Monthly Archives: May 2014

Massless Exploration

atmosphere_sucker

NASA’s various technology programs are investing in a wide range of additive-manufacturing technologies. Some focus on the terrestrial problem of how to manufacture aerospace parts. Others focus on the NASA-unique problem of how to fabricate hardware in space. It’s not just NASA-unique. It’s also revolutionary. If done right, these technologies may revolutionize space science and human exploration.

timthumbThe image above is from Deep Space Industries. It’s a concept in which a sort of 3D printer sucks mass out of an asteroid and prints it into a deep-space habitat. That would be revolutionary, no doubt about it.

There’s a larger point to be made here, one beyond designing a single spacecraft a week (which is this blog’s charter, remember). So, let me start with this bigger picture, something a little polemical, which I think will motivate the spacecraft I have in mind.

A little recent history, first. In 2012 the National Research Council identified the top technology-development priority for NASA: improving access to space. So, late last year Air Force Space Command, the Air Force Research Laboratory, and NASA partnered to sponsor a study at the NRC with the following goals:

      1. Lower the cost of space research and exploration in the long term through targeted, sustained investments that start immediately.
      2. Identify new space-system architectures that can be realized only if in-situ manufacturing is possible.

For this first goal, even with a new generation of lower-priced launch vehicles, e.g. from SpaceX, the economics of space will continue to deter most commercial and government organizations from using space for the nation’s scientific and economic benefit. Unless someone creates a killer app to kickstart commercial use of space, we will need a paradigm shift.

That new paradigm is what some of us call Massless Exploration: change the ratio of mass launched from earth to mass used in space. In the limit, the mass we use in space would all come from space. So, we would be exploring space without mass from Earth, i.e., masslessly—at least from the launch perspective. But you see that this perspective gets around the access-to-space barrier, if it could be made real.

As for the second goal, imagine if we could fabricate in space all the spacecraft components, structures and instruments for human exploration, and expendables (such as propellant, food, and oxygen) to support those efforts. In such a future, what would space-system architectures look like? What would we be fabricating if we had the means to do so? Spacecraft, habitats, even human-specific operations would likely look very different and be composed of unfamiliar materials. And for the U.S. to realize such architectures, what are the advanced-manufacturing technologies we must develop now? That’s the real question, in my opinion.

Massless Exploration provides a purpose, a direction for what are at best uncoordinated technology-development activities in additive manufacturing across various space agencies. At worst, some of them are duplicative or poorly motivated. We need a substantive roadmap that takes us from where we are now—3D printed plastic—to space-systems architectures conceived in a way that exploits what’s novel in an in-orbit manufacturing capability.

So, let me try to state it again succinctly. Massless Exploration is the working title for the paradigm that answers this unique question:

“What science and exploration architectures are made possible by in-situ fabrication and assembly of space systems, whether from new raw material brought from earth, unused components already in orbit, or in-situ material, and what advances in additive-manufacturing technologies must be achieved in order to lower the ratio of mass launched from Earth to mass used in space?”

As we learn to reuse and extract resources from the space environment, we may be able to increase this ratio to the point where access to space is no longer the driver for the size, weight, and power of spacecraft. At that point, we may be launching only people and the particularly hard-to-manufacture components, such as integrated circuits and exquisite components for scientific instruments. I joked about this idea in a Reddit AMA I did a few months ago, and the humor associated with manufacturing humans got taken seriously. It would be an interesting and familiar science-fiction story, though, printing up humans. Or maybe it’s too real to be funny. But I digress.

NASA’s current portfolio has a coordinated vision for an agency-wide path to the future. Additive manufacturing needs connectivity to other activities at the agency and elsewhere at a strategic level. If the NRC points the way, it is my hope that NASA and the Air Force will take on the much harder problem of developing science- and exploration-unique capabilities that will end NASA’s, and the nation’s, dependence on high-cost space launch. In doing so, I expect that transformational new technologies will spin off to the benefit of sectors of the economy beyond aerospace.

In this time of declining budgets for technology research, NASA has to focus and synergize its technology investment dollars on high-priority areas that produce fundamental and required capabilities that NASA cannot acquire through other means. NASA cannot justify investing resources in capabilities already being advanced by others. That need for focus motivates this NRC study.

