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Comment by someuser54541 | Hacker News Reader

How does the aircraft get to 70k feet? Is it launched from a separate vehicle?

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.

Winds up to 20 km are NOT quite calm. They can max out to 60 m/s at height of 10 km, which is more than 3 times than enough to blow a solar aircraft far-far away. You have to choose proper meteo conditions for climbing and descending and plan the trajectory taking into account those winds to be able to land at a given place. I've been analyzing GFS data a couple of years ago for similar project. The problem is that lack of energy demands to design a really low speed aircraft for low densities that you can find in stratosphere. At 10 km winds are stronger than in stratosphere and density is higer. You can find the results in https://journals.sagepub.com/eprint/GVXWXDABPE6A8PNRGTBU/ful.... The whole aircraft model probably is not like yours and is subject to many conservative estimates, but I am pretty much sure that wind model is accurate in this publication and generalizable to different regions.

I'd assume it's like on Mars, where wind speeds are hellacious but there's just not enough 'air' mass to do any real harm.

I think the comment you responded to was saying at 20km ("up at 20km") it is calm, but below this it is not calm.

I suppose an aircraft like this can climb like gliders do, using the streams of air that move upwards. This requires some planning, and likely a skilled glider pilot to help choose the course.

I'm not an expert but I don't think thermals rise that high.

They could help the first few kilometres, where the air density and thus the drag are highest.

I'm similarly curious. Designing for a higher stall speed permits smaller wing area, lower drag, and lower weight. The cost is that takeoffs and landings become troublesome.

Could be catapult launched?

Lee waves? Thermals? Cumulonimbus thermals? :)

I don't think one could just "throw" something 70k feet up in the air.

Climbing to altitude is the straightforward part. The transition from zero knots to the stall speed of the aircraft (minimum speed at which it can remain airborne) is the tricky bit. Designing for a lower stall speed necessitates wings which produce higher drag (by being larger) which requires more propulsion, which means bigger batteries and motors. So launching from a catapult or rocket or mothership or whatever means a lighter plane.

Launch on a hydrogen balloon

Launching by rocket means the plane would have to be pretty rugged to survive it. And that means more weight. You also have the issue of deployment. Folding wings means more weight and more things to go wrong.

Spinlaunch would disgree but here the trouble is getting off the ground; Take-off is the most demanding phase of flight in terms of propulsion demand.

Spinlaunch would crush any aircraft that is light enough to stay aloft indefinitely.

Or balloon launched, parachute landing.

Other options that might work: * Launch from the roof of a vehicle * With a glider winch * Towed by another aircraft * Auxiliary engine / batteries that can be jettisoned & parachuted down

Winch launch?

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