Keywords

16.1 Introduction

Today’s efforts to combat climate change have focused on reducing infrared-absorbing CO2 and other greenhouse gases. Meanwhile, thermodynamic heat engine efficiency of industrial economies has gone down, with 67% of heat input being released into the atmosphere as waste heat (Smil 2021). Scientists and political leaders are increasingly agreeing that some form of direct intervention will be necessary to buy time for other measures to take effect. One intervention is to reduce the amount of solar radiation (insolation) reaching the lower atmosphere. The Glitter Belt architecture, discussed in prior work (Komerath et al. 2021a, b), meets all criteria set out by the US National Academy of Sciences for sunlight-reflecting projects. At the outset, the vehicles described here serve as meteorological platforms ale to loiter over the most remote areas of the planets such as the southern oceans. They thus complement space satellites move at over 400 km altitude and 25,000 kmph, challenging resolution and persistence. Aircraft carry radiosondes and other instruments, with special missions using expendable drop-sondes for vertical profile data of the atmosphere. The Flying Leaflets described below offer unprecedented coverage, persistence, and resolution over remote areas.

16.1.1 Prior Work: FL and FLT

The Glitter Belt architecture (Komerath et al. 2017) will place swarms of ultralight reflector vehicles at 30.5 km altitude. Initially, these will be concentrated where summer is at its peak. The chosen altitude is high enough that solar intensity is at the full value seen in space at Earth’s orbit around the Sun. The Glitter Belt architecture is conceived and built from just two types of vehicles. One is the Flying Leaflet, sized for takeoff from small fields (Fig. 16.1 (left)), a solar-powered flying wing carrying a rolled-up sheet of reflective Mylar and a framework to support the sheet. This vehicle ascends to 30.5 km inside 8 hours, but cannot survive night-time glide without coming below the 18.3 km limit of Class A airspace. The second type of vehicle is the Flying Leaf (FL) in Fig. 16.1 (right), assembled at high altitude by in-flight wing-tip rendezvous and attachment of 11 Flying Leaflets (FLTs). The FLT is a 32 m-span, 4 m-chord flying wing with 4 BLDC motor-driven propellers, carrying a deployable Mylar sheet. It has a solar panel area set at 40% of its wing area, including over the leading and trailing edges and upper surface. The solar panels are assumed to be of 20% efficiency.

Fig. 16.1
2 illustrations. On the left is a 4-propeller flying leaflet that glides over a field. On the right is the flying leaf in the sky.

1 (Left) 32 m × 4 m, 4-propeller FLT with partially deployed sheet above. (Right) 353 m × 32 m FL

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16.1.1.1 The Way to 30.5 km Altitude

A mission profile is shown in Fig. 16.2. FLTs takeoff on a summer morning, climbing to 30.5 km by 4 pm. They join with 10 other FLTs, with 8 of 11 returning, leaving their sheets and sheet-frames attached to the FL. The FLs form into swarms for easier control and for distributed antenna functions. Solar power rises with altitude, from about 1.0 to 1.367 kW/m2. Temperature varies from 85 °C when exposed to the sun, to below −57 °C at night in the stratosphere. Strongest winds are in the lower stratosphere during initial climb; while dynamic pressure is highest close to sea level. Motor cooling in solar-heated, near-vacuum, rendezvous and swarm operation for high-precision distributed antenna applications pose challenges.

Fig. 16.2
3 illustrations. 1. F L Ts take off and climb to height. 2. The F L Ts form swarms and V formations. 3. F L Ts land while moving over the sea.

FLT mission profile. (With permission)

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16.2 Conceptual Design

Figure 16.3 shows three successful HALE concepts: The NASA Pathfinder, Helios-1, and Helios-2 used to benchmark the FLT and FL concepts. Relevant parameters are shown in Table 16.1. Our FLT and FL aspire to far lower wing loading (W/S) than prior concepts. This is because much of the lift is carried by the large area of ultrathin reflective sheets, supported by a carbon fiber truss and grid.

Fig. 16.3
3 photographs. 1. An aircraft glides over a runway. 2. An aircraft glides over a dirty road. 3. An aircraft glides over a sea with an island.

