Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005acaau..57..819m&link_type=abstract
Acta Astronautica, Volume 57, Issue 11, p. 819-828.
Astronomy and Astrophysics
Astronomy
1
Scientific paper
The success of the solar-electric ion engine powering the DS1 spacecraft has paved the way toward the use of low-thrust electrical engines in future planetary/interplanetary missions. Vis-à-vis a chemical engine, an electrical engine has a higher specific impulse, implying a decrease in propellant mass; however, the low-thrust aspect discourages the use of an electrical engine in the near-planet phases of a trip, since this might result in an increase in flight time. Therefore, a fundamental design problem is to find the best combination of chemical propulsion and electrical propulsion for a given mission, for example, a mission from Earth to Mars. With this in mind, this paper is the second of a series dealing with the optimization of Earth Mars missions via the use of hybrid engines, namely the combination of high-thrust chemical engines for planetary flight and low-thrust electrical engines for interplanetary flight. We look at the deep-space interplanetary portion of the trajectory under rather idealized conditions. We study minimum time trajectories with bounded thrust direction and bounded thrust magnitude: the thrust direction α is subject to the inequality -αmax⩽α⩽αmax, while the thrust setting β is subject to the inequality 0⩽β⩽1. We use αmax as a parameter to generate a family of minimum time trajectories. Numerical results show that, as αmax decreases, the flight time increases, while the propellant consumption decreases. Generally speaking, the thrust profile of the optimal trajectory includes three subarcs: the first subarc is characterized by maximum thrust in conjunction with positive (upward) thrust direction; the second subarc is characterized by zero thrust (coasting flight, thrust direction irrelevant); the third subarc is characterized by maximum thrust in conjunction with negative (downward) thrust direction. Two limiting cases have a particular interest. The first limiting case occurs for αmax=180 and has the following properties: (i) the time length of the coasting subarc reduces to zero and the three-subarc trajectory degenerates into a two-subarc trajectory; (ii) maximum thrust is applied at all times and the thrust direction switches from positive to negative just beyond midway; (iii) the minimum time trajectory for α constrained is the same as the minimum time trajectory for α unconstrained. The second limiting case occurs for αmax=0 and has the following properties: (i) the thrust magnitude has a bang-zero-bang profile; (ii) for the powered subarcs, the thrust direction is tangent to the flight path at all times; (iii) the minimum time trajectory for α constrained is the same as the minimum propellant consumption trajectory for α unconstrained.
Miele A.
Wang Tower
Williams P. N.
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