Experimental and theoretical burning of solid rocket propellants near the pressure deflagration limit

Burning of composite solid rocket propellants near the pressure deflagration limit (PDL) was studied experimentally in two different test chambers. The propellant tested was a nonmetallized ammonium perchlorate-based composite propellant (AP 84/CTPB 16). Measurements were taken of the regression rate, oscillations frequency and flame luminosity. Self-sustained oscillations were detected near the PDL that matched reasonably well the predictions of the analytical nonlinear stability theory and of the numerically solved nonlinear mathematical model. Both experimental and numerical results show the burning rate oscillations near the PDL due to statically unstable burning, that is the only combustion regime possible below a certain pressure. When pressure is further reduced the amplitude of the oscillations increases and their frequency decreases, until extinction follows abruptly below a pressure that corresponds to the PDL.     

An approach to the dynamics of modular repetitive structures

The dynamics of modular structures is approached in this paper by means of the discrete Fourier transform. This method, applied to a structure with N bays and ring type boundary conditions, leads to N uncoupled systems of the size of a single bay. For other boundary conditions, it leads to a “spectrally resolved” eigenproblem, that is a form whose dominant terms in each field of frequency are evidenced. Approximate reduced models in narrow frequency fields can therefore be generated by using the “spectral condensation” technique. The method can be applied with general boundary conditions, but the present paper deals mostly with the “clamped edges” boundary condition and shows that numerical advantages can be obtained, particularly for the large space structures.     

Attached payloads on SPACEHAB, growing from the Space Shuttle to the Space Station

Although NASA's Space Shuttle is largely dedicated in the near term to Space Station assembly, 10–16 day flight opportunities still abound for spacecraft technology demonstration payloads, and experiments for the established earth and space science communities. This paper will present the latest developments of SPACEHAB flight systems in order to accommodate the flight needs of these communities on the Space Shuttle today and the Space Station tomorrow. In particular, some examples of payloads from these disciplines will be introduced together with the accommodation and experiment objectives.     

Estimation of probability of occurrence of F1 layer or L condition using tables and electron density profile models

An algorithm is proposed for evaluation of the probability of occurrence of an F1 layer or L condition, based on tables. Observations independent of the tables database are used for comparison between the estimated probability of occurrence, the formulation used at present in IRI, and the occurrence actually observed. The importance of the inclusion of L condition in the electron density profile model is shown.     

The accuracy of data from ionosondes for the estimation of hmF2 and the identification of global change in the ionosphere

A long temporal series of simulated ionograms was generated with a superimposed secular variation of −14 km/century on the h_mF2 parameter. These ionograms were interpreted by the automatic scaling program Autoscala. By applying four different empirical formulas, four artificial series of h_mF2 were generated and then processed with the same methods used by other authors for real data sets. Data analysis of the simulated ionograms revealed the artificially imposed long-term trend. These results lead to the conclusion, that regardless of the empirical formula used, the accuracy of h_mF2 from ionosonde measurements would be adequate to observe a long-term trend of −14 km/century.     

Application of Autoscala to ionograms recorded by the VIPIR ionosonde

In November 2008, the ionosonde station at Boulder, Colorado, USA (40.0°N; 105.3°W) became the host of a new ionosonde (VIPIR, Vertical Incidence Pulsed Ionospheric Radar) developed and built by Scion Associates.     

Electron density profile calculation technique for Autoscala ionogram analysis

An electron density profile model with free parameters is introduced. Initially the parameters are calculated on the basis of the ionospheric characteristics automatically obtained from the ionograms by Autoscala and considering the helio-geophysical conditions. The technique used to adjust the free parameters to the particular ionograms recorded is presented.     

A method to test HF ray tracing algorithm in the ionosphere by means of the virtual time delay

It is well known that a 3D ray tracing algorithm furnishes the ray’s coordinates, the three components of the wave vector and the calculated group time delay of the wave along the path. The latter quantity can be compared with the measured group time delay to check the performance of the algorithm. Simulating a perfect reflector at an altitude equal to the virtual height of reflection, the virtual time delay is assumed as a real time delay. For a monotonic electronic density profile we find a very small relative difference between the calculated and the virtual delay for both analytic and numerical 3D electronic density models.     

Issues and solutions for testing free-flying robots

Space robotics currently has an important role in space operations and scientists and engineers are designing new robotic systems for space servicing missions and extra-vehicular activities. In particular, free-flying robots with extended arms have compelling applications and several prototypes have recently been developed. Testing on Earth free-flying robots is a main issue as the unconstrained environment of free space must be simulated. From the experience acquired by testing a free-flying robot prototype both in a tethered facility and during a parabolic flight campaign, and after several years of experiments using air-bearing planar systems, the authors describe and discuss methods to test free-flying robots. A recent study aimed at designing a free-flying platform suitable for an under-water environment is also presented and discussed.     

On radiative and conductive heat transfer in spacecraft

The “radiative” boundary condition in a heat conduction problem relates the heat flux in a point of the wall to the temperatures in all the other points of the surface system.That is, this condition is of a “functional” type. This “functional” can be solved in discrete terms by using the conventional technique of dividing the body in small discrete elements or “nodes”. This paper instead presents an approach on a continuous scheme, by which the functional is solved in terms of “spacewise harmonics” of the temperature inside the body; the heat conduction problem is thus reduced to an ordinary form differential system whose unknowns are these “harmonics”. An interative linearized procedure to solve this system is also suggested, by which “exact” solutions are obtained. The merits of these solutions, with respect to practical discrete element computations, are in the better spacewise resolution and in the consequent more accurate treatment of both radiation and conduction. The application of the procedure is, however, limited to particular geometries. It is relevant to note that among these possible geometries many are included of practical interest, like hollow cylinders and polygons, cubic boxes etc. A numerical example completes the work.