Traveling To Mars? Just Add Water!
Since the Moon landings, we have eyed Mars as the next step in manned space exploration. In the nearly fifty years since, it still remains a distant location. We have sent many robots there, but sending people is a far more difficult goal. The challenge of reaching Mars is more one of logistics than technology. How do you keep people alive for years in space? How do you launch enough material to do so? at a reasonable cost? What if the key to reaching Mars and other worlds is to rethink what a spacecraft is made of? In short, what if a spacecraft was made mostly of water?
The Apollo program probably would not have happened if it were not for Lunar Orbit Rendezvous. Little known by outsiders, this technique was developed by NASA engineers Tom Dolan and John Houbolt. It made the difference between a mission that was just barely achievable given the technology of the day, and one that would have required a rocket so huge and so massive that the costs would probably have sunk the program. The breakthrough in LOR was logistical, not technological, and that’s an important point because the plans on the books for manned missions to Mars and beyond all have the same fundamental weakness that early moon mission designs had.
What LOR did was to replace one big rocket ship that flew all the way from Earth orbit to the lunar surface and back (as depicted in 1950s era sci-fi), with three smaller craft: the command/service module, which only flew from Earth orbit to lunar orbit and never landed and the lunar module, with its disposable descent stage. Essentially what this did was break one big craft into three smaller craft which each flew a segment of the trip and then were discarded. This greatly reduced the mass of the entire system, which in turn, reduced the size of the rocket required to launch them all from Earth’s surface to orbit at the start of the mission. As it was, the Saturn V was a giant rocket! If they had tried to do the mission with one large spacecraft, as envisioned in 1950s sci-fi films, the launch rocket would have had to be several times bigger, well beyond the realm of possibility then or today.
Manned missions to Mars have a similar logistical problem. To travel from Earth orbit to Mars and back requires a larger change in velocity (delta v) than a trip to lunar orbit, a challenge in its own right. Further complicating the situation is the requirement to provide enough life support and supplies (consumables) to keep the crew alive and comfortable for roughly two years (as compared to two weeks for an Apollo mission). As a rough example, consider a 6 person crew on a mission to the Martian system. This will require roughly two years, so figure about 800 days to include some reserve margin, and about 5 kilograms per day per person allotment for water, oxygen, and food (which is also mostly water). That works out to 24,000 kilograms, or about 24 tons, just for consumables. And here’s why that’s a big problem.
That material is deadweight that has to be pushed along by yet another rocket motor and its propellant. In rough numbers, you end up needing ten to twenty times as much propellant to drive this deadweight mass from Earth orbit to Mars and back (even with half of it dropped off as waste at the destination). The amount of material needed ends up being comparable to the mass of the International Space Station, which took years and tens of billions of dollars to build. Never mind other considerations such as radiation protection. This is a major reason why manned trips to Mars have persistently remained 20 or 30 years in the future.
When Alex Tolley and I started this project, we started with a simple question that we explored in a paper for the Journal of the British Interplanetary Society. What if you could turn all or nearly all of the consumables into propellant? Specifically, what if you could design the entire ship around electric engines that use reclaimed water and waste gases as propellant? It turned out that not only is this possible with existing technology, but it also completely changes the economics of manned interplanetary space exploration, as well as what a spaceship might look like.
Electric propulsion is not a new technology, and has been used on many unmanned spacecraft. The idea is to use an external power source, typically a solar photovoltaic array, to drive an engine that uses an electrical or magnetic field to heat and accelerate a gas stream to great speed (tens of kilometers per second). Because these engines can achieve much higher exhaust velocity than chemical rockets, 10x or better, they can achieve greater change of velocity (delta v) using the same amount of propellant. This means they can venture to more ambitious destinations, carry more payload, or a combination of both. It also turns out these engines can also use a wide range of materials for propellant, including water. Following are some helpful primers on rockets and the rocket equation.
To understand why this is so important, let’s go back to the 24 tons of consumables we would need for our hypothetical six person mission. In a conventional spacecraft architecture, that deadweight, along with the dry mass of the hull, needs to be pushed by an external rocket many times as massive. By turning that material into propellant, you eliminate the need for an external rocket almost completely. This reduces the mass of the system by a factor of 10 to 20 or more, which in turn reduces overall mission complexity and cost by 90 to 95%. For details, calculations and more, we encourage you to read our original JBIS paper, this article on Centauri Dreams, and, if you have the time, our book for Springer Verlag about the spacecoach architecture.
