Spaceships of the Expanse

I have been enjoying “The Expanse” series by James Corey. It’s a space opera set a couple hundred years from now, after humans have colonized and populated the moon, Mars, the asteroid belt, and outer planet moons. Spaceships journey between these worlds, complex engineering projects remake asteroids into habitable stations, and space navies boost from place to place to fight space pirates. I think it’s great because it captures what I wish for humanity’s future: that we will go out and colonize other worlds, that we will be able to undertake engineering projects for the greater good, and that we will become robust enough to weather grand challenges – things we see in the world today as global warming, income inequality, nuclear proliferation, and the like. In many ways, the first three books are about the tension between such grand visions and idealism, and politics and profiteering.

Leviathan Wakes cover, from Orbit

The books are soon going to be a TV series, and I am very much looking forward to see its depiction of space and space travel. (With the exception of parts of the first book, wherein Corey tried to write something horror-ish by being as gross as he could think to be. Whatever. Those are not the good parts of the book.) Corey steered clear of many sci-fi tropes that would have a big impact on the appearance of the series – no artificial gravity here! – and he made sure to build aspects of spacecraft engineering and operations into the cultures he depicted. For example, “Belters” nod and shrug with whole-arm gestures, so that they can be seen when wearing a suit. A good chunk of the books take place in zero gravity. Hopefully that will translate to the screen!

I’m going to take a look at some of the spacecraft engineering concepts in “The Expanse.” Let’s start with the most science-fictional, and therefore least plausible:

The Epstein Drive

Corey very quickly establishes that the powerhouse of his whole solar-system-wide civilization is the “Epstein Drive,” which is some kind of fusion engine for boosting spaceships around. It allows craft to thrust continuously from one planet or asteroid to another, accelerating constantly for half the trip and then decelerating constantly for the second half. This trajectory allows relatively quick travel times between worlds. Conveniently for crew health, and for TV production, the engine also provides “thrust gravity” inside the spaceship. Ships are therefore designed with decks in “stacks” above the engine with a ladder or lift giving crew access between decks, like in a skyscraper.

A fusion engine isn’t a crazy idea, especially not for a civilization a couple hundred years in the future. The problem is propellant. No matter how powerful or efficient your engine is, you will always need to be chucking propellant out the back to sustain this kind of thrust profile.

Picture this: you’re sitting in the middle of a frozen pond. The ice is perfectly frictionless, so you can’t walk or crawl or anything to get back to solid ground. What you do have is a bag full of baseballs. If you throw a baseball away from you, then you have given it some amount of momentum (mass times velocity). Your body gets an equal and opposite amount of momentum: you start sliding in the direction opposite your throw, but much more slowly than the baseball (because its mass is small while yours is big). Great! You have a way to get to shore. But you don’t want to wait out this long slide, so you throw another baseball. This speeds you up a little. Another throw speeds you up a little more. You can keep throwing baseballs until you decide that you’re going fast enough that you can wait it out. That’s basically how spacecraft work now: they thrust for a little, and then coast for a long period of time until they get to their next destination. But what if you wanted to keep thrusting the whole time? You will need more baseballs. Lots more baseballs. You are going to have to keep throwing them, constantly, to keep accelerating yourself.

Writing that a spaceship has a fusion drive instead of a chemical rocket is like replacing yourself in this analogy with a major league baseball pitcher. They will put more momentum into each pitch, and so they’ll go faster across the ice. In other words, their thrust is more efficient. But they will still run out of baseballs at some point, and then they must coast without thrust. The spaceship must stop its burn, cease thrust gravity, and wait several more months before getting to their destination. In the end, high thrust – and, with it, appreciable thrust gravity – should only be active for a short time in any space voyage through the Expanse. As we are learning with ion propulsion nowadays, it can often be most efficient to run at a low level of thrust, but sustain that for a very long time. But that doesn’t give our characters a convenient floor to stand on! So Corey put the word “Epstein” in front of “fusion drive.” “Epstein,” in this case, is short for “magic.” It’s a kind of magic that lets Corey have thrust without propellant, so that he can simultaneously achieve short (astronomically speaking) travel times and keep his crew in thrust gravity.

