External Aerodynamics: Lift of airship created only by buoyancy which doesn t need lift generating surface like an airfoil or a wing

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1 5.1 AERODYNAMICS: The HAA aerodynamic regime could broadly be categorized into External and Internal Aerodynamics. The External Aerodynamics deals with the Shape of airship and the internal aerodynamics deals with the Gas inside the airship. External Aerodynamics: Lift of airship created only by buoyancy which doesn t need lift generating surface like an airfoil or a wing It can stay afloat without moving and maintain geostationary position Drag on airship determines the power required for movement and station-keeping An important factor to consider is the radiant heat transfer between the airship and the outside atmosphere due to exposure to direct sunlight Conventional ellipsoidal shapes have been extensively studied in terms of aerodynamic forces. They show good drag characteristics; considerable experience exists with ellipsoidal shapes and their optimization to minimize the aerodynamic drag Fig 5.1: Lockheed Martin HAA Fig 5.2: BERKUT Airship 5.1.1Lifting Airship: Alternative is to design a 'lifting' airship. This generates aerodynamic lift, reducing buoyant lift and airship volume required. The design and analysis of such envelopes requires sophisticated analysis techniques 106

2 During wind surges, the aerodynamic lift causes the vehicle to climb above its station. The additional altitude is used to accelerate the vehicle in a dive and return to its station. This reduces the propulsion requirement Fig 5.3: Nautlilus Fig 5.4: Skycat 1000 E.g. The Skycat 1000, designed by Skycat Technologies is airfoil shaped and meant for heavy loads at low altitudes; at high altitudes, the large volume would provide lift in the rarefied atmosphere and the flat shape on top could be used for solar panels that would see direct sunlight. 107

3 Fig 5.5: Starlight Advantages of lifting airship: reduction in volume of lifting gas lower propulsive power requirements higher maneuverability of the airship Challenges of lifting airship: Use of sophisticated analysis techniques Problems in manufacturing of the envelope Operational considerations in inflating, launching etc Internal Aerodynamics: The helium in an airship expands in inverse proportion to air pressure as it climbs. A HAA needs only about 5-10% hull volume of helium at sea-level, with the remainder of the volume filled with air. The airship will usually be constructed with multiple ballonets/cells installed inside. When the airship pitches, rocks or rolls, these ballonets can deform/collapse and have fluid sloshing inside them. There will also be gas flow within the airship due to heating and cooling. This internal flow will be very complex; heating and cooling of the external surface can change rapidly depending on orientation with respect to sunlight direction 108

4 5.1.3 Simulation and Testing: There is an extensive role for modeling and simulation to predict airship performance. These simulation tools will require extensive experimental and flight data for validation. Thermal Desktop software developed for modeling temperature of film and gas within. Some experience in the country using high fidelity Computational Fluid Dynamics codes for analysis of external flow past airships. NAVAJO advanced balloon performance and analysis tool developed by Global Aerospace Corporation - validated by comparison to historical balloon flights Also have some experience in modeling sloshing flows. Some experience in the country using high fidelity Computational Fluid Dynamics codes for analysis of external flow past airships. Some experience in the country using high fidelity Computational Fluid Dynamics codes for analysis of external flow past airships. The NAL code RANS3D was successfully used to compute the flow past a rigid aerostat balloon with fins which was developed by ADRDE Agra. While there is some experience in analyzing rigid aerostat, an HAA would essentially be a flexible vehicle hence aero elasticity effects need to taken into account. Tensys limited has analysis package available for simulating structures made of fabric and the corresponding large deformations. For validation of rarefied gas flows computations, IIT Madras has some experimental facilities in the form of a small low-density tunnel, which can simulate 130 to 150 kms altitude. IIT Bombay has significant experience in shape optimization of airships NAL has some experience in modeling sloshing flows. Flow simulation for aerostat was done at NAL. Brief Conclusions: There is sufficient experimental capability within India in terms of both expertise and facilities in academic institutions like IISc and the IITs, as well as in organizations like NAL, VSSC, DRDO, HAL etc. Technology development program for the HAA needs to be initiated at the national level involving multiple organizations. This would involve development of design and analysis tools along with extensive experimental and flight data generation 109

