Can lighter-than-air vehicles make a comeback as air transport for remote areas? Barry E Prentice* and Robert Knotts** provide an overview of cargo airship projects under development and the challenges.
HAV's Airlander is set to fly in the Uk in 2016 (Hybrid Air Vehicles)
Large global areas such as Alaska, the Canadian Arctic and Siberia have isolated northern areas lacking road or rail access. Other nations with infrastructure gaps include Brazil, sub-Saharan Africa and parts of Asia where torrential rains wash out bridges and make roads impassable. Air transport offers the only year-round connections to many of these remote areas.
Conventional aircraft and helicopters are costly with limited cargo size and weight capacity. Larger aircraft can carry sizable loads but generally, landing strips in remote areas cannot accommodate these aircraft. With increased transport needs to remote areas, interest has renewed in the potential use of cargo airships. This article considers the current status of cargo airship developments worldwide and the range of technological challenges and opportunities they present.
Figure 1. Examples of transport airship developments worldwide.
A survey of global airship activity addressing development projects and operations shows that limited but viable worldwide activity is taking place. Approximately 26 airship projects can be identified from conceptual drawings to actual manufacture. In addition, nine companies operate about 40 airships, mostly focused on TV filming and advertising, leisure flights, scientific studies and remote surveillance.
A number of companies are designing heavy lift cargo airships. For remote areas, they could offer year-round point to point transport of large cargoes in vehicles that are energy efficient, environmentally acceptable and requiring minimal supporting infrastructures. No reliable cost data exists but fuel costs are estimated at 35% to 50% of current cargo aircraft costs. Capital costs per ton of lift expectations are lower; airships do not require jet engines or pressurised structures. Moreover, airships, like ocean vessels, enjoy significant economies of size. Doubling cargo capacity less than doubles cost.
Current transport airship designs include a broad spectrum of innovations in design, buoyancy control, ground handling systems, and structure. Figure 1 presents a list of international airship projects.
Figure 2. Illustrations of transport airships under development.
Only a few of these airships are being constructed. Some scaled prototypes have been built and flown but the only full-scale vehicle is the HAV Airlander, scheduled to fly in 2016. Most designs are at a conceptual stage; a prototype could be flown within two years, and certified within three years, if investment was available. Figure 2 shows pictures of the cargo airships currently under consideration.
Large airships operated successfully before WW2. Cargo, if carried, was not a primary consideration; the focus was passenger transport. Consequently, these large airships were not required to offset large weight changes. The largest Zeppelin, the Hindenburg, had approximately 70tons of useful lift, but only carried 100 persons. Most of its lift was used for accommodation, dining areas, etc. Large transport airships face cargo exchanges that could equal the deadweight of the vehicle. Consequently, buoyancy control is a primary concern.
Figure 3. Hybrid airship – methods of dynamic lift.
Buoyancy control is necessary to ascend and descend, accommodate weight changes due to fuel consumption and is critical for cargo exchange. Most airship developers have chosen one of two routes to address load exchange and ground handling issues; a third approach is available.
A hybrid airship is heavier-than-air when empty. It combines aerodynamic lift with static lift of lighter-than-air gas. About 60% to 80% of lift is static; the remainder is produced by the airship’s aerodynamic shape and vectored engine thrust. Manufacturers embracing this approach are Lockheed Martin (US), Hybrid Air Vehicles (UK) and AeroVehicles (Argentina). The idea is shown in Figure 3.
An innovation to control ascent and descent is to compress the lifting gas. Figure 4 illustrates Varialift’s approach to variable buoyancy control. Lift is altered by moving helium between lifting-gas cells and pressurised tanks inside the airship. Compressing the gas makes the vehicle heavier than air for easier ground handling. However, the cost of compressing enough gas to compensate for the cargo exchange is unknown. After loading, the lifting gas is released to expand the gas cells and displaces air inside the vehicle for lift off.
Other companies taking this approach are Worldwide Aeros (US) and RosAeroSystems (Russia). Built with a rigid structure, these designs can control lift at all states with vertical take-off and landing (VTOL) capabilities and carry maximum payload while in hover.
