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jthomaslm
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james@radicalaero.com
- jthomaslm parentThanks for sharing. Estimating air mass and solar irradiance are definitely interesting topics - I'll take a look at what you've put together!
- Our aim is to fly at higher altitudes than Solar Impulse/SkyDweller, which keeps us above the turbulent winds and cloud cover of the troposphere, and makes long endurance flight more reliable/less weather dependent. Our platform is also a lot smaller than SkyDweller, which helps keep costs much lower and opens some interesting new opportunities - for example, deploying lots of aircraft in a mesh network.
The Solar Impulse/SkyDweller lineage is very inspiring - what an amazing way to demonstrate how much you can do with solar power!
- Love the enthusiasm - it's definitely a very fun project to be working on!
Discussed a few of your questions elsewhere in the comments, but here are some thoughts on scaling laws:
Over-simplifying a bit for the sake of brevity: In general, for larger aircraft structural mass scales up more quickly than wing area (check out “indoor free flight models”). By using a span-loaded structure (i.e. having weight distributed along the wing span to match the way lift is spread over the wing) you can avoid some of these constraints and have an approximately constant structural mass fraction.
The aircraft is actually very quiet - it needs to be very aerodynamically efficient so it doesn't require much thrust. You can’t really hear it in flight, but I’m no audio engineer - so I don't know how useful microphones would be. Genuinely curious about this so if you have any pointers let me know!
- Yes, FB were trying something similar to this with Aquila. We actually used to work with a few people on that project, it sounds like the all composite flying-wing design was a bit of a challenge for them. I also understand they weren’t providing internet direct-to-device, just as backhaul for cell towers, which proved to be a difficult business case. In the end they partnered with Airbus on some projects, but I don’t know what came of that.
- Quite a lot has changed in the past decade. Li-ion Battery energy density has roughly doubled, solar tech has come a long way (both in efficiency, but also production scale/costs for commercial silicon cells), and there’s been a wave of improvements for miniaturized drone/UAS hardware. On top of that, there’s 50 years of existing research into solar flight - we’re lucky that we can learn from that!
- The aircraft is able to climb under its own power. We have a diurnal energy cycle - charging the battery up through the day and deploying battery energy in the night. If we launch in the morning with a full battery, we have a whole day's worth of extra solar power to use to climb up to altitude.
Winds will be a bigger issue than energy when climbing. Up at 20 km (70k ft.) winds are quite calm, but we need to ascend through more turbulent winds as we climb. We’re sizing our MVP around this.
- This is a concern for us. As with many disruptive technical advancements (e.g. in Nuclear or AI), there are many ways our technology can be used - both good and bad. It's important to us that what we do is ethical - that means supporting and pursuing use cases with huge positive societal value like rural connectivity or wildfire monitoring. With that said, there are definitely ethically-questionable use cases for this technology and I don’t underestimate how difficult it will be to navigate. We certainly have no intention of monitoring civilian populations and haven’t spoken to any police forces about what we’re doing.
- Wildland forest fires were actually the inspiration for this technology! A year ago Seattle had the worst air quality in the world for a few days, which piqued my interest in the area. Speaking to people in the USFS we learned about the challenges of getting real-time high-resolution data, and realized that an ultra long endurance drone would be a great way to get this.
- Yes, Airbus’s project and team is very cool! What we’re doing shares similarities with Zephyr, but we’re focusing on a lower cost system, and a larger payload which will enable more use cases.
We’re using conventional silicon solar cells, rather than the GaAs cells used by Airbus. We give up some efficiency by doing this, but it keeps costs far lower - which we think is key to iterating quickly, and opening up some of the market use cases.
Similarly, we’re sticking with conventional battery chemistries (Lithium Ion). Battery energy density is by far the biggest driver for this technology - in the past decade we’ve seen huge advancements in battery tech, which is one of the reasons this technology is now possible. As you identify, cycle life is a key challenge and what will limit the aircraft’s endurance.
- Each has merits, but in our evaluation, if you are able to overcome the aircraft design challenges, fixed-wing offers a far more capable platform. The stratosphere has winds that can exceed 100km/h. With balloons, station-keeping is an enormous challenge. We spoke to a lot of people in the Google Loon project, and ultimately this became a defining problem for them. Airships/dirigibles help a bit, but still have to drift in strong winds - beyond that, they also become extremely large and expensive to work with.
- The 24 hour test was with a sub-scale prototype at low altitudes. It turns out that testing a small aircraft in the thicker air at these altitudes matches some important physics of the larger aircraft (Reynolds number).
Perhaps unexpectedly, climbing to 20km altitude isn’t too large a problem from an energy perspective. In a typical energy cycle, the aircraft has low battery in the morning, so if we launch with a full battery charge we have plenty of extra energy to climb up to altitude.
Prime Air - definitely a serious project, but I’m sure Amazon didn’t hesitate in milking the PR! Making a reliable drone delivery service at Amazon scales certainly isn’t easy. That team is still going strong and I’m sure we’ll see more from them as they ramp up commercial deliveries.
- There is definitely some market overlap in wireless telecommunications. But there are some distinct differences with satellite services. One big one is that our aircraft isn’t in orbit, so it’s much easier for us to provide targeted coverage over a specific area without needing a full constellation. Flying at 20 km also puts us much closer to users which makes high-speed direct-to-device service (5G) a lot easier.
In terms of coverage, from that altitude, we have good line of sight coverage over large areas. Obviously, satellites can cover wider areas, but it turns out that in most suburban/rural settings the ground footprint becomes limited by the population density and bandwidth available - so the larger satellite footprint is only really useful in extremely remote locations.