The
Space Shuttle uses liquid hydrogen (LH2) and liquid oxygen (LOX) as
propellants for its main engines. To maximize the amount of propellant
in the Space Shuttle’s External Tank (ET), these propellants
are stored in liquid form. To prevent boiloff of these commodities,
the Space Shuttle’s Main Propulsion, LH2, and LOX systems on
the ground and in the vehicle need to be kept at very low temperatures,
insulated from the external environment. Insulation is accomplished
using special thermal insulating materials and vacuum lines.
The liquid propellants are stored at the pad’s hydrogen and oxygen storage
tanks until fueling of the Space Shuttle starts approximately 10 hours prior
to launch. At that time, they are pumped to the ET using vacuum-jacketed (VJ)
lines. These transfer lines are several hundred feet long, and they reside
at the pad area and Mobile Launcher Platform (MLP). A loss of vacuum in the
VJ lines will cause the propellant to boil off, a condition that is not desirable.
Thermal vacuum gages are used to measure the conditions of these transfer lines.
These gages are permanently installed in the VJ lines. United Space Alliance
(USA) personnel perform periodic checks of the vacuums in these lines.
When loss of vacuum is detected, the line needs to be pumped back to the
desired level. Periodically, an operator manually measures the vacuum in
these lines. A signal-conditioned meter is connected to each of the vacuum
gages, and a reading is obtained and recorded. Since tens of these gages
are located throughout the pad and MLP, this is a very labor-intensive
operation.
The designed system consists of a network of sensors (remote units), located
on the VJ cross-country lines and in and around the MLP VJ lines. This
sensor network connects to the existing thermal vacuum gages permanently
installed in the VJ lines. Each remote station provides current excitation
to the vacuum gage, signal conditioning of the signal coming from the vacuum
gage, and wireless transmission to a central (base) station. The sensor
network utilizes the benefits of wireless communication, and batteries
or solar energy powers it. The designed system provides the following characteristics:
- Signal conditioning
and data acquisition capability in situ to each VJ line sensor.
- Measurement trending
analysis capability.
- Capability to
independently monitor the vacuum in the line and notify user if preset
limits are exceeded.
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Data relay capability
for inaccessible areas, allowing remote read capability (e.g., in trenches,
under gratings on the pad surface, and in tunnels on the MLP).
Requirements of the Space Shuttle program prompted the implementation of several
innovative ideas. First, there is a requirement to maintain electromagnetic
compatibility (EMC) and to keep radio frequency (RF) interferences to a
minimum at the pad. To do that, RF output power was kept to a minimum (10
milliwatts). Since data availability was also a requirement, an innovative
software (Lost Station Algorithm) approach was created to allow alternate
communication routes for the sensors (embedded intelligence in the sensors)
in case of interferences or loss of communication. In addition, capability
to use remote stations as data relay stations was necessary because of
the long distances to be covered. Finally, an innovative power management
algorithm was created to support operations at the pad for 2 years before
replacing batteries.
Each remote unit of the Wireless Sensors Network consists of a transducer module
(providing excitation and signal conditioning), a controller, radio transceiver,
antenna, power supply (battery-powered), and a weather-protective enclosure.
All measurements are transmitted back to a central (base) station and then
sent to the central data-gathering equipment or processed through a host
computer.
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Remote-Station Wireless Module
The
central (base) station runs the polling algorithm that acquires, stores,
analyzes, and distributes the VJ information to the users. The “Master
Configuration Window” display controls all remote station selection,
configuration, data summary, and status information. When a remote station
is selected, the “Remote Station Window” display opens to
allow configuration of data scaling/engineering unit conversion and sample
rate. A list of active remote stations is contained in the “ID
Data Units Window,” along with actual data received and the applicable
engineering units. The “Summary Window” display allows for
a summarized assessment of the number of active remote stations (e.g.,
7 remote stations active). The “Status” window provides a
summarized assessment of the condition of the system and remote station
communication (e.g., 4 lost stations exist, 2 stations not responding).
Remote-Station Test Stand
Key accomplishments:
- Designed and tested
RF transceiver module.
- Designed and tested
analog module.
- Developed and tested
software for remote and base stations.
Key milestone:
- Development of
a field-grade 10-station network for the pad.
Contacts: J.M. Perotti (Jose.Perotti-1@ksc.nasa.gov),
YA-D5-E, (321) 867-6746; A.R. Lucena, YA-D5-E, (321) 867-6743; and
R.J. Beil, PH-G1, (321) 861-3944
Participating Organizations: Boeing (L. Fineberg) and Dynacs Inc. (Dr. C.T.
Mata, B.M. Burns, A.J. Eckhoff, N.N. Blalock, and J.J. Randazzo
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