AskScience AMA Series: We're Cheryl Bowman, Deputy Branch Chief for High Temperature and Smart Alloys, and Sean Clarke, Principal Investigator, X-57 Maxwell Experimental Aircraft. We are part of the NASA team that is developing new technology for Electrified Aircraft. Ask us anything.

nasa · 2020-12-17T21:28:06+00:00

This is an important consideration when designing an aircraft. The design needs to match the mission, so it's important to understand all the different scenarios the aircraft might face. In the case of batteries, the designers need to understand the energy required for the whole flight and understand the instantaneous power required for different segments of the flight. These two factors drive a lot of the design of the rest of the electronics. If your aircraft is intended for long range but does not need a high top speed or climb rate, you could choose batteries that have a high energy density but a high power rating. If you only need to fly for a few minutes, but you are doing high performance aerobatics, you can choose high power cells that don't have as much stored energy. The tricky mission is one where you need high performance some of the time (e.g., an aggressive takeoff or climb) but also want to fly for a long time (maybe more than 30 minutes).

X-57 is in this tricky corner of the requirements. We need to fly for about 30 or 40 minutes, but we also need to use a lot more power for the "high lift" parts of the flight at takeoff and landing. The cells we use are a good compromise between power and energy (Samsung INR18650-30Q) for our applications. This has the nice side effect of letting us study how well systems work that are very high power at takeoff and much lower power at cruise because this is similar to what vertical takeoff and landing vehicles will need to do as well. - Sean

nasa · 2020-12-17T21:05:10+00:00

One of the biggest contributions from the X-57 team is sharing what we are learning with stakeholders, including civil regulation authorities (e.g., FAA), certification standards organizations, industry developers taking the technologies into the marketplace, and academic researchers studying the next-generation technologies. We participate in consensus standards development to help adapt CFR 23 and eventually CFR 25 to accommodate these technologies along with experts from industry.

In order to focus on the research goals of X-57 (the new, high performance wing integrated with electric motors) we are trying to avoid changes to the fuselage to keep our team from redesigning the whole aircraft. The P2006T tail is probably oversized, so we are studying that with computational fluid dynamics to make sure we can control the aircraft in all parts of the flight envelope with the existing tail.

I probably can't dive into all your great questions in the time I have today, but we have a portal linking to the research papers we have written on this and other topics at https://nasa.gov/X-57/technical/ - Sean

nasa · 2020-12-17T21:03:09+00:00

In our "High Temperature and Smart Alloy" branch at NASA Glenn Research Center, we focus on metal for propulsion. Our two main areas of focus are the alloys for the hot sections of turbines and rockets (such as nickel-based super alloys) and unique alloys that enable special applications, such as Shape Memory Alloys and Soft Magnetic Alloys. I honestly cannot remember who came up with the moniker "Smart" for those classes! - Cheryl

nasa · 2020-12-17T21:00:58+00:00

This has been fun; thanks for having us! One surprising thing I saw firsthand was how much energy batteries can release when they experience thermal runaway. We intentionally forced our cells into runaway in an early version of the battery pack and found that 320 of our 3 Ah cells was enough to shake the building! That "test to failure" was a great way for us to collect the data we needed to redesign our battery packs using experts that helped design batteries for astronaut EVA suits in order to make sure a cell going into runaway doesn't take all of its neighbors with it. I'm excited that this technology is in the X-57 battery design now. - Sean

nasa · 2020-12-17T20:57:55+00:00

You are right, battery performance is a crucial design parameter. You can see more information on battery development in this summary from a recent battery workshop, here.

We are also actively pursuing turboelectric solutions for larger aircraft. This allows us to use the energy storage potential of jet fuel but the distributed propulsion benefits of electrical system. One of the great things about this field is that there are many possible solutions to the technical challenge of more efficient aircraft. We don't know which solution will work best because we don't know "yet" the rate at which the underlying technologies will mature. - Cheryl

nasa · 2020-12-17T20:50:28+00:00

Alternative fuels including synthetic fuels are also an important part of the technology development equation. Electrified propulsion research is proceeding in parallel to and not in place of fuels research. - Cheryl

nasa · 2020-12-17T20:48:04+00:00

Hybrid configurations are a great option to explore for many applications. It gives us the benefits of lightweight and reliable propulsors without the potential drawbacks of the less energy-dense batteries compared to carbon fuels (with the drawback of carbon fuel emissions, though). One reason some aircraft may not use hybrid systems is that it's more complex than all-electric powertrains. This could complicate dynamic events like when a pilot pushes the throttles forward to abort a landing or make some other emergency maneuver. The power buses may face challenges if there are delays as the combustion engine is spooling up at a different speed than the electric motors are pulling more power. These challenges can be tackled, but it takes research and testing. NASA researchers are exploring all these options in labs, research test cells, and eventually in flight tests. - Sean

