In my last article, I discussed spaceship weaponry and how I envisioned it to look in the future, once (if) humanity reached out to the stars. This article examines the construction of those ships, specifically the materials used for their hulls. Some of these concepts here you may see mentioned in the Arcworld, but as those are novels and are about the people, there wasn’t space to dig much deeper. Here, both you and I have that luxury.
It is my opinion that in the medium-term future, advances in materials technology will primarily be limited to composites. We have almost exhausted our ability to advance in pure metallurgy (melting and forming of alloys) for aerospace, rather new advances in alloys are currently focused around additive manufacturing. But, composites still offer the most room for improvement, as, believe it or not, we’re still in the infancy of composite design. Take as evidence that we’re only scratching the surface of biomimicry and all the future capabilities that will bring.
Metallic Materials
As I mentioned in my last article, most ships in a terrestrial navy use steel as the primary hull material. That’s primarily because of a) how it dents and bends, rather than tears when damaged, b) the cost and ease of repair, c) initial acquisition and manufacturing costs, and d) combat strength. Steel definitely has its advantages, but it also has disadvantages. It’s heavy, and it rusts, especially when exposed to water. But navies across the world have stuck with it as a primary hull material because of its strengths, as its weaknesses can be relatively easy mitigated.
Aluminum is also used for some hulls, such as that of the Independence Class Littoral Combat Ships in the US Navy. Aluminum has the advantage that it’s much lighter (about 30% of steel’s weight) for the same strength. This means that the ship is faster for the same power plant and more fuel efficient. Aluminum is also easy to work with, but the downside is that there are many more shipbuilders capable of repairing and working with steel than there are those who can weld aluminum (although this is slowly changing). Unfortunately, for many applications, aluminum just doesn’t have the longevity a naval ship requires, as it still corrodes (it is still anodic, and requires a special bottom paint lacking copper) and is subject to stress fractures. Yes, there are some techniques to mitigate this, but it is still a concern that hasn’t been easy to eliminate.
Out of the atmosphere, away from water, the corrosive influences that aluminum is subject to are eliminated, and the weight saving benefits are more apparent in both the construction of ships and their power plant needs. But, for uses where the hull needs to have high abrasion resistance and repairability, it’s still not completely desirable.
Yet another feature of aluminum is relevant when thinking about its use in spaceships. In Vanir, there are two distinct types of engines. For the protagonist, the Entropic Engine doesn’t release a lot of excess heat as it’s not combustion that’s used for propulsion, but rather the catalytically induced rapid entropy of a liquid to its smallest particles (quarks and smaller) and the focusing of the energy released to propel the byproducts out a thruster nozzle. But, for most spaceships in the future and now, combustion is how a ship is propelled. That combustion releases a lot of excess heat, which has to be transferred somewhere off the hull of the ship.
Most science fiction stories ignore the excess heat problems that all spaceships have and will always have. Spaceships produce heat — whether that’s from the engines, heaters to keep the inhabitants warm, or from the inhabitants themselves — or absorb heat from the sun. This excess heat has to go somewhere else, other than the ship. Therefore, future spaceships will have to have large radiators in order to release any excess heat and keep the living quarters habitable. Reducing the heat production, such as with more efficient combustion, would help, but it’s even more important to ensure that whatever heat is produced doesn’t transfer from one part of the ship to another, and aluminum is also very good at that. Aluminum has an emissivity that’s 8 times lower than steel. That means that it will release that heat faster, even into the low density of space.
Another metal with aerospace applications is titanium. Titanium does not corrode (in most aerospace applications, although the A-12 had issues with chlorine in Burbank, CA’s water supply), it doesn’t galvanize, components built with it are lighter than those built from aluminum (technically, titanium is heavier at 50% of steel’s density, yet less material is required for the same strength), and has low acoustic absorption. That makes it perfect for submarines, if the builders can afford it and work with it properly (if not properly welded, titanium loses all its advantages). But, its expense, and the difficulty of sourcing such massive quantities for use as a hull material (not because of presence but concentration) are almost prohibitive. Of course, we’re talking about fantastical worlds where much of what’s really impossible is possible, but I prefer to keep as close to reality as possible so that the reader doesn’t have to stretch much when suspending belief.
