A Stage Coach To The Stars
The manned space program is at a crossroads. There is much confusion about where its going, and what its next destination will be. Will it be the Moon? a near earth asteroid? Mars? What if we changed its focus from picking a destination to building multi-purpose spacecraft that, like maritime research and exploratory vessels, can travel anywhere.
Photo credit: 1950s vintage illustration of a solar electric space craft powered by concentrated solar collectors and an ion drive.
Three years ago, Alex Tolley and I co-authored a paper for the Journal of the British Interplanetary Society that explored the idea of a spacecoach, a fully reusable interplanetary spacecraft whose mass is mostly water. These ships would never enter a planetary atmosphere, and as such would be simpler to build than a craft that has to endure launch and re-entry (we treated the problem of getting to and from earth orbit as a separate issue for a separate system).
It turns out that water is highly desirable in an interplanetary spacecraft, not just for life support (for people and plants), but can be used for many purposes on a long flight, including radiation shielding (its comparable to lead on a per kilogram basis), thermal regulation (its an excellent heat sink), oxygen generation (via electrolysis), attitude control (by pumping water around ring shaped reservoirs) and propulsion (water vapor can be heated to very high temperatures in microwave electrothermal engines to produce thrust several times more efficiently than chemical rockets). Water is also easy to handle, meaning the craft can be resupplied as needed, something that is much more difficult to do with chemical rocket fuels, especially cryogenic fuels.
What we found is that the ability to use water for many purposes prior to using it for propulsion radically reduces the cost of the system while increasing useful payload. This is primarily because most of the dead weight on the ship (consumables, water for the crew and vegetation, radiation shielding, etc) is gradually burned off to generate thrust via the electric propulsion system. As a result, most of the ship’s mass can ultimately be used as propellant.
What we envisioned with the spacecoach is a fleet of ships built largely from off the shelf components, such as Bigelow Aerospace inflatable habs, that would be reconfigured as needed for each mission and would be upgraded as improved technologies come online (for example, more lightweight solar photovoltaic sails that generate more power per unit of mass). These ships, once launched, could remain in service for long periods of time, with some elements such as a central mast, lasting indefinitely. Each one of them would be a named vessel, like the great exploratory ships of history.
They would be continually upgraded as better solar arrays and propulsion units become available. As a result, their mission capabilities would increase with each iteration. Older components would either be discarded or recycled into ships or space stations that don’t require the latest and greatest technology.
These ships would be parked in Earth orbit, above the Van Allen radiation belts, but still be readily accessible to the rockets being developed by firms like Space X. Crews would shuttle to and from the ships on conventional rockets (to minimize the time spent transiting the Van Allen belts). Supply ships would be launched to low earth orbit and slowly spiral out to meet the crewed ship using solar electric propulsion. This would serve two purposes: one to continually test out new electric engine designs using unmanned craft, and second to minimize the cost to launch payloads to high orbit in $/kilogram terms.
The spacecoaches would not need high thrust chemical rockets for many missions, and instead would slowly accelerate to put themselves on trajectories to their destinations (though they could be coupled with higher thrust chemical rocket modules when needed). As the solar arrays and engines are upgraded, their range will increase and trip times will decrease, opening up new destinations. They could be thoroughly tested in Earth orbit (for example by stepping up and down between different orbits to simulate the total change in velocity needed to reach a destination such as the Martian moons). Most aspects of deep space missions, from total delta V (velocity change) to trip duration could be simulated in Earth or Lunar orbit, providing crews with maintenance and abort options during the testing period.
It also turns out that we can solve two of the biggest challenges in long duration flights, the effects of radiation and of long-term microgravity exposure. Water blocks radiation with roughly the same effectiveness as lead (on a per kg basis). Metal radiation shielding is dead weight that contributes nothing in a conventional ship. Water is both a consumable and a propellant in the spacecoach, so the ship or parts of it can be very well shielded. It can also be reconfigured in an emergency, for example by filling a large bladder with water to form a temporary cocoon that the crew can shelter in.
What about gravity? The spacecoach design requires a large area solar photovoltaic sail to generate power. In the designs we looked at, we considered solar arrays measuring a hundred meters across. These would be lashed to a cross shaped main ship that, in turn, could be spun up to generate artificial gravity in the outer modules, which would be tethered to a central node.
None of this is to say that this would be easy to do, but the technology needed to build these ships exists and improves with each year. The International Space Station is proof of that. Building these ships is primarily an engineering problem. In many ways, the design philosophy resembles the “release early, release often” style of software development. Get something with modest abilities flying quickly, then improve its power and engine performance with each upgrade.
The important point is that each one of these ships would fly many missions, and each would be a named exploratory vessel. As the fleet grows in size and variety so too will our exploratory capabilities. Instead of picking a destination, and then building a single use ship, we could focus on improving the ships already aloft and fly them to new destinations as they are ready.
Some of these ships might be kept aloft and preserved indefinitely as historical artifacts, like the USS Constitution. Imagine the USS Roddenberry parked in lunar orbit as a tourist destination a couple hundred years from now. Now that’s an interesting future to contemplate.