摘要:

The structural dependence of Cs adsorption on the &agr;‐Al2O3(0001) surface has been studied using a combination of surface analytical techniques. The unreconstructed or (1×1) surface shows a high initial Cs adsorption probability of 0.9 based on reflection mass spectrometry (RMS). This value decreases rapidly after a critical Cs coverage of 1.5×1014atoms/cm2is reached. Thermally‐induced surface reconstruction to form an O deficient surface results in a decrease in the critical coverage, probability and capacity for Cs adsorption. Low energy electron diffraction (LEED) demonstrates that a predominantly (3&sqrt;3×&sqrt;3)R30° surface yields an initial adsorption probability of 0.5 while a predominantly (&sqrt;31×&sqrt;31)R±9° yields a value of 0.3. Thermal desorption mass spectrometry (TDMS) shows that surface reconstruction suppresses the high binding energy states of Cs, consistent with the observed changes in adsorption probability. X‐ray photoelectron spectroscopy (XPS) provides no direct evidence for formal oxidative/reductive chemistry taking place at the interface. We interpret the facile adsorption and strong binding of Cs on sapphire to result from Cs interacting with coordinatively unsaturated O. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47177

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A semi‐mechanistic phenomenological model was developed to predict the high temperature creep properties of single crystalline binary tungsten alloys for thermionic applications. This new model is based on the limited theoretical and experimental results presently available in the literature for both polycrystalline and single crystalline tungsten alloys. It provides a better understanding on the creep deformation of single crystalline tungsten alloys and helps guide the development of this type of material for long‐term high‐temperature applications. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47179

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Various configurations of high‐voltage metal‐ceramic assembly of thermionic NPP are considered. The results of experimental investigations of metal‐ceramic assemblies are presented. Their characteristics of high serviceability and electric insulation which meet requirements of thermionic reactor‐converter with output voltage 100 V and more are shown. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47180

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This paper investigates means for achieving human expeditions to Mars utilizing existing or near‐term technology. Both mission plans described here, Mars Direct and Semi‐Direct are accomplished with tandem direct launches of payloads to Mars using the upper stages of the heavy lift booster used to lift the payloads to orbit. No on‐orbit assembly of large interplanetary spacecraft is required.In situ‐propellant production of CH4/O2and H2O on the Martian surface is used to reduce return propellant and surface consumable requirements, and thus total mission mass and cost. Chemical combustion powered ground vehicles are employed to afford the surface mission with the high degree of mobility required for an effective exploration program. Data is presented showing why medium‐energy conjunction class trajectories are optimal for piloted missions, and mission analysis is given showing what technologies are optimal for each of the missions primary maneuvers. The proposed surface systems payload manifest is presented, and mission back‐up plans are described. An end to end point design for the Semi‐Direct mission using either the Russian Energia B or an augmented U.S. Saturn V launch vehicle is presented and options for further evolution of the point design are discussed. It is concluded that both the Mars Direct and Semi‐Direct plans offer viable options for robuts piloted Mars missions employing near‐term technology. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47181

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An improved design of the Hall type stationary plasma thruster has been tested in 1994. The test results are presented. The test measures performance, EMI and beam divergence of two models of thrusters from the Russian Keldysh Scientific‐Research Institute of Thermal Processes. The first of these engines, T‐100 produces 80 mN thruster with power of 1.35 kWe. The other thruster, T‐160 is larger and produces 280 nM thrust with 4.5 kWe. Endurance testing of the T‐100 for 2000 hours was completed at NIITP. Post operation wear measurements indicate that the insulator life expectency will exceed the 8000 hour design life objective. Improved efficiencies of 48 to 52% were measured for the T‐100 and 58‐62% (with elevated tank pressure) for the T‐160 at specific impulse Isp of 1600 seconds and 2000 seconds respectively. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47182