nasa · 2020-12-17T20:37:07+00:00

Yes!! Machine learning is a useful tool in many aspects of aeronautics research. We use it for analyzing alloy chemistries specifically designed for Additive Manufacturing, for example. - Cheryl

nasa · 2020-12-17T20:36:09+00:00

There are a lot of tradeoffs for these new motors. They are a lot lighter than the combustion engines they are replacing, so we can mount them in places that would not be practical with engines. We studied a lot of configurations when we were designing the X-57 wing, including whether to use a pusher or traction configuration. We ended up with the traction configuration to get better mechanical mounting structure to the motors without interfering with the ailerons. A side effect is that the aileron is a little more effective than the cruise propeller is at higher power, so that's an interesting design challenge (because sometimes the cruise propeller is at lower power!). - Sean

nasa · 2020-12-17T20:35:07+00:00

We do not have a definitive answer for time frame. There are a number of companies that are exploring the options for air-taxi type services and those small aircraft could be all electric or hybrid electric. I can say larger regional jet or single aisle passenger aircraft would be later. Here is a link to NASA studies on this for reference. - Cheryl

nasa · 2020-12-17T20:29:16+00:00

Electric propulsion technologies certainly help make autonomous vehicles more practical, and batteries work well for short flights (around the size of a city, for example). Aircraft that use wings for lift are a lot more efficient than multicopters, so it may be possible there will be delivery vehicles that transition between vertical takeoff and wing-borne flight for cruise. There are other challenges that NASA researchers are tackling to help here also in the areas of autonomy and air traffic management. Those might be good topics for another time! - Sean

nasa · 2020-12-17T20:21:18+00:00

Thanks. That is our home electronics lab, mostly working from home these days. - Cheryl

nasa · 2020-12-17T20:19:37+00:00

The magnetic properties of materials are driven by the alignment of dipoles in the atomic structure. You can "demagnetize" a "permanent" magnetic by overheating it. Soft magnetic materials help shape magnetic fields in motors and electronic devices. When you operate a soft magnetic material at too high a temperature, it no longer provides the additional shaping. We have been developing new alloys and processes for soft magnetic materials in the "nanocrystalline" class of soft magnets and have a patent on an alloy that operates at 300C with the same electrical losses as a conventional alloy that tops out at 150C. - Cheryl

nasa · 2020-12-17T20:18:54+00:00

The silly answer is "zero miles," because in our research flights, we will always be landing back on the main runway at Edwards Air Force Base where we took off from. If we flew in a straight line instead of conducting our flight test maneuvers, we could probably fly about 100 miles, depending on what altitude we chose. The Tecnam P2006T is rated to about 2700lb gross takeoff weight in the FAA certified configuration, but the X-57 with the new wing and lots of analysis by our researchers has increased the limit to around 3200 lb. We are working to remove anything we don't need so that we can fly a little longer, but there isn't really any cargo capability left over between our battery system, instrumentation system, and test pilot that are required for all of our operations. - Sean

nasa · 2020-12-17T20:13:03+00:00

We do use Additive Manufacturing (AM) extensively in prototyping (as everyone does). I believe that there are already certified AM parts flying as subcomponents in commercial turbine engines. Several consensus standard organizations (like SAE and ASTM) are looking at certification standards. NASA is also looking at AM for designing complex components that can enable features not available from conventional machining. We are doing things like looking at combustors with internal swirl designs that can be made through AM. - Cheryl

nasa · 2020-12-17T20:12:31+00:00

We're actually pushing the design in the opposite direction where possible. If we can design the motor and propellers to spin slower then the noise from the prop is greatly reduced, and the forces are more manageable. We also choose odd numbers of propeller blades so that as each blade sweeps past the leading edge of the wing there isn't another one sweeping past at the same time. This also makes them quieter. - Sean

nasa · 2020-12-17T20:09:23+00:00

We wouldn't be able to speak on behalf of the industry or companies when it comes to pricing and costs, but it's possible we may see the same benefits in aircraft that automobile users see. That's one of the things we want to study! We already have some experience with this after running different motors in our labs and test benches for hundreds of hours. As we get into flight testing on X-57 we'll start to learn whether there are operations and maintenance challenges we don't know about, and we'll share what we learn as we go. - Sean

nasa · 2020-12-17T20:05:39+00:00

Shape Memory Alloys and Super Elastic Alloys are materials (most frequently metals) that have unique shape changes associated with their crystal structure. These shape changes can be driven by heat or magnetic fields. In the 2019 Boeing ecoDemonstrator research aircraft, NASA and Boeing worked together to design, build, and test a vortex generator, which improves air movement at low speeds but folds flat at higher altitudes. This passive folding was enabled by a shape memory alloy that drives a torque tube in response to the temperature change. Shape memory alloys could be used in future electrified propulsion aircraft to take advantage of waste electrical heat. We also use these alloys in a lot of space applications--but that is a subject for another time!! - Cheryl

nasa · 2020-12-17T20:04:25+00:00

Electric motors can do lots of interesting things for aircraft, not just drive propellers! NASA has researched using pressure (and suction) to change how wings perform in flight by increasing laminar flow or reducing the boundary layer. I hope we see more interesting applications as these technologies get out there. We also have researchers looking at heating the skin of the aircraft to change how the aerodynamic effects work in flight. - Sean

nasa · 2020-12-17T20:02:40+00:00

Lots of questions here!