What_Is_What_Could_Be_Massless

Image Courtesy of NASA

This image shows just a few of the ingredients of Massless Exploration that are already being brought together. Whether it’s painting lunar habitats into existence with a sort of toothpaste-like lunar-regolith cement or simply fabricating CubeSat components with the goal of doing so in orbit, there is a wide range of applications and solutions. I’m not alone in thinking about this problem.

I believe that the Maker Community has a big role to play here. We’ve reached a point in history where an individual can hope to build and launch his or her own spacecraft. The impulse to make space, or make space work for us, informs Massless Exploration as well, whether through business-development incentives as well as simply the desire for adventure. The role of government is to establish and nurture these grand visions for the sake of the nation’s citizens and businesses. If the right policies are in place, if we can consider space more of a national park (a “land of many uses”) than a sacred shrine, an object of mere detached scientific study, or the military high ground, we’ll see Massless Exploration all the sooner.

So, that’s the big picture. Now here’s an idea for a spacecraft I don’t think you’ve seen before.

Let’s agree that it’s hard to gather mass from the moon, say, even though it’s relatively nearby. You would need enough energy to change the velocity of a spacecraft by thousands of meters per second to go to the moon, secure regolith, and bring it back to low Earth orbit (LEO). It’s probably about as hard as gathering it from asteroids, although for different reasons. So, let’s pull it out of Earth’s atmosphere instead.  How about a spacecraft that slowly collects ions hovering in low Earth orbit until there’s enough mass to feed into a 3D printer?  Or a greenhouse?

Here’s roughly what a spacecraft rams into at 300 km altitude, just below the International Space Station’s orbit, if the sun activity is low.  If the sun is acting up, there’s about ten times more:

  • 1×1014 oxygen atoms (O) per cubic meter
  • 5.6×1012 hydrogen molecules (H2) per cubic meter
  • 3×1012 helium atoms (He) per cubic meter
  • 1.8 x1012 nitrogen molecules (N2) per cubic meter
  • 5.6×1010 oxygen molecules (O2) per cubic meter
  • A little argon, too, but I don’t really care about argon.  Do you?

The ratio is about right for making ammonia (NH3), e.g. by the Haber process. Then we would have fertilizer. And there’s more than enough oxygen to collect and use as an oxidizer, with the goal of nitrification. So, our spacecraft would provide water, which may be the key to everything. The image at the beginning of this post shows a satellite (adapted from IBEX, in this case, courtesy of NASA, just to show something modest in size). It extends a boom that collects ions sort of like dipping a candle in molten wax, or maybe making rock candy. Electrostatic charge attracts some ions. A chemical catalyst might help, too.

As this wick—this ion collector—orbits the Earth, a combination of ram effects and electrostatic attraction pulls in particles from the ionosphere. There’s a lot to this. Let’s not worry about the details of the plasma dynamics right now. For simplicity, let’s say that this collector sweeps out 10 square meters’ worth of area as it goes. A year’s collecting mass in this orbit yields mass that corresponds to between 80 and 800 grams of ammonia, depending on solar activity. That’s enough nitrogen for a large garden.

For this design to be worth the effort, it needs to produce enough fixed nitrogen to justify launching the satellite in the first place. If you can simply launch the nitrogen fertilizer to your orbiting colony, don’t bother with the satellite to collect it from Earth’s atmosphere, right?  If a 3U CubeSat (traditionally 4 kg) with a 10m long, electrically charged tether can accomplish this task, the spacecraft would justify its launch cost in about 9 years. If there’s a market for helium in space, and/or a way to use that extra oxygen, the break-even point arrives sooner.

And we’ll have to come up with a way to make this satellite stay in orbit, not drag down into and burn up in the very atmosphere it is trying to collect. But that’s a post for another day.

The larger point here is that we should be exercising our imaginations to come up with new ways to explore sustainably. All the mass we need to establish permanent human colonies throughout the solar system is already in orbit. It’s just in the wrong shape.

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Corona Light

corona_light_cafe

Spacecraft extend human consciousness into space. We see galaxies through their optics, touch other planets with their robotic arms, and dance virtually through the solar system as they propel themselves ever farther from Earth. Communications technology connects us to these spacecraft. Images of Earth, Mars, what have you, reach us by radio, usually encrypted. Without this flow of data between Earth and our distant robotic avatars, the spacecraft might as well not be there. At least, that’s the principle that governs the design of most contemporary spacecraft.

