A related possibility is multi-rotor helicopters where the main power source is a jet engine, but the power distribution is electrical. One of the huge headaches of the Osprey is the mechanical linkage by which one engine can power both props. There are flexible shafts, U-joints gears, and a big clutch. This might provide a way to get to VTOL without insane mechanical complexity.
NASA has built a big model tiltrotor craft with 10 props to test this.[1]
There's also the Latitude VTOL drone.[2] This simply has electric quadrotors for VTOL, plus a gas engine, wings, and prop for sustained flight. So it has the range of a winged drone, without the need for a runway.
The gearbox is an extremely heavy, failure prone, and complicated part of all helicopters. The gearbox alone typically weighs between 10 and 30% of the total dry weight of the helicopter. So replacing it with electrical drives does sound very attractive - and not just for the Osprey.
NASA built several "one big engine on a gimbal" lunar landing trainers for Apollo. Over the short life of those special-purpose craft, three pilots had to eject - two astronauts and a test pilot.
The real breakthrough in electric aviation will come when it is combined with beamed energy. Rather than storing power onboard the vehicle, you have ground-based stations which power the vehicles with tracking microwave tightbeams. That way, you only need enough onboard battery power to take off, land, and bypass one broken ground station -- maybe 15-20 minutes of total flight time. This would allow electric aircraft to be considerably lighter than hydrocarbon vehicles, even considering the current energy-density of batteries.
Beamed-energy flight systems are being developed for space launch[1], but I don't know of anybody yet doing this for plain old aviation. It's a gap waiting to be filled.
First, you can place the beaming stations on towers -- most birds stay within a few hundred feet of the ground (which is why skydivers don't have to worry about smacking into them). That'll rule out the majority of bird conflicts. Second, put a high-definition video camera aimed down the barrel of the beam, with enough field-of-view that you get a second of warning before a bird crosses the beam. With automated image recognition you should be able to switch off the beam within a few hundred milliseconds of detecting a bird. Switch it back on again when the bird leaves the beam path. Because the plane isn't being directly powered by the beam -- it's just topping up its onboard batteries -- the disruption should not be consequential.
If you think about it (or run the numbers), you'll realise that our present system of jet engines actually dumps far more heat into the atmosphere than this would.
I'm a paying customer, and I'm using your link because logging in on my computer will log me out on my iPhone, along with a security error. This, for content I've paid for. I hate the future.
The batteries have also another problem not mentioned in the article - you have to carry their weight for the entire duration of the flight, while with regular fuel you burn it up during the flight, making the aircraft lighter and more efficient. Not to mention, that many aircraft can't land with their tanks full - but electric aircraft will have to be able to land with the same weight they took off with.
I don't think that's an issue, especially with the projected use cases. The article says that they're aiming for battery storage of about an hour of flight time, which is plenty for takeoff, final approach, and other situations where you'd want a lot of power in a hurry. Most of the flight, unless it is a very short commuter-type flight where you can conceivably charge with shore power between flights, is going to be running much like a hybrid car, with the jet engine powering a generator working at the best point on its performance/economy curve. So it'll still use fuel, and still get lighter as the trip goes on. Also, those batteries mean you don't need a set of engines that need to handle the power needed at take off, so those can be lighter as well. So I imagine that a 1:1 replacement capacity-wise would probably have at worse the same empty weight, taking into consideration the lighter engines and the smaller fuel tank that would be needed.
You have to consider the system as a whole. The kerosene tank with kerosene, plus the motor and gearboxes = X kg.
The battery + The electrical motors and driveline = Y kg.
Example take a small aircraft as an example:
- Combustion: Motor 150kg (1kW per kilo) fuel 100kg (43MJ/kg) = 250 kg, 4300MJ
- Electrical: Motor 20kg (5kW per kilo) batteries 230kg (1MJ/kg) = 250kg, 230MJ
The amount of batteries carried is more than twice the weight of the fuel, so the difference in carried energy between those planes is now no longer 43x, its < 20x. Still a big gap, but the point is that it's much smaller when all things are considered.
Now consider any additional weight we didn't count here (gearboxes or other drive line components), the complexity of those planes, e.g. the number of moving parts. Also consider noise level, fuel economy, environment, reliability. For multi-engine craft it becomes even more pronounced. I don't think batteries will ever need to come close to the energy density of combustible fuels for electric flight to be viable.
On the other hand batteries don't get that much lighter as you use the energy stored within. Also you're going to have to design quick capacity change battery packs or always fly with the maximum weight penalty.
That's true, the average weight of the fuel is probably 60-70% of the takeoff weight or something, varying wildly depending on aircraft type and flight. Takeoff weight is probably the limiting design factor, so there you can compare the batteries to the full tank, which is what I have above.
