# Could robotic ornithopter have all energy it needs to be able to fly from solar power?

I read from here that there’s this material called Organic photovoltaic (OPV) solar cells that could be used, for example, as wing material for an ornithopter. How much energy would it be able to provide and would that alone be enough for a robotic ornithopter to fly on Mars, and possibly carry a small amount of payload (like 1kg)?

• Well in I meant to find solutions for Mars' conditions but it should be able to work on Earth also 30km from the ground. – Paula-R. Feb 6 '17 at 21:50
• Mars, unlikely, weaker sunlight & very thin atmosphere. Earth at 30 km. That's around the height that a U2 spy plane would operate. The height record for a conventional helicopter on Earth is 42,500 feet (12,954 meters), so a solar powered helicopter would have no chance of reaching 30,000 meters! – Andrew Thompson Feb 6 '17 at 22:13
• To the close voters: it's my opinion that this question is answerable as stated. The Nature article includes weight and power conversion numbers, some of our other Mars-flight-vehicle questions link to articles with power-required calculations, and the amount of solar flux at Mars is known. It's feasible to provide a back of the envelope calculation from these things, though I don't have time to do so at work. – Bear Feb 9 '17 at 13:35
• @Bear Could you help me find sources so I could keep searching answer for my question, where is this Nature article you mentioned? Thank you :) – Paula-R. Feb 14 '17 at 13:56
• @Paula-R I actually meant your own first source, which includes a figure of 10 W g−1 for the authors' best performing OPV devices (first paragraph of the discussion section). I'll see if I can write you up something here shortly. – Bear Feb 14 '17 at 15:03

I'm going to try to answer a more specific question than your title asks, which is Could OPV solar cells plausibly supply enough energy to maintain a flapping-wing aircraft on Mars?

The only well-studied Mars flapping-wing concept that I am aware of is the Ohio Aerospace Institute report on an entompoter that you had asked about in a previous question. Unfortunately, that vehicle isn't suited for purely solar operation because it requires consumables:

Even though positive net lift is obtained from both the CFD and analytical solutions, the force is not sufficient for flight in Mars’ lower atmosphere. Hence, active flow control technology will be used to augment the lift, and thrust along with reducing the drag of the Entomopter... Blowing of the wing in a proper way is critical not only to the stability and control of the Entomopter, but in the Mars application, its ability to fly.

One of the clever part of the entomopter concept is using exhaust gasses from its "muscle" to achieve additional lift, and (by blowing on its own wings) to steer. Without it, the vehicle won't work in the rarefied Martian atmosphere - the craft needs to expel gas to function. And you can't replenish the gas with solar energy alone.

(I suppose one could imagine a solar-powered condenser light enough to fit on the craft. I'm skeptical that it's possible, but I invite someone to ask a follow-up question to find out!)

This study suggests, then, that the answer to the question is no: any solar cell will fail to supply enough energy for a flapping-wing aircraft on Mars, because flapping-wing vehicles can't fly on Mars at all without some sort of expendable propellant.

That answer isn't very satisfying. Let's play with some numbers and see if we can glean anything else.

In your first source, Kaltenbrunner et. al. present their results from testing a variety of configurations of Organic photovoltaic (OPV) solar cells, which are extremely thin devices that under investigation because they can be made flexible and lightweight.

From the article's discussion section:

The OPV devices constructed on PET presented here have a per-area mass of $4 gm^-2$, and 4% efficiency, giving $10 Wg^-1$.

As best I can tell, this figure represents the authors' most successful prototype. It's plausible that the efficiency could actually be improved, but we'll start with these numbers.

The figure of $10W/g$ (roughly) appears to have been computed as:

$$({1050W/m^2}) / ({4g/m^2}) * 0.04 = {10.5W/g}$$

where $1050W/m^2$ is a common figure for solar irradiance at the Earth's surface. For our purposes, we need to use solar irradiance on the Martian surface instead. This report on Mars entomopters suggests that this is about $590W/m^2$ on average.

Our quick-n-dirty calculations then suggest that (neglecting any and all effects on the OPV's materials by Martian surface conditions) the panels in the study would give us $5.9W/g$ to play with on Mars.

Is this enough to fly?

The OAI study's primary design point on page 49 requires $883W$ to fly and appears to have $0.107m^2$ of wing area per wing set (of which it has 2). Coating the wings in OPV panels only costs $0.856g$ - no wonder Nature Communications was excited about these! If we do that, we buy ourselves:

$${0.214m^2}*{590W/m^2}=126.26W$$

Keep in mind you only get this during daylight. When you factor in the power needed to keep the electronics and instruments from freezing (and the fact that you need to store energy so they don't freeze overnight, either), this really isn't much to work with. Maybe you could make a short flight every once in a while after charging your batteries, if we completely neglect the gas issue.

Unfortunately, this (admittedly lacking) analysis also suggests that the answer is no, solar cells cannot plausibly power a flapping-wing vehicle in the Martian atmosphere.

Consider trying a large fixed wing craft for the greatly increased surface area.

• Thank you so much for your effort! It's certainly much more than I could hope to find on my own... Now I just need to find out how much energy the ornithopter would need to be able to fly (on Mars) and compare that result to your calculations. – Paula-R. Feb 14 '17 at 21:18
• Finally had time to take a closer look. Sorry that took so long. Is this for the NASA challenge you mention on your profile? I think your team should start looking at propellant transfer, generation, and storage solutions if you want to keep the entomopter idea. – Bear Feb 15 '17 at 1:50
• "(I suppose one could imagine a solar-powered condenser light enough to fit on the craft.)" - I can imagine a solar-powered condenser on a ground rover accompanied by the flying craft, serving as "refueling station" in between flights. – SF. Feb 15 '17 at 8:24
• @SF. That seems like it would be the better solution, of course, but unless the condenser is part of the entomopter itself, it doesn't seem to fit the "OPV panels alone allow flight" criterion to me. – Bear Feb 15 '17 at 13:21
• @Bear Sorry it took such a long time to answer you. Yes, this is for the NASA challenge. I wonder if these calculations that you so kindly made are suitable for an ornithopter too? As I have understood the flying mechanism of an ornithopter is a little bit different than in entomopter. I managed to found this OAS's research about solid-state aircraft (SSA): niac.usra.edu/files/studies/final_report/836Colozza.pdf -I wonder if this could help, I haven't have time yet to read it and I'm not quite sure if this SSA uses flapping-wing mechanism. I'll need to look in to this. – Paula-R. Feb 19 '17 at 4:46

I think I found my answer (at least partly) when I found a study about Solid-State Aircraft (SSA) which is a non-traditional ornithopter that uses a flapping-wing mechanism and flies like a bird. The study of SSA suggests that solar arrays (which consists of organic photovoltaic solar cells) which are implanted to the wings of the SSA need lithium battery (thin-film lithium batteries in this case) to preserve and supply the energy continuously, but the solar power is the only source of power the SSA uses. In the study there were separate sections for Mars purposes and based on the study the SSA should be able to fly there too.

That proves that at least this type of ornithopter can actually have its energy for flying from solar power, but as far as I understood it doesn’t consider carrying any payload with it.