10 ms·
Supersonic Electric Flight
- DJBunnies 6y ago> ...and having batteries with 500 Wh/kg could... This seems to be the big if, but I don't speak battery. How close are we to such things?
- Shivetya 6y agoTwo stories I found, that both provide insight https://cleantechnica.com/2020/06/10/usas-battery500-initiative-has-achieved-li-metal-battery-energy-density-of-350-wh-kg/ https://cleantechnica.com/2020/06/10/usas-battery500-initiat... https://www.nasa.gov/sites/default/files/atoms/files/650_whkg_1400_whl_recharg_batt_new_era_elect_mobility_ymikhaylik_0.pdf https://www.nasa.gov/sites/default/files/atoms/files/650_whk...
- ejolto 6y agoThere is a company called Sion Power that claims to have 500 Wh/kg batteries [0]. "With supply dates beginning in mid-2021 for UAV markets. Broader distribution for the Licerion-HE is scheduled for early 2022." [0]: https://sionpower.com/2020/sion-power-demonstrates-key-electric-vehicle-ev-battery-performance-requirements-in-its-lithium-metal-rechargeable-battery-cell-technology-2/ https://sionpower.com/2020/sion-power-demonstrates-key-elect...
- Tade0 6y agoWe're there: https://www.amprius.com/technology/ https://www.amprius.com/technology/ Just not at scale and naturally such batteries are expensive.
- simonebrunozzi 6y agoCommercially and at a reasonable cost? Probably at least 8-10 years from now.
- semi-extrinsic 6y agoI think Musk recently said they are going to have 400 Wh/kg in 3-4 years. So maybe 2030.
- elihu 6y agoI believe lithium iron phosphate tends to be around 120-150 Wh/kg, pretty decent regular lithium ion tends to be around 150, fairly recent Tesla battery packs are a bit over 200. (These are types of batteries that have been around for years that you can easily buy as a normal person for a reasonable cost. There's probably a lot of fancy/new battery technology that is somewhat better but either not mass-produced yet or otherwise prohibitively expensive, or trades off some other desirable quality like safety or durability to get more capacity.)
- jacobush 6y agoSounds like SpaceX light. San Francisco to LA in 5 minutes. Neat.
- NovemberWhiskey 6y agoAny time someone says they have a design for a prototype that includes aerodynamic aspects, and at the end they say "now scale up by 10" ... there is reason for a strong degree of skepticism.
- jvanderbot 6y agoSkepticism which, admittedly, author pre-loaded for us in the first few paragraphs.
- scottlocklin 6y ago>The underlying reason that I believe electric aircraft can break the sound barrier is that electric motors can deliver far higher power-to-weight ratios than piston engines, jets, or turbines. That's a completely worthless figure of merit (I assume he means thrust to weight; because electric helicopters have abysmal power to weight ratio -shit like this matters) and no reason to believe in this nonsense. By the same reasoning, one could build a piston engined supersonic aircraft because there are piston engined helicopters. Another thing that has a high thrust to weight ratio; those water rockets I used to play with as a kid.[0] Pretty sure you can't go supersonic with those either unless you fire them out of a howitzer. Electric airplanes are a terrible idea without battery technology comparable to hydrocarbon power to weight ratios. Even then, they're a terrible idea unless the motivating technology is lighter than a jet engine and tanks for reasons that should be obvious to anyone with high school physics educations. [0] https://en.wikipedia.org/wiki/Water_rocket https://en.wikipedia.org/wiki/Water_rocket
- jeffreyrogers 6y agoThis guy has a physics PhD from CalTech, so his sloppy reasoning is doubly shocking.
- scottlocklin 6y agoSo much for my CalTech fanboyism: https://news.ycombinator.com/item?id=25118150 https://news.ycombinator.com/item?id=25118150 Cipolla was right Law 1: Always and inevitably everyone underestimates the number of stupid individuals in circulation.