There is another approach. In the early 1960s, three decades before President Clinton declassified the existence of the National Reconnaissance Office, the United States operated the Corona series of earth-imaging satellites. The spacecraft would fly overhead, take pictures on Eastman Kodak film, and release a canister of this film–called a “bucket”–to reenter the Earth’s atmosphere. As it fell, an aircraft captured it. The film was developed and images printed in basically the same way that it had been done since the 19th century. Not only was this Corona architecture an elegant adaptation of the photographic technology of the time, it was also quite secure. No one could intercept Corona’s images simply by listening in.

Let’s consider a spacecraft that communicates its discoveries Corona style. And not just any spacecraft. Let’s look at a spacecraft that does science at an exoplanet. Cornell astronomy professor Jamie Lloyd and I have been thinking about this idea lately.

Our version of the Corona architecture begins with a spacecraft-on-a-chip: like a Sprite but a single application-specific integrated circuit (ASIC). Really. A single silicon or gallium arsenide chip that weighs only milligrams. Most of a Sprite’s electronic guts reside in a single IC already: the wonderful CC430 chip by Texas Instruments. So, it’s not asking too much for the rest of the circuitry to be collapsed into a single semiconductor device. If an ASIC proves to be too hard, maybe we would use a Field Programmable Gate Array (FPGA), which is a sort of all-purpose reconfigurable IC that can be naturally radiation tolerant. Tiny solar cells, nanowire antennas, and a CMOS camera round out the key subsystems.

Traditional radio communication from such a tiny device is hopeless on the scale of solar system distances, let alone interstellar distances. There just isn’t enough power available through solar cells or within an on-board energy-storage technology to transmit a signal powerful enough to be received back on Earth. For example, the current Sprite can manage roughly 1 bit per second from low Earth orbit for very low power, thanks to a subtle trick known as matched filtering. Even if continuous communications at this rate were possible from much farther away, downlinking every bit of this SD card’s data would take 31 millennia. Hence the Corona architecture. Let the golden light radiating from its angelic countenance illuminate our imaginations.We’ll save the data on a memory device and retrieve it physically.

Thanks to 21st century technology, our Corona Light can carry a few milligrams of silicon to house terabytes of information. Back in 2010 SanDisk offered a 2 terabyte SD card, a version of which serves as Corona Light’s bucket. We’ll count on this SD card, or something like it, to retain the spacecraft’s discoveries.

We would speed Corona Light on its way out of the solar system. How? Well, that’s another post. No more than one miracle per week, I think. But to fix ideas, let’s say we’ll accelerate it out of an orbiting railgun. The U.S. Navy apparently can launch 3.2 kg to 2.6 km/sec from its experimental railgun. They continue to make progress. So, let’s say they’ll exceed 700 megaJoules some day. Although the dynamics of launching something tiny differ, that energy would propel a 30mg spacecraft-on-a-chip to 7,000 km/sec, i.e. 2.3% of the speed of light. The spacecraft will need that sort of speed if it is to make it to Alpha Centauri, about 4.3 light years away, on a human time scale. The simplest version of the celestial mechanics involved says that the spacecraft reaches the exoplanet NASA has found there in 184 years. Maybe we can do better, but (again) that’s for a future post.

The trick is to get this spacecraft-on-a-chip back to Earth, or at least within the solar system, where we can pop the cap on this tiny satellite and drink in its knowledge. It will need a beacon, and I’ll propose that we simply replicate what we know we can do on Sprite today: a specific pseudorandom noise sequence that we would be looking for (accounting for all that Doppler shift, of course) when the spacecraft completes its ring around our spiral arm of the Milky Way and returns home.

Sometime in year 184, Corona Light performs a fly-by of Alpha Centauri. As it approaches this star, the power available to the spacecraft through its tiny solar cells increases, enabling some very minimal science: capturing a few photons from the environment and recording a time stamp in the form of counts of radioactive decay of a small on-board sample of an isotope. It would be carbon-dating itself, in some sense. A realtime on-board clock would be asking a lot—power that requires mass, slowing it down. But if possible, that would be great; these days an atomic clock on the scale of a microchip is mainstream technology. But the isotope-decay trick trades precision for even lower mass, which I think we want.

