30,027 research outputs found
Integrated launch and emergency vehicle system
A heavy launch vehicle is discussed. The launch vehicle is comprised of an expendable, multi-container, propellant tank that has a plurality of winged booster propulsion modules at one end and a payload supported by adapter structure at the other end. The preferred payload is an entry module that can be adapted for docking to the space station and used as a return vehicle for the space station crew. Additionally, the payload may include communication satellites, supplies, equipment, and/or structural elements for the space station. The winged propulsion modules are released from the expendable propellant tank, in pairs, and they return to Earth in a controlled glide. After a safe landing, at or near the launch site, the modules are prepared for reuse. The rocket engines for each propulsion module are dual-fuel, dual-mode engines and use methane-oxygen and hydrogen-oxygen from the multi-containers of the propellant tank. When the propulsion modules are released from the expendable propellant tank, the rocket engines are moved into the module cargo bay for the return glide flight
Dual-fuel, dual-mode rocket engine
The invention relates to a dual fuel, dual mode rocket engine designed to improve the performance of earth-to-orbit vehicles. For any vehicle that operates from the earth's surface to earth orbit, it is advantageous to use two different fuels during its ascent. A high density impulse fuel, such as kerosene, is most efficient during the first half of the trajectory. A high specific impulse fuel, such as hydrogen, is most efficient during the second half of the trajectory. The invention allows both fuels to be used with a single rocket engine. It does so by adding a minimum number of state-of-the-art components to baseline single made rocket engines, and is therefore relatively easy to develop for near term applications. The novelty of this invention resides in the mixing of fuels before exhaust nozzle cooling. This allows all of the engine fuel to cool the exhaust nozzle, and allows the ratio of fuels used throughout the flight depend solely on performance requirements, not cooling requirements
How to squeeze the toothpaste back into the tube
We consider "bridges" for the simple exclusion process on Z, either symmetric
or asymmetric, in which particles jump to the right at rate p and to the left
at rate 1-p. The initial state O has all negative sites occupied and all
non-negative sites empty. We study the probability that the process is again in
state O at time t, and the behaviour of the process on [0,t] conditioned on
being in state O at time t. In the case p=1/2, we find that such a bridge
typically goes a distance of order t (in the sense of graph distance) from the
initial state. For the asymmetric systems, we note an interesting duality which
shows that bridges with parameters p and 1-p have the same distribution; the
maximal distance of the process from the original state behaves like c(p)log(t)
for some constant c(p) depending on p. (For p>1/2, the front particle therefore
travels much less far than the bridge of the corresponding random walk, even
though in the unconditioned process the path of the front particle dominates a
random walk.) We mention various further questions.Comment: 15 page
Stationary distributions of multi-type totally asymmetric exclusion processes
We consider totally asymmetric simple exclusion processes with n types of
particle and holes (-TASEPs) on and on the cycle . Angel recently gave an elegant construction of the stationary measures
for the 2-TASEP, based on a pair of independent product measures. We show that
Angel's construction can be interpreted in terms of the operation of a
discrete-time queueing server; the two product measures correspond to
the arrival and service processes of the queue. We extend this construction to
represent the stationary measures of an n-TASEP in terms of a system of queues
in tandem. The proof of stationarity involves a system of n 1-TASEPs, whose
evolutions are coupled but whose distributions at any fixed time are
independent. Using the queueing representation, we give quantitative results
for stationary probabilities of states of the n-TASEP on , and
simple proofs of various independence and regeneration properties for systems
on .Comment: Published at http://dx.doi.org/10.1214/009117906000000944 in the
Annals of Probability (http://www.imstat.org/aop/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Evaluation of Proposed Rocket Engines for Earth-to-Orbit Vehicles
The objective is to evaluate recently analyzed rocket engines for advanced Earth-to-orbit vehicles. The engines evaluated are full-flow staged combustion engines and split expander engines, both at mixture ratios at 6 and above with oxygen and hydrogen propellants. The vehicles considered are single-stage and two-stage fully reusable vehicles and the Space Shuttle with liquid rocket boosters. The results indicate that the split expander engine at a mixture ratio of about 7 is competitive with the full-flow staged combustion engine for all three vehicle concepts. A key factor in this result is the capability to increase the chamber pressure for the split expander as the mixture ratio is increased from 6 to 7
The Role of Osteocytes in Targeted Bone Remodeling: A Mathematical Model
Until recently many studies of bone remodeling at the cellular level have
focused on the behavior of mature osteoblasts and osteoclasts, and their
respective precursor cells, with the role of osteocytes and bone lining cells
left largely unexplored. This is particularly true with respect to the
mathematical modeling of bone remodeling. However, there is increasing evidence
that osteocytes play important roles in the cycle of targeted bone remodeling,
in serving as a significant source of RANKL to support osteoclastogenesis, and
in secreting the bone formation inhibitor sclerostin. Moreover, there is also
increasing interest in sclerostin, an osteocyte-secreted bone formation
inhibitor, and its role in regulating local response to changes in the bone
microenvironment. Here we develop a cell population model of bone remodeling
that includes the role of osteocytes, sclerostin, and allows for the
possibility of RANKL expression by osteocyte cell populations. This model
extends and complements many of the existing mathematical models for bone
remodeling but can be used to explore aspects of the process of bone remodeling
that were previously beyond the scope of prior modeling work. Through numerical
simulations we demonstrate that our model can be used to theoretically explore
many of the most recent experimental results for bone remodeling, and can be
utilized to assess the effects of novel bone-targeting agents on the bone
remodeling process
Two-stage reusable launch system utilizing a winged core vehicle and glideback boosters
A near-term technology launch system is described in which Space Shuttle main engines are used on a manned orbiter and also on twin strap-on unmanned boosters. The orbiter has a circular body and clipped delta wings. The twin strap-on boosters have a circular body and deployable oblique wings for a glideback recovery. The dry and gross weights of the system, capable of delivering 70klb of cargo to orbit, are compared with a similar system with hydrocarbon-fueled boosters and with the current Shuttle
Roughening and inclination of competition interfaces
The competition interface between two growing ``Young clusters'' (diagrams),
in a two-dimensional random cone, is mapped to the path of a second-class
particle in the one-dimensional totally asymmetric simple exclusion process.
Using the asymptotics of the second class particle and hydrodynamic limits for
the exclusion process (Burgers equation), we show that the behavior of the
competition interface depends on the angle of the cone: for angles in [180^o,
270^o) the competition interface has a deterministic inclination, while for
angles in [90^o,180^o) the inclination is random. We relate the competition
model to a model of random directed polymers, and obtain some partial results
for the fluctuations of the competition interface.Comment: 4 pages, 3 figure
Can Government Deal with Science?
An adaption of a speech delivered by Prof. Martin at the Sixth Life Sciences Symposium at the Los Alamos Scientific Laboratories in New Mexico. It was also published in the American Industrial Hygiene Journa
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