- FlyMoreRockets 6y ago> By the same reasoning, one could build a piston engined supersonic aircraft... Now that's a thought provoking engineering problem! Assume a supersonic propeller is required. Using the Thunderscreech as a baseline, which admittedly didn't quit go supersonic, but it gets us close. Its powerplant produced nearly 6,000 horsepower. That is certainly doable with a piston engine, top fuel dragster engines produce roughly 10,000 horsepower. For about ten seconds. If this plane burned nitromethane, it wouldn't even need much atmospheric oxygen, nitromethane required roughly one tenth the amount of oxygen per mass of jet fuel. Of course, this means it has to carry a lot more fuel https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech#Specifications https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech... https://en.wikipedia.org/wiki/Top_Fuel#Performance https://en.wikipedia.org/wiki/Top_Fuel#Performance
- NikolaeVarius 6y ago> The underlying reason that I believe electric aircraft can break the sound barrier is that electric motors can deliver far higher power-to-weight ratios than piston engines, jets, or turbines. The F-4 Phantom is a textbook example of high thrust, being able to (just) achieve a vertical climb. In contrast, for $100 I can buy a racing drone that can accelerate vertically at 10 gs. There are other factors at play but a power-to-weight ratio of 10 screams possibility. You can't just ... assume that
- Gravityloss 6y agoJet engines have a thrust to weight ratio of about 10. The power is not so interesting as a lot of the thrust comes from heating the air. Ie the compressor might work at 40 MW and turbine also then of course extracts 40 MW from the airflow. I'm not sure why one couldn't build an electric supersonic aircraft. It would have very short range. The design presented, could be made more aerodynamic by fairing the motors. Leave a hole in the tip for cooling air. Add a compressor for low speed cooling if needed. Alternatively have liquid cooling. You could also have a design where the airflow is slowed down in a duct, compressed by a fan and then expanded. The duct would be heavy though. But it might obviate some problems with the free stream propeller.
- NikolaeVarius 6y agoWhere are you getting this 10 number from. Wikipedia shows Eurofighter Typhoon as the highest T/W for a Jet airplane at 1.15 and The Concorde at 5.4 https://en.wikipedia.org/wiki/Thrust-to-weight_ratio https://en.wikipedia.org/wiki/Thrust-to-weight_ratio You WANT wings for supersonic flight (For a normal non missile aircraft). Sure it sucks that its hard to optimize for supersonic flight and subsonic, but you go Supersonic at extremely high altitudes to reduce drag on the plane. This requires a shit ton of power to go forward, without even counting the effort to stay in the air vertically.
- Gravityloss 6y agoThat's the plane. I was talking about the engine.
- mrfusion 6y agoInteresting point that VTOL actually lets you design your wings only for supersonic flight and avoiding the larger wings (and drag) you’d need for traditional landing and take off. I hadn’t considered that before.
- marvin 6y agoYou might not even need wings. You could just shape the fuselage like a wing. Not sure if that would be the ideal for all applications, though. Maybe the smaller cross-section of small wings and a more narrow fuselage would be better in some situations.
- jeffreyrogers 6y agoThis is like the lifting body aircraft NASA worked on in the 60s[0]. Their downside was high landing speeds which made them dangerous and hard to fly safely, but with VTOL that problem is mostly eliminated. Edit: That said, I just read the article and his proposal seems completely infeasible to me barring some sort of massive technological breakthrough. I don't think supersonic electric flight is impossible, but I don't think it would look like what he described and I don't think it has any commercial relevance given the current state of technology and its likely future developments. [0]: https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-011-DFRC.html https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-01...
- skykooler 6y agoA big reason that jet turbines are useful for supersonic aircraft is that their exhaust speed is (pretty much) constant regardless of airspeed; this allows a jet to continue accelerating until the drag force exceeds its thrust. An electric fan could perform similarly if it could provide enough static pressure to achieve supersonic flow through a de Laval nozzle. If that could be done (whether it can is something I'd love to see data on!), the best electric power source would probably have what looks like a regular jet engine intake on the front, a subsonic fan, and a de Laval nozzle in the back like a rocket engine.
- bob1029 6y agoCouldn't you just use an electrically-driven turbo compressor to produce a source of hot, high-pressure gasses? Not saying the efficiency or mechanics would play out at scale, but hypothetically you should be able to generate any arbitrary source flow required to feed through this type of nozzle.
- larrydag 6y agoI think this sounds like a ramjet or something similar
- ben_w 6y agoI thought ramjets didn’t have any turbines, they relied on ram compression? https://en.wikipedia.org/wiki/Ramjet https://en.wikipedia.org/wiki/Ramjet
- TylerE 6y agoIn theory? Maybe, but the motor would probably weight more than the whole plane does now. A typical jet engine is producing the equivalent of tens of thousands of horsepower.
- WalterBright 6y agoThe only reason that jet turbines work for supersonic flight is the inlets to the turbines slow the air to subsonic.
- jjk166 6y agoThere's a big difference between a propeller with supersonic tip speeds and a supersonic propeller. A supersonic propeller is not strictly impossible, but even after decades of active research remains elusive. You can't simply handwave that you'll use a 3 meter propeller spinning at mach 3 just because it's technically below the limit where it would tear itself apart from centrifugal forces. It doesn't matter what battery energy density you can achieve if the propellers you're using to drive them don't work.
- FlyMoreRockets 6y agoYou should check out this: https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech Yes, an aircraft employing a supersonic propeller first flew in the 1950's. It wasn't terribly efficient, but it was terribly loud .
