AIR POWER DEVELOPMENT CENTRE BULLETIN

LIGHTER-THAN-AIR AND HYBRID AIRSHIPS

Contrary to the views expressed by Rear Admiral Melville, lighter-than-air (LTA) and hybrid airships have continued to be part of aviation even after a hundred years. In the field of aircraft design, the need for cost-effectiveness and increased persistence are becoming crucial drivers, leading to increased development pressures for LTA and hybrid airships. Their combination of heavy lift and vertical/short take-off and landing (V/STOL) capabilities is an attractive attribute for air mobility operations; while in the intelligence, surveillance and reconnaissance (ISR) role, the enhanced endurance of LTA platforms offer the prospect of providing geo-stationary, pseudo-satellite capabilities.

The use of stratospheric airships as pseudo-satellites was explored in a series of workshops in the US during 2013, with participants from NASA, academia and industry. The aim of this study was to explore the benefits of such high-altitude airships for scientific purposes. It was found that these airship concepts have considerable potential for cost-effective applications for earth and atmospheric sciences as well as for space observation platforms. Such long-endurance, high-altitude platforms would enable atmospheric measurements for monitoring carbon emissions and air quality over mega-cities with far

less atmospheric distortion than ground-based sensors. For such applications, airships offer the additional benefit that they would enable much easier upgrades of instrumentation and communication systems than would be the case for satellites. It was recognised, however, that these concepts are currently technologically immature, and a prize-driven challenge was recommended to spur the development of affordable high-altitude airships for these scientific purposes. The first challenge concept recommended was a million-dollar-class prize for demonstrating a 20kg scientific payload being maintained at 20km (65kft) stationary for 20hrs. It was also noted that the US Defence Operationally Responsive Space (ORS) Office may be willing to invest in these capabilities should they be demonstrated to be technically viable.

The potential for more efficient use of energy is a fundamental consideration in the growing interest in airships, especially in the ISR role. Global energy challenges are leading to airship concepts that evaluate the use of renewable energy. For example, a tethered aerostat is currently being tested to carry wind turbines aloft that could exploit winds at altitude for power generation. They are also being evaluated for use in capturing solar power generated energy beamed from satellites in a two-stage transmission process for use on the ground. These concepts are expected to assist further developments in airship applications in the ISR role where long-endurance and energy efficiency are coveted characteristics. A July 2012 patent describes a system for the use of airships in commercial communication relays, which is claimed to have significant economic benefit in comparison to satellites.

Employment in any of these roles would require the generation of power to run the onboard systems as well as to manoeuvre the airship. Given the large surface area of airships, the use of photovoltaic cells is being considered both for power generation and also to create hydrogen for use in fuel cells to power the airship at night. This development also provides options for the onboard production of hydrogen to replenish the lifting gas within the airship. Two recent patents have described multi-

Issue 225, July 2014

There is no basis for the ardent hopes and positive statements made as to the sae and successful use of the dirigible balloon or flying machine, or both, for commercial transportation or as weapons of war.

attributed to Rear Admiral George Melville Engineer-in-Chief, USN, December 1901

Concept art of the Aeroscraft ML866. Its potentially revolutionary Control of Static Heaviness system compresses and decompresses helium in the 210-ft.-long envelope, changing this proposed sky yacht’s buoyancy during take off and landings.

ship concepts to increase the persistence of airships. One involves the use of a ‘logistics’ airship to replenish the power and other requirements of an on-station platform. The other has two tethered airships with an altitude difference of around 5km between them with the shear layer arising from the wind speed difference between the altitudes being exploited to generate onboard power and assist station keeping.

Hybrid airships that combine their natural buoyancy and aerodynamic lift to stay aloft have particular application in the air mobility role. A 2011 Masters thesis from the Canadian Forces College, released to the public only recently, considers the feasibility of employing such a hybrid airship to carry a payload of 200 tonnes in the strategic airlift role. The conclusion of the research is that if hybrid airships are developed beyond the current prototype stage, they could become a niche capability between sealift and traditional strategic airlift capabilities. Further, a US Naval Postgraduate School report found the use of airships to be particularly viable for time-critical shipment of payloads up to 2500 short tons.

Recent patents indicate that there is a wide range of potential hybrid airship concepts being developed. One that is particularly innovative is the concept where a UAV that takes off in a conventional aircraft mode and configuration releases helium or hydrogen into an envelope to provide it with sufficient buoyancy to enhance its on-station endurance. Significant research and innovation is being undertaken in the configuration design of hybrid airships.

Another area where concerted attempts at innovation are being carried out is in the design of the controls where dynamic modelling of unconventional airships and their control is being developed. It is expected that the development of equations of motion will assist with the efficient design of control laws for these platforms. Contemporary literature provides information on the direction of research. Two recent publications examining the development of control laws for airships point towards the employment of fuzzy logic-based methods—one using visual sensor data and the second using ultra-sonic sensors. The processes described in these two papers have the potential to be employed in hybrid and LTA vehicles when they are used amongst buildings. The potential for use in the urban environment for ISR is considerable.

As with all futuristic developments, fiscal constraint will be a limiting factor in hybrid airships achieving their full potential. An example is the US Army’s Long Endurance Multi-Intelligence Vehicle (LEMV), which undertook its maiden flight in August 2012. It was

estimated by the US Government Accountability Office that the Department of Defense had spent USD 7 billion on 15 different airship programs between 2007 and 2012. The report also concluded that the programs were not sufficiently co-ordinated, that they had experienced considerable cost and schedule overruns, and that there were significant technical challenges to be overcome. It has been recently reported that the LEMV program has been cancelled. In the current global financial circumstances, this could well be the harbinger of even greater constraints on airship developments.

Recently published literature indicates that LTA and hybrid airship concepts are an area of active research interest and limited development activity. Even though their advantages have been clearly enunciated, current economic concerns make it unlikely that any significant developments will take place in the immediate future. It is to be hoped that increased civilian applications for these technologies would drive further developments and that the military will benefit in the ISR and air mobility roles from these initiatives.

Key Points• The ability to take off vertically and lift heavy

payloads are unique capabilities of LTA airships.

• The use of stratospheric airships as cost effective pseudo-satellites is an area where research and development is currently concentrating.

• Hybrid airships that combine their natural buoyancy and aerodynamic lift to stay aloft have particular application in the air mobility role.

Air Power Development CentreF3-GF, PO Box 7932, Department of Defence

Canberra BC ACT 2610Ph: 02 6128 7041 Fax: 02 6128 7053

Email: airpower@defence.gov.auWeb: www.airforce.gov.au/airpower

Disclaimer: The views in this Pathfinder are not necessarily those of the RAAF

The Lockheed Martin P-791 LEMV on a test flight. In 2006, the prototype vehicle P-791 flew in Palmdale, California, successfully completing all flight test objectives.