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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.

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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.

Throw a Probe into Orbit

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We send astronauts into space for many reasons. Science, for instance. Listen to what Steve Squyres, who heads up the science on the Mars Exploration Rovers thinks about human exploration. In 2009 he famously said, “What Spirit and Opportunity have done in 5 1/2 years on Mars, you and I could have done in a good week. Humans have a way to deal with surprises, to improvise, to change their plans on the spot. All you’ve got to do is look at the latest Hubble mission to see that.” I agree. I would go farther to say that we’ve barely scratched the surface of what humans can do in space. When people interact with the space environment, they find ways to adapt to and exploit the surprising physics of microgravity. Remember how the Apollo astronauts learned to hop on the surface of the moon, as a quick way to travel in that one-sixth gravity? Let’s consider an idea that may seem a little frivolous at first but, I think, makes this point even more clearly. How about an asteroid rover that an astronaut can launch simply by throwing it into orbit?

First, a few comments about asteroids. They come in many sizes, but in fact nature makes no clear distinction among minor planets, planetoids, dwarf planets, and many other names that keep students busily memorizing taxonomies in astronomy classes. To make this discussion simple, let’s agree that we’re considering celestial bodies small enough that their gravitational field would allow someone to throw a space probe into orbit around it.

How small?  Well, remember that the more mass, the greater the gravitational field. And the farther from the celestial body, the weaker the gravitational pull. So, the gravitational field of two spherical asteroids of the same mass would be the same at a given distance from the center, but the surface of one asteroid might be closer to its center if that asteroid is denser than the other. That’s because the same mass takes up less space, and the volume (therefore the radius) of the asteroid would be lower. The density of an asteroid composed of mostly rocky material could be less than 2000 kg/m3. A nickel-iron asteroid may be triple that density, and it would have less than 70% the radius of the more rarefied one and about double the gravitational pull at its surface.

A spherical asteroid of about average density (just over 3000 kg/m3) and 25 km in diameter would do it. At about that size, an astronaut standing on the surface would have to toss this probe at 25 mph for it to enter a circular orbit just above the surface. The period of this orbit is about 111 minutes. She should set an alarm on her watch so that she remembers to duck an hour and 51 minutes later when it comes around again. Throwing it a little harder—about 37 mph—allows it to escape the gravity of the asteroid entirely. I normally use metric units of measurement, but I think of baseball pitches in mph. I found out recently at the Ithaca Sciencenter that my fastball is now down below 70 mph. But even I could toss a spacecraft into orbit this way.

Let’s take a look at 433 Eros, which happens to be about the right size. Here’s a picture of it, taken in 2000 by NASA’s NEAR spacecraft. It’s also the asteroid referenced in the novel Ender’s Game.

earth-asteroid-433-eros

Image courtesy of NASA

Yes, it’s lumpy. It’s about 11 km in the narrow direction and about 34 in the other. That makes the orbit mechanics more subtle. The rover wouldn’t really execute a nice circular orbit. It would wander around, probably smacking into the surface at some point. It might also escape at even lower speed. But consider the opportunities: an astronaut could toss equipment, such as science sensors, prospecting hardware, communications-network nodes, transmitters, and many other useful components to virtually anywhere on the asteroid’s surface. An astronaut looking for valuable materials, such as water, might never even have to leave her landing site.

This idea suggests that human exploration of the cosmos will likely be conducted in a way that the Apollo astronauts would find very unfamiliar. These days we carefully script the activities of astronauts, safely planning their extravehicular activities and scientific investigations aboard the International Space Station. And that’s perfectly appropriate—for now. In the decades to come, we should expect our natural creativity, resourcefulness, and adventurousness to once again determine how we make the cosmos our own, like our ancestors did when they left Africa hundreds of thousands of years ago.

Ego Trip

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Arthur C. Clarke’s novel Songs of Distant Earth and Don Bluth’s film Titan A.E. both feature some sort of “seed ship.” The idea is that humans are too heavy and require too many resources—and don’t live long enough—to travel to other worlds, let alone colonize them. So, we send some samples, some DNA perhaps, instead and build the humans from those seeds when the ship lands. Now, I have to admit I haven’t quite figured out how to do that. Maybe by the next post. Instead, let’s see what can be done to personalize space travel in a similar way. What with all the enthusiasm for putting oneself into space, could we create a one-person seedship of some kind?  And is there a business case for such a product?

A sort of Viking burial has been done, in fact: not a seed ship that brings life but a final resting place after death. The ashes of Star Trek creator Gene Roddenberry, his wife (and voice of the Enterprise computer) Majel Barrett, and actor Jimmy Doohan (Scotty) were launched into space in 2009 by a company called Celestis. Many others’ remains have taken a similar trip. And Celestis has made a business out of that service.

The Sprite spacecraft-on-a-chip offers a means to do something analogous. Instead of carrying ashes, a Sprite might carry a person’s genome on board, in chemical form or in digital form. Both might be best. The data embedded in a single human’s genome can be stored as about 1.5 GB of digital information. The genome per se disregards the epigenetic information, which may well be essential to the identity of a person. But a genome is certainly a start. The Sprite’s current design has limited on-board data storage: 32KB, which lives in its CC430 brain. So, storage external to this chip would be required, ideally some sort of non-volatile memory. From there, the genome sequence could be transmitted continuously while the Sprite survives. After that point it no longer announces its presence; it’s a passive sample.