This fly-by puts the spacecraft on a trajectory that uses the pull from the galactic center to redirect the spacecraft back into our solar system. Make no mistake: this maneuver is not easy. Missing the Earth is likely. Fortunately, Corona Light is cheap, a few pesos each. We’ll fabricate thousands, maybe millions of them (they would fit in a suitcase) and fire them toward Alpha Centauri as a halo of tiny exploration vehicles. If one makes it back, we will have visited the nearest star. The SD card with a couple of centuries of data will represent our most distant physical interaction with the cosmos. Let’s toast to the original Corona mission and raise our bottles of Mexican beer to the possibility of interstellar discovery.

Solid Motor Idea

cubesat_MAV

We’ve spent decades developing technologies for space exploration. Some great innovations have come from government programs–yes, the famous spinoffs that lead to new jobs, new products, and a better life for us all. How would we take those indispensable selfies without the JPL-developed CMOS cameras in our smart phones?

But what about the original inventions? Those all-but-forgotten heat shields, optics, avionics, or solid rocket motors that someone spent a lot of effort designing? Despite that spacecraft program managers seek “heritage” components for the systems they’re designing, i.e. historical precedent and provenance for the technologies they use, every new spacecraft project still seems to be designed to demand many custom solutions. It’s as if no single entity is really in control of both the mission objectives and the engineering that delivers them. Too often the former drives the latter, no matter what time and money could be saved by adopting a holistic view in which what’s available informs the project’s goals.

A quick example. Honeywell builds reaction wheels, spinning discs that torque a spacecraft, making it able to rotate in space without propellant and point its instruments precisely. These devices are robust and well-engineered. As electromechanical systems, they are fairly straightforward. Here’s the thing. These wheels can come with any of eight different electrical and data interfaces, a complex situation that is not straightforward at all. Honeywell has done a great job of accommodating its many customers but, instead, if all spacecraft were designed to accept a single standard interface, this expensive proliferation of nearly identical hardware would vanish like the extinct branches of an evolutionary tree.  In the extreme, imagine a USB-style plug-and-play satellite. Government and commercial customers would save time and money.  Honeywell would have a more straightforward product line to manage. Achieving these goals requires leadership from the government or an industry consortium, like the JPEG working group. We’re not there yet.

Spacecraft often incorporate exquisitely engineered, uniquely sized bits of technology to solve problems. Let’s turn that tendency on its head. Let’s ask what we could do by repurposing what has come before. Instead of seeking a redesign of familiar components each time, let’s build on what we’ve already got. Doing so demands that we find common ground between what we really want to achieve (the space science) and how we achieve it (the space technology). As an example, how about we strap a small satellite to a famous old rocket motor and see how far it will go?  Specifically, let’s try to launch nanosatellites from the surface of Mars into orbit with hardware that has been around for decades: solid rocket motors.

Spend a few minutes browsing ATK’s fantastic catalog of rocket motors, and you’ll have plenty of Spacecraft a Week ideas of your own. It’s nothing less than a history of space exploration from the perspective of the chemical propulsion technology that made that history possible. You’ll find a rocket motor there for any application, mostly because of–or maybe despite–our nation’s abortive and often uninformed attempts to establish spaceflight standards, and its tendency to associate grand visions with specific people and abandon them when those people move on.

Take the Star 4G, for example.  What a great idea: a nanosatellite-scale solid motor that takes advantages of the benefits of small size to provide a compact, mass-efficient propulsion system for a CubeSat. Well, almost. It’s just a hair too big to fit in a CubeSat. That’s a real missed opportunity. From the look of it, ATK executed this project brilliantly, even taking this design from concept to test in a matter of months. It’s a pity that the requirements did not originate from a perspective of what it takes to infuse technology into how we explore space.

