Design of cycler trajectories and analysis of solar influences on radioactive decay rates during space missions

Blake A Rogers, Purdue University

Abstract

This thesis investigates the design of interplanetary missions for the continual habitation of Mars via Earth-Mars cyclers and for the detection of variations in nuclear decay rates due to solar influences. Several cycler concepts have been proposed to provide safe and comfortable quarters for astronauts traveling between the Earth and Mars. However, no literature has appeared to show how these massive vehicles might be placed into their cycler trajectories. Trajectories are designed that use either V∞ leveraging or low thrust to establish cycler vehicles in their desired orbits. In the cycler trajectory cases considered, the use of V∞ leveraging or low thrust substantially reduces the total propellant needed to achieve the cycler orbit compared to direct orbit insertion. In the case of the classic Aldrin cycler, the propellant savings due to V∞ leveraging can be as large as a 24 metric ton reduction for a cycler vehicle with a dry mass of 75 metric tons, and an additional 111 metric ton reduction by instead using low thrust. The two-synodic period cyclers considered benefit less from V∞ leveraging, but have a smaller total propellant mass due to their lower approach velocities at Earth and Mars. It turns out that, for low-thrust establishment, the propellant required is approximately the same for each of the cycler trajectories. The Aldrin cycler has been proposed as a transportation system for human missions between Earth and Mars. However, the hyperbolic excess velocity values at the planetary encounters for these orbits are infeasibly large, especially at Mars. In a new version of the Aldrin cycler, low thrust is used in the interplanetary trajectories to reduce the encounter velocities. Reducing the encounter velocities at both planets reduces the propellant needed by the taxis (astronauts use these taxis to transfer between the planetary surfaces and the cycler vehicle) to perform hyperbolic rendezvous. While the propellant expenditure for the cycler vehicle increases, trade studies over seven synodic periods show that the low-thrust Aldrin cycler is effective in reducing the total (i.e., cycler plus taxi) initial mass in low-Earth orbit. A mission is proposed whose architecture is a series of stopovers, unlike conventional cycler trajectories that string series of flybys together. The vehicle would be captured into orbits about the Earth and Mars without landing on either planet. The zero hyperbolic-excess velocities with respect to the planets keep the mass of the taxis low. To allow a mission in every launch opportunity, the cycler vehicle is required to make a complete round trip in less than the synodic period of the two planets (i.e. 2 1/7 years). A high level of acceleration is required to satisfy the itinerary, which results in a large mass (90 metric tons) for the power generator. Fortuitously, the high (11 MWe) power level of the propulsion system would also be effective in hauling the cargo payload via a spiral trajectory about the Earth. Because one synodic period is not enough for the cycler vehicle to fly both the interplanetary trajectories and the Earth-spiral trajectories, it is suggested that two nuclear power generators be developed, which could alternate flying the interplanetary trajectories and the Earth-spiral trajectories. Once these power generators are launched and begin operating in space, the mass requirement in seven subsequent missions (over a period of 15 years beginning in 2022) would be modest at 254 to 296 metric tons to low-Earth orbit per mission. Two launches of NASA's Space Launch System for the cargo and one launch of the Falcon 9 Heavy for the crew would be more than adequate to maintain support for each consecutive mission. Previously, cycling trajectories have been constructed by finding the solution to Lambert's problem between two planetary encounters that occur some multiple of a synodic period apart. In this work, the relative equations of motion are investigated to determine if they can be used to find new cycler trajectories, as well as those previously discovered. First order approximations to the relative motion equations are unfruitful for Earth-Mars cyclers because the variation in radial distance from the Sun is too large. However, using optimization techniques, cycling trajectories are found for the Earth-Mars, Earth-Ceres, and Mars-Ceres systems. Experiments showing a seasonal variation of the nuclear decay rates of a number of different nuclei and decay anomalies— apparently related to solar flares and solar rotation— have suggested that the Sun may somehow be influencing nuclear decay processes. Recently, there have been searches for such an effect in 238Pu nuclei contained in the radioisotope thermoelectric generators on board the Cassini spacecraft. In this work, that analysis is modified and extended to obtain constraints on anomalous decays of 238Pu over a wider range of models, but these limits cannot be applied to other nuclei if the anomaly is composition-dependent. It is also shown that it may require very high sensitivity for terrestrial experiments to discriminate among some models if such a decay anomaly exists, motivating the consideration of future spacecraft experiments which would require less precision. A mission on which such an experiment could be run is proposed. The proposed mission will take various isotopes on a spacecraft that has a large variation in radial distance and return them to Earth. Two different types of trajectories are considered: one with intermediate Venus flybys and one that injects directly into an Earth-resonant orbit. It is shown that each of these types of trajectories have their relative merits with regards to the scientific objective. The suitability of the upcoming Solar Probe Plus and Solar Orbiter missions to perform this experiment is also investigated.

Degree

Ph.D.

Advisors

Longuski, Purdue University.

Subject Area

Aerospace engineering|Nuclear engineering|Molecular physics

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