On the other hand jet airliners are designed to be lighter when they land, landing gear and brakes don't have to manage a landing at full weight (which is why they have to dump fuel before making an emergency landing immediately after takeoff). An electrical plane would have to have brakes and landing gear designed for the heavy batteries, as it will be as heavy at landing as it was at takeoff. This might add quite a lot of weight.
Lithium ion might not be the battery tech for high energy density uses once we work out some other, non battery technologies.
Aluminum-air batteries [1] are well studied in the past decade and there is lots more research going on there. Currently the state of the art for such a battery is about 4.7 MJ/kg and the theoretical maximum is as high as 28.8 MJ/kg, at which point the mass of the turbopumps and other fuel handling parts might make up the difference.
Of course there are issues, primarily the fact that they're not rechargeable and have to be chemically recycled. It's currently too expensive to do that now, but once we can do it economically, each airport will just have to have a stock of batteries and a robot to swap them on the jet and we'll have electric flight.
There might not be a need to equal the energy density of jet fuel in a battery. If the efficiency of the engine and propulsion improve significantly compared to fuel jets, an electrical system might not need that much energy after all.
See the hybrid car: the efficiency of a regular engine (VW Diesel not taken into account ;-) is relatively low. If an electrical engine can be twice as efficient, you only need half the energy...
The electric motor is around 90% efficient, but the thermal efficiency of a jet engine is (if I recall correctly) only around 30%. So the batteries only need to contain 1/3 of the energy of the jet fuel. Batteries are still a long way off from this, but that factor of 3x makes it somewhat nearer than it would otherwise be.
If the density doubles every 10 years that means we're 50 years to parity. And we don't need to get to parity in order for there to be at least one economically feasible design. there might be some cases where reliability and availability of fuel are at a premium and an aircraft could be viable in half that. Which means now is exactly the right time to start experimenting if you're a plane manufacturer.
How about solar-powered planes? I would assume not having to carry an energy source (except for emergency use) is a huge efficiency win. And it's always sunny in the stratosphere (well, not at night).
There's a very strict limitation on solar planes that depends on the panel efficiency and other material science avances as well as the shape and, especially, its height.
The bigger you make the jet, the more of its surface you have to cover with solar panels. However, while the mass grows geometrically with the size because of volume, the surface area of the panels only grows exponentially (x^3 vs x^2). Just like with single celled organisms: at some point the surface area can no longer absorb enough energy to support the massive machinery.
There are tricks to increase surface area but the panels still have to face the sun so you're forced to make planes short vertically but very long horizontaly. I don't think existing airports would be able to support such a different design.
I suspect that you can also add a thin layer of solar panels with wiring stretched between wings and tail, increasing usable surface significantly, without big changes in volume, mass, or drag.
Yeah, battery technology really needs to improve. In fact, there needs to be some revolutionary discovery in order for electric planes (and a lot of other awesome things) to be feasible...
Well, that depends on the purpose of the airplane. We already have electric sailplanes which have basically just enough battery power to get the sailplane into the air, after that you don't need an engine. Plus battery storage has been improving at roughly 10% every year. Right now a flight of around 25 minutes is the point at which fuel makes more sense than batteries but as batter tech improves that number keeps going up. We are at the very early stages of electric flight and while it may never be a viable option for commercial cross country flight there is a good chance that it will take over a lot of the smaller, general aviation aircraft.
Well, sure, but you could have said the same thing about modern airliners when Louis Blériot flew across the channel for the first time in 1909.
Electric aircraft are a technology that does exist, already. Two different aircraft have been built and flown over the channel already. Electric airliners will require significant additional tech development, but the fact that planes have already crossed the channel multiple times is a pretty good start.
I would be curious whether electric planes significantly reduce the carbon footprint of flying. My intuition is that you wouldn't be dumping CO2 and H2O (both fairly potent greenhouse gases) directly into the upper atmosphere, so it would be a huge win, but I am neither a climate scientist nor an aerospace engineer, so maybe I'm missing something.
Not only water (vapor) is a greenhouse gas, it's by far the most important. Per wikipedia, its contribution to the greenhouse effect is 36-72%, while the next contribution, of CO2, is 9-26%.
We have developed superconductivity. MRI scanners wouldn't work without it. The problem is that it needs to be cooled to very low temperatures, which requires complicated plumbing and expensive cryogenic gases. Not really something viable on an aircraft. Superconductors have been slowly improving in operating temperature though - perhaps the temperatures found at aircraft cruising altitude are within reach?
NASA has built a big model tiltrotor craft with 10 props to test this.[1] There's also the Latitude VTOL drone.[2] This simply has electric quadrotors for VTOL, plus a gas engine, wings, and prop for sustained flight. So it has the range of a winged drone, without the need for a runway.
[1] http://www.extremetech.com/extreme/188338-nasas-electric-ver... [2] https://latitudeengineering.com/products/hq/