While displaying Mothership Aeronautics' latest solar powered autonomous airship at the Commercial UAV Expo 2019 in Las Vegas this October, as seen at 6 on this CNE listicle many attendees asked us the following questions:
“Can you tether it?”
“Is this one tethered?”
"Can you take it off the tether?"
While Mothership Aeronautics offers both free-flying eBlimp airships and tethered aerostats as products, the two are distinct and almost mutually exclusive both in their physics of operation and in how they are regulated.
Aerostats and airships both primarily use buoyancy forces from lighter-than-air gases to lift them. Both aerostats and airships also benefit from secondary aerodynamic lift from relative wind against the balloon. However, this is where the similarity ends. Some of the differences described in the sections below:
Most airships are flown slightly heavier than air (partial buoyancy) or flown such that the buoyancy forces equal the weight of the craft (neutrally buoyant). This is done for controllability and safety reasons.
Aerostats on the other hand must be flown positively buoyant such that they pull upward tightly on their tether lines so that they can maintain their position in wind.
Similarly to on the tail of an airplane, the tail of an airship has rudders to control yaw mounted and hinged on the back of the vertical fins and elevators to control pitch, hinged on the backs of the horizontal fins.
Most aerostats, on the other hand, do not have any hinged control surfaces on their fins. The fins act only as horizontal and vertical stabilizers.
Both airships and aerostats requires electrical power to operate payloads such as cameras, sensors, computers, LED Screens, and to communicate via radios or datalinks. In many cases where the aerostat is used as a communication tower, the communication system is also the primary payload.
Additionally, since airships must produce thrust for maintaining altitude and velocity. This thrust is often produced by electric motors, which require an order of magnitude more electrical power than the other onboard electrical. And as mentioned above, airships use movable control surfaces which also require electrical power to actuate.
In general, because of the above, aerostats are a less power-intensive solution than airships making them well-suited to persistent operations not requiring mobility.
In the United States, small unmanned airships can be considered sUAS and operated under FAA’s Part 107. Tethered Aerostats of all sizes, along with kites, are regulated under Part 101.
When to deploy an Aerostat VS a Erect a Tower
Aerostats can be seen as a replacement solution to erecting a tower. The cases in which this solution is a practical one are as follows:
The time to erect a tower is too long
The cost to erect and engineer the tower is too high
The required height of the tower is too high
The terrain is non-conducive to construction
When to use an Aerostat VS a Tethered Quadcopter
Several companies offer a drone tethers for long duration quadcopter surveillance over a target. These solutions can be costly and dangerous and not always necessary. In many cases with mild weather and in absence of high precision positioning requirements, an aerostat offers an easy economic alternative.
When to use an Aerostat VS a UAV in Surveillance
Target surveillance area is small or constant
Required surveillance duration is long (>20 minutes)
Decicated manpower/oversight is not consistently available for the required surveillance period
Propeller/aircraft noise is to be avoided
Regulations prohibit use of free-flying UAVs under FAA Part 107 in surveillance area but Part 101 operations, as shown in the table above, are more workable.
Hybrid Aerostat/Airship Designs
There may arise cases where the functionality of a tethered aerostat is needed, but with the option to be mobile. This SBIR solicitation by the US Army for example offered $150,000 in funding in 2019 for a small business to design and deploy a system capable of both. The request reads:
While Aerostat systems are an essential tool for support to ongoing combat operations their utility is limited during high intensity conflicts due to their static nature, inability to rapidly redeploy within theater and inability to provide persistent Intelligence
Due to the differences in buoyancy requirements as described above, designing a transitioning hybrid Aerostat/Airship is not elementary. Advanced buoyancy control would be required to achieve this, hence the call for bids from the Army.