Prior HALE vehicles: The NASA Pathfinder, Helios 1, and Helios 3. From Ravikovich et al. (2021) and Wikipedia

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Table 16.1 Benchmarking
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Figure 16.4 (left) shows necessary performance parameters to stay above Class A airspace. The zone between red and green vertical dashed lines is feasible, with CD0 below 0.025, and wing loading below 1.5 Pascals. Figure 16.4 (right) shows how descent profile varies with profile drag coefficients. The notion of joining wings at the tip to increase aspect ratio, decrease induced drag coefficient, and increase range, has been discussed in Quinlan (2019). Wu et al. (2021) have explored joining 2–6 single-propeller wing-tail UAVs at the wingtips. We note in passing that with sufficient control to perform rendezvous, it is also possible to form swarms of sufficient precision to act as synthetic aperture antennae.

Fig. 16.4
2 line graphs. On the left, C D 0 versus wing loading has a descending curve. On the right, hours after the start from 30480 meters M S L versus altitude reached has 3 descending curves for (1.25, 0.022), (1.25, 0.02), and (1.25, 0.018).

(Left) Highest value of profile drag coefficient at each wing loading, to stay up above 18.3 km in 12-h glide. (Right) Effect of profile drag coefficient on glide timeline, at W/S of 1.25 Pascals. From Komerath et al. (2021b), with permission

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Very large antennae can be formed with FLs, a topic for future discussion. In the FL, the main power source is solar panels (assumed 20% efficient) (Ackermann et al. 2021; Kleemann et al. 2020) covering 40–85% of the FLT wings including around the leading edge to capture low-horizon sunlight, and some on the bottom surface to pick up diffused cloud reflections. A small battery (100 Wh) is provided to operate instruments and communications. A generator charges the batteries. Propeller windmilling can recover some power, keeping the battery charged, and enabling a burst flare maneuver for landing at night. The lift and drag coefficient data for a thin cambered flat plate were used to model the lifting sheet (Gilbert 2020). Maximum lift coefficient is 1.2. We restrict operations to CL of 1.0. Structural weight was modelled assuming the technology of the benchmark vehicles of Fig. 16.3 to estimate the total weight of the FLTs and known strength/weight profile of carbon fiber beams supporting the thin (0.05 kg/m2) aluminized Mylar sheets. Figure 16.5 shows that FLT models smaller than 30 m span may not be able to stay above 18.3 km through the night.

Fig. 16.5
A scatterplot of wing loading versus F L T span. It has a decreasing trend for 11-sheet F L. 2 dots at wing loading of 2 and 1.2 from 15 to 32 meters are joined by a line. Values are approximated.

A diagram showing why sheet spans of over 30 m are needed to achieve 12-h glide above 18.3 km

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Space limitations prevent detailing other aspects such as power system (Fazelpour et al. 2013; Isaienko et al. 2020; Dantsker et al. 2020), motors and propellers (Anon EPI 2020; Serrano et al. 2021), and the use of propellers instead of deflectable control surfaces for attitude control, winds, radiation, and other constraints. The risk-reduction process has gone from conceptual design to wind tunnel (gauging wing/sheet positioning and measuring aerodynamic coefficients), ground and low-altitude glide, and powered tests for strength, stability of Mylar sheets and flying wings, and robustness to gusts and sunlight/weather exposure. Flight simulation with present vehicle designs is being used to demonstrate performance and identify problems. Thermal and aeroelastic verification (Voß et al. 2020) and control strategies remain to be done.

16.3 Conclusions

Electric-aircraft aspects of Flying Leaflet (FLT) and Flying Leaf (FL) vehicles that comprise the initial Glitter Belt system are summarized in this short paper. The 30.5 km altitude and slow glide requirements provide unique challenges. Conceptual design, small scale design-build-fly tests, and dynamic flight simulation are used to remove uncertainties and derive properties of this system. The FLT, a 32 m-span, 4 m-chord, 4-propeller flying wing, carries a variably-deployed Mylar sheet to 30.5 km, and joins with 10 other similar craft to form an FL with 12 BLDC motors driving low-inertia propellers. The rise of solar power with altitude is a unique feature. Motor cooling in near-vacuum, rendezvous and swarm operation for high-precision distributed antenna applications pose unique challenges. Rendezvous and swarm operation for high-precision distributed antenna applications pose unique challenges and opportunities.