Flow in space
Automation, control, instruments all play part in testing aboard space station
By Jim Strothman
With the goal of understand the dynamics of fluid flow in the absence of gravitational forces, a Fluid Science Laboratory is now part of the International Space Station (ISS) orbiting earth 200 miles above earth.
Riding aboard the European-built Columbus space laboratory, which blasted off 7 February aboard NASA’s Space Shuttle Atlantis and then attached to the ISS days later, the Fluid Science Laboratory shares precious space in Columbus with a host of other experiments.
Scientists said they believe the Fluid Science Laboratory’s experiments, which aim to understand and control fluid flow in a spherical geometry under the influence of rotation, also will be useful in a variety of engineering applications, such as improving spherical gyroscopes and bearings, and centrifugal pumps.
Named after the famous explorer from Genoa, the Columbus laboratory is the major cornerstone of the European Space Agency’s (ESA) contributions to the ISS.
Multiple lab facilities
The 23-foot-long Columbus laboratory consists of a pressurized cylindrical-shaped cabin 115 feet in diameter, closed with welded end cones. It has a mass of 10.3 tons and an internal volume of 98 cubic yards.
The Columbus Control Center (Col-CC) in Oberpfaffenhofen, Germany, on the premises of the DLR’s German Space Operations Center, is responsible for control and operation of the Columbus laboratory.
All the European payloads on Columbus will transfer data directly to Col-CC. Relevant data will go from Col-CC to the different user support and operations centers across Europe responsible for either complete facilities, subsystems of facilities, or individual experiments.
Fluid Science Laboratory
The Fluid Science Laboratory is a multi-user facility designed to study the dynamics of fluids in the absence of gravitational forces.
Under weightless conditions, such forces are almost entirely eliminated, resulting in significant reductions in gravity-driven convection, sedimentation, stratification, and fluid static pressure. This allows the study of fluid dynamic effects normally masked by gravity.
The first fluids experiment to take place is Geoflow, which will be carefully studied by scientists researching areas such as flow in the atmosphere, the oceans, and the movement of Earth’s mantle on a global scale as well as other astrophysical and geophysical problems having spherical geometry flows shaped by rotation and convection.
Other experiments include the heat and mass transfer from free surfaces in binary liquids; a study of emulsion stability; studies of electric fields on the boiling process; and a study to improve the processing of peritectic alloys.
The Geoflow experiment will investigate the flow of an incompressible viscous fluid (silicone oil) held between two concentric spheres. A central force field comes together after applying a high voltage difference between the two spheres. Maintaining the inner sphere at a higher temperature to the outer sphere also creates a temperature gradient from inside to outside.
Researchers said the geometrical configuration can be seen as a representation of the Earth, where the role of gravity ends up being the central electric field. These experiments require a weightless environment in order to turn off the unidirectional effect of gravity on Earth.
Researchers will observe thermal convection between the two spheres, measuring the temperature distribution with the spheres revolving around a common axis at low, medium, and high rotation rates and also while stationary. In the case of a high rotation rate, high centrifugal effects are expected.
Measurement of the temperature distribution will occur using Wollaston Shearing Interferometry, though additional optical diagnostics may also be used (Schlieren or shadowgraphy).
The science team said the study of centrifugal effects, which serve to simulate the central gravity field, would find applications in areas such as high-performance heat exchangers and in the study of electro-viscous phenomena.
It should also help researchers understand the motion of liquids in several ground-based industrial applications where injected ions are a source of charge.
The right side of the Fluid Science Laboratory contains functional subsystems for power distribution, environmental conditioning, and data processing and management. The core element on the laboratory’s left side consists of the Optical Diagnostics Module and Central Experiment Module, into which the experiment containers are inserted. The Optical Diagnostics Module houses equipment for visual, velocimetric, and interferometric observation, related control electronics, and the attachment points and interfaces for special front mounted cameras.
The Central Experiment Module comes in two parts. The first contains the suspension structure for the experiment containers, including all the functional interfaces and optical equipment. That structure can pull out from the rack to allow inserting and removing containers where they can integrate the experiments.
The second part contains all the diagnostic and illumination equipment, together with the control electronics to command and monitor the electromechanical and opto-mechanical components.
Automation plays a role in the Biolab facility aboard Columbus, which supports biological experiments on micro-organisms, cells, tissue cultures, small plants, and small invertebrates.
The major objective of performing life sciences experiments in space is to identify the role weightlessness plays at all levels of an organism, from the effects on a single cell up to a complex organism including humans.
The first experiment to take place in Biolab is investigating the effect of weightlessness on the growth of seeds. Its goal is to better understand the cellular mechanism, which impairs the immune functions and aggravates the radiation response under spaceflight conditions. NASA and ESA said the experiment is important in view of future, long-term human space missions. Further experiments will try to unravel the influence of gravity on cellular mechanisms, such as signal transduction and gene expression.
Biolab is divided physically and functionally into two sections—the automatic section on the left side of the rack, and the manual section in the right side of the rack. In the automatic section, known as the Core Unit, the facility performs all activities, after manual sample loading by the crew.
By implementing such a high level of automation, the demand on crew time drastically goes down. The manual section, in which the crew performs all activities, is mainly devoted to sample storage and specific experiment-handling tasks.
The main element of the Core Unit is a large incubator, a thermally controlled volume where experiments take place.
Inside the incubator are two centrifuges that can each hold up to six experiment containers, which contain the biological samples, and can be independently spun to generate artificial gravity in the range of 10-3g to 2g. This allows for simultaneous performance of 0g experiments with 1g reference experiments in the facility.
During processing of the experiment, the facility handling mechanism will transport samples to the facility’s diagnostic instrumentation where, through tele-operations, a scientist on the ground can actively participate in the preliminary in-situ analyses of the samples. The handling mechanism also provides transporting samples into the ambient and temperature-controlled automatic stowage units for preservation or for later analysis.
