DIYDrones member and SUAS News author Gary Mortimer wrote an interesting article about the first flight tests of the Boeing Hydrogen-Powered Phantom Eye UAV. The story has also been syndicated at Slashdot.

The platform will no doubt generate interest outside of military applications because a hydrogen energy storage presumably offers a higher energy storage potential than current battery technologies. I look forward to learning more about why hydrogen energy storage was selected. For those of you who follow discussions about alternative sources and storage methods for transportation, Dr. Richard Muller has a number of excellent, publicly available lectures on the realities of energy density, the (simplified) numbers behind gas, cookie powered propulsion, battery energy density and losses, and why hydrogen isn't necessarily an environmentally friendly alternative to gasoline in his outstanding Physics for Future Presidents lectures. 

While Boeing is principally targeting military applications for this platform, NASA, NOAA, and dozens of other scientific teams will no doubt have similar application requirements. If this makes sense for Boeing's perceived military applications (and I'd personally like to learn more about what design targets were met with this alternative approach) it almost certainly makes sense for atmospheric, weather, disaster assistance, search and rescue, and similar applications. 


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Comment by Chris Huitema on June 4, 2012 at 7:12pm

hows it going to land? im assuming there is a big belly full of hydrogen as a liquid? and no landing gear? looks like its deployed from a cart

Comment by Michael Pursifull on June 4, 2012 at 7:20pm

A good question. I've seen trailer landing done during emergencies for Helicopters, but that cart does not look particularly suitable for a fixed wing recovery. I expect with those wings it would be a rather fast approach, and the trailer looks too precise to make a good recovery. 

I also wonder about CG changes from fuel use, from a designer standpoint. I expect that at least some of that large fuselage is used for fuel storage. A water landing might be planned, there might be air pontoons/bags hidden about. Something like that would certainly be less risky if the CG shifts rearward and the props were not spinning. It would be very interesting to learn how it was landed (for this flight, and the long term plan.)

Comment by Michael Pursifull on June 4, 2012 at 7:40pm

Flight Global has just run a story about findings in a January predator crash. The reason I mention it is this is one example (though not a primary one) of why you might decide to use Hydrogen, rather than jet fuel. It seems the predator crashed due to an engine coolant failure. Or rather, at high altitude, there are risks for some fuels of freezing, so you sometimes need fuel heaters. The various challenges for electronics, power systems, freezing affecting sensors are, no doubt, topics of interest for our balloon and rocket amateurs here.

Aside from high altitude, a lot of scientific research is conducted near the poles, and similar challenges exist for our members who design and fly in those extreme environments. But there must be more to the story, since the challenges for handling highly pressurized hydrogen are not offset simply because it tolerates the cold well...


Comment by Ellison Chan on June 4, 2012 at 7:44pm

The only way I can see this thing landing, without tearing apart the undercarriage, is a water landing.  I would suppose that the H2 tank would make this thing quite buoyant, when empty.  But, is liquid hydrogen and oxygen lighter than water?

Comment by Michael Pursifull on June 4, 2012 at 7:56pm

I don't know the pressures in use, obviously, but in general liquid hydrogen us 67.80g/cm^3, while water is 1g/cm^3. Did you read somewhere that they are also carrying oxygen? I know some rocket systems do that, but I thought that many ground-based hydrogen systems use available oxygen? I don't follow it that closely because I've never been convinced that hydrogen, given the current (expensive) methods of preparing it as an energy storage medium, is really a very efficient plan. It seems to me that, at least right now, it is a less dense and more expensive (and most likely less ecologically sound) form of energy. In general, it is my understanding that the production of the hydrogen takes sufficiently more energy (and carbon footprint) and it is only in releasing that energy that it is a "clean" fuel, essentially a shell game of "hide the carbon/cost." But then there are advances across the industry that may solve these issues.

Comment by Ellison Chan on June 4, 2012 at 8:03pm

This video indicates its a oxygen, hydrogen reaction powering it.  But maybe he means oxygen comes from the air, instead of being carried, on second viewing:

That would make more sense, unless they need a higher volume than they can extract from the air alone.

Comment by Ellison Chan on June 4, 2012 at 8:12pm

Quebec has plenty of hydroelectricity, so hydrogen production would be pretty clean for us.  A hydrogen driven economy would benefit us a lot. ;-)

Comment by Michael Pursifull on June 4, 2012 at 8:39pm

The topic of energy storage is important to all of us, but it's a complex topic. Modern newsertainment sound bytes do not do it much justice, with easy quips about "clean" fuel, where the "only byproduct is water." And it can be said that liquid hydrogen as a higher specific energy than gasoline. However, this ignores the energy that was required to create (and transport) the liquid hydrogen, a sizable portion of which is from fossil fuel. So we burn more fossil fuels to "store" a portion of the energy as hydrogen, in essence, every neuton of hydrogen fuel represents a larger amount of energy generated in the near past (along with the attendant pollution/carbon load.) 

It also bears mentioning that while the specific energy is higher for liquid hydrogen, the density is also much less, so you have a serious volume difference between the two. To put some numbers to it:

Energy Form                     - Specific Energy - Energy Density

Liquid Hydrogen (storage)  - 39,410 W·h/kg    - 2,359 W·h/L  

Gasoline (fuel)                  - 12,700 W·h/kg    - 9,700 W·h/L

Li-Po (storage)                  - 130-200 W·h/kg   - 300 W·h/L

For comparison; however, note that the conversion efficiency and discharge efficiency for lipo is around 99.8%. By comparison, the production of hydrogen varies, depending on which method is used, and whether you count upstream energy costs as part of the loss to between 60% and 70% efficiency. So if you have the electricity already, storing it in a lipo for use in the near term (there is a loss over time) is an attractive option. But of course energy density is directly tied to weight, and finding a more efficient (i.e., lighter per watt) method of storage is critical for our flying robots. 

The reason I find this specific platform so interesting is that research in lithium sulfur battery technology suggest a [reusable] storage method that has an energy density of between 1800 - 2100 W·h/L, with few of the safety, handling and specialized requirements (try finding a hydrogen refueling station in distant parts of the world where this UAV is likely to operate...) of refueling. And it would have the same density. So there must be reasons, aside from this, that Boeing seeks to gain by this approach. Or perhaps it is simply a safety, supply or cost issue (a dozen tones of lithium sulfur is also not exactly safe, or inexpensive.) 

Comment by Michael Pursifull on June 4, 2012 at 8:42pm

So perhaps with a platform this large, it isn't an attractive alternative. But for our use, I would love to get my hands on lithium sulfur. Imagine a drop in replacement (mostly, the lithium sulfur battery is a simple manufacturing change in existing materials, like swapping brown sugar for white) but the resulting batteries expand up to three times their volume when charged... but you can get 8 times the energy density.... quad flights go from 8 minutes to 64 minutes! A 45 minute bixler flight.... becomes an all day event. 

Comment by Ellison Chan on June 4, 2012 at 8:56pm

I don't think anything non-nuclear rivals gasoline, in energy density.  But the energy in oil took millions of years to store.  What we really need is a better and cleaner way to extract the energy than just burning it, which is highly inefficient.  Also the energy required to refine it into gasoline is also a waste, and unclean. 

I never heard of Lithium Sulphur batteries.  Need to do some reading, but sound promising.  

The other side of endurance is efficiency.  For electric flight we need something better than running electricity through copper wires.  


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