774 research outputs found
Cryogenic Moisture Uptake in Foam Insulation for Space Launch Vehicles
Rigid polyurethane foams and rigid polyisocyanurate foams (spray-on foam insulation), like those flown on Shuttle, Delta IV, and will be flown on Ares-I and Ares-V, can gain an extraordinary amount of water when under cryogenic conditions for several hours. These foams, when exposed for eight hours to launch pad environments on one side and cryogenic temperature on the other, increase their weight from 35 to 80 percent depending on the duration of weathering or aging. This effect translates into several thousand pounds of additional weight for space vehicles at lift-off. A new cryogenic moisture uptake apparatus was designed to determine the amount of water/ice taken into the specimen under actual-use propellant loading conditions. This experimental study included the measurement of the amount of moisture uptake within different foam materials. Results of testing using both aged specimens and weathered specimens are presented. To better understand cryogenic foam insulation performance, cryogenic moisture testing is shown to be essential. The implications for future launch vehicle thermal protection system design and flight performance are discussed
Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency
Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency <I>f<sub>H</sub></I> was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-static HF radio scatter on the London-Tromsø-St. Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was near and then above the critical frequency of the F2 layer by up to 0.4 MHz. An increase of the electron density up to 25% accompanied by strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation of striations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization
Measurements of Proton, Helium and Muon Spectra at Small Atmospheric Depths with the BESS Spectrometer
The cosmic-ray proton, helium, and muon spectra at small atmospheric depths
of 4.5 -- 28 g/cm^2 were precisely measured during the slow descending period
of the BESS-2001 balloon flight. The variation of atmospheric secondary
particle fluxes as a function of atmospheric depth provides fundamental
information to study hadronic interactions of the primary cosmic rays with the
atmosphere.Comment: 21 pages, 11 figures, 4 table
Analysis of the coronal green line profiles: evidence of excess blueshifts
The coronal green line (Fe XIV 5303 A) profiles were obtained from
Fabry-Perot interferometric observations of the solar corona during the total
solar eclipse of 21 June 2001 from Lusaka, Zambia. The instrumental width is
about 0.2 A and the spectral resolution is about 26000. About 300 line profiles
were obtained within a radial range of 1.0-1.5 R\odot and position angle
coverage of about 240\circ. The line profiles were fitted with single Gaussian
and their intensity, Doppler velocity, and line width have been obtained. Also
obtained are the centroids of the line profiles which give a measure of line
asymmetry. The histograms of Doppler velocity show excess blueshifts while the
centroids reveal a pre-dominant blue wing in the line profiles. It has been
found that the centroids and the Doppler velocities are highly correlated. This
points to the presence of multiple components in the line profiles with an
excess of blueshifted components. We have then obtained the(Blue-Red) wing
intensity which clearly reveals the second component, majority of which are
blueshifted ones. This confirms that the coronal green line profiles often
contain multicomponents with excess blueshifts which also depend on the solar
activity. The magnitude of the Doppler velocity of the secondary component is
in the range 20-40 km s-1 and they show an increase towards poles. The possible
explanations of the multicomponents could be the type II spicules which were
recently found to have important to the coronal heating or the nascent solar
wind flow, but the cause of the blue asymmetry in the coronal lines above the
limb remains unclear
Time Evolution in Superstring Field Theory on non-BPS brane.I. Rolling Tachyon and Energy-Momentum Conservation
We derive equations of motion for the tachyon field living on an unstable
non-BPS D-brane in the level truncated open cubic superstring field theory in
the first non-trivial approximation. We construct a special time dependent
solution to this equation which describes the rolling tachyon. It starts from
the perturbative vacuum and approaches one of stable vacua in infinite time. We
investigate conserved energy functional and show that its different parts
dominate in different stages of the evolution. We show that the pressure for
this solution has its minimum at zero time and goes to minus energy at infinite
time.Comment: 16 pages, 5 figures; minor correction
Vortices on Higher Genus Surfaces
We consider the topological interactions of vortices on general surfaces. If
the genus of the surface is greater than zero, the handles can carry magnetic
flux. The classical state of the vortices and the handles can be described by a
mapping from the fundamental group to the unbroken gauge group. The allowed
configurations must satisfy a relation induced by the fundamental group. Upon
quantization, the handles can carry ``Cheshire charge.'' The motion of the
vortices can be described by the braid group of the surface. How the motion of
the vortices affects the state is analyzed in detail.Comment: 28 pages with 10 figures; uses phyzzx and psfig; Caltech preprint
CALT-68-187
Multiâinstrument Observations of IonâNeutral Coupling in the Dayside Cusp
Using data from the Scanning Doppler Imager, the Super Dual Auroral Radar Network, the EISCAT Svalbard Radar and an auroral allâsky imager, we examine an instance of Fâregion neutral winds which have been influenced by the presence of poleward moving auroral forms near the dayside cusp region. We observe a reduction in the time taken for the ionâdrag force to reâorientate the neutrals into the direction of the convective plasma (on the order of minutes), compared to before the auroral activity began. Additionally, because the ionosphere near the cusp is influenced much more readily by changes in the solar wind via dayside reconnection, we observe the neutrals responding to an interplanetary magnetic field change within minutes of it occurring. This has implications on the rate that energy is deposited into the ionosphere via Joule heating, which we show to become dampened by the neutral winds
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