The power of aerostats

October 2018

They don’t look like impressive flying machines, but aerostats have the advantage of persistence. Retired U.S. Air Force Col. Charlie Lambert explains how the company he founded, SkySentry LLC, combined existing components to produce an aerostat system for agencies with thinner wallets.

Picture smoke signals from a bluff high above a remote prairie. That’s just one way humans have tried to communicate over remote, rugged terrain for millennia. Humans intuitively understand that they can see and communicate farther from a higher place. Hence the primitive smoke signals from high on a bluff.

When I retired from the U.S. Air Force in 2003, I formed SkySentry near Colorado Springs to help advance the High Altitude Airship project that I worked on during my last decade in service. The North American Aerospace Defense Command wanted to see if airships could expand the radar picture for missile defense. My staff and I embarked on a lengthy effort to explain the potential benefits of a High Altitude Airship to the military. Eventually, we collected over $100 million dollars to build and fly one in a technology demonstration. As development continued, we realized that payload development was every bit as challenging as the vehicle itself, so we acquired large aerostats as a means of field-testing payloads. Those payloads were supposed to be mature enough for deployment on airships, but they were, in fact, rarely ready. Communications nodes required different antenna configurations; radars needed widely separated magnetic sensing; cameras need stabilization and steering. We helped the various engineers resolve these issues by mounting payloads on aerostats in different orientations; varying the power system inputs and whatever else was required.

This experience showed us that aerostats could be tremendously useful in their own right as surveillance and communications platforms. Each can be unreeled to a desired altitude on a synthetic tether about the diameter of a school pencil. When necessary, the tether can house fiber optic strands to download data from the payloads without risk of interference and copper wires to deliver power to the payloads. An aerostat provides persistence and never inconveniently passes over the horizon like a satellite does. Plus, its flight can be controlled more readily than that of an airship. Indeed, when measured on a cost-per-hour basis, there is no cheaper alternative for elevating payloads to improve their ranges. As an example, an aerostat at 500 feet above ground will give a radio nearly 50-kilometer line-of-sight range to the horizon; that exceeds the range of most radio payloads commonly used by field operators.

For five years, SkySentry led a major Army contract task order to specify, order, purchase and operate aerostats for the Army Space and Missile Defense Command. Simultaneously, SkySentry expanded its lines of business to include design and integration of complete aerostat systems for other customers.

SkySentry is a lead system integrator. Our innovation is to find the best off-the-shelf equipment and assemble it into affordable communications, sensor and surveillance platforms.

The right stuff

The heart of such a system is the aerostat, and SkySentry’s quest to find a more affordable, properly performing aerostat was lengthy and thorough. Our first few aerostats were not the right solution for agencies with budgets smaller than the U.S. military’s. They were traditionally shaped models, looking like Goodyear blimps at about 25 meters long. These were flown by the U.S. military during operations in Afghanistan and Iraq, but these aerostats cost a lot to buy and operate. We postulated that we needed smaller aerostats and alternative designs for these potential new customers.

The laws of physics readily tell us the most efficient shape for maximum lift is a simple spherical balloon, so we focused first on that shape and found several models. However, when exposed to winds, spheres tend to spin and flop around. That behavior isn’t conducive to effective payload operations. We assessed manufacturers of small aerostats worldwide, each with different accoutrements aimed at stabilizing their aerostats in strong and turbulent winds. While the designs were often similar, we found a large disparity in the quality, weight and durability of aerostat fabrics and fabrication techniques.

After a detailed trade study, we settled on the patented Helikite design fabricated by Allsopp Helikites in the United Kingdom. These balloons are spherical at the front, which is the most efficient lifting shape in no-wind conditions, and each has a kite-shaped tail and keel for stability in the wind. They have been through extensive U.S. government assessments with good results. They vary in size from a tiny 2-cubic-meter version to one that is about 8 meters in diameter with a volume of 250 cubic meters. The customer buys the size needed to lift the payload and tether. In no wind, the helium volume lifts the balloon, with a rule-of-thumb planning factor of 1 pound of net lift per each cubic meter of helium volume. As wind velocity increases, the underlying kite increases the lift by four times or more over the no-wind lift. The keel below the aerostat acts like a boat rudder, providing considerable stability in the wind flow.

We field tested a common alternative design in which a sail rides behind the balloon to try to keep it from spinning in the wind. In high winds, we found the sail pulled the aerostat down toward the ground, once even bouncing it off the surface. Vendors offering this design try to mitigate the pull-down tendency by specifying a great amount of excess helium lift above desired payload weight. Ultimately, SkySentry adopted the Helikite aerostat and synthesized it with winches, tethers, power supplies and mooring components to provide stable, persistent elevated functionality, while reducing aerostat volume from about 800 cubic meters to 100 cubic meters.

After choosing the Helikite as the basic building component of this SkySentry line of business, SkySentry shifted focus toward design of a rugged, compact, turnkey system. Design is more complex than one might perceive by looking at a complete system. For example, winches must have a minimum core diameter to prevent bending and breaking copper wires and fiber optic strands in the tether. The winch must have a minimum pull strength to launch and retrieve an aerostat and should run at a speed of about 30 meters per minute or more, so launch and recovery don’t seem to take forever. Tethers offer similar challenges. Electricity running up copper wires in a tether meets resistance. Large wires lower the resistance from the copper, but the larger the wires, the more weight the aerostat has to lift. So a detailed trade analysis is done for each customer to accurately calculate the smallest wire size needed to power the payload, resulting in the smallest, most cost-effective aerostat size.