Composites
Composites already have a firm presence in current and prior spacecraft design, being used for the space shuttle’s carbon-carbon HRSI panels, the ablative Avcoat composites used 50 years ago in the Apollo program, and the current fairings from Space X’s Falcon program. Composites will continue to be relevant in the near and far future, but as a primary hull material, it’s limited in its application.
In aerospace, composites have only in the most exotic of situations been utilized to their full potential. The Airbus A350’s fuselage is primarily comprised of CFRP. So too is the fuselage of Boeing’s 787, but how that CF is manufactured and used structurally differs considerably. The Airbus uses CF panels to replace aluminum panels, whereas the 787 goes one step further by the fuselage being made of several joined continuous CF barrel structures, with only the stringers a carryover from traditional aluminum manufacturing. Both techniques are disdainfully referred to by Burt Rutan as “black aluminum”. And Burt should know. He’s the man who envisioned and built SpaceShipOne which won the X-Prize in 2004.
But it’s when weaving an airframe or space frame that the full potential is realized, because plate and sheet CF suffers from anisotropy and is only strong in two dimensions, whereas weaving can produce isotropic components strong in all three directions. Look towards the difference between the most exotic of CF-bodied automobiles and those more common. Therefore, I’d argue that composites will play an extremely important role in spaceship design, but not necessarily as a primary hull material, at least for most ships.
Rather, just like composite use today, CF will be used where it’s needed most and its strengths are preferred. For automobiles, an aluminum or steel frame can be repaired, albeit perhaps with some additional metal, but for CF, it’s pretty much “back to the oven”. CF can be resin-filled, patched, or new layers glued to the old, but it loses its advantages rapidly after damage, even with repair. Therefore, CF will not be a primary hull material for more than in a few specialty military or exotic applications. Instead, like its sister, ceramics, it will be used as armor, for radar absorption, heat absorption (reentry), and other uses, and it will be used in plates mounted outside a metallic hull.
Note: Advancements in additive manufacturing, carbon-metal-composite structures could change these calculations a bit, but the arguments that follow will nonetheless prove it an unworthy material for the majority of ships.
And the Winner Is…
Steel.
Why am I introducing the winner in the middle of the article? Because the material has already won for the future of reusable spaceflight, and there aren’t any other arguments why it shouldn’t win for the future of spaceflight either. Especially considering the longevity that’s required of craft that are built for spending their lives 100% in outer space.
The rest of the article will discuss why steel couldn’t have lost in the first place.
Impact Resistance
Current spacecraft use aluminum for their skin, and cover the most sensitive areas with MLI, an insulating layer that protects against micrometeorites and solar radiation. This design is adequate until we’ve reached full Kessler syndrome, whereby lower LEO and GEO are unavailable for unarmored spacecraft and satellites due to the sheer volume of high-energy microparticles.
Space is vast, but habitation will always cluster. Around these clusters will be conflicts, or accidents. And, those accidents, or “accidental disassembly” of ships, will lead to a lot of space junk hurtling itself at 25,000 kph or more throughout any solar system. And, as it disperses from those clusters of habitation, it will become less dense yet ubiquitous, meaning that just about everywhere a spaceship could travel, it would be highly probable that in the journey that ship’s trajectory would intersect with many of these high-speed particles.
This debris will pose a hazard to all spacecraft and satellites, and there is no amount of proposed cleanup that will render this risk inert. The future of spacecraft will require an extremely resilient and repairable hull, and there’s no amount of imagination that will get around this eventuality. (Yes, stuffed Whipple shields could be used to minimize the risk, but those could also be installed on steel-hulls as easily as aluminum or even easier than on composites.)
Manufacturing
The primary concern with building spacecraft is the question of where to build it. For larger ships that are never expected to enter the gravity well of a planet (meaning the area where gravity dominates and orbit is neigh impossible), the answer has to be in orbit, for there’s no need to build the extra strength into the superstructure, and therefore no need to supply the materials for that extra strength.
