ChipSat Atmospheric Entry

ChipSats are gram-scale spacecraft. A single PCB (see “Chip Satellites” page for more information) includes all the essential components of a functioning satellite. While previous missions have successfully demonstrated ChipSats, research is currently underway to increase their capabilities. Of particular interest is atmospheric entry. If ChipSats are able to survive and safely land back on earth, they can physically transport large amounts of data and bypass the bottleneck of traditional radio communication. Successful landing and an understanding of their distribution could also facilitate sensor networks on the surface of planetary bodies.

Atmospheric entry is all about energy dissipation. For spacecraft in low Earth orbit, this entails decelerating an object from hypersonic velocities (~Mach 22) down to its terminal velocity. For most spacecraft designed to survive this journey, various approaches are taken to either (1) absorb heat while insulating the spacecraft, (2) dissipate energy while burning away excess material, or (3) slow down prior to the denser part of the atmosphere where maximum heating occurs. Alternatively, we can go small.

Due to their low ballistic coefficient, we hypothesize that ChipSats will not generate as much heat while slowing down in the atmosphere and, therefore, maintain survivable temperatures. Behavior of very tiny meteoroids supports this hypothesis, as well as behavior of larger hollow objects (such as satellite propellant tanks) that survive reentry. To further limit aerothermal heating to the operating range of the electronics, past researchers have proposed concepts such as a water-based heat shield (pictured left), as well as more traditional approaches (e.g., ceramic coating and ablative layers).

Modelling Hypersonic Deceleration

While the intuition is there, ChipSat atmospheric entry has yet to be modelled and tested. Hypersonic testing (both flight and ground-based) is costly and hard to access. Mathematical models for aerothermal heating along entry trajectories are useful for obtaining ballpark figures but are not specific to the ChipSat’s geometry. In conjunction with these simplified models, current work develops a Computational Fluid Dynamics (CFD) simulation.  The simulation will use initial conditions that reflect that maximum point of aerothermal heating from the trajectory model. This work will analyze two cases: (1) a fixed-orientation, similar to traditional re-entry vehicles, and (2) tumbling, a more realistic model given the ChipSat’s current design. A comparison of these results will assess whether tumbling improves chances for survival.

Landing Distribution

Once the ChipSats reach terminal velocity, a significant portion of their descent remains. The way they fall matters. Falling flat plates (paper, business cards, etc.) is a specialized research interest in Fluid Mechanics. Depending on the geometry (size, density, and thickness-width ratio), the plate can drop, tumble, flutter, or perform combinations of these behaviors. In 3D, a spinning ChipSat could demonstrate frisbee-like behavior. Such intricate motions have a significant effect on the landing spread. For example, a gliding or tumbling ChipSat would travel much farther than one that is fluttering back and forth. However, ChipSats are not perfect flat plates. The circuitry has various protrusions on the surface and affects the mass distribution. These features induce additional torques on the ChipSat that would deviate from the predicted motion, or even cause spiraling/gyration, like a maple seed falling from a tree.

If the effect of these perturbations could be understood, they could be used to both predict and design future landing spreads. To model these, we adapted a 6-degree-of-freedom CFD simulation to the ChipSat problem. We ran a parametric analysis to demonstrate how small torques (represented by perturbations in center of mass) affect the ChipSat motion. Current work develops  force models to fit the data collected.

Designed to Survive

All the insights gained from ChipSat-entry modelling will be used to design the next-generation of ChipSats: ones that can survive. The geometry, materials, and circuity will be designed to maintain operating temperatures and achieve a desired spread. The 6-degree-of-freedom trajectory models developed will predict the landing zone for future ChipSat missions.