A study of feasibility is performed for the development of a nowcasting method of aircraft icing conditions. The supercooled cloud water content is a key parameter for the determination of hazardous regions. The production of supercooled water in clouds is possible only where sufficiently strong updraft produces supersaturation with respect to water. However, to maintain the presence of supercooled droplets, the generation of moisture excess during the air uplift must exceed the rate of vapor deposition on snow and the rate of snow riming. The "storm of the century" (March 13-14, 1993) has been chosen as a good case of stratiform precipitation event and its microphysics was simulated using a high resolution three-dimensional kinematic cloud model with a full microphysical scheme. The winds driving the development of hydrometeors are obtained by a three-dimensional wind retrieval technique applied to single Doppler radar data. The output of the cloud model provides the field of supercooled (since T $<$ 0$ sp circ$C) cloud water. In this way a potentially operational method for detection of icing conditions is suggested. / Once implemented, the method was used on a second case (October 21, 1993), where aircraft observations provided a mean of evaluating its performance. The comparison of retrieved cloud liquid water with in situ measurements, shows a good potential for the retrieval method.

The interception of a tactical ballistic missile (TBM) represents a great challenge to the guidance and control community as the maneuver potential of the TBM is almost in the same order as the interceptor missile. The maneuver advantage of the interceptor over the TBM being reduced, a new approach for estimation and guidance for the interception of a maneuvering TBM is presented in this research. The target is assumed to use its maneuvering potential by performing a single evasive bang-bang maneuver. A simple detection algorithm for an evasive maneuver of the target combined with the use of a multiple model estimators like the interactive multiple models (IMM) led to a new adaptive guidance law, the DGL/Adapt law. Simulations results using this approach gave a reduction in the miss distance in comparison with a static guidance law.

<p> The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic responses (creep strain, creep rate, and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). The nanocomposite test articles were fabricated by high shear mixing, casting, curing, and post-curing in an open face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at prescribed combinations of temperatures (23.8 - 69.2 C), applied stresses (30.2 - 49.8 MPa), and VGCNF weight fractions (0.00 - 1.00 parts of VGCNF per hundred parts of resin, phr) determined from the CCD. The response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction marginally increases the creep resistance of the VGCNF/VE nanocomposite at low temperatures (i.e., 23.8 - 46.5 C). However, increasing the VGCNF weight fraction for temperatures greater than 50 C decreased the creep resistance of these nanocomposites. The latter response may be due to a decrease in the nanofiber-to-matrix adhesion as the temperature is increased. The RSMs for creep strain, creep rate, and creep compliance revealed the interactions between the VGCNF weight fraction, stress, and temperature on the creep behavior of thermoset polymer</p>

<p> NASA has had a decades-long problem with cost growth during the development of space science missions. Numerous agency-sponsored studies have produced average mission level development cost growths ranging from 23 to 77%. </p><p> A new study of 26 historical NASA science instrument set developments using expert judgment to re-allocate key development resources has an average cost growth of 73.77%. Twice in history, during the Cassini and EOS-Terra science instrument developments, a barter-based mechanism has been used to re-allocate key development resources. The mean instrument set development cost growth was -1.55%. Performing a bivariate inference on the means of these two distributions, there is statistical evidence to support the claim that using a barter-based mechanism to re-allocate key instrument development resources will result in a lower expected cost growth than using the expert judgment approach. </p><p> Agent-based discrete event simulation is the natural way to model a trade environment. A NetLogo agent-based barter-based simulation of science instrument development was created. The agent-based model was validated against the Cassini historical example, as the starting and ending instrument development conditions are available. The resulting validated agent-based barter-based science instrument resource re-allocation simulation was used to perform 300 instrument development simulations, using barter to re-allocate development resources. The mean cost growth was -3.365%. A bivariate inference on the means was performed to determine that additional significant statistical evidence exists to support a claim that using barter-based resource re-allocation will result in lower expected cost growth, with respect to the historical expert judgment approach. </p><p> Barter-based key development resource re-allocation should work on science spacecraft development as well as it has worked on science instrument development. A new study of 28 historical NASA science spacecraft developments has an average cost growth of 46.04%. As barter-based key development resource re-allocation has never been tried in a spacecraft development, no historical results exist, and an inference on the means test is not possible. </p><p> A simulation of using barter-based resource re-allocation should be developed. The NetLogo instrument development simulation should be modified to account for spacecraft development market participant differences. The resulting agent-based barter-based spacecraft resource re-allocation simulation would then be used to determine if significant statistical evidence exists to prove a claim that using barter-based resource re-allocation will result in lower expected cost growth.</p>

This thesis refers to windshear recovery systems, designed to increase the survival capability of an aircraft. It is known that optimal trajectories in take-off are difficult to implement because of the lack of global information on the wind flow field and the lack of enough time and computing capability onboard. Consequently, one is forced to employ local/prior information on the windshear and the downdraft. An investigation of the gamma guidance scheme is presented with reference to the take-off problem. Attention is focused on the feedback control form of the gamma guidance law. This law is modified so that not only the aircraft can react to unfavorable shears, but can take advantage of favorable shears. Indeed, every unfavorable shear (central core of the downburst) is both preceded and followed by favorable shears. Therefore, a systematic investigation of the effect of parameters on survival capability is presented for two windshear models. The results indicate that, through a proper choice of the parameters, the windshear efficiency of the gamma guidance law is within 5% of that of an optimal trajectory. (Abstract shortened with permission of author.)