There was also the question of how to deliver the aerostat systems to customers. Many of our staff are former U.S. military service members who have experience delivering equipment to sites around the world via C-130s. We decided that the total aerostat system had to be packable into standard shipping containers for sea or airlift to a customer’s operational area or for home storage. The complete aerostat system had to require no more than two people to set up, launch and recover.

In early 2011, we announced the result of this research, a system called TEA (TEE-uh), short for Tactically Expedient Aerostat. Each TEA is highly customizable, with trailer, winch, mooring platform, and other supporting components sized to the customer’s needs. The design process typically starts with defining the size and weight of the payload. Then a trade study on the power supply assesses whether continuous power up the tether is required, or whether the aerostat can be brought down periodically for changing out the payload batteries without unacceptable impact on the mission. As the lifted weight increases, the aerostat grows in size, which dictates the breaking strength and weight of the tether, in turn influencing the size and performance of the winch and mooring trailer. Should a customer not want to tow a trailer to the flight location, the aerostat and components can be carried in a truck and the TEA can be flown from a ground mooring base.

The versatility of TEAs is limited only by one’s imagination. Their small size makes them hard to detect, so they can be deployed in unexpected areas to keep adversaries off balance. For example, a TEA on a trailer can be placed at a location in just an hour, so border intruders may well encounter a team with aerostat-based communications networks and surveillance where none existed just moments before. Further, if a large surveillance asset malfunctions, these small systems can be deployed quickly as gap fillers. For security operations at large sporting events, the TEAs can carry advertising logos, disguising that they are actually enabling a wide area surveillance and communications sphere. We’ve even mounted them on boats.

Payloads

No one really buys an aerostat just to be able to say, “Look, I own an aerostat!” A customer buys an aerostat to lift a payload. While gyro-stabilized cameras are popular for some relatively limited-range surveillance, we have generally found our aerostats to be most impactful when they carry wide area network communications for operations in austere environments. During the early days of our search for wide area network payloads, we flew and tested radio nodes weighing hundreds of pounds, with huge power draws and awkward antennas for broadcasting radio transmissions. The goal was to establish wide area networks for operators on the ground. But these large payloads were typically priced at more than a quarter million dollars and required huge aerostats to lift. Within the last few years, manufacturers have made quantum-leap improvements to these nodes. They are now smaller and less power hungry with longer ranges.

Each node creates a coverage area that can be visualized as a hemisphere extending below the aerostat and creating a 15-kilometer-radius coverage footprint on the ground. We call this coverage area a Tacti-Sphere. Anyone equipped with a properly programmed smartphone can verbally communicate with everyone else in a wide area network, by talking, texting, or push-to-talk chatting. Each participant in a Tacti-Sphere network can be a sensor out to the far edges of the network, using his or her phone to pass photos and videos to other participants or team headquarters. Greatly enhancing mutual support and situational awareness, each participant’s location can be shown to all other participants. By linking the aerostat node to either a network SATCOM or a commercial cellphone tower, all participants can reach back to the internet or commercial communications. In addition, we recognized that customers might want to preserve legacy investment in their land mobile radios, so if an agency wants to use LMRs instead of smartphones, we can accommodate that as well.

Learning about payloads was an arduous effort for us, given the dozens of types, makes and models. We found unexpected idiosyncrasies when integrating with an aerostat, usually boiling down to antenna performance. Two general categories of networks have proved most useful. The most popular is a 4G LTE network. An ultralight 4G LTE node is lifted on the aerostat, with the smallest node capable of simultaneous transmissions by about 32 operators. Using a 30:1 planning ratio for the number of registered users to simultaneous transmissions, this smallest node can serve a population of about 1,000 registered users with all the functionality listed above. Disaster response may require more registered users, dictating that a larger node and larger aerostat be employed. 4G LTE networks have been proven in rugged territory over areas as large as 300 square miles. Even larger areas can be covered by linking two 4G LTE nodes together with high bandwidth radio links, with each node surrounded by its own registration of smartphones. Importantly, these 4G networks require pre-deployment spectrum coordination, which is usually not too difficult.

The second category involves mobile ad hoc networks, using peer-to-peer nodes in self-forming, self-healing architectures. These are most useful in smaller areas, with deep ravines or significant surface obstructions. These mesh networks are often a bit less expensive than the 4G LTE. In both technological categories, all the functions of the Tacti-Sphere are fully operative.

For decades, we’ve observed that wide area austere communications were regarded as a “holy grail” among remote operators. LMRs are blocked by terrain. Cellphones allow communications only when the phone is relatively near and in line-of-sight of a tower. As a most pointed example, shortly after 19 firefighters died in a forest fire near Yarnell, Arizona, we were contacted to assess how firefighters in the middle of the wilderness could stay in touch over ridges. The need for long-haul communications in remote environments prevails for numerous operations as varied as forest firefighting, military actions, search and rescue, security and border patrol, and even disaster response to devastated urban locations. Tacti-Spheres can help resolve this shortfall.

Charlie Lambert. Credit: Courtesy

An inflatable aerostat mooring base, winch and pulley. Credit: SkySentry