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.

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