Distributed multimedia applications require a variety of communication services. These services and different application requirements have to be provided and supported within (1) end-systems in an efficient and integrated manner, combining the precise specification of Quality-of-Service (QoS) requirements, application interfaces, multicast support, and security features, and within (2) the network. The Da CaPo++ system presented here provides an efficient end-system middleware for multimedia applications, capable of handling various types of applications in a modular fashion. Application needs and communication demands are specified by values in terms of QoS attributes and functional properties, such as encryption requirements or multicast support. Da CaPo++ automatically configures suitable communication protocols, provides for an efficient run-time support, and offers an easy-to-use, object-oriented application programming interface. While its applicability to real-life applications was shown by prototype implementations, performance evaluations have been carried out yielding practical experiences and numerical results.

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The d esign a nd co nstru ction of a lar ge-scale hydr ogen liqu efi er by t he N a tion al Bureau of Standards r eq uired a concurre n t developmen t of large, transpor table liq uid-hydrogen con tainers. These co n tainers, known as D ewar vessels after t he in ven tor, J a mes D ewar, ha d to be capa ble of road a nd air t ranspor t, whi ch implied ruggedness a nd ligh t weig ht . This ar t icle reports o n lar ge-scale liquid-h ydrogen D ewar development condu cted by t he N BS Cr yogenic E ngineerin g L a bor atory. A 440-li ter a nd a n 840-li ter D ewar a re discussed. E ver y effort has been made to design D ewars wi t h ver y low hydrogen-l oss rates within t he limi tations of t he t ransp or t r equire men t . Dewar ConfigurationThe op timum D ewar configuration is spherical, because the sphere has the smalles t area-to-volume ratio of any shape. I n practice, however, such Insulation MethodsOnly two m ethods of D ewar insulation appeared feasible a t the time of the design : the use of powder insulation, su ch as silica aerogel, at a moder ate vacuum, and th e use of an unfilled sp ace at a high vacuum . Although the moder a te vacuum associa ted with powder insulation brings m any advant agessuch as r educing th e impor tance of ver y minute leaks and making th e achievem en t of a static vacuum easier-the insula tion value when used in moder ate thickn ess does not appear to b e as gr ea t as that of an unfilled space a t a high vacuum (up to abou t 0.01-}.L H g pressure) with surfaces having a high refl ectivity (low emissivity). As one of the primary goals was the achievem ent of a low hy drogen loss withou t making the D ewar too bulky, high va cuum r ather than evacuated powder was chosen for t he D ewar insula tion.With high-vacuum insulation, h eat energy flows to th e inner surfaces by three m ech anisms : th ermal radiation, r esidual gas conduction, and solid conduction through piping and insulating suppor ts.Withou t any shielding, thermal r adiation from room temper ature directly to the liquid-hydrogen con tain er would b e excessive and would m ak e it difficul t to develop a low-loss hydrogen D ewar. I n order to r edu ce this thermal r adiation, a liquidni trogen-cooled r adiation shield was planned. This shi eld completely surrounds the liquid-hydrogen 1 Work SUI)ported by the U. S. A tomie E nergy Commission an d t he U. S. Air F orce., Now with Beech Aircraft COrl). a J<' igures in brackets ind icate the li terature references at t he end of t his paper.container and intercepts all ambien t temperat ure radiation from th e outer shell. Thermal radiation to t he liquid-hy drogen container is th en r educed by a factor of at least 200 and b ecomes of the same order of magnitude as the r esidual gas conduction and the solid conduction . Wh en considering hea t t ransfer to the ni trogencooled shield, r esidual gas condu ction for a vac uum of th e order of 0.01-}.L H g is relatively unimportan t compared with thermal r adiation . A low h eat leak to th e nitrogen-cooled shield th er...

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