Storing a sample of the DNA itself on board could work well. The DNA molecule is naturally robust. At an ideal, cold temperature (quite easy to maintain beyond the orbit of Mars), it would be readable for over a million years. In a hotter environment, such as Earth orbit, the DNA would degrade with a half life of about 500 years, during which time the chemical bonds would break down and make the DNA unreadable. But with the digital version on board, the chemical instantiation of the molecules and the digital one could sort of back each other up. The transmissions serve as a third level of redundancy, from the receiver’s point of view anyway.

Why do this? I speculate that the sort of person willing to pay the roughly $300 required to personalize one of the original Kicksat Sprite spacecraft would pay more to immortalize his or her unique genome and transmit it into space. Maybe such a person would count on the benevolent, curious, alien civilization that finds this personalized Sprite to try to reconstruct that human from the data and from the DNA sample. Maybe it’s just an ego trip. But I believe that this idea has just as good business prospects as Celestis, and it offers a more optimistic perspective: your body can live on, and maybe even seed the solar system.

After all, life may have started on Earth thanks to an infection from an asteroid impact. DNA may be a very common thing throughout the universe. Here’s our chance to pay it forward.

Silicon Carbide Microdirigible

SiC_Sprite_Venus_Clouds

Venus has a terrifying atmosphere. The pressure is 93 times what we experience on Earth. It’s as hot as a pizza oven. And the air itself is wickedly corrosive, at least near the surface. Spacecraft that land on Venus don’t survive long; a couple of hours is the current record. Still, there’s a motivation to explore this toxic furnace.  Knowing more about the atmosphere can tell us what it takes for an Earth-size planet to support what we think of as life, within the so-called habitable zone of another solar system. With that in mind, consider a new way to conduct planetary science with a pocket-size robotic explorer: a robust, pocket-sized dirigible equipped with a satellite-on-a-chip.

The sort of balloon we’re all familiar with holds some gas, like Helium, at a pressure just above that of the atmosphere around it.  That positive pressure keeps the balloon inflated. It keeps the thin, rubbery material taut enough to maintain a volume of lightweight gas that displaces heavier air. So, the balloon is lighter than what surrounds it, and it rises. It reaches an equilibrium altitude when the weight of the gas inside the balloon, plus that of the balloon and its payload, matches the weight of the air it displaces.  Simple enough.

Never content with the simple and convenient answer, Spacecraftlab is interested in what else could be done to produce a balloon-like vehicle. Let’s start by noting that the gas inside the balloon does weigh something. Could we get rid of that gas entirely? Save that weight? The result would be a vacuum chamber, a sort of bell jar. Rather than inflating something, we’d need a structure stiff enough to withstand atmospheric pressure.  In other words, instead of a balloon that expands under internal pressure, let’s design a rigid sphere that does not contract despite the external pressure of the atmosphere.

Silicon Carbide (SiC) is one of the best materials for such a vehicle.  It has extraordinarily high compressive strength, around 3.5 billion Newtons per square meter, or gigaPascals (GPa).  That strength allows even thin SiC structures to withstand forces that seek to crush it. In Earth’s atmosphere, a 3.6 centimeter radius, 2 micron-thick sphere would do the trick, a ceramic bubble the size of a baseball. In fact, that’s twice as thick and another 15% more buoyant than necessary, in theory.  It also meets buckling requirements, at least at a glance. So, there’s margin in this design. Add 100 mg of electronics and an antenna, like what’s on board the Sprite femtosatellite, and you’ve got a little atmospheric explorer.

This basic idea becomes even more attractive if we take it to Venus. Not only would SiC resist the hostile chemistry of the environment, but the dense atmosphere makes an evacuated balloon even more buoyant. In fact, a 1.5 cm radius sphere, smaller than a ping-pong ball, would be big enough to lift the Sprite’s electronics. I’d prefer to keep the electronics on the inside, away from malevolent Venusian chemistry. The spherical SiC shell also has to be thicker to withstand that 93 Earth atmospheres worth of pressure, but only about 80 microns. That’s thick enough that a 3D printer could build these microdirigibles, turning them out in large numbers. And, again, that’s a design with 100% structural margin and 15% more lift than strictly necessary.

By the way, the sphere itself consists of only about 7 mg of ceramic material, and that’s a lot lighter than a typical latex balloon. So, although the SiC itself is denser than the various polyimides or other materials we might use, and as strange as this idea may be, the evacuated structure we’re considering is a real improvement. Yes, you’ve got to treat it as the delicate structure it is. But if you can handle it carefully, it may survive in an environment that has got the better of every one of our more traditional spacecraft designs to date.

This microdirigible is scalable, too. We could fabricate spheres of many different sizes, which would settle at different equilibrium altitudes. So, a swarm of these microdirigibles could explore a range of altitudes and, with the help of whatever winds prevail on Venus, spread out to explore a wide territory. With the right ballistic coefficient, they may even be able to withstand entry into Venus’s atmosphere from orbit and then communicate their discoveries to an orbiter. Maybe one of these microdirigbles will detect the life in the atmosphere of Venus.

There’s so much to be learned among the planets of our solar system, and we’ve only just scratched the surface with our decades-long exploration of Mars.  Maybe it’s time for science to move on, to broaden the scope of planetary questions we ask.  Our neighbor, Venus, may be where we find some astonishing answers.