For a more physically realistic depiction of relationship between fuel, propellant, and thrust, consider Neal Stevenson’s spaceship Ymir in Seveneves.

The Way Ships Move

In the Expanse universe, spaceships are flipping around all the time to vector their engines in the correct direction to change their velocity. And we often read references to what the thrusters are doing on ships. This is all good. But the ships don’t really move the way real spacecraft move.

First of all, orbits barely enter the picture. One scene in Leviathan Wakes involves a character plotting out the likely trajectories of a certain ship, but other than that, the characters can go just about anywhere they want to go as long as they have a good ship to call theirs. Absent the Epstein “magic,” that behavior isn’t really plausible.

Second, though, is that Corey imagines his spaceships rotate themselves around in the same way just about all science fiction authors do: with thrusters. That’s not what most modern spacecraft do! They actually use wheels. Spin a wheel clockwise, and Newton’s third law kicks in: there’s an equal and opposite reaction. The spacecraft spins counterclockwise. Devices that function as I just described are, therefore, called reaction wheels. Other wheel-based devices that take advantage of gyroscopic torques can give satellites quite a lot of agility – without using any propellant. I suspect that the reason why these realistic actuators don’t often appear in science fiction is that there are no obvious cues to their operation: no thruster spurts, no blue glows shining out of emitters, nothing. The ship just starts to rotate.

I was happy to read that Corey’s spaceships are all native to space. There are not many cases where a ship lands, and in those cases, it’s always a small one. The heroes’ ship does once, on Ganymede. With surface gravity comparable to Earth’s moon, that’s not such a stretch for a fusion-drive starship.

28 June 15 Edit: Darn it, I just started Cibola Burn and about the fourth thing that happens is that the Rocinante lands on an Earth-size planet and immediately takes off again. Minus points for that!

The Battles

Space combat plays a big role in the plot of the Expanse books. And it’s a generally great depiction of space combat! Lots of the tactics and technologies are grounded in plausible physics. Ships shoot missiles and guns at each other, the effective range of a torpedo is determined by how close your ship needs to be to make sure the enemy ship doesn’t have time to shoot your torpedo down, the crew all gets into space suits at the beginning of the battle, there’s a ton of electronic warfare activity, and the battles wax and wane in intensity as the spaceships maneuver and orbit.

I’ve long thought that the most effective weapons in a space battle would be simple kinetic slugs or flak shells. My reasoning is simple: the speeds of objects in space are fast enough that a relatively small piece of junk can easily blast a hole through sensitive components. This is exactly why present-day spacecraft engineers – like me – worry about micrometeoroid strikes, space debris, and the Kessler syndrome. In the Expanse, the ships all fire torpedoes or guns at each other. And the results of weapon strikes are devastating: it only takes one torpedo or a few well-placed railgun slugs to take out a ship. Ships blast electronic garble at each other to screw up their targeting systems, but in the end the best defense is not getting hit – so we see the pilot do a lot of evasive maneuvering. I think this is all on the right track from a physics standpoint, though a real space battle with “Expanse-style” ships would probably take a lot longer, involve more orbit dynamics, and require a lot more computerized coordination.

There are two rather implausible elements to the battles. First is the Epstein Drive, which makes the combatants’ maneuvering matter a lot more than orbit dynamics. Second is the “juice,” a drug cocktail that keeps people alive and functioning when exposed to high gee forces. As a way to deal with high gees, the “juice” is just about as good a science-fictiony way to do it as any other, including immersing people in fluid as in The Forever War or inventing some kind of mythological inertial dampener. In the end, though, humans are squishy, precious cargo, and fighting full-on battles with them inside your spaceships doesn’t make a whole lot of sense.

Stealthy Spaceships

(There are some minor spoilers in this section!)

