Why the Stratosphere?
By flying in the stratosphere above the weather, a relatively lightweight, persistent, solar-powered system becomes viable. This can provide persistent flight operations for up to several months at a time without landing, recharging the onboard batteries each day using the solar cells fitted to the platform, then continuing to power both the aircraft and payload throughout the night from the batteries.
Operations in the stratosphere can offer a number of advantages over systems operating at other altitudes. The persistent nature of these platforms allows for 24/7 coverage of a wide area with sensors or communications systems, where a more traditional system would cover a smaller area with the payload fitted, and need to repeatedly land, requiring a greater number of more expensive platforms to deliver a similar capability. Additionally, operating at stratospheric altitudes can avoid latency and intermittent coverage issues which are often present with low earth orbit satellite based solutions.
In many situations, a stratospheric, solar powered solution can present a more cost effective approach than use of numerous conventional aircraft, or low earth orbit satellites.
Thus, a solar HALE solution can either replace conventional systems, or augment them, providing a layered solution to communications provision or sensor coverage.
Challenges in the Stratosphere
Solar HALE operations present a number of significant challenges over those which can be undertaken by more traditional air platforms. The lightweight nature of Solar HALE platforms presents structural challenges compared to more traditional platforms, meaning launch and recovery of a platform needs good weather conditions, from take-off, up to operational altitude in the stratosphere.
Once in the stratosphere, conditions continue to be challenging, with platforms needing to hold altitude above the jet stream, where winds can reach 140mph. Temperatures in the lower stratosphere can typically be -50°C or lower. Additionally, systems in the stratosphere are impacted by solar radiation and neutron bombardment. Together, all of these conditions present a very hostile environment, which places high demands on any platform flying there.
Any stratospheric system needs to be able to work with or around these challenges, in order to be able to provide an operationally viable solution to customers.
Two main approaches can be taken to the design of a Solar HALE platform, namely fixed wing heavier than air solutions, and lighter than air solutions such as balloons or airships. A fixed wing solution, such as PHASA-35 presents the most flexibility in operations, especially as regards the ability to re-task or relocate a platform.