One of the attractive features of these STAR motors is their exceptional propellant mass fraction. That’s the fraction of the engine’s mass devoted to propellant. (Remember that a solid rocket motor comes with a slug of solid propellant; you don’t fill or, generally, refill it.) If the rocket motor has no tanks, combustion chamber or nozzle, only propellant, the mass fraction would be 100%. So, it’s remarkable that some of ATK’s motors approach 95%. The STAR 13 motor has a mass fraction of 87%. If one were to spin-stabilize and light up this motor with nothing attached to it, the resulting impulse would impart over 5,700 m/s of velocity (we call that increase delta-V). You could send this motor by itself from low Earth orbit all the way to the moon. What for? I don’t know, but I’ll bet a lot of ATK engineers have wanted to give that a try.

The STAR 13 motor weighs about 35 kg, the mass of an elementary school kid. You could pick it up. So could an astronaut. Imagine a 9 kg spacecraft attached to this roughly 42 cm diameter motor. That’s nine 1U CubeSats or roughly a single 6U CubeSat. Make it five 1U CubeSats along with some hardware to eject them from the motor, which would be necessary to avoid the heat from so-called thermal soakback after the propellant has burned away. That combination could launch from the International Space Station and acquire 3200 m/s delta-V. It would escape Earth’s gravity. It could head out on an interplanetary trajectory. And these motors are not expensive. Some cost less than $1M. Think of the opportunities to explore the Cosmos that are right there in front of us. And we’ve had these parts for years.

I expect that the ISS Program Office would hesitate to put a fully fueled solid rocket motor on board the ISS for simple reasons of safety. That risk posture probably makes sense, although I would point out that rocket motors were launched as part of satellites deployed by the Space Shuttle.

Finally, how about that unmanned Mars application? I would propose a launch pad on Mars that spins up the stack of 3 1U CubeSats and a Star 13 motor to about 120 RPM. These motors have seen that sort of speed before, according to the catalog. The spin stabilizes the attitude—like a Frisbee or a football. Distributing these CubeSats around the perimeter adds inertia to the spin axis, which aids stability. The spin also prevents small center-of-mass misalignments from causing the spacecraft to flip over as the motor thrusts. So, fire the motor. The delta-V is about 4,100 m/s, enough to overcome the drag of the thin Martian atmosphere, overcome gravity, and insert these CubeSats into a low Mars orbit. Maybe they can carry samples of Mars soil back to Earth.

I have immense respect for the spacecraft engineering of the ‘60s and ‘70s.  It was innovative, robust, and usually elegant in its simplicity. And even the fonts in their drawings were cool. The good news is that these achievements are still with us. They haven’t gone anywhere. They are ready to fulfill the promise of the Apollo era.  Let’s bring them back to help us finish what we started.

Sometimes Even a Low Ballistic Coefficient Needs a Little Help

Cubesat_Water_Reentry

This week has many of us thinking about Kicksat and its imminent reentry into Earth’s atmosphere. Here’s an idea for reentering a small spacecraft more gently, maybe Kicksat’s size or maybe the size of one of its 104 Sprites: fill a heat shield with water, and as the heat of reentry converts it to steam, expel it out of tiny rocket nozzles to slow the descent and simultaneously remove that heat.

The key to successful entry into a planetary atmosphere is somehow intentionally dissipating all the kinetic energy of the vehicle. That dissipation will happen one way or another, whether you want it or not. If you don’t plan ahead, the vehicle enters the atmosphere and aerothermal heating raises its temperature until, most likely, it burns up. There are several ways to avoid this fiery end, as we currently understand thermal protection systems. You can cover your spacecraft with ceramic tiles or ablative material. The former heats up but insulates the spacecraft. The latter burns away, taking energy with it. Another approach is to enter slowly, using rocket engines, Buck Rogers style. The lower the speed, the less the vehicle heats up. The technology involved in this slow reentry, known as supersonic retropropulsion, may be a great solution, but it’s not very mature. Maybe helicopter-style autorotation would work. That hasn’t panned out yet, either.

But there’s another option: go small. Very tiny meteoroids, as well as larger manmade hollow objects (such as satellite propellant tanks) survive reentry because of their low ballistic coefficient. That’s a number that tells us the importance of aerodynamic drag in comparison to the reentering body’s own momentum.  A low number means that the body is light for its area, like a leaf on the wind.  A heavy number means it’s denser, like a bullet. Higher drag slows down the reentering body before its speed reaches the point where aerothermal heating destroys it.