A plot point early on in the first book, Leviathan Wakes, revolves around the appearance of a stealth spaceship. This doesn’t involve any cloaking devices like in the Star Trek universe. Rather, a few spaceships avoid detection by (1) being painted black, which hides them in the visible spectrum, (2) having surfaces that absorb or scatter radar, which hides them in radio wavelengths, and (3) radiating heat out the side of the ship facing away from the enemy, which hides them in infrared. Much as it might give some people heartburn, this is all fairly plausible! The first two points are easy to imagine based on what we know about about the present-day Air Force. Though its not as familiar to the general public, the third item is actually something that comes up all the time in spacecraft design: especially if your satellite has sensitive electronics – like an infrared telescope – the design will include coolers, heaters, baffles, insulation, and radiators designed to emit heat in directions pointing both away from the precious detectors and away from the sun. Even the International Space Station has radiators that rotate to keep them pointing away from the sun most of the time. (The reason is that if the radiators face the sun, they’ll start to absorb heat into the station instead of emitting it out!) Such a “thermal management system” could be designed to, with the other stealth elements, give one side of a spacecraft the appearance of a cool, black spot indistinguishable from the rest of empty space.

A stealth spaceship wouldn’t be easy to build, and it wouldn’t be perfectly invisible – just harder to detect than normal to a lone adversary. And, in fact, both those points are relevant to the spaceships in the Expanse. One crewmember is able to spot a stealth ship on his sensors, but he doesn’t know what it is or how to respond to it. And that’s really all it takes for the stealth ship to accomplish its mission, after all! The very difficulty of constructing a stealth spacecraft actually makes the stealthiness more effective. The characters who cannot conceive that somebody could field a stealth spaceship end up more prone to falling prey to it.

Spaceship stealth makes an appearance other times, as well. At least twice in the series, the heroes’ ship hides itself by masquerading as something it’s not.

Spin Gravity

Lots of space stations in the Expanse, including some embedded in asteroids, spin to provide their inhabitants with centrifugal “gravity.” This is an idea that’s been around the aerospace and science fiction communities for decades, and Corey executes it well. In fact, one of the things I enjoy about the books is how the plot moves between the different environments of planetary gravity, low lunar gravity, spin gravity, and (“Epstein”-based though it may be) thrust gravity. The different gravitational environments contribute to different cultures, and they put the characters in interesting and different situations. If the TV series sticks to the books, we’re going to see low-gee gunfights and damage control teams solving problems in microgravity. Regularly.

All the stations with spin gravity are large, which is the right choice. It means they don’t have to spin at a dizzying rate to get a comfortable level of gs for their inhabitants. There is another benefit, in that the weird non-intuitive kinematics of rotations – Coriolis forces – have less of an effect the larger the rotating space station. These effects can be truly weird, and it can be hard even for physicists to bookkeep all the terms correctly to model them. Something that might happen if you were standing in a spinning space station is that if you drop something, you will actually see it follow a curving path to the floor, and it won’t land at your feet. You will also see it fall at a different speed than you would expect based on the gravity you experience. (I’m planning to write something up separately to go into all the details.)

Anyway, suffice it to say that spin gravity is a strange environment and Corey, like most science fiction writers, doesn’t go into all the details. But spin is the right idea for giving gravity to spacefarers, and I can’t wait to see how the visual effects team on the Expanse interprets all the spinning structures.

All in all, I’m thinking that The Expanse will be good for science fiction on TV. It will be a show with a time period a bit closer to us than, say, Star Trek. And the show will have a wide diversity of environments to challenge the characters. I am looking forward to seeing their depictions of spacecraft and how they move around in space!

Posted in Books, Movies, Science Fiction, Space | 6 Comments

Space programs are not ambitious enough

This week, NASA announced the selection of nine instruments for a proposed mission to Europa. Europa is probably the best place we know about to find alien life, and the discovery of alien life would surely be an achievement rivaling the moon landing in NASA – and human – history. I have an issue with the thinking presented by NASA in its press releases, though. Agency spokespeople say things indicating that the purpose of the Europa mission is to determine whether or not Europa “could be habitable.” The exact phrase on the web site linked to above is that this mission is part of “our search for oases that could support life” (emphasis mine). That’s not what I want from a mission to Europa. Probes to outer planets come decades apart, so I want to get as much done in a single shot as possible. What I want is to determine whether or not there is life on Europa.

The important difference between those two statements – determine whether Europa could support life and determine whether Europa has life – betrays a slight difference in ambition. I want the big-risk, big-reward activities and objectives of a true moonshot. NASA is hedging its statements, and lowering the bar of its mission goals.