Apart from contributing to solar and steller physics, knowledge of the interaction between the solar energy flux and Earth’s atmosphere is of great importance for atmospheric modeling, atmospheric chemistry, and climatology.
The SOLAR experiment will study the sun with unprecedented accuracy across most of its spectral range. Currently scheduled to last two years, the experiment will be located on Columbus’ External Payload Facility zenith position (i.e., pointing away from the Earth). The SOLAR payload consists of three instruments complementing each other to allow measurements of the solar spectral irradiance throughout virtually the whole electromagnetic spectrum—from 17 nm to 100 µm—in which 99% of the solar energy is emitted.
The three complementary solar science instruments are:
SOVIM (SOlar Variable & Irradiance Monitor), which covers near-UV, visible and thermal regions of the spectrum (200 nm – 100 µm)
SOLSPEC (SOLar SPECctral Irradiance measurements), covering the 180 nm - 3,000 nm range
SOL-ACES (SOLar Auto-Calibrating Extreme UV/UV Spectrophotometers, measuring the EUV/UV spectral regime
SOVIM and SOLSPEC are upgraded versions of instruments that have already accomplished several space missions. SOL-ACES will test a new generation of atomic clock in space.
ABOUT THE AUTHOR
Jim Strothman is a freelance writer based in Florida. His e-mail is Jstrot@comcast.net.
Engineers troubleshoot engine cutoff sensor woes
Open circuits in the Shuttle Atlantis external tank’s external electrical feed through a connector caused of false readings in the Engine Cutoff (ECO) sensor system, which led to the December launch delays of the European-built Columbus space laboratory.
Prior to the 7 February lift off, NASA formed a combined troubleshooting team involving multiple NASA centers to find the root cause and develop plans to fix the sensor system. The ECO system is one of several electronic sensor systems that protect the shuttle’s main engines by triggering an engine shutdown in the event either liquid hydrogen or liquid oxygen fuel levels in the external tank run unexpectedly low.
The 6 December and 9 December anomalies were not the first time shuttle engineers and launch team encountered ECO system problems. Similar head-scratching faulty readings occurred prior to three other space shuttle launches in 2005 and 2006.
Reports low fuel levels
The ECO sensors operate much like the “gas low” warning light in an automobile. When the fuel level drops below a sensor, that sensor sends a message to the orbiter’s computer that it is dry.
The orbiter’s computers poll the ECO sensors about 8-12 seconds prior to its planned main engine cutoff, which occurs about 8.5 minutes after launch.
If two of the four ECOs indicate dry, which means the tank is almost empty, the space shuttle main engines will be immediately shut down. If the main engines shut down prior to normal operating time, it could affect whether or not the shuttle reaches the appropriate orbit. Other motors—namely the Orbital Maneuvering System engines—have the capability to make up for a slightly early main engine cutoff, NASA said, but not one that occurs very early.
In the history of the Space Shuttle Program, the liquid hydrogen ECO system has never initiated an engine shutdown. NASA’s own launch rules require three of four sensors working at launch time, although NASA said the shuttle has never lifted off without all four fully functioning.
Sensor system detailed
The ECO sensors in the liquid hydrogen section of the external tank include wiring, harnesses, a series of connectors, and point sensor box electronics in the orbiter. Sensors in the tank send electronic signals through wires to the point sensor box in the orbiter, which in turn transmits data signals to the orbiter’s onboard computer system. The sensor wires lead to a feed through connector in the side of the tank. External cables run up the external tank (ET) vertical strut to the liquid hydrogen ET/orbiter interface. The circuit then routes inside the orbiter to the point sensor electronics box.
Four ECO sensors in the liquid hydrogen tank mount on a single, shock-isolated carrier plate four feet from the bottom of the tank. Similar ECO sensors on the liquid oxygen side are in the main propulsion system liquid oxygen feedline inside the orbiter.
Once propellant loading begins on launch day, the liquid hydrogen ECO sensors will read ‘wet.’ To demonstrate if they are functioning properly, the sensors are tested during tanking operations. When the fast fill stage of tanking begins, NASA’s launch team sends an electronic command to force the sensors to read “dry.” This simulated “dry” command is held until just after NASA enters a built-in “T-9 minutes” hold (nine minutes on the countdown clock). At that time, the “dry” command is removed, and the sensors are monitored to assure they are reading “wet.”
The Space Shuttle Atlantis (STS-122) 6 December launch attempt was postponed after two of the four liquid hydrogen tank ECO sensors gave false readings while Atlantis’ external tank was being filled.
On 9 December, one of the four ECO sensors inside the liquid hydrogen section of the tank gave a false reading, causing another delay. NASA then modified its launch commit criteria to require all four sensors function properly.
After removing a layer of external tank foam insulation at the launch pad, the external plug and feed through connector were removed from the tank and shipped to Marshall Space Flight Center in Alabama to determine whether the failure could be recreated in a test facility using focused and limited nondestructive and destructive physical tests. The tests were configured to replicate tank chill down temperatures, loading pressures, and environmental conditions during the two launch attempts.
NASA engineers said all circuit anomalies experienced during testing were able to be repeated as seen during the two launch attempts and tanking test. Open circuits in the part that connects wires from the interior to the exterior of the liquid hydrogen tank, known as the feed-through plate, were identified as the culprit that caused false readings.
A modified connector was designed with the pins and sockets soldered together. A similar, but slightly redesigned, connector was to be used. Both the original and modified connector configurations were subjected to temperature, pressure, and vibration environments identical to those experienced during a shuttle launch.
The tests verified the adequacy of the new configuration, and the pesky ECO sensor system problem was finally solved.