The third approach is to exchange ballast equal to the weight of cargo. The CargoLifter design proposed using water ballast, an approach being considered again by some developers, like BASI.
The advantage of the hybrid design and variable buoyancy methods of buoyancy control is that the airship can operate independently of any prepared ground handling facility. However, a ballast-dependent airship needs access to the ballast on the ground where the cargo is exchanged; this is a function of use. It raises the next issue of ground handling.
Ground handing and loading/unloading cargo remains a challenge. (Aeroscraft)
Loading/unloading and handing cargo on the ground raises interesting questions about the future market for transport airships. Military requirements have driven modern airship development. In a time of war, or even natural disaster, cargoes must be delivered to locations that are by definition unprepared with respect to ground infrastructure. Several transport airship designs illustrate a loading ramp that drops from the airship allowing cargo to roll on and off. While this is one possible solution for military/emergency situations, it may not be appropriate for civilian transport use; heavy ground handling equipment would have to be carried, reducing payload capacity.
A similar problem faces proposals involving the carriage of ISO containers on airships. A 40ft steel container reduces the paying cargo opportunity by about 3t. The container also needs ground equipment to transfer it from an airship to a truck.
Most civilian transport operations differ greatly to military emergencies. Trains do not just stop anywhere along the tracks to unload cargo, any more than ships drop anchor at random spots along the coast to load and unload. Civilian transport operates from established bases (stations, ports, etc) to established bases. Given that the majority of cargo is of civilian nature, the emphasis of the airship developers on zero ground infrastructures is more a function of military investment, than the potential civilian freight market.
Efficient civilian cargo operations have established loading docks, for handling and transshipment. Typically, the last mile involves a truck. Consequently, most ground handling systems feature loading docks where tractor-trailers back up with forklift trucks transferring palletised freight, or even bigger container lifters to transfer 20 and 40ft long steel boxes from one mode of transport to another.
Clearly, in remote areas the availability of ground infrastructure adds to cost, but most remote locations have small airstrips. Positioning enough ground handling infrastructure at a remote location to handle an airship is not that daunting.
Finally, ground handling involves the safety and stability of the airship on the ground. Most airship developers offer some solution to steady the airship while cargo is exchanged but none have been demonstrated. The notion of modified hovercraft skirts with reversible fans acting like suction cups to hold the airship steady is appealing, except for a few realities. First, this equipment would not be inexpensive or light. Second, the hovercraft skirts need an energy source (fuel) and engines that add weight and expense. While the airship is on the ground, this equipment must be in operation. As with all moving parts, there is more maintenance cost. Finally, the airship must keep its nose into the wind. How these systems allow stability and flexibility when the wind changes, is yet to be demonstrated.
Anchoring the airship to the ground via cables and docking it to a mast is another proposed ground handling system. This approach also has challenges. The mast requires a cleared circular area whose radius is the length of the airship. A masted airship lacks stability in yaw and pitch, although some Zeppelins attached a rail car on a circular track to hold it stable.
At the 2015 Paris Air Show, Lockheed Martin signed an agreement with Hybrid Enterprises for it to be the reseller of its LMH-1 airship – aimed at the freight market. (Lockheed Martin)
Concerns about airships operating in adverse weather conditions have frequently been aired. British airships in WW1 and American blimps in WW2 successfully coped with weather extremes. During the 1950s US Navy airships conducted extensive and prolonged trials to prove airship all-weather capability. Conclusions included: airship ground handling operations can be accomplished in virtually all weather conditions and maintaining a continuous station over the Atlantic Ocean was feasible under all weather conditions. Furthermore, while weather posed major airship operating problems in the 1930s technology now offers effective mitigation against such hazards. Modern weather prediction technology enables operators to avoid severe weather and find favourable winds.
One weather issue that may prove more difficult is the build-up of ice and snow. The hybrid designs with wider, flatter tops present more area for ice and snow to accumulate. It is not that this cannot be overcome by various means but the issue cannot be ignored for operations in the northern remote areas. Also, wide variations in temperature occur in relatively short time periods. Semi-rigid and non-rigid designs will have a more difficult time maintaining their shape than rigid airships that do not depend on internal pressure differentials.
