Feasibility Study of Stratospheric Airship  As High Altitude Platform For High Integrity Psuedolite Based Precision Navigation System

Principal Investigator: Prof. R. S. Pant

Aerospace Engineering Department

IIT Bombay  

 

OBJECTIVE :

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.

                                   

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.

 

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 realize the airships in India.

4. Arriving at the Baseline Requirements and Specifications of the airship.

5. Deciding the roadmap to realize the airships

 

PROJECT COMMENCEMENT DATE: 6th July 2004

PROJECT COMPLETION DATE: 26th April 2005

PROGRAM ON AIRSHIP DESIGN AND DEVELOPMENT

PADD

Research And Development Projects