Mass-Optimal Transit Time for Acceptable Effective Radiation Dose on Manned Deep Space Exploration Missions

As future deep space exploration missions venture beyond Earth’s protective magnetosphere, astronauts will be exposed to higher ambient radiation doses than ever before. Radiation mitigation that provides sufficient protection for astronauts while also keeping total mission mass and cost within budget presents a significant engineering challenge. To meet established constraints on total allowable mission dose, mission planners can either use fast orbital transfers with little radiation shielding or slow transfers with heavy radiation shielding. The objective of this study is to determine the optimal transit time that minimizes the initial mass in low-Earth orbit (IMLEO) required to deliver a vehicle to Mars with enough radiation shielding to not exceed the maximum allowable mission radiation dose. Lambert’s problem is solved for a range of Earth-Mars transfers to determine the total delta-V required as a function of transit time. For a given transit time, the radiation shielding mass required to meet dose constraints is determined. Then, the IMLEO required to deliver a transit habitat with the added radiation shielding and consumables mass is computed using the delta-V corresponding to that transit time. The sensitivity of IMLEO to transit time is reported and the minimum IMLEO is identified. The minimum IMLEO is shown to be obtained at a faster‐than‐Hohmann transfer because reducing the transit time down to the optimal point reduces the shielding mass more than the loss in payload capacity. The mass and time savings of the optimal transit relative to a standard minimum-energy transfer is reported and discussed. Even when the mission is optimized solely for radiation mitigation, the fastest transfer is clearly not always favored. The sensitivity of these results to the radiation shielding model used and allowable exposure limits is explored along with the potential for using existing ECLSS water as the shielding material.