VentureStar… reaping the benefits of the X-33 program

Major X-33 flight hardware has been delivered, and assembly of the vehicle is well underway in anticipation of its flight test program commencing in the summer of 1999. Attention has now turned to the operational VentureStar^TM, the first single-stage-to-orbit (SSTO) reusable launch vehicle. Activities are grouped under two broad categories: (1) vehicle development and (2) market/business planning, each of which is discussed. The mission concept is presented for direct payload delivery to the International Space Station and to low Earth orbit, as well as payload delivery with an upper stage to Geosynchronous Transfer Orbit (GTO) and other high energy orbits. System requirements include flight segment and ground segment. Vehicle system sizing and design status is provided including the application of X-33 traceability and lessons learned. Technology applications to the VentureStar^TM are described including the structure, propellant tanks, thermal protection system, aerodynamics, subsystems, payload bay and propulsion. Developing a market driven low cost launch services system for the 21st Century requires traditional and non-traditional ways of being able to forecast the evolution of the potential market. The challenge is balancing both the technical and financial assumptions of the market. This involves the need to provide a capability to meet market segments that in some cases are very speculative, while at the same time providing the financial community with a credible revenue stream. Furthermore, the market derived requirements need to be assessed so as not to impose unnecessary requirements on the vehicle design that add unreasonable cost to the development of the system, yet provides the right capabilities for new markets that could be triggered by dramatically lower space transportation prices.     

疲劳S-N曲线预测的三维断裂力学方法

根据旋转弯典圆棒疲劳断口分析,给出了裂纹几何参量及应用强度因子随裂纹扩展的变化规律,采用考虑三维应力约束效应的断裂力学理论及常幅载荷下材料的疲劳裂纹扩展da/dN-ΔK曲线,得到了独立于试件几何形状和应力比的材料da/dN-ΔKeff基准数据,结合断口分析所得的应力强度因子以及材料da/dN-Δkeff数据,以三种旋转弯曲圆棒S－N（应力－疲劳寿命）曲线进行了预测,结果表明,本文预测的疲劳S－N曲线和试验的结果较为吻合,本文的S－N曲线断裂力学预测方法对确定结构全寿命具有实际指导意义,同时也揭示了发展疲劳断裂统一理论的可行性。     

Principal components, covariance matrix tapers, and the subspaceleakage problem

A new class of robust space-time adaptive beamforming techniques is introduced to address a broad range of subspace leakage phenomena that arise in many sensor array applications. When present, these leakage phenomena can significantly increase the effective rank of the dominant colored noise interference spectrum, thereby reducing the appeal of techniques that exploit low-rank dominant interference (such as principal components (PC) or diagonal loading) to reduce sample support (training) requirements. By combining the covariance matrix taper (CMT) approach with either PC or diagonal loading, the minimal sample support properties of these techniques can be preserved     

金属零件的一种快速成型制造方法

金属零件的快速制造是快速成型技术的最终目标之一,也是当今快速成型领域的一大研究热点。本文在介绍快速成型技术的一个重要分支－选区激光烧结的基础上,提出了一种基于小功率激光快速成型系统的金属零件成型工艺。这种方法首先烧结金属与有机粘结剂的混合粉末,生成“绿件”,而后除粘、熔渗金属,最终得到致密的金属件。最后,文中给出了加工实例,并对这种工艺的影响因素进行了详细的分析与讨论。     

组合式三相逆变器负载特性的分析与研究

分析了组合式三相逆变器，该逆变器三相分别独立控制，易实现功率器件的软开关、无输出变压器的三相四线制输出和模块化的结构；具体分析了影响该三相逆变器输出电压不平衡度的因素，讨论了其带对称与不对称负载的特性，着重分析了不对称负载对每个单相逆变器的影响，得到了组合式三相逆变器中单模块设计与普通单相逆变器的不同点，并给出了实验数据和相应波形。实验结果表明，组合式三相逆变器具有良好的带不平衡负载的能力和优良的电气性能。     

An experimental study of flame turbulence

An experimental study of the turbulence generated by an enclosed, premixed, propane-air flame has been carried out in a combustion chamber of 25 × 20 cm cross section. Care was taken to reduce any effects of the axial pressure gradient. By suitable changes in the grid geometry, the turbulence intensity and scale of the approach flow were varied independently. The results of these experiments show that a strong link exists between the mechanisms of turbulent flame propagation and flame-generated turbulence. Thus three distinct regions may be identified, each having different characteristics in regard to the effects of turbulence scale on flame-generated turbulence. For each region, a physical mechanism for flame-generated turbulence is proposed. In particular, it is observed that over a wide range of intensity and scale of the approach turbulence, (a) the relative turbulence intensity in the flame zone varies in the range 1–2 times the relative turbulence intensity of the cold flow, (b) in the region of intermediate turbulence levels ( ) the flame-generated turbulence intensity reaches a minimum value which is equal to the approach stream turbulence intensity, (c) the flame-generated turbulence intensity reaches a maximum value when the rate of production of turbulent vorticity is equal to about half its rate of viscous dissipation.     

具有无源控制空腔时正激波/湍流附面层干扰的数值模拟

采用雷诺平均N-S方程和B/L代数湍流模型计算了具有无源控制空腔时正激波/湍流附面层干扰流场。计算与实验结果的比较表明,本文方法可较准确地预测激波结构、激波与附面层干扰区流动基本特征及波后流动分离状态、激波位置、波前马赫数等参数。      

电火花成型机数控系统的研究

通过研究开放式数控系统的体系结构，结合开放式的运动控制器，开发了基于PC的四轴联动电火花数控系统，本研究通过采用神经模糊控制智能技术，并结合数据库，依据放电间隙状况对电火花成型加工过程进行实时控制，保证加工过程处于优化状态，文中还详细研究了高精度双闭环控制以实现工件的高精度加工。     

The organic matter of comet Halley as inferred by joint gas phase and solid phase analyses

During encounters with comet Halley, the experiment PICCA onboard GIOTTO measured the gas phase organic ion composition of the coma and the experiment PUMA onboard VEGA-1 measured the dust composition. Joining both results, we obtain a consistent picture of the parent organic matter from which dust and gas is produced. One recognizes a complex unsaturated polycondensate, which splits during coma-formation into the more refractory C=C,C-N-containing dust part, and the more volatile C=C,C-O-containing gas part. The responsible exothermal chemical reactions, which are triggered by the sunlight, may play a major role in the dynamics of coma formation.This paper is a shortened and upgraded version of Krueger, F.R., Korth, A., and Kissel, J.: 1989, in S. Chang (ed.) Proc. of the ROSETTA Conf., Milpitas CA, January 1989, submitted.