Size matters here. Small objects of a given density offer greater surface area for the mass. That’s why the tiny water droplets in mist stay airborne, while raindrops head straight for the ground. Aerodynamics governs the flight of those droplets, but gravity governs raindrops, all because of the ratio of surface area to mass. If you prefer, here’s a mathematical expression of this principle. A sphere’s volume V (therefore its mass, M, for some constant density) increases with the radius R to the third power. Its surface area A increases with R2. The ratio of mass to area therefore increases with R3/R2, which simplifies to R. This ratio determines the ballistic coefficient. So, the smaller R is, the lower the ballistic coefficient, all other things being equal. This principle motivates NASA’s current work on inflatable heat shields.

left_hand_of_lightness Justin Atchison is now a Senior Aerospace Engineer at the Johns Hopkins Applied Physics Lab. Back in 2006, when he was a grad student at Cornell he took a look at this question of how flight dynamics scale, laying the groundwork for what would become the Sprite project. Here’s the output of one of his simulations, which shows how the peak temperature that a simple vehicle encounters during reentry depends on its shape–the thickness and area of a plate, in the case of a Sprite. Any errors in the output are my fault, having revived his simulation years after he last had it working.

slide length vs thickness vs temperature

The three-dimensional graph above shows that a sheet-like object, whose surface area is high for its mass, reenters at low temperatures. Consider a spacecraft comprised of electronics printed on a thin film. Such a sail-like object would reenter without even breaking a sweat, so to speak. Even commercial-grade electronics could continue to operate.

But sometimes even a spacecraft with a low ballistic coefficient needs a little help. So how about this. Go ahead and let the thermal-protection system heat up, reaching 200 °C, the temperature of commercially available high-temperature electronics. That would happen for a body of any reasonable dimensions. Water initially stored in the heat shield at about 1 atmosphere of pressure boils, and reaches higher pressure. Heating up from 273 kelvins (that’s 0 °C) to 473 kelvins (that’s 200 °C) increases the pressure about 68%, according to the Gay-Lussac law. The steam bursts a diaphragm at this pressure, and it vents through rocket nozzles distributed around the perimeter of the heat shield, near the top. This thrust and the deceleration that the vehicle has already been experiencing keeps the water low, near the hottest surface, separating it from the steam that flows out through these rocket nozzles.

Let’s look at the specifics for a CubeSat-size system. We’ll devote 500 grams to water and about 500 grams to the spacecraft and its structure, including this heat-shield-as-pressure-vessel. Let’s say that each nozzle has a 1 mm throat diameter and a 1 cm exit diameter. From basic rocket equations, the steam’s exit velocity is 178 m/s, with a specific impulse of about 18 seconds. That’s not fantastic, but it’s comparable to low-end cold-gas propulsion systems. The total impulse out of this system is 94 kg-m/s. But at least as important is the 438,000 J of energy removed with the steam, as well as the 1,183,000 J of energy required, separately, to vaporize that much water, and which therefore never reaches the spacecraft itself.

So, this system offers four ways to keep the spacecraft cool as it reenters: (1) water as an insulator; (2) water-to-steam as a way to absorb energy via vaporization; (3) steam as a storage medium for energy; and (4) a little supersonic retropropulsion to slow down the reentry during what would be the worst time for aerothermal heating.

There remain some trade-offs. Better propulsion is possible if we let the heat shield become even hotter before the diaphragm bursts and the steam is expelled. But we don’t want a hot vehicle. A more exotic propellant might do better, but we might prefer the safety and simplicity of water for reasons that outweigh propulsion efficiency per se.

The lesson learned here is that some innovation may be possible by rethinking what we reenter and how. Maybe commercial uses of space, such as sending manufactured goods back to Earth as downmass from the International Space Station, would be more cost-effective if very small capsules were returned, rather than large ones: protein crystals grown as advanced pharmaceuticals and dropped back to Earth with a tiny transponder so they can be gathered up and sold. There would be no need for a team of technicians to recover and unpack a large capsule full of supplies and equipment from ISS, with the cost and schedule impact of that activity on commerce.

Maybe this is a way to return scientific samples from asteroids or other planetary bodies. Do-it-yourself atmospheric entry may be the way that citizen scientists learn about the origins of life on earth and the prospects of life elsewhere.