Yes, regeneration turns out to be a key feature to enable controlling the glide path of X-57 at landing. We have so much extra propulsion that we need to make sure the aircraft isn't going too fast when we land, so we can control the drag by putting energy back into our battery. It only really makes sense at landing and descent, though, because regen is a very effective way to descend quickly without letting the airplane speed up beyond structural limits.

Hybrid configurations make a lot of sense for longer range missions. X-57 doesn't use a hybrid power plant because we needed to keep the traction system as simple as possible in order to focus on the research wing and motors, but a lot of our technology and lessons learned would benefit vehicles with range-extenders as well.

We are using passive air-cooling on all the systems in X-57 to keep everything as simple as possible. Our batteries will be pre-chilled to around 10°C before takeoff and might be as high as 50 or 55°C by the time we land, which is around the limit for comfort for our test pilot, who will be sitting in the cabin with the batteries. Our motor nacelle cooling is also really tricky because we have the motors to keep cool as well as the power inverters and instrumentation system.

One big challenge with packaging the electric propulsion technologies for X-57 has been getting everything to fit! Our new research wing is about 40% the size of the original P2006T wing, and we need to move that 300 kW from the cabin out to all the motor nacelles. We worked with a wire manufacturer to develop custom, highly-flexible cables (4 AWG wires made of of 40 AWG strands!) that we can snake through conduits in the wing along with all the rods and pulleys used to control the flaps and ailerons and the research instrumentation system we use to measure how everything performs in flight. - Sean

nasa · 2020-12-17T19:55:35+00:00

It is not a simple question and there is not a simple answer! Yes, batteries are heavy and kerosene (jet fuel) is very efficient. Using batteries only for power storage is only being considered right now for small, short range aircraft. For instance, you might be able to design a safer "urban rescue helicopter" by having propulsion driven by multiple electric motors rather than one main engine shaft.

For larger aircraft, the solution has to be more than just batteries. We already generate electricity from the turbine shaft on aircraft. People have looked at whether all the turbine power could be converted to electricity and then distributed throughout the aircraft.

In the near term, some hybrid combination of turbine shaft power and distributed electric power can provide new propulsion-airframe integration opportunities that we hope will make aircraft more efficient and lay the ground work for future all-electric aircraft.

You can learn more about this by checking out NASA EPFD Battery Industry Day!
-Cheryl

nasa · 2020-12-17T19:48:11+00:00

Electric propulsion has a few benefits that we are hoping to explore. The lower emissions in flight is one of them, but electric motors are also much easier to maintain than complex combustion engines, so operating cost and maintenance downtime could potentially be reduced. The noise level could also be improved for a couple reasons: first, the sound from electric motors is quieter and higher frequency (so it might not carry as far -- we have acousticians studying this!), and second, we can distribute the propulsion across more of the aircraft so we end up with many little propellers instead of one or two big ones. - Sean

nasa · 2020-12-17T19:46:31+00:00

I love to work with test pilots. We have all sorts of flight research projects that rely on test pilots at NASA Armstrong where I work, as well as the other NASA Aeronautics centers at Glenn, Langley, and Ames. You can take a look at applying here, and jobs at NASA are posted at usajobs.gov. We also have info on internships available at intern.nasa.gov. One of our X-57 pilots is actually the branch chief for the Armstrong test pilots. - Sean

nasa · 2020-12-17T19:44:12+00:00

Ah yes, the Edison vs. Tesla question!! Actually, there are pros and cons both ways. It is definitely an entire system optimization question. I think the winning electrical architecture solution will depend on which subsystems improve faster. Electromagnetic Interference (EMI) is an important issue, and we are looking at power cables as an entire subsystem - Cheryl

nasa · 2020-12-17T19:32:54+00:00

I certainly can't predict, but I'm excited about the opportunities that electric propulsion technologies are bringing. It's important to keep in mind that in-flight emissions are only part of the story, so lower emissions in power generation is also key to reducing the impact of aviation. At NASA we're working to explore as many avenues as possible to make aircraft more efficient, including some zero in-flight configurations like X-57. Battery power density has a ways to go for commercial all-electric vehicle configurations, but there are already some interesting vehicle concepts that could work for shorter trips less than around 100 miles. - Sean

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