Airships (airships or dirigible balloons are types of aerostats or lighter-than-air aircraft) are currently enjoying renewed interest, as evidenced by ongoing programs in France or abroad. This very old aeronautical solution presents an interesting alternative from an economic point of view, because of its sobriety in terms of fuel consumption. Another advantage is its hovering capacity.
The main disadvantages of this solution are its vulnerability to aerological conditions, its low manoeuvrability, and the delicate control of manoeuvres on the ground or near the ground. The two main applications considered, dual, are freight transport and surveillance. The integration of technological advances in the design of these machines (in terms of materials, control, hybrid or all-electric propulsion…) suggests new perspectives.
To see these machines fly one day on a large scale, a major challenge will have to be taken up, that of certification, formed by its three main pillars: airworthiness, pilot training for these very specific aircraft, and their integration into the airspace.
The most promising perspectives for dirigible balloons
The first application concerns the transport of point-to-point freight, heavy and/or bulky loads. This solution can also make it possible to open up isolated areas while avoiding building infrastructures. For example, there is the possibility of a Sahara air link between the oil extraction points and the Mediterranean coast, or the East (where the economic centres are located) and West (where the resources are located) link in China. In France, the LCA60T program run by FLYING WHALES helps to improve logging in mountainous regions. This concept of unprecedented employment is established in collaboration with the French Office National des Forêts.
Another application of interest is surveillance or long-endurance tropospheric observation (five to ten days). In fact, the airspace permanence capabilities of dirigible balloons, potentially far superior to any other aircraft, and in addition to low energy costs, offer interesting prospects. Historically, this application has spawned several developments, including Good Year soft airships in the United States of America. These dirigible balloons were operational until 1962 in the U.S. Navy. Regularly, concepts stand out on these applications. The latest notable achievements in this area are the Blue Devil 2 airship (United States of America) program, stopped in 2012, while the aircraft was almost finished, and Lockheed Martin’s eighty-five meters ling LEMV.
Finally, the dirigible balloons could be used to great advantage for high altitude long endurance applications (twenty kilometres, one year) in the field of monitoring observation, or telecommunications relay. Several projects have been conducted in the United States of America, a program is underway in China (Yuanmeng). In France, the Stratobus project sponsored by Thales Alenia Space is under development. It should be noted that these applications are dual, that is to say, usable in a framework of military employment as well as civil employment.
Promising opportunities for dirigible balloons
Dirigible balloons are mainly used for long-term surveillance purposes. Several categories (volume class) exist, and permanence on the zone can vary from a few days to a month. To hold the wind sufficiently efficiently, it is necessary to use a shaped form that is oriented in the wind bed, and to have a ground station that accompanies the gyration of the dirigible balloon. For these surveillance applications, the United States of America has been using dirigible balloons for four decades: for border surveillance (like the Tethered Aerostat Radar System, or the Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System) or advanced databases (like the Persistent Ground Surveillance Systems). In France, several companies manufacture such dirigible balloons; we can cite for example the companies Airstar and A-NSE.
Specificities of the aircraft
The airship belongs to the family of aerostats, that is to say that it uses the principle of buoyancy to ensure its lift, unlike aerodynes (planes, helicopter…) that use the principle of aerodynamic lift. To do this, the airships consist of an envelope of a very large volume in which is imprisoned a gas lighter than air. Helium, an inert rare gas, is used for the most part in current applications. This rare gas is largely preferred to dihydrogen, lighter and more abundant, for safety reasons, because dihydrogen is indeed a highly flammable gas. However, it is not excluded to consider dihydrogen in the future, provided that the safety of the solution is demonstrated.
Flying with a dirigible balloon is therefore based on the control of Archimedes’ principle. Several parameters influence the variability of this thrust. The buoyancy of Archimedes is proportional to the density of the air. This density varies in particular according to altitude and atmospheric conditions. Airships generally operate at constant envelope volume. The difference in altitude will cause a modification of the thermodynamic properties of the gases, resulting in a variation of the volume of the carrier gas. Classically, this is achieved by a regulation based on exchanges, filling/evacuation with the outside air.
