Monthly Archives: April 2014

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.

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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.

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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

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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.

Lunar Xistera II

Our Lunar Xistera provides a means for a spacecraft to land on the moon with no propellant—just brakes. Can we take off the same way?

Not quite. Braking is comparatively easy. Dissipating that energy is a thermal problem, at worst. Taking off again is another matter. It requires energy to be stored and then imparted to the spacecraft. Both of those requirements lead to a heavier spacecraft or some more subtle infrastructure on the moon. So, can we avoid expending all this energy? Can we store up momentum in the spacecraft as we land and pay it back as we lift off without expending energy? It may just be possible.

First let’s take a look at a straightforward design that includes batteries and electric motors. Wire up the motor to the battery and accelerate like an electric car. A lunar Prius.

Now, remember that to use that same lunar runway, the spacecraft needs to accelerate at 5g. It would begin its run-up to liftoff at the more tightly curved end of the runway, which is where the vehicle stopped after landing. (That makes a certain intuitive sense.) This 20,000 kg spacecraft has to reach 1679 m/s, the velocity for a circular lunar orbit that is just barely above the surface of the moon. For a constant 5g, the power is at its maximum at the end of the runway, where the speed is enough for the spacecraft to be in orbit. At that instant, the power is over 1.6 GW, more kick than Doc Brown needs for his flux capacitor. And the spacecraft has acquired over 28 GJ of kinetic energy during its short trip. Even if you wanted to use that many batteries and an appropriate electric motor, that hardware alone would weigh more than the 20,000 kg vehicle. Sorry, but this design is just too far out of reach of current technology. Even for Spacecraftlab.

Before giving up entirely on motors, let’s consider a more subtle approach based on gyroscopic effects. It turns out that you can impart angular momentum (i.e. apply torque) to a mechanical system in many ways. Some use less energy than others. This fact is quite profound, drawing a really useful distinction between energy and momentum, and it’s at the core of how power-starved spacecraft rotate quickly in orbit.  A space technology that is particularly good at minimizing that energy is the control moment gyroscope (CMG). A CMG consists of a spinning rotor that tilts on a gimbal. That tilting motion imparts torque with hardly any change in kinetic energy—in other words, without much power. For the power of a few light bulbs, a CMG can produce enough torque to tip over a car. Seems promising, right?

For this design to work, that torque has to be parallel to the axle that spins the wheels. However, as a single CMG gimbals, the direction of its torque rotates too. That effect could cause the spacecraft to flop around on the runway like a fish on land as the CMG rotates, which is not at all what we want.

A pair of CMGs that tilt in opposite directions can cancel the off-axis component and leave only the torque we want. “Scissored pair” is the name for such an arrangement. This video shows a scissored pair.

The red lines are the angular momentum vectors of each individual CMG and their sum. Notice that the sum of those vectors is always along a single line (vertical in the video, horizontal in the case of our spacecraft). That’s also the direction of the torque. Unlike a single CMG, whose torque tilts with the gimbal in an inconvenient way, a scissored pair applies torque along this single, constant axis, such as the axle of a vehicle with wheels. If we ditch the motors that drive the gimbals, replacing them with a torsional spring that pushes the pair of rotors apart, there would be no electrical power required during takeoff other than to keep the CMG rotors spinning. And that’s certainly a desirable feature, since power is in such short supply on the moon, and since batteries to store it are heavier than we would like. Incidentally, that spring is beefy. I’d say about 320 kg of carbon steel.

Here’s some engineering that describes what a CMG-powered spacecraft would have to look like if it is to lift off the lunar surface on the runway we’re thinking of.

