in the lines would simply cause water to boil, a not particularly hazardous problem even for an spacesuited astronaut caught in the leak. Beyond the snow-line, where water stays frozem, the water would be transported as ice and any simple warming approach could be used to melt it – solar concentartors, heating elements, etc.

    Finally, in a pinch, one could simply fill watertight containers and lug them from teh refueling station into the spacecoach and discharge them inside the craft with pumps.

    The old idea of refueling deep space ships with tankers becomes much simpler when the propellant is water, compared to any cryo-propellant for chemical or nuclear engines. Space docking has become almost routine since Gemini, and the recent Progress failure an exception that proves the rule that of its routine nature. Pumping water in micro-g is not a issue, neither is getting teh water to orbit, other than cost. Should water be available on te moon, lauching it as water rather than processing it into cryo-fuels for orbital fuel depots is going to require less effort and cost, and water is going to be able to be kept stable for far longer in storage.

  • Rüdiger Klaehn May 3, 2015, 15:36

    @Peter Popov : at the energies involved in electric propulsion, it does not really matter if the propellant is chemically inert or reactive. Even completely non-reactive argon or xenon will erode the grid of e.g. a gridded ion thruster. However, it seems that erosion is well under control in modern electric engines. Even gridded ion thrusters can run for years without any maintenance.

    In hall effect thrusters, there is no grid or anything that could be eroded by the propellant, and the walls are made from refractory ceramics that are not sensitive to hydrogen embrittlement. So I would not expect any issues.

  • Brian McConnell May 3, 2015, 17:00

    I have created a spreadsheet that implements a simple parametric model for costing round trip missions — it’s available on request. In it I assume that the entire ship returns, and that you don’t drop off part of the structure at the destination (if so you can reduce the fueling cost by 30-50% with that tactic).

    You can try the following delta V’s to get a sense of how costs vary by mission (it also breaks out how costs vary depending on engine specific impulse). Try:

    10,000 m/s for a proving run in cislunar space and back to EML2
    15,000 m/s for a tour of Martian moons and back to EML2
    20,000 m/s for a tour to low Venus orbit and back to EML2
    25,000 m/s to go to Asteroid Belt or Ceres and back to EML2

    What you’ll notice is that cislunar space and Mars is absolutely do-able even with pretty crappy performance, and as newer engines become available, you’d just swap them in to step up the operating capabilities with each upgrade. Once you get to an Isp of 3,000s (upper end of Hall Effect performance), even Ceres is cheap (the refueling cost for a 40 tonne ship would be about $120 million, so figure another $100M for the crew launch via F9H plus amortization, and its still around $250M, cheap by spaceflight standards).

    I think that’s the really mindbending part about it is that the cost reductions are so huge. It doesn’t get you into space tourism territory, but many countries should be able to afford to operate spacecoaches for research and exploration.

  • Dave Moore May 4, 2015, 0:25

    I would suggest as a first step into space that you design a reusable, unmanned space tug that can take satellites of up to 10 ton into GEO from LEO and take a similar-sized space probe to Earth escape. It would rendezvous and refuel from an automated refueling station in LEO that in turn would be refueled a SpaceX Falcon. This would enable NASA to launch Flagship class missions cheaply.

    Particularly, if SpaceX makes both stages of its Falcon reusable, this could be a very cheap arrangement as SpaceX could use their old equipment again and again until it failed. The payload is cheap to replace.

    Another possible mission to show this technology off would be to design a Mars sample return that collects the samples from Mars orbit and parks by the ISS so they can be retrieved by astronauts.

  • ijv May 4, 2015, 1:54

    I love the concept, but I have one question about the use of water as both propellant and radiation shield.

    Doesn’t using water for both mean that you have excellent radiation protection at the start of the mission, moderate protection after the departure “burn” and minimal protection after the arrival “burn”?

  • John May 4, 2015, 5:38

    I love the concept and invention. This seems like the kind of ground breaker that would be favoured by the Elon Musks or Jeff Bezo’s rather than government space exploration organisaions that tend to be conservative in how they view space exploration.

  • swage May 4, 2015, 6:22

    Systems that are not specialized to a single purpose but can instead be used for a multitude of applications certainly the right approach. Its a fundamental leap in design philosophy. I’d go so far to say this line of thinking is equal to second generation space technology.