Another question we asked was “Do we really need to send people to a planetary surface on most missions, or is getting them to the general vicinity good enough?”. This is another major flaw common to Mars missions. The primary motivation to go there initially will be scientific exploration, most importantly the search for evidence of past or present life. Mars colonies make for an exciting story, but realistically those will come well after initial exploration because we don’t want them to end up like Jamestown.
Sending people to the Martian surface is risky and expensive compared to putting them in orbit overhead. First there is the cost of all of the equipment needed to get people to and from the surface from Martian orbit, as well as the physical risk of flying to the surface and back. More importantly, there is the risk of biological contamination, not from Martians infecting us, but from us contaminating the Martian environment with Earth microbes. Humans are literally walking piles of bacteria and will contaminate any area they explore. Because of planetary protection concerns, early Mars expeditions will avoid locations thought to have liquid water. If that’s the case, what’s the point of going? And lastly, there is the risk that people just won’t care very much about “boots on Mars” and will be reluctant to fund an Apollo scale program. Pop quiz! Name the astronauts currently flying aboard the International Space Station.
Sending people to the vicinity of Mars doesn’t mean you can’t do anything on the surface. It turns out you can do quite a lot. Instead of people, you send robots to the surface which will be tele-operated by astronauts from orbit overhead. From Mars orbit, the communication delays induced by the speed of light are negligible, so astronaut operators would be able to control them in real-time. With VR and tactile feedback, they will have fine control over these robots, and will experience the surface environment and interact with it as if they are there, all while in a shirt sleeve environment and without the expense and risk of flying to the surface.
Small sample return rockets could deliver surface material back to local orbit for more detailed study. As an added bonus, astronaut operators will be able to control multiple robots at multiple locations on the surface, which means that mission planners will be able to hedge their bets by exploring many interesting locations in a single mission. A manned surface expedition will only be able to go to one location per sortie. This approach reduces risk and expense by another order of magnitude. Al Anzadua’s paper, “From Moon To Mons To Mars”, is worth a read, as it describes this pathway in detail.
The point isn’t that people should never go to the Martian surface, just that we can get a lot of mileage out of remote exploration from orbit before we need to do so and before we are ready. The argument for sending people straight to the surface in the immediate future just isn’t there if we can accomplish the same goals at less cost via tele-operation.
What Would A Water Based Spacecraft Look Like?
Alex and I started with the basic idea of using consumables as propellant, and let the analysis guide us toward a system design. We started with a few basic constraints, both to simplify the design process, and to avoid tying the design to a specific mission.
Among the main constraints were:
- The ships are purely interplanetary vessels. Once built, they never land on a surface or enter a planetary atmosphere, but instead travel between orbit around Earth and other worlds. Any descent to a planetary surface is done via a separately designed module. This also means the ship can be reused for many missions, so its construction and launch cost can be amortized accordingly.
- The ships are powered by large solar photovoltaic arrays. While other power sources, such as nuclear electric power plants, could be used, we assume that solar is the primary power source, for technical and political considerations, especially because solar PV is a well understood and continually improving technology.
- The ships are propelled mostly by electric propulsion technology, and use water, carbon dioxide and gasified waste as propellant, essentially they convert the crew waste streams and reclaimed water into propellant after first pass use by the crew. Water and water rich material is used for other purposes, such as radiation shielding and heat management, while in passive storage.
- Habitable areas are derived from inflatable structures, such as Bigelow Aerospace units, to allow large structures to be fit into existing launch systems and then be self-assembled in space with less manual intervention.
- The system as a whole is modular, so external landers, chemical propulsion units, and other modules can be attached and detached as needed based on mission requirements.
This design pattern leads to a ship that looks like the rendering below. While this isn’t the only possible configuration, this “kite” pattern minimizes the materials required while providing a sizable habitable area, and while generating enough electrical power to generate useful amounts of thrust via electric propulsion. We coined the name “spacecoach” to describe them, as a nod to the Prairie Schooners of the Old West.
Because the ships are purely interplanetary, they can gradually change their orbit under electric propulsion rather than use high thrust chemical rockets. These engines generate low thrust for long periods of time, so the ships never experience extreme stresses or vibration, but rather gently accelerate and decelerate. They are more like the Mars Cycler concept proposed by astronaut Buzz Aldrin than a conventional rocket ship, and I suppose you can think of them as active cyclers. Because of this, the ships can be big, with a large surface area for solar arrays, and can be large enough to generate artificial gravity. They will also be able to fly many missions, with a useful life comparable to the ISS (20+ years) so their construction and initial launch cost can be amortized across 5 to 10 missions.
