On the dynamics of a novel manipulator with slewing and deployable links

Using a novel space platform-based manipulator with slewing and deployable links, the paper addresses two issues of considerable importance: (a) How important is it to model flexibility of the system? (b) How many modes are needed to adequately represent the elastic character? Results suggest that the fundamental mode is able to capture physics of the response quite accurately. Due to its massive character, the platform dynamics is virtually unaffected, even by severe maneuvers of the manipulator. Hence, treating the platform as rigid would save the computational cost without affecting the accuracy. Although the link flexibility does affect the manipulator's tip vibration, the joint and platform vibrations remain negligible. The revolute joint flexibility appears to be an important parameter affecting both the joint as well as tip responses. The information should prove useful in the design of this new class of manipulators.     

RLV economics: fiscal evaluation of NASA''s reusable launch vehicle effort

The US RLV program aims to stimulate commercial development of a next-generation heavy-lift launcher and lower launch costs by one order of magnitude from the Space Shuttle. This paper discusses the incentives needed to encourage private investment — income tax relief, investment mitigation, financing assistance — in the venture and uses a specifically developed case study model to evaluate their effectiveness. It finds that an R&D tax credit would be the most practical incentive. Directions for future work are provided.     

Thrust modulation of solid propellant rockets by a fluidic vortex valve with secondary combustion

An analytical model for calculation of the thrust and pressure modulation of a solid propellant rocket by means of a fluidic vortex valve with secondary combustion has been developed. Thrust control by the vortex valve method was found superior to the axial injection of control flow. Addition of oxygen in the injected flow improves the energetic performance of the system as well as the thrust modulation capability. Experiments have been conducted using a mixture of nitrogen and oxygen as the injection gas. The two main parameters investigated in a series of experiments were the oxygen percentage in the injection gas and the ratio between the mass flow rates of the control gas and the solid propellant combustion products. The results show an increase of thrust by a factor of 2 for a 25% addition in mass flow rate by secondary injection at optimal conditions.     

Economy of modular low-technology launch vehicles

The paper comprises an assessment of the design and the economics of so-called “low-cost” simple modular launch vehicles. It is shown that the performance is very marginal and that the cost per launch cannot compete with technically more advanced fully reusable vehicles. Especially a private-funded development cannot be amortized economically in case of an expendable launch vehicle.     

On the structure of the floating zone in melting

Floating zone melting is used in crystal growth and purification of high melting materials. The use of a reduced gravity environment will remove the constraint imposed on the length of the zone by the hydrostatic pressure. The equilibrium of the floating zone may involve, (1) Hydrostatic forces, when the zone rotates as a whole. (2) Convective driving forces, when the zone is stationary but fluid property gradients appear. (3) Hydrodynamic forces, when some parts of the zone are set into motion with respect to others. The last effects are considered in this paper. The flow pattern of a floating zone held between two discs in relative motion is complicated, and thence the solution of the problem is difficult even assuming a constant property-newtonian liquid. Nevertheless, when a small parameter appears in the problem, the complete flow field can be split into zones where simple solutions are found. To illustrate this approach, the spin-up from rest of an initially cylindrical floating zone is considered with detail. Here the small parameter is the time elapsed from the impulsive starting of motion. Since the problem which has been considered, as well as some others which can be tackled by use of similar methods, concern the viscous layer close to either plate, they can be simulated experimentally in the ground laboratory with short floating zones. Procedures to produce these zones are indicated.     

A sulfur-based survival strategy for putative phototrophic life in the venusian atmosphere

Several observations indicate that the cloud deck of the venusian atmosphere may provide a plausible refuge for microbial life. Having originated in a hot proto-ocean or been brought in by meteorites from Earth (or Mars), early life on Venus could have adapted to a dry, acidic atmospheric niche as the warming planet lost its oceans. The greatest obstacle for the survival of any organism in this niche may be high doses of ultraviolet (UV) radiation. Here we make the argument that such an organism may utilize sulfur allotropes present in the venusian atmosphere, particularly S(8), as a UV sunscreen, as an energy-converting pigment, or as a means for converting UV light to lower frequencies that can be used for photosynthesis. Thus, life could exist today in the clouds of Venus.     

Gravitational Interaction of Celestial Bodies and neutron flux bursts from Their Surfaces

The gravitational interaction between the Sun and the planetary solar system gives rise to the well-known tidal waves at the planets. The tidal wave originating in the Earth's crust perpetually transforms the microstructure of the Earth's crust leading to a variation of the concentration of natural radioactive gases in the terrestrial air and to changed conditions of their leakage to the Earth's atmosphere. These variations give rise to bursts of thermal and slow neutrons in the vicinity of the Earth's crust, because the radioactive gases are sources of energetic alpha particles that induce neutron production upon the interaction with the nuclei of elements of the Earth's crust and atmosphere. In this work, the idea of neutron production in the ground coat is extended to the other celestial bodies interacting with one another.     

Single-Axis Solar Orientation of a Satellite of the Earth

The possibility of using the mode of single-axis solar orientation is considered for a satellite placed into a nearly circular orbit with an altitude of 900 km and bearing a solar sail. The satellite (together with the sail) has an axisymmetric structure, its symmetry axis being the principal central axis of the maximum moment of inertia. The center of the sail pressure lies on this axis and is displaced with respect to the satellite's center of mass. The symmetry axis of the satellite is set to the Sun so that its center of mass would be located between the Sun and the pressure center and would rotate around this axis with an angular velocity of a few degrees per second. The satellite's axis of symmetry makes a slow precession under the action of the gravitational moment and the moment of light pressure forces. Though the maximum magnitudes of these moments are comparable, the moment of the light pressure forces dominates and controls the precession in such a way that the symmetry axis orientation to the Sun remains unchanged.     

Promoting research and education in basic space science: the approach of the UN/ESA workshops

UN-affiliated regional centres for space science and technology education are being established or are in operation in Africa (Morocco, Nigeria), Asia and the Pacific (India), Latin America and the Caribbean (Brazil, Mexico), and Western Asia (Jordan). Education curricula at the university level, embracing remote sensing, satellite communications, satellite meteorology, and space science have been developed for these centres. This article briefly reports on the structure of the most recent updated education curricula in the four disciplines that have been made available for implementation in 2002 and 2003, in the six official languages of the United Nations. This is also an effort to bridge the gap between such education curricula as they vary significantly between nations and among educational institutions in nations.     

Dynamics and control of variable-geometry truss structures

Variable-geometry truss structures are likely to be used extensively in the future for in-orbit space construction. This paper considers dynamics formulation and vibration control of such structures. The truss system is modelled as a collection of sub-structures consisting of truss booms, prismatic actuator elements, and in some cases a manipulator at the end. Each truss boom is treated as a separate ‘link’ and its flexibility is modelled using the finite element method. Equations of motion for individual sub-structures are obtained which are then assembled. The non-working constraint forces are eliminated to obtain the equations governing the constrained dynamics of the entire system. For vibration control, the singular perturbation method is employed to construct two reduced-order models, for quasi-static motion and for modal co-ordinates, respectively. Computed torque with PD control is applied to maintain the quasi-static motion, while an optimal LQR method is used for vibration control. Typical simulation results are presented for the planar case.