  • Robert May 4, 2015, 10:57

    This seems a natural venue to bring up the controversial, but I believe true, breakthrough that Dr. Randall Mills has achieved at BlackLight Power. In a sentence that breakthrough is this; hydrogen has a series of stable states more tightly bound to the nucleus accessible only through resonant collisions with an appropriate catalyst that release huge amounts of energy of about a hundred times that of burning hydrogen with oxygen. The energy is released as light but at tens of thousands times more intense than natural sunlight and is converted with concentrated solar cells at about 40% efficiency.

    Given that technology applied to this scenario, water is used as the source of hydrogen generating about 10MW of electrical energy at a rate of 10 grams of water per second. Basically you replace a huge solar array with a small device with much less mass. In fact about 100MW worth of these devices called SunCells could power a jumbo jet on pure electricity using about 100 gallons of water an hour.

  • Alex Tolley May 4, 2015, 13:54

    @Robert. It is more than a bit worrying that this may be a fraud [ http://en.wikipedia.org/wiki/BlackLight_Power ].

    There was a similar issue with sonoluminescence when the prime investigator claimed fusion was happening with deuterated water, but in fact nothing of the sort was shown and I think scientific misconduct was the result. Also we should be reminded of the cold fusion episode.

    People can be mistaken, or mislead, particularly in the short term. But when claims go on for a decade or more without independent validation, especially when there are conflicts of interest, it is best to be more than a little skeptical of any extraordinary claims.

  • Rüdiger Klaehn May 4, 2015, 16:52

    @ijv: Good question. You would use up the consumables and radiation shielding during the mission, but keep the crew quarters and sleeping quarters shielded until the end of the mission.

    So at the start of the mission the entire pressurized volume would be shielded, whereas at the very end of the mission just the crew quarters would remain shielded, and the freedom of movement of the crew would be somewhat reduced. You could still move freely in the entire pressurized volume, but should spend most of your time in the still-shielded section.

  • Joy May 4, 2015, 17:00

    Thank you Brian and Alex,
    I have no quibbles with the concepts you have advanced. My doubts about a human future in space concern two issues:

    1) Cost to LEO. I am very pleased that the cost will be dropping down to $1700/kg with Falcon 9 heavy. This is a great improvement over the greater than $10,000/kg costs of ULA boosters and Ariane. However, IMO, this is still too expensive to allow for placement of sustainable infrastructure in space. Even the primitive trailer park called the ISS masses 420,000 kg. With Falcon 9 heavy, a new technology orbital outpost of that mass still would cost $714 million to freight to orbit. And every kilo of milk powder (worth $2.50 on Earth) would still cost $1700, not a trivial freight bill for the groceries. It is very difficult to model how massive an orbital outpost would have to be in order to feed and repair itself, but I am thinking orders of magnitude more massive than ISS. IMO, we can not establish a sustainable off world presence at $1700/kg to LEO. We need more revolutionary advances in cost reduction to LEO.

    2) Profit from space colonisation. The Americas were successfully colonised by Europe because the colonisation funded itself. The investment in ships was more than paid off with the profits from slave trading (the “Indians” were traded as slaves right from the start – prior to the trans-Atlantic trade in Africans), sugar, tobacco, furs, and of course gold and silver. Space is full of valuable resources, but there is great doubt that anything (save possible lunar H3 if reactors existed that could burn it) could be returned to Earth for a profit. Without profits from space colonisation how can sustainable off world infrastructure be established?

  • Geoff May 4, 2015, 20:50

    Nice ideas, but I think (like ijv above) I see a flaw in the plan. Water used as radiation shielding can’t really be re-used as propellant, because it needs to still be there at the end of the trip! You wouldn’t board a slow flight to Mars if you knew the radiation shield was going to disappear along the way.

    Maybe if the spacecoaches (like their near-namesakes) were carrying colonists on a one-way trip, the water radiation shield could be re-used as propellant on the (uncrewed) return journey.

