Journal of Aerospace Engineering

The pointing control system of the Hubble Space Telescope (HST) represents the current state of the art for the precision control of a large spacecraft. The proposed Next Generation Space Telescope (NGST) will require an order‐of‐magnitude increase in pointing resolution over that of HST. The use of active optics in the form of a steerable secondary mirror has been proposed for NGST in order to satisfy these requirements. An introduction to some of the design concepts associated with the design of the NGST, including a description of the proposed vehicle, a description of the pertinent design parameters, and a description of the types of disturbances to which it may be subjected, is given herein. A companion paper provides a description of a feasibility study conducted to evaluate the use of a control system for the implementation of “beam steering” to compensate for the motion of the vehicle without necessitating the reorientation of the whole structure.

The pointing control system of the Hubble Space Telescope (HST) represents the current state of the art for the precision control of a large spacecraft. The proposed Next Generation Space Telescope (NGST) will require an order‐of‐magnitude increase in pointing resolution over that of HST. The use of active optics in the form of a steerable secondary mirror has been proposed for NGST to satisfy these requirements. The primary motivation for this study was to demonstrate the feasibility of satisfying the pointing‐stability requirements by sensing the guide‐star position and steering the optical path of the telescope with the active secondary mirror. To study the requirements of the control system, a two‐degree‐of‐freedom model that retains the rigid‐body mode of the telescope as well as its first oscillatory mode was constructed. The corresponding optimal control law was developed and implemented in a discrete manner to examine the behavior of the system subject to typical spacecraft excitations.

Design and construction of a structure on the Moon requires addressing a host of issues not encountered on Earth. Since there is no atmosphere on the Moon, a lunar structure must contain an artificial atmosphere. One critical design issue is the magnitude of the pressure of this atmosphere. Much of the current literature on the design of lunar structures assumes a pressure of 101.3 kPa (14.7 psi), corresponding to that at sea level on Earth, which is an order of magnitude larger than any other loading on the structure. An assessment of the outcome of lowering the internal pressure for a lunar structure is presented that accounts for human physiology, plant growth, mechanical equipment for gas circulation, structural aspects, leak rate, decompression, flammability, combustion, and economic issues. Options for the magnitude and content of an internal atmosphere for a lunar structure are given. Results clearly show that there is a great savings if the pressure is lowered by an amount that does not greatly affect the inhabitants' physiology or safety.

Instead of a pessurized cylinder serving as the basic module for space stations, certain polyhedra are studied having the all‐space filling property, permitting the construction of pressurized volumes of cylindrical or virtually any conceivable shape. Rotatable forms for artificial gravity become an option because loads are evenly distributed through a triangle‐based grid work. This grid work may be left open, or panelled and pressurized partially or totally. The isosceles tetrahedron is an especially useful member of a class of polyhedra that may be stacked face to face in three dimensions without leaving space among the units. Since only a few simple parts must be duplicated, the approach is economical as well as flexible. Components may be shuttled into orbit in the disassembled state so that a substantial space station may be established with a single flight. Reconfiguration is possible as requirements change. Conversion to translunar or interplanetary craft is also conceivable. On arrival such craft may again be reconfigured to fulfill several roles simultaneously: orbital support craft, lander, and surface base.

The construction of an outpost on the Moon in which humans can live and work for periods exceeding six months will require special countermeasures to adapt to the hostile environment present at the lunar surface. Various inherent dangers such as meteoroids, galactic cosmic radiation, solar proton events, and large thermal extremes will drive the design configuration of the outpost. Other considerations such as lunar soil mechanics, equipment performance, mass delivery, risk, reliability, and tele‐operability act strongly as constraints that shape and control the design alternatives. Analysis of these fundamental relationships have resulted in lunar civil engineering guidelines, which are unique to this domain, and these in turn have pointed to research areas needing further attention. A preliminary design is presented for a lunar outpost shelter. Additionally, the design methodology is explored, and early enabling technologies are identified to facilitate an understanding of lunar shelter designs from an integrated system standpoint.

Distributed parameter modeling may offer a viable alternative to the finite‐element approach for modeling large flexible space structures. In addition, the introduction of the transfer matrix method to the continuum modeling process provides a very useful tool to facilitate the distributed parameter model applied to more complex configurations. This paper proposes a piecewise continuous Timoshenko beam model, which was used for the dynamic analysis of a tapered beam‐like structure. Instead of the arbitrarily assumed shape functions used in a finite‐element analysis, the closed‐form solution of the Timoshenko beam equation was used. Application of the transfer matrix method was used to relate all elements as a whole. Using the corresponding boundary conditions and compatibility equations, a characteristic equation for the global tapered beam was produced and natural frequencies derived. The results from this analysis were compared to those obtained from a conventional finite‐element analysis. While comparable results are obtained, the piecewise continuous Timoshenko beam model significantly decreased the number of modal elements required.

The finite‐element method has proven to be an invaluable tool for analysis and design of complex, high‐performance systems, such as those typically encountered in the aerospace or automotive industries. However, as the size of the finite‐element models of such systems increases, analysis computation time using conventional computers can become prohibitively high. Parallel processing computers provide the means to overcome these computation‐time limits, provided the algorithms used in the analysis can take advantage of multiple processors. The writers have examined several algorithms for linear and nonlinear static analysis, as well as dynamic finite‐element analysis. The performance of these algorithms on an Alliant FX/80 parallel supercomputer has been investigated. For single load‐case linear static analysis, the optimal solution algorithm is strongly problem dependent. For multiple load cases or nonlinear static analysis through a modified Newton‐Raphson method, decomposition algorithms are shown to have a decided advantage over element‐by‐element preconditioned conjugate gradient algorithms. For eigenvalue/eigenvector analysis, the subspace iteration algorithm with a parallel decomposition is shown to achieve a relatively high parallel efficiency.