Is the US ban on using hydrogen for lighter-than-air vehicles restricting development of an useful niche in aviation?
Following the 1922 Roma accident, the US banned the use of hydrogen as a lifting gas in airships. This accident, like most of the rigid airship accidents prior to WW2, was caused by a structural failure. The hydrogen burned, as did the engine fuel, envelope and many flammable components of the airship; however, only hydrogen received this attention. The ban on hydrogen was easy for the US because it had ample supplies of non-flammable helium. The rest of the world carried on with hydrogen up to the Hindenburg accident which was the last of the large rigid airships in commercial use.
Following WW2, the technological development of jet aircraft made both airships (and flying boats) obsolete. With continued military and civilian aerospace investment, the US became the undisputed technology leader, as well as the largest commercial market. Consequently, US Federal Aviation Administration (FAA) regulations became the industry standard. Most countries around the world adopted these regulations, including banning the use of hydrogen as a lifting gas for airships.
In 2015, a curious situation prevails. Hydrogen gas is permitted as a fuel in forklift trucks, cars, buses and even for experimental aircraft but using hydrogen for airship buoyancy is subject to a 93-year old ban. Conceptually, no difference exists between a low-pressure hydrogen fuel tank and a hydrogen gas cell but this regulation, based on neither scientific proof, nor engineering evidence, has held back the use of a superior lifting gas in transport airships.
Naturally, a hydrogen-filled airship design would have to pass an airworthiness certification but no builder is going to use cow intestines glued on to linen sheets as a gas cell in a 21st century airship. Modern materials and sensors could make a hydrogen airship as safe as any helium airship.
The benefits of returning to hydrogen gas are multiple. First, hydrogen provides about 10% more gross lift. Assuming that the deadweight of the airship is about half the total this results in a 20% increase in cargo lift. Second, helium is a rare gas and quite expensive. Hydrogen can be produced anywhere from water, at a small fraction of the cost of helium. Hydrogen can also be used as a fuel. Consequently, only one gas would be needed for lifting and propulsion of the transport airship. A hydrogen-powered airship is the only practical zero-carbon emissions transport aircraft.
It is suggested that there is a sound business case to use hydrogen instead of helium. However, by banning the use of hydrogen, airships are sub-optimal and far more expensive to build and operate than would otherwise be the case.
HAV is planning a demonstration tour with Airlander that could spark off real interest and sales. (Hybrid Air Vehicles)
Cargo airships offer the only year-round connections to many remote areas in the world. However, they face a range of technological challenges. Major ones are buoyancy control and ground handling. Hybrid airships or variable buoyancy technology are being developed to accommodate these challenges which, if successful, will allow airships to land in areas without the need for ground handling infrastructure. An alternative that exchanges ballast such as water equal to the weight of cargo needs access to the ballast on the ground where the cargo is transferred.
Military cargo handling requirements need to be part of an airship’s self-contained equipment, whereas for civilian transport ground handling equipment is needed as part of the infrastructure of a remote location handling an airship. Such an approach increases an airship’s payload potential.
While extensive and prolonged trials in the 1950s demonstrated a blimp’s all-weather capabilities, the issue of the build-up of ice and snow on the wider flatter hybrid designs has to be considered; airship operations in arctic areas will face this problem. Also wide variations in temperature occur in relatively short time periods, causing greater difficulty for semi-rigid and non-rigid designs to maintain their shape than rigid airships. Modern weather prediction technology will also enable operators to avoid severe weather and find favourable winds.
Despite a 93-year old ban on using hydrogen for airship buoyancy there is a sound business case to use hydrogen instead of helium. Hydrogen offers 10% more gross lift and can be produced anywhere from water, at a small fraction of the cost of helium. Furthermore, with hydrogen used as a fuel, only one gas is needed to lift and propel an airship resulting in a zero-carbon emissions vehicle.
Airships offer year-round point-to-point transport of large cargoes in vehicles that are energy efficient, environmentally acceptable and requiring minimal supporting infrastructures. Their future success will enable sustained support of remote communities in many areas of the world.
*University of Manitoba
**Former Chairman, Airship Association