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The high specific impulse (Isp) and engine thrust‐to‐weight ratio of liquid hydrogen (LH2)‐cooled nuclear thermal rocket (NTR) engines makes them ideal for upper stage applications to difficult robotic planetary science missions. A small 15 thousand pound force (klbf) NTR engine using a uranium‐niobium ‘‘ternary carbide’’ fuel (Isp ∼960 seconds at ∼3025 K) developed in the Commonwealth of Independent States (CIS) is examined and its use on an expendable injection stage is shown to provide major increases in payload delivered to the outer planets (Saturn, Uranus, Neptune and Pluto). Using a single ‘‘Titan IV‐class’’ launch vehicle, with a lift capability to low earth orbit (LEO) of ∼20 metric tons (t), an expendable NTR upper stage can inject two Pluto ‘‘Fast Flyby’’ spacecraft (PFF/SC) plus support equipment—combined mass of ∼508 kg—on high energy, ‘‘6.5–9.2 year’’ direct trajectory missions to Pluto. A conventional chemical propulsion mission would use a liquid oxygen (LOX/LH2‘‘Centaur’’ upper stage and two solid rocket ‘‘kick motors’’ to inject a single PFF/SC on the same Titan IV launch vehicle.For follow on Pluto missions, the NTR injection stage would utilize a Jupiter ‘‘gravity assist’’ (JGA) maneuver to launch a LOX/liquid methane (CH4) capture stage (Isp ∼375 seconds) and a Pluto ‘‘orbiter’’ spacecraft weighting between ∼167–312 kg—with chemical propulsion, a Pluto orbiter missin is not a viable option because of inadequate delivered mass. Using a ‘‘standardized’’ NTR injection stage and the same single Titan IV launch scenario, ‘‘direct flight’’ (no gravity assist) orbiter missions to Saturn, Uranus and Neptune are also enabled with transit times of 2.3, 6.6, and 12.6 years, respectively. Injected mass includes a storable, nitrogen tetroxide/monomethyl hydrazine (N2O4/MMH) capture stage (Isp ∼330 seconds and orbiter payloads 340 to 820% larger than that achievable using a LOX/LH2‐fueled injection stage. The paper discusses NTR technology and mission characteristics, shows NTR stage and payload accommodations within the 26.2 m long Titan IV payload fairing, and discusses NTR stage performance as a function of assumed cryogenic tank technology. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47183

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The results of an analysis into the thermal absorption effects in the silica wall of a closed‐cycle gas core nuclear rocket are presented. Emphasis is placed on determining the fraction of radiation heat flux emitted from the fuel and lying below the Transmission Cutoff of the silica wall. A mean wall temperature is obtained from the correlation for the absorbed heat flux. Three different silica compounds are analyzed: industrial glass, fused silica, and SuprasilTM300. For fuel emitting temperatures greater than 50,000 K, more than 90% of the radiation heat flux is absorbed in the wall. Corresponding wall coolant mass flow rates are in excess of 500 kg/sec per m2of wall surface area. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47237

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A technically viable option for low‐cost minimum‐fuel Lunar transfers with short trip times is the use of nuclear thermal rockets. However, little work has been done on the effects the associated thrust transients have on these optimal trajectories. The nominal thrust level of an engine is not immediately reached when the rocket is turned ‘‘on.’’ Similarly, when the engine is turned ‘‘off’’, the thrust and specific impulse levels decrease over a period of time which is directly related to both the flow effecs of the engine and cooling requirements. This paper presents an analysis of these effects on a typical optimal Lunar transfer. Several different models simulating the transient effects are used. They range from simple ‘‘mass dumps’’ to account for the extra required propellant to curve‐fits of actual engine characteristics obtained from the NERVA nuclear rocket program. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47184

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In the report the principles of nuclear power and propulsion complex (NPPC) construction are presented. NPPC considered can ensure the time of spacecraft transfer to geostationary orbit (GSO) within 1 year under electric power level no less than 40 kW at prolonged nominal mode on GSO for spacecraft loads feeding. The main power and mass and dimension performances of such NPPC are summarized. Analysis of relationship between spacedraft mass, its mission payload and transfer time also and a number of main NPPC parameters is performed. The conclusion is made about considerable promises of the NPPC for creating future satellite multi‐purpose systems by using PROTON‐ and TITAN‐class launch vehicles.

ISSN:0094-243X    

DOI:10.1063/1.47185

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Previously, detailed analysis was conducted to assess the technology developed for the Advanced Thermionic Initiative reactor. This analysis included the development of an overall system design code capability and the improvement of analytical models necessary for the assessment of the use of single cell thermionic fuel elements in a low power space nuclear reactor. The present analysis extends this effort to assess the nuclear criticality safety of the ATI reactor for various different scenarios. The analysis discusses the efficacy of different methods of reactor control such as control rods, and control drums. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47187

出版商:AIP    

年代:1995

数据来源: AIP