The aeroassisted flight experiment (AFE) is to simulate a transfer from a geosynchronous Earth orbit (GEO) to a low Earth orbit (LEO). Specifically, the AFE spacecraft is released from the Space Shuttle and is accelerated by means of a solid rocket motor toward Earth, so as to achieve atmospheric entry conditions identical with those of a spacecraft returning from GEO. During the atmospheric pass, aerodynamic and thermal data are gathered for use in designing aeroassisted orbital transfer (AOT) vehicles. Following the atmospheric pass, the AFE spacecraft ascends to the specified LEO either directly or indirectly via an intermediate parking Earth orbit (PEO). The optimal trajectories are determined by minimizing the postatmospheric characteristic velocity if the entry path inclination is fixed or the total characteristic velocity if the entry path inclination is free. The final maneuver includes the rendezvous with and the capture by the Space Shuttle. The entry and exit orbital planes of the AFE spacecraft are identical with the orbital plane of the Space Shuttle. Under following two assumptions: (i) the instantaneous orbital plane is nearly identical with the entry orbital plane; (ii) the Earth's angular velocity is relatively small, the motion of the AFE spacecraft of order six (combination method) can be decoupled into two subsystems of order three, the longitudinal motion and the lateral motion (decomposition method). The results of optimal trajectories are nearly the same by using the combination method and the decomposition method. The optimal control is bang-bang if the entry path inclination is fixed and is constant if the entry path inclination is free. Finally, the optimal trajectories are advantageous over a nonoptimal reference trajectory in terms of the main quantities of interest, namely, the characteristic velocity, peak dynamic pressure, peak heating rate, and peak wedge angle.

Superelastic Nitinol Shape Memory Alloy (SMA) as a shielding material for hypervelocity impact is investigated. Impact peak pressures from Rankine-Hugoniot equations and empirical figures of merit, which give a general evaluation of this material as a nickel titanium alloy, have been derived. Superelastic Nitinol was impacted at oblique and normal incidence using a two-stage light-gas gun and was evaluated as an intermediate layer in the Stuffed Whipple configuration and as bumper in the Whipple configuration. Its performance during oblique impacts is better than that of Kevlar and Al2024-T3. Superelastic SMA can dissipate impact energy through a martensitic phase transformation and through plastic yielding. Differential scanning calorimeter studies demonstrated that stress induced Martensite (SIM) is present in the hypervelocity impacted samples. Optical metallography and electron microscopy studies confirm the presence of SIM.

The 10 kilowatt prototype of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, abbreviated as VX-10, is designed to eject plasma at exhaust velocities of tens of kilometers per second. In this device, energy is imparted to the plasma ions by two mechanisms: ion cyclotron resonant heating (ICRH), and acceleration in an ambipolar electric field. Measurements from two different electrostatic probes are combined to determine how much each mechanism contributes to the total ion energy. The first probe is a gridded retarding potential analyzer (RPA) that incorporates a multi-channel collimator to obtain precise measurement of the ion and electron parallel energy distributions. The second is an emissive Langmuir probe that measures the DC and RF components of the plasma potential. The plasma potential obtained from the emitting probe allows calculation of the parallel velocity distribution once the parallel energy distribution is obtained from the energy analyzer data. Biasing the RPA housing is shown to minimize the plasma perturbation, as monitored by an auxiliary probe. When this minimization is done, the RPA measurements become compatible with the emissive probe's measurement of plasma potential. The collimated RPA and emissive probe have been used to examine the effects of a double dual half-turn (DDHT) antenna encircling the plasma. When power at the ion cyclotron frequency is applied, changes are seen in the saturation current and mean ion energy of the collimated RPA characteristic. The evolution of these changes as the RPA is moved downstream from the antenna is interpreted as firm evidence of ion cyclotron heating, albeit at absorbed energies of less than 1 electronvolt per ion. The emissive probe shows that, within experimental error, all of the increased ion energy is accounted for by an increase in the plasma potential that occurs when the ICRF power is applied. The combined RPA and emissive probe data also show that there is a jet of flowing plasma in the VX-10 when operated with the helicon source alone but that the signal from this jet is overwhelmed by a rapidly growing stationary plasma within the first second of the discharge.

An innovative methodology to design a robust Control Momentum Gyro (CMG) momentum manager for the International Space Station (ISS) is developed. Unlike most other momentum manager designs, which use either a Linear Quadratic Regular (LQR) or mu-synthesis technique, the proposed methodology is based on the Nash differential game theory. This approach accounts for both a robustness specification in terms of the gain/phase margins, and a time domain requirement, such as the maximum CMG momentum. An inverse solution procedure is used to recast the original coupled Algebraic Riccati Equation (ARE) problem to a constrained optimization problem. Further, the dynamics used in this momentum manager design are linearized for any operating point, including the Torque Equilibrium Attitude (TEA), as opposed to only the Local Vertical and Local Horizontal (LVLH) frame. Design examples are provided to demonstrate the practicality of the proposed methodology, and the results are favorable.

This thesis is concerned with the study of pseudospectral discretizations of optimal control problems governed by ordinary differential equations and with their application to the solution of the International Space Station (ISS) momentum dumping problem. Pseudospectral methods are used to transcribe a given optimal control problem into a nonlinear programming problem. Adjoint estimates are presented and analyzed that provide approximations of the original adjoint variables using Lagrange multi pliers corresponding to the discretized optimal control problem. These adjoint estimations are derived for a broad class of pseudospectral discretizations and generalize the previously known adjoint estimation procedure for the Legendre pseudospectral discretization. The error between the desired solution to the infinite dimensional optimal control problem and the solution computed using pseudospectral collocation and nonlinear programming is estimated for linear-quadratic optimal control problems. Numerical results are given for both linear-quadratic and nonlinear optimal control problems. The Legendre pseudospectral method is applied to formulations of the ISS momentum dumping problem. Computed solutions are verified through simulations using adaptive higher order integration of the system dynamics.