- 0_____0 6y ago> Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[11] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[18] Holy hell
- zaroth 6y agoPer TFA; > Yes, it would be very noisy.
- jjk166 6y agoThat's not a supersonic propeller, that's a propeller with supersonic tip speeds. A supersonic propeller is a propeller that can work in a supersonic airstream. The XF-84H had a top theoretical speed of Mach 0.9, though in testing it was evaluated to be more like Mach 0.83, and even that's disputed, with another estimate being Mach 0.7. It never achieved these speeds. The only plane to fly with a propeller at supersonic speeds was the XF-88B, though that was actually a jet aircraft that just had a propeller on the front for testing purposes - the vast majority of the thrust was still coming from the jet engines.
- ajuc 6y agoSupersonic electric flight could be a propeled glider that goes stright up, hides the propellers and engines bhind some aerodynamic covers and glides at supersonic speed, then repeats the procedure as many times as needed. Another idea is to make a turbo-jet engine heated by electricity instead of fuel.
- FlyMoreRockets 6y ago> Another idea is to make a turbo-jet engine heated by electricity instead of fuel. I was going to decry the inefficiency, then remembered that a resistor is only device with 100% efficiency.
- ajuc 6y agoYou were right though - much better to use a heat pump ;)
- mrfusion 6y agoElectric jet engine powered by a heat pump. I wonder if anyone has looked into that? You could collect a lot of heat from the airflow over the body. Meaning there would be a lot of air with some level of heat that’s constantly being replaced.
- blunte 6y agoI guess it could depend on your definition of "glide", but I doubt that anything gliding at supersonic speed would have a very good glide ratio. Thus, it wouldn't really cover much ground even thought it might travel quite a bit vertically. If we consider the reason one wants to travel at supersonic speeds, we might arrive at something involving the value of a human's time. Given that, slow but ultra efficient, luxurious (and internet-connected - "workable") travel might be something. Then gliding, floating, etc. with ample space and creature comforts could be the focus.
- Ekaros 6y agoI actually wonder if it is even possible to reach sufficient height with electric power... Ofc, it is possible with lift like balloon. And if it were, wouldn't it be just more efficient to glide at lower speeds?
- avmich 6y agoWhen I read "supersonic electric flight", I assumed something propeller-less, like this - https://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121 https://news.mit.edu/2018/first-ionic-wind-plane-no-moving-p... .
- FiatLuxDave 6y agoI'm surprised that no one is discussing electrohydrodynamic engines for this application, for example as used by https://en.wikipedia.org/wiki/MIT_EAD_Airframe_Version_2 https://en.wikipedia.org/wiki/MIT_EAD_Airframe_Version_2. These don't have the same limitations as rotors do with vectoring relative to high speed gas flow. Most of the ones which have been built are small, but that is not an intrinsic limitation; mainly its the battery weight that is the limiting factor. Of course, the whole idea of electric field is depending on the battery breakthrough, but assuming you have that and want to go supersonic, I'd figure you would go EHD.
- pluto9 6y agoFor reasons I can't entirely recall, there are pretty serious limitations on the thrust these can generate. I believe it has something to do with the air becoming "saturated" with ions. The limit on the density of charged particles limits how much mass you have to push on. The aircraft in your link has a top speed of 17km/h. Heating the air with microwaves or an electric arc could work, since it would just be replacing burning fuel with another heat source. It would still require moving parts though (unless it's only used as a ramjet), since a jet engine has to compress the air before heating it.
- handol 6y ago>It would still require moving parts though (unless it's only used as a ramjet), since a jet engine has to compress the air before heating it. You could use a tesla valve for an electric pulse jet, that would make for no moving parts.
- gusgordon 6y agoHere's a fairly in-depth analysis of supersonic electric VTOL aircraft: https://github.com/gusgordon/electric_jet https://github.com/gusgordon/electric_jet tldr; they become "practical" with a battery specific energy of around 1500 Wh/kg. However, if you just want to do a demonstration, they are practical now (as this post suggests).
- blackrock 6y agoIs it possible to have a liquid hydrogen and liquid oxygen airplane? Essentially a rocket on wings. The fuel (hydrogen and oxygen) can be produced electrically from renewable sources.
- dragonwriter 6y ago>Is it possible to have a liquid hydrogen and liquid oxygen airplane? Yeah, you just usually don't carry oxygen with you when you can get it free from the atmosphere, since it subtracts directly from your payload capacity.
- trhway 6y agoyou don't need wings. For high-speed long distance the most efficient trajectory is ballistic - thus passenger variant Starship carrying 100-500 passengers at $2-5M/launch (thus $5-20K/ticket) will, i think, be the next transcontinental mode of transportation (less than 1 hour SF-Melbourne).
- Olumde 6y agoElectric supersonic flight you mean.