4,052 research outputs found
Evolution of Earth-Lunar Transportation Systems
Space science - earth-lunar missions - high-energy propellants and nuclear propulsion systems in manned space fligh
The influence of lunar propellant production on the cost-effectiveness of cislunar transportation systems
It is well known that propellants produced at the points of destination such as the Moon or Mars will help the economy of space transportation, particularly if round trips with a crew are involved. The construction and operation of a lunar base shortly after the turn of the century is one of the space programs under serious consideration at the present time. Space transportation is one of the major cost drivers. With present technology, if expendable launchers were employed, the specific transportation costs of one-way cargo flights would be approximately 10,000 dollars/kg (1985) at life-cycle cumulative 100,000 ton payload to the lunar surface. A fully reusable space transportation system using lunar oxygen and Earth-produced liquid hydrogen (LH2) would reduce the specific transportation costs by one order of magnitude to less than 1000 dollars/kg at the same payload volume. Another case of primary interest is the delivery of construction material and consumables from the lunar surface to the assembly site of space solar power plants in geostationary orbit (GEO). If such a system were technically and economically feasible, a cumulative payload of about 1 million tons or more would be required. At this level a space freighter system could deliver this material from Earth for about 300 dollars/kg (1985) to GEO. A lunar space transportation system using lunar oxygen and a fuel mixture of 50 percent Al and 50 percent LH2 (that has to come from Earth) could reduce the specific transportation costs to less than half, approximately 150 dollars/kg. If only lunar oxygen were available, these costs would come down to 200 dollars/kg. This analysis indicates a sizable reduction of the transportation burden on this type of mission. It should not be overlooked, however, that there are several uncertainties in such calculations. It is quite difficult at this point to calculate the cost of lunar-produced O and/or Al. This will be a function of production rate and life-cycle length. In quoting any cost of this nature, it is very important to state the cumulative transportation volume, since this is a very sensitive parameter. Nevertheless, cost models must be developed now to understand fully the interdependencies of a large number of parameters and to provide the best possible data for planning purposes. Without such data, mission modes and vehicle designs or sizes cannot be selected intelligently
A Numerical Treatment of the Rf SQUID: I. General Properties and Noise Energy
We investigate the characteristics and noise performance of rf
Superconducting Quantum Interference Devices (SQUIDs) by solving the
corresponding Langevin equations numerically and optimizing the model
parameters with respect to noise energy. After introducing the basic concepts
of the numerical simulations, we give a detailed discussion of the performance
of the SQUID as a function of all relevant parameters. The best performance is
obtained in the crossover region between the dispersive and dissipative
regimes, characterized by an inductance parameter \beta_L'\equiv 2\pi L
I_0/\Phi_0\approx 1; L is the loop inductance, I_0 the critical current of the
Josephson junction, and \Phi_0 the flux quantum. In this regime, which is not
well explored by previous analytical approaches, the lowest (intrinsic) values
of noise energy are a factor of about 2 above previous estimates based on
analytical approaches. However, several other analytical predictions, such as
the inverse proportionality of the noise energy on the tank circuit quality
factor and the square of the coupling coefficient between the tank circuit and
the SQUID loop, could not be well reproduced. The optimized intrinsic noise
energy of the rf SQUID is superior to that of the dc SQUID at all temperatures.
Although for technologically achievable parameters this advantage shrinks,
particularly at low thermal fluctuation levels, we give an example for
realistic parameters that leads to a noise energy comparable to that of the dc
SQUID even in this regime.Comment: submitted to J. Low Temp. Phy
3-junction SQUID rocking ratchet
We investigate 3-junction SQUIDs which show voltage rectification if biased
with an ac current drive with zero mean value. The Josephson phase across the
SQUID experiences an effective ratchet potential, and the device acts as an
efficient rocking ratchet, as demonstrated experimentally for adiabatic and
nonadiabatic drive frequencies. For high-frequency drives the rectified voltage
is quantized due to synchronization of the phase dynamics with the external
drive. The experimental data are in excellent agreement with numerical
simulations including thermal fluctuations.Comment: 5 pages, 4 figures -- Fig.4 revise
Microscopic Analysis of Low-Frequency Flux Noise in YBaCuO Direct Current Superconducting Quantum Interference Devices
We use low-temperature scanning electron microscopy combined with SQUID
detection of magnetic flux to image vortices and to investigate low-frequency
flux noise in YBaCuO thin film SQUIDs. The low-frequency flux noise
shows a nonlinear increase with magnetic cooling field up to 60 T. This
effect is explained by the surface potential barrier at the SQUID hole. By
correlating flux noise data with the spatial distribution of vortices, we
obtain information on spatial fluctuations of vortices on a microscopic scale,
e.g. an average vortex hopping length of approximately 10 nm.Comment: submitted to Applied Physics Letter
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