The use of water and waste gases as propellant, besides reducing the mass of the system by a factor of ten or more, has enormous safety implications. 90% oxygen by mass, water can be used to generate oxygen via electrolysis, a simple process. By weight, it is comparable to lead as a radiation shielding material, so simply by placing water reservoirs around crew rest areas, the ship can reduce the crew’s radiation exposure several fold over the course of a mission. It is an excellent heat sink and can be used to regulate the temperature of the ship environment. The abundance of water also allows the life support system to be based on a one-pass or open loop design. Open loop systems will be much more reliable and basically maintenance free compared to a closed loop system such as what is used on the ISS. The abundance of water will also make the ships much more comfortable on a long journey.
The design pattern also leads to extensive redundancy, and a ship that is safe by default. Take the electric engines as an example. In most of the configurations we looked at, there would typically be dozens of relatively small units grouped into arrays that collectively generate usable thrust. Similarly the electrical system would consist of many smaller units that collectively generate a large amount of power. The failure of a few units out of dozens would lead to a minor recalibration instead of a crisis.
Some of the design patterns we explored will also enable the ship to rotate to generate artificial gravity while cruising. Microgravity, along with radiation exposure, are the two primary health hazards associated with multi-year missions. The abundance of water, an excellent radiation shielding material, will make radiological exposure much less of an issue. Artificial gravity will enable the crew to spend most of their time in Earth like conditions, and will also simplify many aspects of ship design (furniture, showers, toilets, etc).
The design pattern also borrows an important lesson from the computer industry, that many small, incremental improvements in component technologies can compound to produce overall huge improvements in capabilities and costs. The basic technology used in computing and communication, the solid state transistor, was invented decades ago. The dramatic improvements we’ve seen resulted from making them smaller, faster and less energy intensive with each manufacturing cycle. The space coach will enjoy a similar effect as solar photovoltaic and electric propulsion technology improves, especially if ships are designed so that these components can be swapped out with each new mission.
Testing and Development
People typically assume that this spacecraft architecture will be prohibitively expensive to develop. In fact, it will be possible to validate most of the mission architecture without building a full scale spacecraft, and the most important work can be done without leaving the ground.
Electric propulsion technology is the fundamental driver in the system design and its cost model. In our research, we identified several technologies that should work with water and waste gases, and that will offer the performance required for missions throughout the inner solar system. Some of these systems are already in advanced stages of development, and a few, such as Hall Effect thrusters, are already flying. The next step then is to organize an engineering competition to test these engines with water, carbon dioxide and other waste gases, and to measure how they perform. This competition can be done in ground based facilities, and can be done very cheaply in the context of spaceflight.
The winners from that competition would move on to build small satellites that test these engines in actual spaceflight conditions, and that simulate the overall duration and profile of an interplanetary mission while remaining in near Earth orbit. They would do this by stepping up and down in orbital altitude to accumulate the total change in velocity and flight duration required for an interplanetary mission. Everything short of the crew habitat could be tested via small unmanned platforms like this.
The first crewed ships could simulate an interplanetary mission while remaining in near Earth space. A spacecoach would fly out to cislunar space to spend two years gradually changing its orbit around the moon to simulate both the delta-v and the duration of a subsequent mission to the Martian moons. During the mission, the crew would be able to fully test ship systems, tele-operate surface robots on the moon, and perform other activities anticipated in an actual Mars mission. Meanwhile they would be able to return to Earth within a few days via a crew return vehicle should an emergency arise. Once fully tested, the first ship would be ready to venture out from the Earth-Moon system to the Martian moons and eventually far beyond.
A Real World Starfleet
These ships will not be destination specific. They will be able to travel to destinations throughout the inner solar system, including cislunar space, Venus, Mars and with a large enough solar photovoltaic sail, to the Asteroid Belt and the dwarf planets Ceres and Vesta. They’ll be more like the Clipper ships of the past than the throwaway rocket + capsule design pattern we’ve all grown up with, and their component technologies can be upgraded with each outbound flight.
In effect, they will form the basis for a real world Starfleet, one whose range of operation will grow as their component technologies evolve and improve.
Further Reading
“Reference Design For A Simple, Durable, and Refuelable Interplanetary Spacecraft”, Alex Tolley; Brian McConnell, Journal of the British Interplanetary Society (2010)
“Spaceward Ho!”, Alex Tolley, Centauri Dreams (Jan 9th, 2015)
“A Stagecoach To The Stars”, Brian McConnell, Centauri Dreams (May 1st, 2015)
“A Design For A Water Based, Reusable Spacecraft Known As The Spacecoach”, Springer International Publishing (2015), ISBN 978–3–319–22677–4