If that’s the case, then larger inter-stellar and inter-solar ships will be built in special space docks (unlikely) or in the microgravity environment of captured asteroids (extremely likely).
The vertical integration of material-to-manufacturing will be the primary concern in ship factories, and therefore placing the site of manufacturing next to the mines will be of paramount importance.
On Earth, iron is one of the most common elements. Iron is also one of the most common elements found in some types of asteroids (S- and M-types). In order to create stainless steel — because a pure iron hull will do nobody any good — carbon, nickel, and chromium are also required. Those elements and more can also be found on those same asteroids. In fact, the iron found is usually a nickel-iron, and chromium is actually easier to find on some asteroids than it is on Earth.
Therefore, except for fuel or energy to heat the kilns, just about every material required to lay down the hull of a spaceship can be found on various asteroids. (Well, solar panels and electrolysis of water mined from the same asteroid, or another one captured and docked side-by-side to the shipbuilding asteroid, would also work.)
Does the “ease” (we’re talking in the future here) of vertical-integration-asteroid-manufacturing signify that steel is the best choice by itself? Not necessarily. Technically, titanium is easier to find on the moon than on Earth, but it’s still more difficult to work with. And, we have already ruled out aluminum for the abovementioned structural and longevity reasons.
Therefore, I would argue again that based on the feasibility of manufacturing on an asteroid that stainless steel will be the hull material of choice in any future spacefaring civilization.
But, we’ve still got that heat issue. In my next article, I’ll discuss more about heat management, and how stainless steel poses a particularly specific issue due to its high coefficient of thermal expansion.
Alternatives
I do still believe that all sorts of materials will be used for spacecraft in the near and far future. What I’m arguing here is that steel will be used predominantly for the majority of those ships, as most of those ships can be expected to be in LEO or GEO the majority of their lives, and the risk of micrometeorite damage will be exponentially higher in the future than it is today.
Just like aircraft are built with a rated number of airframe hours, so too will spaceships, although sitting on a tarmac doesn’t add to a spaceship’s airframe hours, yet sitting in LEO, exposed to space debris, will.
Rather, I think that the other materials used for hulls will be for non “working” type of spacecraft and a small subset of Earth-to-Orbit craft. Shuttles will just be made smaller for their power plants, in order to handle the extra weight of steel over aluminum or CF/titanium. But, these exotic spacecraft, with predominantly military usage will use some of these more exotic materials because it’s not a corporation that’s funding the build of those spaceships, but rather governments, and they can afford lower airframe hours for their fast-movers, terra-assault dropships, and stealth destroyers.
Conclusion
It is in my opinion that composites will be limited to use in hulls manufactured terrestrially and for ships that could be expected to launch and land repeatedly from a planet, and that for those ships, the hulls will be woven rather than just honeycombed or paneled in composites. This holds especially true for ships that will spend most of their lives earthbound and away from space debris.
For the rest, especially the loaders and trucks we can expect will make the journeys from the asteroids to the planets, or even between the stars, their hulls will be steel.
Therefore, I argue that steel, not aluminum, nor titanium, nor even CFRP, will be the dominant material used in hull and superstructures of spaceships in the theoretical future in which my characters exist.
If you disagree, do so in the comments. Not only are these articles entertaining to write (and read, hopefully), they also offer me a medium in which to fine tune my worldbuilding ideas.
As a final note, I’ll add a soon-to-be-obligatory Burt Rutan quote, after he was asked, “How is it that you’ve been able to come up with so many odd and innovative designs for so long?” (Freeman001, now suspended Reddit account)
“There is nothing else to do in Mojave, California. I lived in that small desert area for 46 years.” — Burt Rutan, Sept. 20, 2016.
Gotta love that man.
This is the second of my articles describing the technologies behind the worlds I build. There will be others. There will also be articles on other topics I am passionate about.
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The Arcworld: From ancient Rome to distant solar systems, follow the adventures of the inhabitants of Earth as they escape their humble beginnings and reach out to the stars.
Text Copyright 2022 GJ Herman
