A Spacecraft For the Future

DARPA (Defense Advanced Research Projects Agency) recently issued an RFI to support a project to last at least 100 years concerning building a spacecraft able to reach a nearby star. Naturally, this sparked my imagination to wonder what it would take to build such a craft. It quickly became obvious that the key focus would be stunning breakthroughs in propulsion technology–without that, nothing else would matter.

As I didn’t want to just toss out science fiction-y ideas, I decided to instead think about the other necessary technologies that would still be applicable with interplanetary travel. The new technologies focus on two broad areas: protection, and resources.

Protection

There are two major areas in the realm of protection that need breakthroughs for manned interplanetary travel: space energies (e.g. radiation, cosmic rays) and biological–notably, 1g simulation.

Space is a very harsh environment. Outside of some natural protection–such as the Earth’s magnetic shield deflecting solar radiation, and the Sun filtering some of the worst of open space radiation–Humans don’t stand much of a long-term chance when bombarded by the plethora of high-energy particles that abound in the harsh environment of space. As a result, whether stationed on a planetary colony or travelling on board a spacecraft, there’s a very real and immediate need to surround the space people with reliable, cost-effective, and (ideally) low-mass shielding.

Obviously, between a planet-side base and a space-faring ship, the base has many more options. If it can be situated in a cave or tunnel, then nature will have done a fair amount of work for us. Also, we being builders, we can construct shelters using local, natural materials to give us the safety we need–not unlike interplanetary hermit crabs. Many of these items have been the basis of on-going studies. There is no one solution since every environment will present its own challenges. The Moon’s regolith, for example, is very different from the oxidizing soil on Mars. Different gravity/atmospheric issues will also factor into what construction methods would be the best to use. Despite this, it’s clear that constructing land-based shelters is something from which we can build upon our millennia of practice and experience.

Protecting a spacecraft is a whole different kettle of metaphors. As there is a direct relation between mass and the energy needed to affect that mass (aka acceleration — courtesy of Mr. Newton), there is a very real need to consider every option carefully so as to achieve the required effectiveness with an efficient use of materials.

The first requirement for our spacecraft is the habitat itself. Without some sort of shell to provide living space…well, it makes everything else moot. Traditionally, we build our shells out of aluminum, titanium, steel, ceramics, and a variety of polymer systems. Lately, private company Bigelow Aerospace has orbited inflatable habitats that so far have demonstrated the viability of non-traditional constructions. Because of their inherent advantages (lower mass, slightly improved protection), I think that inflatable construction will become the basis of future long-term missions.

Because the crew will be very far away from a repair facility, the smartest course is to build a craft with a large amount of redundancy. This suggests that surrounding a multi-port docking module will be “petals” of inflatable modules–any one of which can have a loss-of-integrity failure without affecting overall spacecraft functionality. One of the advantages of an inflatable system is the possibility of either patching the affected node or, perhaps, replacing it with a stowed, deflated, spare.

Whatever the shell, there is still the need to provide protection from the wealth of cosmic radiation that will be bombarding the ship. The most obvious first step is to have each module be double-walled, the space in between being filled with water, which is a very effective radiation shield (notice how important it is in nuclear reactors beyond the cooling function). While helpful, this water sleeve doesn’t protect against everything. Having plate shielding outside the vehicle might also be necessary. I advocate individual plates as they can be replaced/repaired/removed, as necessary, en route. Again, for a long-duration trip of six-months or more, the ability for the crew to improvise in situ is critical.

A useful non-material shielding that is currently being researched is the idea of generating a magnetic “bubble” around the spacecraft. This would help deflect charged particles around the ship where they would be deposited at the poles. While the field itself is effectively massless, the generator to create the necessary magnetism might be prohibitively large–and would likely need to be integral to the propulsion system.

As important as the habitable shell is, the major breakthrough needs to come in the area of protecting the orthopedic/muscular integrity of the Human travelers. If, after only a “brief” one-year journey, they are physically incapable of landing on a gravity-rich world, then the mission as a whole seems rather pointless. Despite all sorts of exercise nostrums–which have so far all fallen short of the goal–the fact remains that there is no substitute for living in a 1-g environment.

Ideally, we could go all science-fiction and manipulate energy fields in order to create artificial gravity. While a laudable goal, in any foreseeable future that isn’t going to be a practical goal. This essentially leaves us with only one alternative: centrifugal force. Again, this isn’t a new concept. You have a rotating structure that has been accelerated to provided an effective 1-g environment to whatever is trapped inside. In order to have successful long-duration missions, I see this as the only reasonably feasible solution. Even so, it isn’t without its own set of not-insignificant problems.

When considering the complications of angular momentum, structural centripetal considerations, torque on the spacecraft itself, and other variables, it quickly becomes apparent that this isn’t as easy as the concept sounds. The more effective the rotating habitat, the larger in radius it needs to be. The larger it is, the stronger the materials and construction need to be in order to prevent the structure from flying apart. Also, as it rotates, it will torque the vehicle into rotating in a contrary fashion. Even with contra-rotating rings, there will be a need for secondary systems to adjust the ship’s orientation.

I propose a simpler solution than rotating the entire habitat: only have sleep modules rotate around the station. As the human occupants are going to be spending 1/3 of their time effectively doing nothing, why not let them passively reap the benefits of being in a 1-g environment during that period? Lower-mass sleep pods can extend (actively or permanently) from the central structure and provide the necessary acceleration. This also provides a diminished need to provide a radial position sufficient to mitigate any “tidal” differences that could manifest themselves in a larger structure. The crew would then return to the micro-gravity core for the rest of their day. If 8hr/day at 1-g is insufficient, this plan can be modified while not sacrificing too many of its advantages–for example, in addition to the sleep pods, some 1-g pods with standing workstations or perhaps additional exercise equipment.

