First-Hand:Low-thrust Interplanetary Vehicles

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submitted by Gerald Ash

I did work related to the space program, however, it was not directly related to Project Apollo. During my graduate program at Caltech from 1965-1968, I did my PhD thesis work on Optimal Guidance of Low Thrust Interplanetary Space Vehicles, which solves the problem of how to guide a low thrust interplanetary space vehicle, propelled by an ion engine, to achieve a minimum probability of miss at the destination planet. This work grew out of two summers of work (1966 and 1967) at the Jet Propulsion Lab (JPL), a NASA Center, in Pasadena, CA. My work at JPL resulted in a funded contract for my advisor Professor Sridhar’s group at Caltech. Working at JPL was an outstanding opportunity and life experience: during the late 1960’s, while I worked there, JPL is landing Surveyor landing vehicles on the moon and preparing to send Voyager spacecraft on the “Grand Tour” of all the outer planets.

JPL successfully flew a mission using an ion engine on the Dawn space probe launched by NASA in September 2007, with the goal of studying two protoplanets in the asteroid belt, Vesta and Ceres. The Dawn mission was designed to answer questions about the formation of the Solar System, as well as to test the performance of its ion engine in deep space. The Wikipedia article on the Dawn mission,, gives interesting statistics on the performance of the ion engine.

At Caltech, I specialize in modern control theory in the EE department under Professor Sridhar. As part of my thesis project, I develop computer algorithms to solve the optimization problems required to achieve optimum guidance of low thrust space vehicles. These results are published in my thesis and in two technical papers. I presented one paper entitled A Study of Low-Thrust Guidance, co-authored with my boss at JPL, Boris Dobrotin, at the AIAA Guidance and Control Conference in Huntsville, Alabama in August 1967. A fringe benefit to attending this conference is that I get to witness the live testing of an F-1 rocket engine at the Marshall Space Flight Center in Huntsville, where the F-1 rocket engine is one of the five enormously powerful rocket engines that propel the Saturn V rocket and our astronauts to the moon. I present the second paper entitled Low-Thrust Guidance - An Optimal Stochastic Control Problem, co-authored with Professor Sridhar, at the Joint Automatic Control Conference in St. Louis, MO in August, 1971.

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