Two decades of spacecraft attitude control

Abstract

Introduction T HIS paper does not fit into the usual pigeonholes of technical works, so it is necessary to start by telling the reader what it is and what it is not. It is a very personal perspective on spacecraft attitude control since the subject was first formalized in 1957, given by one who has followed it from the beginning. Special emphasis is on the period of concept formation, mainly prior to 1965, and on topics that fall within my own areas of special interests. The paper is obviously not an exhaustive literature survey of attitude control; the archive literature alone, with several thousand items, is simply too vast. Nor has there been any attempt to make the coverage complete, in the usual scholarly sense, even within its restricted scope. Although I have tried to maintain a balanced viewpoint, the reader must recognize that problems are always possible when history is recounted by an active participant. At the end of two decades of space flight it is appropriate to take a retrospective look at some of the major functional ingredients of spacecraft.! Attitude control is one of these. In the literature of astronautics it was scarcely recognized as an area worth studying. In 1977 I heard it said that in extreme cases the attitude control subsystem can represent up to 30% of the cost of the spacecraft. This much change in the subject's perceived importance is reason enough to review its evolution over the past two decades. If its role could be so underestimated then, what might have changed in our viewpoint toward the structure of the discipline itself and the methods available to perform the attitude control function? We would hardly expect attitude control to have suddenly emerged as a new discipline exactly 20 years ago, at the time space flight began. On the basis of the published literature, it is not unreasonable to pick 1952 as its nominal birth year. A history of the subject prior to 1952 already has been given , in which it was pointed out that the first systematic study of spacecraft attitude control in its own right began that year. (This study was documented only in unpublished form , i.e., as a company report whose original classification was Secret.) Some forerunners existed (described in Ref. 1), extending from the technology of spin-stabilized projectiles in the 16th century, through the gyroscopic stabilization proposals in early speculative studies of space flight done in the late 1920's, to several secret studies of spacecraft sponsored by U.S. government agencies (under the euphemism High Altitude Test Vehicles) in the second half of the 1940's. But as regards the specific subject of attitude control, published work was sparse, neither comprehensive nor intensive, and invariably formed an incidental part of broader system studies. Not until the mid 1950's did a trickle of publications begin in which spacecraft attitude control was the explicit central theme. Gravitational torque on an artificial satellite was the motivation of one work in 1956, although its wording had to be very carefully couched to avoid hinting at such a vehicle. In 1957 another addressed the effect of the Earth's magnetic field on satellite spin. Finally, in 1957 a third described for the first time in the open literature the general problem of actively controlling an artificial satellite so that one of its axes remains pointed downward toward the Earth. In the USSR that same year, Beletskii made two contributions' to problems of classical type (see later) having implications to the uncontrolled behavior of artificial satellites. Thus the open, archive publication of works on artificial satellites began at almost exactly the same time as the first space flight (in October, 1957), giving us a double motivation for choosing 1957 as the initial point of the two-decade period with which we are dealing. The purpose of this paper is to give an overview of the attitude control discipline as we see it at the end of 1977, and to put this into perspective with the 1957 viewpoint. Certain subsequent developments could be foreseen fairly well at that time, and these are reviewed. Perhaps more interesting is to identify those unforseen developments which were essentially new to the period. Finally, a few words are ventured about the future. The real substance of the two decades lies, of course, in the attitude control systems themselves, those that actually have been put into space. A review of those and their observed performance would be very appropriate at this time, but this