Resources

Also on tap for any long-duration voyage is the need for resources. Again, we aren’t talking about taking along “just enough”. There has to be more than enough to handle emergencies. The loss of ammonia coolant on the ISS causing a bit of a crisis in its low Earth orbit easily shows the need to never assume that you aren’t going to lose some critical material to space, contamination, mechanical failure, or any of a myriad of other reasons. The prudent long-duration designer must not only assume that Murphy’s Law^[1] is correct, but hold the opinion that Murphy was an optimist^[2].

When talking about resources, this includes not only the necessary nutrients required to keep the crew alive (e.g. water, food, air), but also the needs for medical emergencies as well as maintaining the health of the spacecraft (e.g. fuel, coolant, lubricants, etc.) Add into this the need to supply power to just about every system, and you begin to see why manned spaceflight is very complicated. Factor in the inability to re-stock, and you have made a difficult situation all that much harder.

Just as the shell was the foundation of protection, so too must power be considered to be the foundation of the resource equation. It is so critical that redundancy MUST be factored in. This doesn’t necessarily mean taking along a spare, but it does mean building in sufficient emergency systems unconnected to the primary system that the ship can survive for a significant time (we’re talking no less than months) for other solutions to be brought to bear. Again, the time for repairs to the ISS tells us how necessary being able to extend a power-availability timetable can be for mission survival.

Unless something unexpected comes up, the primary power source for an extended mission has to be nuclear. Nothing else is as reliable or as capable of providing power/heat as this option. While solar is a nice alternative when available, that pesky inverse square law makes it less viable the farther you travel away from the inner solar system. While fusion would be great to have, it still hasn’t been sustained on Earth, so it’s reasonable to anticipate that our ship’s primary power source will be based on nuclear fission.

What of our secondary source(s)? You use whatever technology you have available. Fuel cells are always an option, as are batteries, chemicals, and flywheels. There is also the human component–in an emergency, there is no reason why some the crew can’t take shifts manually powering a generator. It won’t be easy. Without the primary power source, any mission will likely grind to a halt eventually.

After power sources are taken care of, it’s time to consider the chemicals that must be hauled with the ship. These will be varied and many. The atmosphere will require oxygen plus a substantial amount of a noble gas…likely nitrogen. Water is a given. While it’s tempting to think about simply electrolysising  the water to get oxygen, that’s probably not the most efficient use of the material or storage space. If nothing else, you need to do something with the hydrogen. As resources are precious, you don’t want to simply dump the hydrogen. I’m thinking that much of the electrolysis will be to provide fuel for the fuel cells, not for the atmosphere…but I could be wrong. The problem is that storing oxygen and nitrogen is a nasty business. While keeping the tanks shaded from the sun will help maintain the cryogenic temperatures necessary for LOX and LN storage, the sheer numbers becomes daunting. Since they must be overstocked to compensate for leaks, ruptures, and general system inefficiencies, these potential bombs will have to populate much of the ship’s exterior.

Now we also have to get into the problem of recycling these gases. Conventional hypotheses say that plants can do much of this job. This biosphere approach has been experimented with on numerous occasions, but we’ve yet to achieve a truly closed system with plants and large animals where symbiotic equilibrium is achieved. Even so, as the crew will have to eat, some sort of hydroponic setup must be established. The types of plants grown will have to be somewhat limited to those species that can be the most fully utilized–so, expect a lot of salads. In addition, it’s probable that yeasts and algae will be additions to the diet.

That’s the thing about long-duration missions: unless you are planning on bringing a grocery store with you, pretty much everything has to be part of an efficient cycle. Body wastes must be processed for water, nutrients, and minerals. Respiration gases have to be processed so that livable proportions are maintained.

Thought must be given that the ship and its internals are essentially living things as well. Their power has to be clean. Parts that die must be replaced. This means an ability to manufacture parts (likely via 3-d printing/fabrication) as well as to repair/replace existing components–specifically electronic components. To that end, it might be necessary to dial back some of the miniaturization so that macro-sized humans can work on them. This means stepping back from some surface-mount ICs.

Summary

The gist of it all is that for a long-duration mission, any crew size greater than 2 or 3 will require what is essentially a small space station with a big engine. Considering that a useful crew will have at least five individuals–all cross trained so the loss of any one person isn’t a failure node for the mission–then sufficient living space, as well as the space necessary to make the craft more-or-less self-sustaining, requires a large assembled-in-orbit ship.

One only has to look at a modern nuclear submarine to see that the single limiting factor is likely to be food. You either have to be able to grow what you need, or you need to haul everything. It’s often said the the only practical reason many nuclear subs have to return to port is because they run out of food.

Whether you are travelling to Mars, Europa, Titan, or to the nearest habitable world in another solar system, the basic needs don’t change: protection and resources. The only thing that changes is the scale: an interstellar mission (or even a deep interplanetary mission) will likely need to carry a small community, not just a handful of crew; and the main power-plant/engine will change depending on the mission.

In the short term, I see the two areas of most critical need being: the ability to supply a 1-g environment, if only for sleep periods; and the ability to grow enough food to sustain a crew. After that, propulsion technology will need the greatest innovation. Other technologies need to evolve and mature, but there doesn’t seem to be anything we can’t reasonably expect to be able to do given what we currently know.

¹ “If anything can go wrong, it will.” Murphy’s Law and other reasons why things go wrong!, Arthur Bloch, Price/Stern/Sloan, 1981

² “O’Toole’s commentary on Murphy’s Law” [ibid]