  • Joy May 4, 2015, 21:51

    typo, meant He3

  • Alex Tolley May 4, 2015, 22:46

    @Joy – although out of the scope of the Spacecoach proposal, you are raising THE question concerning the commercialization of space. The plan du jour is tourism, based on the model of island tourism, where nothing is really traded, just the value of the location. If orbital. as opposed to sub-orbital, tourism is to get going, tourists will want more than a cramped capsule for several hours. A hotel, all amenities, a variety of activities will all be desired to improve the experience. That is the start. And once you have a hotel, the cost of sourcing consumables will need to fall, resulting in a need for low cost water and food, which could drive NEO water extraction and orbital farms. Every $1700/lb of Earth sourced infrastructure will be amortized over a larger mass of recycled, ET resources, every gram of which will be sweated as an asset. Obviously for the super rich at first.

  • Chris May 4, 2015, 22:55

    Is it possible the water vapor expelled for thrust could be used by a later generation of ramjet propelled spacecraft?

    The water economies this type of craft would support and rely upon are very exciting.

  • Michael G. Gallagher May 4, 2015, 23:11

    I don’t want to spend 15 months going to Ceres, or eight months to Mars. I want my 39 days to Mars and VASIMIR! Don’t get too green, please!

  • Volucris May 5, 2015, 6:22

    Joy, I think the best answer to 2) is space based solar power, built from asteroid materials, with the factories residing in the asteroid (for all of current earth electricity needs you need only one asteroid, and not even very big at that), and ready solar power satellites shipped back to GEO orbit with solar electric propulsion.

  • swage May 5, 2015, 9:50

    Yeah, wilderness refiling seems to be definitely on the table for this one. Perhaps you could refill the tanks at the ice-fields of Saturn’s rings or by skimming through water vapor plumes orbiting the outer icy moons.

  • Robert May 5, 2015, 16:02

    “@Robert. It is more than a bit worrying that this may be a fraud [ http://en.wikipedia.org/wiki/BlackLight_Power ].”


    Your reaction is typical of those unfamiliar with Mills’ work but no, it is not a “fraud” in any way. I have a physics background and have studied quantum mechanics as an undergraduate and as a graduate student at the University of Illinois. Mills is a Harvard and MIT trained scientist whose work I have followed since 2000 and I am convinced the results are real and reproducible as several people have now replicated the reaction including someone at my old school U. of I. It is very controversial to be sure because it challenges the conclusions of quantum theory which after a century has been raised to the status of unquestionable fact and not what it really is, a one hundred year old theory that has its limits.

    Readers here also need to understand that Wikipedia, while in general a wonderful resource, in matters involving controversy the pages can sometimes be edited by people who are not always informed, fair or accurate. In this case the Wiki page is not accurate. In fact it goes well beyond being merely not accurate to intensionally misleading to the point of making libelous statements. I know that because many of us interested in Mills’ work have been following the page for years. The Wiki page editors like to highlight casual quotes from physicists who I believe have not actually taken the trouble to study the theory or data, some quotes decades old, and remove links to actual data, replications and supportive comments concerning the work. The page does mention numerous published papers which critique Mills’ work. Typically, physicists do not publish peer reviewed papers critiquing ideas they consider based on fraud. Wrong, yes, fraud, no. Note that the page mentions a 2005 European Physical Journal D article critical of Mills’ theory but completely ignores a 2011 article by Mills in the same journal in which the editors highlighted and while admitting its controversial nature, mentioned its potential importance. That is intellectually inexcusable. Statements have surfaced that the editors felt it was their duty to paint BLP in a negative or even criminal light. Mills has brought a defamation lawsuit against these editors in order to defend himself. Here is an alternative site with another point of view;


    The work has been extensively published, including above mentioned the European Physical Journal D vol. 64 in 2011 as a highlighted article. Here is the publication list;


    Three public demonstrations were given in 2014 to show the reaction energetics and discuss development plans to ultimately design and build power generation devices. Outsiders who replicated the reaction have publicly spoken out at the demonstrations. There is a BlackLight YouTube channel. Videos of the reaction and crude devices to demonstrate the concepts are available.

  • swage May 6, 2015, 14:48

    Is it possible to acquire the mathematical framework for this concept?

  • Robert May 8, 2015, 13:24


    To whom are you addressing this request?