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Focusing on aircraft, not balloons, I found this question that led me to some basic research on atmospheric satellites. I imagine the technology has evolved since NASA's research more than 10 years ago.

Given the following points:

  • The aircraft cannot stay in flight forever, as it must land for maintenance (as would any aircraft).
  • In 2001, NASA planned a 40h long trip based on solar powered technology.
  • A fuel-powered UAV can stay up to 33h in flight.
  • The Qinetiq Zephyr stayed aloft about 2 weeks.
  • Redundancy can be added to continue operation in case of equipment failure (as in any commercial aircraft).
  • 'Real' satellite can stay airborne for several years (payloads seem to be able to operate several years without maintenance)
  • If the aircraft can land, the payload and equipment do not have to be as reliable as they would on 'real' satellites, as they could be changed or repaired.

For an atmospheric satellite based on solar-powered airplane technology such as the NASA's Helios, operated in normal conditions, what is the most restrictive element that make landing compulsory?

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    $\begingroup$ Probably because there is no application that needs that, why to create an overdesigned airplane? $\endgroup$ – Trebia Project. Mar 30 '15 at 9:57
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    $\begingroup$ Although it isn't an exact match, this question covers a lot of relevant points $\endgroup$ – Pondlife Mar 30 '15 at 10:01
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    $\begingroup$ Related, if not an outright duplicate: What are the factors limiting how long an aircraft can stay aloft? - All of these (including fuel: availability of sunlight / battery capacity) would apply to a solar aircraft. $\endgroup$ – voretaq7 Mar 30 '15 at 16:20
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    $\begingroup$ Balloons can stay aloft indefinitely. But they don't because there's really no need to $\endgroup$ – rbp Mar 30 '15 at 18:08
  • $\begingroup$ I believe that currently, even being at the very frontier of pholtovoltaics and battery technology, those things simply have barely enough power to lift their batteries off the ground, let alone carry some substantial payload. We would either need a significant breakthrough in battery technology (storing much more energy per mass) or photovoltaics (much more efficient solar cells reducing the need for batteries). Breakthroughs in material science (lighter materials) probably won't help much, since the weight is dominated by the batteries anyway. Although, more resilient airframes may help. $\endgroup$ – Jörg W Mittag Apr 1 '15 at 14:22
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The most common failures in planes start with: a human mistake or a mechanical failure. Where, of course, mechanical failures are much more likely to happen on moving parts.

Satellites have very little moving parts: you get couple servo motors to adjust the angle of your equipment such as the solar panels and the antennas. Both happen in units of full rotations a day. You also often have some small rocket engines to correct the satellites course, but they are rocket engines whose moving parts comprise more or less just one valve.

In a plane, however, you have pieces that move very fast and fluids of all kinds (not only fuel which you need not have on a solar-powered plane, but also lubricants, hydraulics etc.), also very sensitive mechanical sensors whose non-stop operation is crucial for the flight. This all needs to be inspected, oil levels checked, hydraulics checked. Last but not least, the weather in the space is quite calm, compared to the atmosphere. Depending on how high you fly, you meet clouds, storms, birds, temperature changes etc., which all can add to the wear of things and make things require maintenance.

I'm not sure which would be the limiting factor, but I think that either some equipment wouldn't be operational without maintenance, or you would get a mechanical failure of some kind.

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    $\begingroup$ Considering Helios flies around at 96,000 feet, clouds and storms usually aren't an issue. Also, birds tend to stay a bit lower than that... $\endgroup$ – reirab Mar 30 '15 at 15:05
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    $\begingroup$ @reirab Yes, I know, but these were minor points in my answer, and after all, you still get weather and winds up there, just not as violent. $\endgroup$ – yo' Mar 30 '15 at 15:14
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Other than mechanical or human factors, the other key considerations would be the weather and the season.

The weather would affect even a high-flying solar powered aircraft because they descend (albeit slowly) during the night cycle. In addition, their design requires a very carefully managed flight envelope: an unstable air mass could easily destroy the aircraft. Note that thunderstorms, jetstreams and even mountain wave can reach into daytime flying heights as well. There needs to be a trade off between how high the aircraft can fly in the thin air and the size, weight and fragility of the resultant design. This would put the aircraft at eventual risk during one or more phases of the flight.

The season (in combination with the aircraft's position) would affect the amount of energy it could receive from the sun. Whilst a solar powered UAV may fare well in the summer months in the northern hemisphere, it may not be possible to fly it continuously in the winter months, for example. As most applications are location specific, and typically over industrialised regions, this precludes moving the aircraft to the equator or flying a sinusoidal pattern between the two tropical circles.

In your mechanical assumptions it would also be worth accounting for ionising solar radiation (which may affect onboard computer and memory chips) and stronger UV light, which is likely to have a degrading effect on most of the materials used to build the aircraft.

Finally, just a mention that redundancy may be a sensible approach for when there is spare payload and/or energy, but would be less desirable when these were both at a premium.

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One that I can think of is the charge/ discharge effectiveness of the battery. the more charge/ discharge cycle applied to a battery, the less effective the battery can hold its charge, down to 70%. Eventually, the batteries need to be replaced before running out of power at night.

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There are some practical matters which factor into this issue.

With satellites there is a high launch cost, and servicing them in space is also very expensive. So the systems are designed for an economic life which is derived from the expected failure rates of all the systems, and things like the depletion rates of RCS used for station keeping, etc. So there is a high economic incentive to design systems that have a long life.

With solar powered UAS, the costs are much lower, and certainly the launch cost is a couple of magnitudes or so less. The cost to service is low, if the aircraft can be retrieved, maintained and relaunched. So really there is little economic incentive to make the investment to have a UAS which can stay on station for years.

So in my opinion, it is possible to have solar powered UAS which will station keep for very long times, but there is little economic incentive to develop the high reliability systems (perhaps including redundancy) because the cost to retrieve and relaunch is low.

One such application for aircraft like this is to provide data connectivity over cities or even remote areas, kind of like launching an airborne hotspot.

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