I’m coming to believe that the statement about Europa Clipper’s objectives is symptomatic of a general lack of ambition in NASA’s modern thinking. You can see it in other statements the agency makes: Mars Science Lab Curiosity‘s mission was to determine whether Mars, at some point in its past, could once have been an environment that supported life. The oft-repeated purpose of the “proving ground” activities in the human spaceflight program’s “Journey to Mars” campaign is to “learn how to live and work in space.”

I don’t want to do those things. I want to find out if there is life on Europa; similarly I want to find out if there is (or was) life on Mars, and I want people to live and work in space.

Ironic that a space program – of all things – would lack ambition, isn’t it?

You might think that this is just the public relations spin. NASA is trying to manage expectations, so that they know they can achieve the first objectives of any mission and claim success immediately. Then they can parade that success in front of Congress, while the scientists go after their real scientific objectives in the “extended mission.” But I think the underlying philosophy here is penetrating beyond the publicity level into the actual mission design. It’s easy to find statements from scientists, engineers, and NASA spokespeople that Curiosity couldn’t actually find life on Mars unless that life walked in front of its camera and waved hello. To me, those statements beg the question: why not? We sent a nuclear-powered jetpack-landed laser-toting robot all the way to Mars, why wouldn’t we put some instruments on it that can identify basic things like amino acids? Similarly: NASA sends a probe to Jupiter approximately once per decade (and slowing). Since that rate keeps dropping as time passes, why wouldn’t we try to answer the big questions as soon as we can?

The way NASA now formulates its missions, I can just imagine a variation of Kennedy’s famous moon landing speech: “Our nation should dedicate itself to the goal, before this decade is out, of lifting a man five inches above the surface of the Earth. If that is achieved, this mission is a complete success. As a stretch goal, we might have that flight go to the Moon.”

The great thing about opening up the ambitions of our space program is that it would enable engineers to implement known solutions to the problems we face in space. For example: we know that humans have health problems after spending long periods of time in microgravity. Do we need to keep answering the question of whether or not humans have health problems after spending long periods of time in microgravity? Or can we instead think about the details of building spacecraft that spin to provide artificial gravity? Similarly, we know that there are extreme logistical challenges in sending people to Mars. Do we think about long a mission we could run given the amount of food we can send up with our astronauts, or can we think about the details of having them grow food on Mars?

The difference between those questions is the difference between “learning to live and work in space” and “living and working in space.”

It’s also the difference between the space program we have, and the space program we imagine.

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What is the nature of the STEM crisis?

There is a recent National Science Foundation report out that says, over the decade from 1993 to 2013, the number of college graduates in science and engineering fields grew faster than the number of graduates in any other fields. By 2013, we got up to 27% of college graduates getting their degrees in science or engineering. Hooray! STEM crisis solved, right?

I actually see something in this report that I find quite worrying, and a sad commentary on the state of science and engineering in the United States.

The report says that only 10% of all college graduates got jobs in science or engineering fields. That statistic means that, although 27% of our graduates are in STEM fields, at least 17% of graduates got their degree in science or engineering but couldn’t find a job in any scientific or engineering field. Put another way, at least 63% of STEM graduates couldn’t get a job in STEM fields!

The STEM crisis, in my opinion, isn’t about the number of graduates. It’s about the support our country and society gives to science and engineering. Our government has forsaken basic research in favor of maintenance-level defense tasks and austerity. Our companies have forsaken applied research in favor of “killer apps” and next-quarter profits. In light of those actions, it’s no wonder that we’re now worried that other nations might leapfrog us technologically.

If we want to get out of this hole we dug, we need to dramatically increase our support for science, engineering, and innovation.

Posted in Politics, Science, Social commentary | 1 Comment

How to Get to Mars

NASA wants to go to Mars. Great!

