Aerocapture, Entry, and Co-Delivery in Uncertain Planetary Atmospheres

Abstract

Aerocapture, the method of entering orbit via a single pass through the atmosphere of a planet, is an enhancing or enabling technology for a range of interplanetary missions. Compared to propulsive maneuvers, aerocapture can reduce cruise duration while decreasing the total mass expended for orbit insertion, thus leaving more time and mass for the primary science mission. Two mission classes in particular both benefit from aerocapture and are of high priority for the next decade of planetary science: exploration of the ice giants, Uranus and Neptune, and low-cost small satellite platforms. However, despite its potential benefits, aerocapture has never been implemented in flight. This is primarily because of the large uncertainties involved, which must be modeled and adequately mitigated by closed-loop autonomous guidance onboard the spacecraft.

Aerocapture guidance has been well-studied for vehicles that control their atmospheric flight by changing the orientation of a lift vector, but is not as well developed for a class of flight vehicles that achieve control by releasing a drag device mid-flight. Known as drag modulation, this control mechanism is significantly less complex in terms of hardware and avionics than lift modulation, and is thus appealing for small satellite missions. However, the state of the art guidance solutions have a computational demand that is both high and difficult to bound. This dissertation contributes a novel guidance algorithm for drag-modulated aerocapture that achieves equivalent performance to the state of the art, but with reduced computational demand.

One of the most pernicious sources of uncertainty that aerocapture guidance must mitigate is atmospheric density, which varies over space and time. While scientific and engineering atmosphere models are available and well-characterized for on-the-ground studies, models that retain this fidelity while being significantly more compact and analytically tractable are desirable for onboard use. This dissertation develops reduced-dimensionality models of uncertain atmosphere for use onboard a spacecraft, derives a method for updating the model based on noisy measurements, and demonstrates the ability to accurately predict future state uncertainty resulting from these environmental dispersions without requiring the use of random sampling. These contributions have a range of potential applications, including incorporation into future stochastic guidance algorithms.

Many of the mission concepts most relevant to aerocapture, such as the Uranus Orbiter and Probe, involve more than one flight vehicle. These missions benefit from the ability to deliver both spacecraft to their destination with minimal disruption to the overall concept of operations. While a number of missions have successfully executed multi-vehicle architectures in the past, this ``co-delivery'' method has not received dedicated systematic attention. This dissertation addresses the concept as a topic in its own right, investigating the ability to co-deliver an orbiter and probe from a single approach trajectory without the need for a divert maneuver. Co-delivery of an entire network of probes from a single, non-maneuvering mothership is also investigated. Finally, expressions for relative motion in the velocity frame are derived in order to provide a mathematical model that is more intuitive than the typical rotating orbit frame for highly-elliptical orbits, as are common for aerobraking, entry, and aerocapture.

To illustrate the unifying motivation for this work, the contributions of this dissertation are applied to an example problem: the concept of reducing atmospheric uncertainty for aerocapture by including a fly-ahead probe that enters the atmosphere some time before the orbiter. While this idea has been proposed several times, the benefit conferred to the orbiter by the probe has not been quantified. The contributions of this dissertation naturally lend themselves to addressing this problem, as well as other entry, aerocapture, and co-delivery scenarios for future interplanetary missions.