Each of the CMGs lies on an axle that freely rotates. The rotors start out with angular-momentum vectors parallel to each other and parallel to the wheels’ axis of rotation. Each gimbal then tilts about 180 degrees as the spacecraft travels along the runway. As they do so, they impart angular momentum along that axle to drive the wheels, which we assume are about 1 m in radius, as discussed in the earlier Lunar Xistera post. When the scissored pair has gimbaled to its 180 deg limit, the angular momentum in the pair of CMGs has been imparted to the spacecraft. Here, the spacecraft can be considered to have angular momentum referenced to a point like the end of the runway. The point doesn’t really matter, but picking one makes the calculations simple. The angular momentum consists of the spacecraft’s mass traveling at its velocity, at a radial distance from the ground equal to the wheel radius. That’s a mouthful. Have a look at this picture to see what I mean about the angular momentum (H) of the vehicle.

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To accomplish this extraordinary goal of an unpowered liftoff from the moon, we’ll need two 1.45 m radius, rim-weighted rotors that weigh over 4500 kg each (that’s nearly 50 times the size of the Space Station’s CMG rotors), much larger than any CMG I’ve ever encountered. And because they’re larger than the wheels, we’ll need a transmission that offsets a central part of the axle from the wheels, allowing the CMG assembly to rotate without hitting the ground. The CMG assembly drives the wheels with some sort of belt or gear arrangement. Each rotor needs to spin at 12,000 RPM, which is very sporty, and probably beyond the capability of the best steel: the tensile stress in this material will be high, and the rotors are likely made of some exotic material. Let’s also say that the rotor constitutes 66% of the mass of each CMG, which is also quite optimistic, given that the Space Station’s CMGs are only 37% rotor, 63% other hardware.

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Front view of the CMG drivetrain (sectioned through the middle)

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Side View of the CMG Drivetrain (Wheels and Axles Shown As Semitransparent)

Spacecraft with Side View of Drivetrain Shown

The top figure (above) is a sketch of this drivetrain, viewed from the front or rear. It consists of a scissored pair of CMGs, interconnected with a worm gear and two spur gears. Some other mechanical solution may be better; this is simply what occurred to me. The next figure is a side view, and the last shows that side view where it belongs in the spacecraft. You’ll notice that the CMG assembly is offset from the wheels’ center of rotation. You’ll also see a torsional-spring arrangement that connects the worm gear to a support structure of the CMG assembly. A pulley & belt connects the CMG assembly to the wheel axle. Maybe gears would be better there, considering the mechanical power involved. Whatever. It’s the big picture that matters.

The animation below includes some gray material that indicates how the spacecraft chassis would mount to these rotating assemblies.

With these admittedly sketchy assumptions, about 6300 kg of mass remains for the rest of the spacecraft: payload, wheels, structure, guidance components, and so on. Yes, you’ll need to spin up the rotors (it could take hours, even days, from a standstill). You’ll need to plug in the spacecraft at some point, or let its own solar panels provide that power. Also, I should point out that this design has virtually no margin to account for friction and other losses from rolling on the runway and from gears meshing within the drivetrain, which could well double the requirements.

HOWEVER, if you could build such a contraption, you would be able to lift off of the moon without significant electrical power (remember the spring), without propellant, and probably with fairly small batteries. That’s incredible!  Seems like this concept deserves a closer look.

Consider the upside: a spacecraft could land on the moon and use a kind of mechanical wind-up toy technique to tension that spring. As the vehicle slows, the spring winds up, and it tilts these huge CMGs as they suck up the momentum of the vehicle on the runway. As long as the rotors continue to spin, the spacecraft is ready to take off at a moment’s notice (well, maybe with a little rocket-motor or battery-driven help to account for friction that assists landing but retards take-off).

Stepping back, I’d guess that this concept is on the ragged edge of possible, certainly not easy. But that’s what this blog is about, after all. Innovation demands that we reject conventional wisdom, such as the belief that we need rockets to land on the moon and take off again. It happens when ideas collide in an unexpected way. It happens when we know the rules just well enough to break them but not so well that they become dogma. And in my opinion, we innovate when we sense a worthwhile goal just out of reach, a goal like economically viable lunar commerce. That sense of possibility inspires us to investigate the unfamiliar.

No fuel, little power, and indefinitely reusable. Now that’s sustainable lunar transportation.