The approach, as the agency has been publicizing with fancy graphics like the one below, seems to consist of the following:

NASA’s tentacles travel to Mars

  • Send astronaut Scott Kelly to the International Space Station for one year, to learn about the effects of zero gravity.
  • Perform the Asteroid Redirect Mission, moving a near-Earth asteroid into lunar orbit, to prove that solar electric propulsion works in space.
  • Assemble a Mars transfer spacecraft in distant Earth orbit out of components launched on the Space Launch System.
  • Pack everything astronauts need for a round trip to Mars on the new spacecraft and send them on their way!
  • Keep the spacecraft in space for future trips to Mars. Bring the astronauts and supplies back and forth separately.

Not that these aren’t good brainstorming ideas, but they are not how I would get to Mars. I am too confident and impatient for this plan.

For one thing, we can probably skip this one-year mission. In fact, NASA, I can help you out by zipping straight to the conclusions: Being in zero gravity for a year results in bone loss, muscle atrophy, a compromised immune system, radiation exposure, and changes to the shape of the astronauts’ eyes. We know all that already. Similarly, we already know that solar electric propulsion works – quite effectively, robustly, and scalably – in space. Commercial satellites are flying solar electric propulsion right now, with more on the way. Heck, NASA itself has been flying solar electric propulsion, on missions like Dawn, since the turn of the millennium! Nothing needs proving here. We can take the known technology and use it.

Now, assembling a Mars transfer spacecraft, sending it onward, and reusing it for further exploration – that I like. Here is how I would do it.

First, get one of the companies developing solar-electric propulsion satellites to build a number of spacecraft buses. They will probably run a few tens of millions of dollars each, and they can ride up to space on Falcons, Arianes, or Atlases. (That’s bargain basement stuff for NASA!) Then, tie them together. All I really want are the propulsion systems. Each spacecraft has a propulsion system with something around 10 kW power, and NASA wants to get up to around 100 kW to go to Mars. So, by my rocket science calculations, we need…ten satellites. Or maybe, if we strip out all the telecommunications payloads that these satellites usually carry but I don’t care about for this application, maybe we can get the number down to five-ish.

Four of 'em stuck together

Four of ‘em stuck together (with obligatory blue ion engine exhaust)

Somebody would probably have to do some thinking about the best way to support all these stuck-together satellites. Maybe a truss of some kind. But I’m not too worried about that, because NASA has two decades of experience building modular things and sticking them to trusses in space. They can just do what they do best, using their own well-proven techniques.

Now we need a place to put our astronauts. Preferably a place that has some accommodations for solving the problems that Scott Kelly will be confirming. Many of the major physiological issues with space travel have to do with being in zero gravity. Too bad our Mars transit vehicle can’t bring gravity along with it.

Oh, wait! Science fiction knows the answer. It’s known the answer for decades! Spin the spacecraft. The astronauts get to live with a force akin to gravity, pulling them outward along the spin axis.

But building a giant ring-ship takes a lot of time, effort, energy, and resources. I have something different in mind. Something simpler:

My Mars transit vehicle is finished!

My Mars transit vehicle, finished!

On the right, that’s supposed to be an inflatable, cylindrical habitat. (Inflatable things would be terrific for space construction, because they only need a small launcher. Since everything on my vehicle is made of small components, we can launch them once a month instead of once every two years, if they needed a super-heavy launcher like SLS.) This inflatable habitat is tied to the central propulsion core by tethers, or maybe trusses of some type. The astronauts would feel “gravity” pulling them toward the right-hand side of this image (and a little bit downward, because of the thrust). On the left is a dumb counterweight: I’ve drawn it to evoke the empty upper stage of a rocket. It could maybe be long-term storage, but its main purpose is simply to be dead weight to make the spinning easier. The whole vehicle would rotate about the thrust axis, rapidly enough to give the crew at least lunar or martian gravity levels. (The illustration isn’t to scale!)

I’d do one last thing before I send this to Mars with a crew. I’d pack the transit vehicle with enough food, water, and air to get the astronauts to Mars, and for their surface stay.

Not enough to get back, though.

Instead, I would bring seeds. When the astronauts land on Mars, the first thing they will do is become high-tech space farmers. They are going to grow all the food for their return trip on Mars’ surface.