5 5.1.4 Envelope sizing and optimization: Airship Sizing: In terms of sizing, for an operating altitude of 20km, some estimates based on the payload are given below: Organization Length Max Dia Payload Volume Lockheed Martin HAA 150 m 46m 1800 kg m3 Lockheed Martin HALE 73 m 21 m 20 kg m3 StratSat HAA 200 m 48 m 1000 kg m3 Berkut ET (0-30 la) 150 m 50 m 1200 kg m3 Berkut ML (30-45 lat) 200 m 50 m 1200 kg m3 Berkut HL (45-60 lat) 250 m 50 m 1200 kg m3 JAXA, Japan 245 m 61 m 1000 kg 480,000 m3 KARI, Korea 50 m 12.5 m 100 kg 4091 m3 Table 5.2: Estimate of dimension based on payload On the basis of these estimates, a required volume of about 500,000 m3 to carry a payload of 2000kg appears reasonable. Although the final shape of the airship will be decided based on aerodynamic shape optimization, for the initial estimates of size the shape can be estimated to be a prolate ellipsoid 110

6 of revolution. Using the standard formula (shown below), the calculated volume of an airship in the shape of a prolate ellipsoid and with the dimensions of the Lockheed-Martin HAA is 196,350 m3 which is quite similar to the reported volume. (Volume of prolate ellipsoid of revolution = 4π a b c/3, where a, b, c are the three semi-axes) A possible low drag shape for the airship is the G N V Rao designed shape that is a combination of an ellipse, circle and parabola. The final shape will, of course, be determined using multi-disciplinary optimization, based on the operational requirements at stratospheric altitudes. Fig 5.6: G N V Rao shape for airship 111

7 An airship of length 240m and max. dia 60m will have a volume of about 450,000 m3 and this volume will increase to about 470,000 m3 if the length is increased to 250m. This is estimated to be the maximum size of the airship that should be considered for the preliminary design study. For the technology demonstrator, an airship of length 70m and max. dia. 20m will have a volume of about 15,000 m3. This should be able to carry a small payload of about 20kg to high altitude Design Issues & Requirements Capture: Based on literature survey, following design drivers in stratospheric airship design and development were identified as listed below: 1. Stratospheric Atmospheric Conditions 2. Envelope Configuration 3. Aerodynamic considerations 4. Onboard power Systems 5. Vehicle Launch and Recovery Processes 6. Thermal and night cooling compensation 7. Lightening & Electrostatic Charge protection 8. Navigation/Positioning Service Using Airship-Based GPS Augmentation System Requirements Capture: Literature review and communications with NAL, Bangalore, following preliminary requirements for the airship platform are captured. 1. Operating altitude of 19 to 21 km 2. Payload of 50 kg 3. Onboard power requirement of 1 KW 4. Ambient winds of m/s or as prevailing at the said operating altitude 5. ISA + 20 atmospheres 6. Discharging time can vary between 8-15 hours 7. Average Solar Irradiance for India to be taken into account 112

8 Another case study for payload of 2000 kg and onboard power requirement of 25 KW was also to be carried out. Various envelope geometry profiles, like GNVR, SAC, TCOM, Optimum, NPL and Oblate Spheroid (or Lenticular) are incorporated in the methodology Key technologies for Indigenous Technology Demonstration: Development, procurement of critical technologies, ground tests and flight tests are crucial steps towards realization of the technology demonstrator of prototype airship. From above mentioned program strategies, the technology required for indigenous development of individual component for realizing such airship is listed below. Advanced high strength low weight materials for envelope to counter temperature, pressure, and ultra-violet radiations, development of Manufacturing techniques for airship envelope fabrication Flexible efficient mono-crystal solar cells Regenerative fuel cells with energy management systems Sensor package, Integrated electrical power systems Customized Flight management system for unmanned airship for safe flight operations with buoyancy control Thermal management of a Helium inflated hull. 113