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RESEARCH AND DEVELOPMENT PROJECTS
Feasibility Study of Stratospheric
Airship As High Altitude Platform For High
Integrity Psuedolite Based Precision Navigation System
Principal
Investigator: Prof. Rajkumar S. Pant
Aerospace
Engineering Department
IIT Bombay
BACKGROUND
& INTRODUCTION:
While GPS is slowly becoming the default
system for guidance and navigation of terrestrial and airborne systems for
Civilian applications, serious questions are being raised about its
applicability for the same roles in strategic applications. This is due to
the fact that there are risks involved related to the integrity, accuracy and
availability of reliable GPS signals. This project aims to establish the
feasibility of a Psuedolite based High Integrity Precision Navigation
System.
The most suitable candidate for the Psuedolites in
such a system is a constellation of remotely-controlled airships. This is
because as a high altitude long-endurance platform, airships score over
other aerial vehicles (such as aircraft or helicopters) due to their
extremely low operating cost and ability of station-keeping in a small area
without massive fuel consumption.
The past few years have seen a resurgence of interest
in aerostats and airships, with technology developments such as new plastic
envelope materials that are strong, UV resistant and leak-proof to Helium,
which is now almost universally used instead of Hydrogen. Such hi-tech
airships have featured in high-profile attempts to circumnavigate the globe
(for e.g. the Breitling Orbiter). Several high
technology airship programs have been launched recently in industrially
advanced countries such as USA, UK, Germany, Japan, France, The
Netherlands, and Russia. Several other countries such as Korea, Taiwan,
Malaysia, Indonesia and Brazil have also set-up research laboratories and
R&D centres in this area. Studies have indicated that airships
are the vehicle of choice as a high-altitude long endurance platform for
provision of next-generation communications platform. Such airships are
normally called �Stratospheric Airships�, since they are
designed to operate at altitudes between 20 km and 22 km in the
stratosphere, where the atmospheric disturbances are known to be the least.
However, there are a number of open technical issues
associated with the design and development of Stratospheric Airships which
are being actively pursued by companies and research laboratories all over
the globe. These include platform station-keeping, hand-off considerations
even for fixed stations due to platform movement and payload power. These
technological challenges can be summarized as follows:
a)
Stratospheric environment and thermal condition
The atmospheric pressure at altitudes between 20 km
to 22 km is approximately 40 hPa in the middle latitudinal regions. Air
density at this altitude is about 1/20 of that at sea level; therefore, the
airship envelope needs to be large enough to yield necessary buoyancy. If
the atmospheric temperature and the buoyant gas temperatures fluctuate
drastically diurnally and annually, it will directly affect the buoyancy;
buoyant gas expands or contracts with temperature fluctuations. If thermal
variation is so large that the platform has to vent excessive helium gas at
high temperature conditions, then the platform will lose buoyancy during
sunset (lower temperatures) and possibly descend to the ground. In this
context, thermal analysis and thermal limitation are important to design
the vehicle and to determine the thresholds of structural capacity to cope
with these thermal fluctuations and to circumvent such an unexpected
operational abortion.
b)
Energy source for propulsion
Solar energy can be harvested continuously in daytime
in the relatively continuous �fair weather� stratosphere. However,
if night-time becomes longer, as in the Polar Regions in winter, the
platform requires ground-based wireless power transmission systems for
continuous thrust powering due to the lack of solar energy. In this case,
however, the platforms can be designed into much smaller sizes compared to
solar-powered platforms. Such platforms also suffer from the eclipse
problem (similar to satellite) with regards to payload power due to the use
of solar cells.
c)
Energy storage
Rapid progress in development of electrical automobile
batteries lends itself to carriage of on-board secondary batteries for
nocturnal propulsion, and there are good prospects that more energy
efficient and lighter batteries can be developed in the very near future.
d)
Propulsive efficiency
Propulsive efficiency is one of the most important
parameters affecting total vehicle weight. Rigid airships in the past have
had total volume drag coefficients of 0.022 - 0.023. It is assumed in this
study that an optimized laminar flow body equipped with an aft propulsor
achieves 0.020 as the total volume drag coefficient, considering recent
research on the optimized laminar flow body.
e) Aerodynamic design
The airship is a pressurized
envelope, i.e., the envelope skin is made of strong fabric that confines
gas expansion and prevents buoyancy fluctuations from the buoyant gas
temperature rise. The hull shape design is adopted from a study on minimum
drag hull shape optimization. The empennage sizes are determined by
existing airship data. To get maximum propulsive efficiency, an aft
propulsion three-bladed propeller is normally used.
f) Ground handling, launch
and recovery
Launch and recovery is perhaps the most difficult phase
of airship flight. One important factor is the real estate required, which
will depend on the size of the airship, its controllability and type of
launch. The number of airships required to be moored or hangared at any one
site would also affect the acreage. Many companies are planning large
fleets but the build, launch and recoveries will need to be scheduled to
optimize the ground area and manpower available. However, the wind profile
during the year may force all launch and recovery operations to be
conducted in a relatively short period. A restricted zone could be
established around the launch site to reduce airspace management issues but
this will still be an issue during transit and operation. Before starting
discussions on ground handing methods, the intrinsic characteristics of
stratospheric airships are to be identified. Thereafter, requirements for
ground handling have to be identified.
g)
Hull structure
Challenges related to technology developments to actually
construct gigantic pressurized thin fabric structures for these airship
hulls are to be precisely identified and overcome, especially; those
related to the technology of fabricating gas-tight ultra-light membrane
pressurized structures and of constructing the tail wing assemblies as
lightly as possible.
OBJECTIVE OF THE PROJECT:
The objective of this project
is to carry out conceptual design studies related to the stratospheric
airship which will act as a platform for a Psuedolite based High Integrity
Precision Navigation System.
SPECIFIC TASKS TO BE CARRIED
OUT IN THE PROJECT:
1. Literature Survey
and Critical Review of Stratospheric Airship Programs.
2. Requirements Capture
and Breakdown of the various tasks to be carried out.
3. Identification of
various resource agencies, regulatory bodies and industry
partners to realise the
airships in India.
4. Arriving at the
Baseline Requirements and Specifications of the airship.
5.
Deciding the roadmap to realise
the airships
PROJECT
COMMENCEMENT DATE: 6th July
2004
PROJECT
COMPLETION DATE: 26th
April 2005
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