Why would I want to do that? Well, for one thing, seeds are smaller and less massive than full-grown food products. They are probably less expensive – in an energy sense – to get to Mars than those food products would be. Then, on Mars, we can get water and carbon dioxide from the atmosphere, to fuel plant growth. So, over the whole mission, I’m actually saving time and money. There’s also a second reason, one I find more compelling. What’s the point of this whole endeavor if we don’t come out of it knowing how to colonize and explore other planets, and keep colonizing and exploring them? Learning to use the resources on other worlds is fundamental to the future of space exploration. We know Mars has water, we know it has oxygen, and we even know that we might be able to grow crops in its soil. We should focus on that idea and advance it. In other words, I think that – both pragmatically and philosophically – it would be shortsighted and silly to attempt Mars exploration using only what supplies we can bring from the Earth.

We need a space program that focuses on developing the technology to use the resources on Mars to support further Mars exploration. We need to do this in a modular, reusable, scalable manner. We need to make sure our astronauts – no, our pioneers – have the tools, the materials, the infrastructure, and the autonomy to solve their own problems. In other words, we need to stop thinking about how to put a few guys in spacesuits on Mars, and stop thinking about how to have astronauts do science on Mars, and instead think about how to colonize Mars. That requires a lot of little things to come together, with more than a few big things in the mix as well. But, for the most part, we have the technology. We’ve had it for my entire lifetime. We need a space program with the right stuff to use it.

That’s how to take a journey to Mars.

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New map! “The Scientik Age”

Giant's Causeway on steroids!The culture of this world is having a bit of a renaissance: they are discovering frontiers of architecture, science, commerce, and engineering. The terrain of rolling hills gives way to basaltic columns – which the cartographer emphasizes, likely beyond plausibility, as an artifact of this renaissance. Those columns shape the flow of nearby waterways. Scattered around the region are representations of monuments and landmarks: the White Palace, the Water-Whele, the Observatorie, the Derrick, the Beacone.

I’ve had this idea lodged in my head for a while, to do a map along the lines of, oh, every hexagon-dominated sci-fi map ever. I wanted to caricature the Giant’s Causeway, and I had the goal of using the hexagonal columns to play with the flow of water. That turns out to be only a minor feature of this map, playing second fiddle to the color. I’m particularly happy with the color blending I got in the peninsula just below center-left. The labels are something I’ve done before, incoherent scribbles that try to give the impression of glyphs, but pushed further than I’ve done before to include bullet lists and parenthetical notes.

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Space to get excited about

Last month, NASA repeated an accomplishment they first checked off in 1964 – with some few improvements hardly worthy of the intervening half-century. But tomorrow morning, assuming the weather is a “go,” we will get to see a space travel event that has me far more excited. SpaceX is going to launch a rocket…and they’re going to turn the first stage around from high altitude and hypersonic speed to land on this:

SpaceX drone barge

in the Atlantic Ocean.

I love this.

I love this because of the technical meaning of the capability: being able to reuse a rocket would be seriously cool. It has the potential to alter forever the economics of spaceflight. And it’s not as crazy an idea as you might think at first – SpaceX has actually pretty much done it already, albeit without the barge underneath the landing rocket.

But I also love this because of what the event represents! Elon Musk estimates a 50/50 chance of success. SpaceX is trying this because nobody has tried it before. They are trying it because there’s no way to convince people it’s possible except by doing it. They are being incredibly ambitious, and they are willing to accept failure in order to learn from it.

In an industry increasingly defined by incrementalism and risk aversion, SpaceX recognizes that sometimes reward comes from risk. They are truly innovating; trying things that are new. It seems that while NASA’s human spaceflight programs once had the “right stuff,” they lost it in bureaucratization – but the “right stuff” didn’t vanish. It just moved – to the robotic explorers like Curiosity or New Horizons and to the “new space” companies like SpaceX.

Best of luck to the SpaceX team tomorrow. I know that even if you don’t succeed, you’ll be proud of your achievements and you will try again. But I’d bet you’ll see that first stage again!

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A brief review of “The Martian,” by Andy Weir

THIS IS THE SPACE PROGRAM I WANT. NONE OF THIS PANSY CONGRESSIONALLY DESIGNED INCREMENTALIST SLS/ORION CRAP. THIS ONE. RIGHT HERE.

Fantastic, Mr. Weir.

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