Journal of Aerospace Engineering

As the number of space‐faring nations and orbiting spacecraft increases, it is desirable to develop an international traffic‐management strategy to coordinate, monitor, and control the interactions of spacecraft in orbit. Successful strategies will facilitate cooperative missions while still supporting each nation's unique goals and objectives in space. The potential benefits of such a strategy include: reductions in future program costs and increases in mission success through the standardization of space operations and equipment; increased safety through development of a coordinated collision avoidance strategy for active spacecraft and debris; and establishing a basis for legal and economic compensation agreements. One means of implementing such a strategy is to utilize a control zones technique that assigns different types of orbital operations to specific regions of space surrounding a vehicle. This paper considers the issues associated with developing a control‐zones technique to regulate the interactions of spacecraft in proximity to a manned vehicle. It includes discussion of technical and planning issues, flight hardware and software issues, mission‐management parameters, and other constraints. It addresses manned and unmanned vehicle operations, and manual versus automated flight control. A review of the strategies utilized by the Apollo‐Soyuz Test Project and the Space Station Freedom Program is also presented.

All large spacecraft are susceptible to impacts by meteoroids and pieces of orbiting space debris. These impacts occur at extremely high speeds and can damage flight‐critical systems, which can in turn lead to catastrophic failure of the spacecraft. A long‐duration spacecraft developed for a mission into this environment must include adequate protection against perforation of pressurized components by such impacts. This paper presents the results of an investigation into the effects of projectile shape and material on the perforation of aluminum dual‐wall structural systems. Impact damage is characterized according to the extent of perforation, crater, and spall damage in the structural systems as a result of hypervelocity projectile impact loadings. Analysis of the damage data shows that there are distinct differences in impact damage from cylindrical and sherical projectiles. Projectile density is also found to affect the type and extent of damage sustained by dual‐wall structural systems.

Development and study of mechanical properties of engineering materials from locally available materials in space is a vital endeavor toward establishment of bases on the Moon and other planets. The objectives of this study are to create a lunar simulant locally from a basaltic rock, and to design and develop a new vacuum triaxial test device that can permit testing of compacted lunar simulant under cyclic loading with different levels of initial vacuum. Then, triaxial testing is performed in the device itself without removing the compacted specimen; this is achieved by a special mechanism installed within the device. Preliminary constrained compression and triaxial shear tests are performed to identify effects of initial confinements and vacuums. The results are used to define deformation and strength parameters. At this time, vacuum levels up to ${10}^{-4}$ are possible; subsequent research should involve higher vacuum levels, e.g., ${10}^{-14}\mathrm{torr}$ as they occur on the Moon. The research can have significant potential toward development of methodology so as to develop compacted materials for various construction applications, and also toward stress‐strain‐strength testing of lunar simulants with different vacuum levels.

This paper investigates the possibility of generating a time‐domain modal identification algorithm that does not need measurements of the input signals. Such a technique is useful when complete input data acquisition cannot be performed. The approach is based on the Yule‐Walker equations, extended to the case where the input signal is not white noise. The algorithm is written recursively both to minimize data acquisition and to be flexible enough when time‐varying modal parameters are tracked. Natural frequencies and damping ratios are extracted with an error magnitude inferior to impulse response techniques but superior to methods using the input time history. From a computational aspect, the algorithm only introduces scalar inversions, which presents an important gain of time and stability. Examples and comparisons with other techniques are presented. The case where a change in the modal parameter values occur is also highlighted. A normalized error, taking into account the quality and the swiftness of the new estimation, is introduced.

The effect of time‐delayed control signals on the stability and the dynamic response of actively controlled structures is investigated. The necessary optimal control law that takes the effect into account is derived using the variational approach. With the help of an example of a moment‐controlled simply supported beam subjected to a concentrated load, it is shown how time delay cannot only degrade the system response, but can also destabilize actively controlled structures. A gain‐compensating technique is proposed, and its use is illustrated with the help of the example. The results indicate that the effect of smaller values of time delay can be compensated for by applying additional energy, but the effects of higher values may not be compensated for, because of the prohibitively larger amounts of energy required in those situations.

The effect of transverse shear deformation on the supersonic flutter of composite panels has been investigated using the finite element method. First‐order shear‐deformation laminated‐plate theory and quasi‐steady aerodynamic theory are employed for the analysis. The total displacement of the plate is expressed as the sum of the displacement due to bending and the displacement due to shear deformation. Thus, the aerodynamic pressure induced by the plate motion is also the sum of the pressure induced by bending deformation and the pressure induced by shear deformation. Numerical results show that the transverse shear deformation may have a significant effect on the flutter boundary if aerodynamic damping were small or neglected in the determination of flutter boundary.

Sandwich panels made of thin skins and a lightweight core expand and/or bow when subjected to temperature changes. The significance of induced thermal stresses in the panels depends on material properties. The effects of bonding layers on these stresses were not investigated in available works on the structural analysis of sandwich panels. This paper presents elasticity solutions for thermal stresses in sandwich panels with interlayer slip. The effects of finite bonding stiffnesses on the structural behavior of the panels are investigated. The numerical results show that the bonding stiffness, up to a certain level, has a strong effect on panel structural response. The answer to what constitutes perfect bonding is best answered in terms of the ratio of the core stiffness to the bonding stiffness. A heat chamber is designed and used to test sandwich specimens under different temperature changes. The experimental values for normal stresses in the skins are in good agreement with the present theory.

With an external electrical stimulus, a method of controlling the elastic/viscoelastic behavior of a cantilever beam embedded with a bulk viscoelastic material having ``smart'' properties, is addressed. The test smart material used is constituted of a heteorogeneous mixture of barium titanate and ferrite (in powder forms) dispersed in a sol-gel phase of polyacrylamide with a specific aqueous content. The barium titanate content possesses the piezoelectric property that enables the material to exhibit electroelastic characteristics. The test beam made of plexiglass material keeps the smart material constrained in its core along its axis. When subjected to transient and static load conditions, and electrically stimulating the embedded smart material, the free-vibration damping characteristics of the test beam are elucidated. It is shown that the measured response of the cantilever beam excited to vibrate in its first natural mode can be controlled with the application of the voltage stimulation. Relevant experimental data are furnished.