It will also be necessary to ensure permanently the vertical balance of the machine, that is to say the first order to balance the weight, and aerostatic lift. The machine mass variations are therefore to be studied with great care: load transfer case (transport application), compensation of fuel consumption. It is therefore necessary to put a gas management system adapted to the use of the airship, which does not exist, or little in the current aeronautical world. It is therefore difficult to find qualified equipment on the shelf, and it will be necessary to develop appropriate equipment (performing from the point of view of mass).
The legal status of dirigible balloons
HAPS are aircraft, usually unmanned airships or airplanes. Stratospheric flights above fifteen kilometres altitude were already made in the 1930s, in balloons with pressurized gondolas, manned by pioneers such as the Swiss Auguste Piccard. In the 1990 and 2000 decades, several projects were launched in order to explore the potential application of high altitude platforms for telecommunications and remote sensing. Large projects were started in the United States of America, Japan, and South Korea. A remarkable fact for the HAPSs concept was the initial definition of a frequency band for its telecommunications services on the World Radiocommunication Conference 1997 (WRC-97), organized by the International Telecommunication Union (ITU), which deals with the regulation of the use of radio frequencies. At this conference, the term “High Altitude Platform Station” (HAPS) has been established, defined as a telecommunications station located at an altitude of twenty to fifty kilometres and at a specified fixed point relative to the Earth. This fact shows that, at the time, there was a growing interest in HAPS utilization as a complement to terrestrial and satellite-based communications network. Over the years, several terms have been used for this type of aircraft, such as: “High Altitude Powered Platform”, “High Altitude Aeronautical Platform”, “High Altitude Airship”, “Stratospheric Platform”, “Stratospheric Airship” and “Atmospheric Satellite”. The term “High Altitude Long Endurance” (HALE), which has sometimes been used to label HAPS, is generally more associated with conventional unmanned aerial vehicles (UAVs), with service ceiling of about eighteen kilometres, as the Northrop Grumman RQ-4 Global Hawk. Currently, the expression “High Altitude Platform” (HAP), adopted by the ITU, has been the most commonly used. The most common types of aircraft used as HAPS are: airplanes, airships and balloons.
The main HAPS applications are in telecommunications and remote sensing, both civilian and military. In the area of telecommunications some of the advantages of HAPSs in relation to terrestrial networks (relay towers) are larger coverage area, less interference caused by obstacles (buildings, ground elevations) and shorter time to deployment. Compared to satellites, HAPSs have the advantages of lower latency (transmission delay) and the possibility of return for maintenance or payload reconfiguration. For remote sensing, HAPSs have as an important advantage over satellites, mainly the low orbit ones, and the ability to remain continuously over an area for very long periods (persistence). Another advantage is to permit better resolution images, because they are closer to the covered areas. Designing aircraft to operate in the stratosphere as HAPS imposes major technological challenges, with the main ones being: lightweight structures, energy generation and storage, thermal management, operation at low altitude and reliability. Some aspects of each of these challenges will be discussed next. The HAPS projects, both airplanes and airships, are optimized for the stratosphere conditions, at altitudes close to twenty kilometres, where the thin air is relatively calm and the wind speed is low.
Another important aspect to be considered in HAPS operations is the coordination with the airspace control organizations. Most of the time of flight of a HAPS is above the air control altitude limit, usually defined at twenty kilometres. The launch and recovery phases, which occur at lower altitudes, should be planned in conjunction with the airspace control agency, with the definition of specific segregated areas for that operation. The integration of unmanned aircraft in not segregated airspace is a subject not yet regulated, mainly due to the issue of avoiding air collisions (“sense and avoid”). Aspects in the field of International Law related to the overflight of other countries also need to be analysed. From 2013, Airbus Defence and Space, Thales Alenia Space, Google and Facebook began investing in HAPS projects mainly aimed to supply Internet in areas without telecommunications infrastructure, bringing new hope to achieve the establishment of a HAPS industry. The future of HAPSs will be driven mainly by the evolution of technologies of potential competitors, such as microsatellites constellations, and the availability of financial resources to overcome the HAPS technological challenges.
As a conclusion, we can believe that those dirigible balloons will be considered aircrafts and will fall under aircrafts legal status.