5 ms·
The equation isn't just energy density; it's also the efficiency of harnessing that energy. Good electric motors are way more efficient than a turbofan.
by tankenmate 10y ago
The equation isn't just energy density; it's also the efficiency of harnessing that energy. Good electric motors are way more efficient than a turbofan.
- dredmorbius 10y agoThat buys you roughly a factor of three. Subtract again for the fact that you're not shedding half your takeoff weight in fuel over the course of a flight. You land with as much battery as you started with. More if you're using metal-air batteries.
- koolba 10y agoNot necessarily! Imagine a system where the plan actually jettisons the spent batteries along the way. They could glide/parachute down to a collection depot to be recharged and sent back to the airport for installation.
- JustSomeNobody 10y agoThat sounds like something that could go very wrong and either cause the plane to crash or hurt/kill people on the ground.
- J-dawg 10y agoSpeaking as someone with no aviation knowledge at all this seems like an awesome idea. The plane could take off with battery packs slung under the wings which are jettisoned and become drones, gliding down to a depot. I'm now waiting for someone who actually knows what they're talking about to point out the flaws. One that springs to mind is the logistics of collecting the drone-batteries and transporting them to be reunited with the parent aircraft.
- kgabis 10y agoI would also add safety and the fact that 70% of our planet is covered in water/not easy to retrieve batteries from.
- dredmorbius 10y agoThe aircraft in question is being offered for a relatively short-range flight (London-Paris), 345 km (215 mi). Odds are good this flight configuration wouldn't be used on long-distance trans-oceanic flights. Though it might be best-suited to short hops between islands, islands and mainlands, or across deep peninsulas. That's a somewhat limited set of markets. For travel between heavily-populated continental points, ground-based rail would almost certainly be more efficient and practicable, and even in the case of the suggested London-Paris route, there is a ground-based alternative.
- jsjohnst 10y agoThe risk of puncture would likely cause the safety precautions required to make this cost prohibitive. That's just one battery specific reason, there's hundreds more why jettisoning isn't done except with military aircraft and even then only rarely.
- dredmorbius 10y agoKeep in mind that all protrusions are going to generate significant drag. So if you can find ways to incorporate the batteries within the airframe, you'd be better off. There's also the problem of mass-transfer. In general, aircraft should keep the center of mass behind the center of lift, and ... bad things happen when this isn't maintained. With liquid fuel, tanks are actively pumped to retain both forward-aft and left-right balance, something difficult to achieve with solid battery packs. There's the problem of both energy consumption profiles and battery delivery/drain cycles. An aircraft generally needs maximum power to get off the ground (hence: all batteries delivering at or near their maximum output), but only partial power once airborne. What you'd like to do is to drain a few batteries completely in the take-off and ascent stages, then jettison them, but this doesn't match the batteries' own power delivery capabilities. You might be able to switch to jettisonable packs after TOaA, to completely drain those. If fit between wing spars, you might end up with a roughly rectangular package which could be ejected aft of the aircraft from the wing, with a door sealing off the cavity. The battery itself would require some sort of deployable wing itself, as well as guidance and control systems and surfaces, possibly a small propulsion unit. A guided descent stage might actually be one of the more viable options. It's also possible that jettisoning additional batteries on final approach would make landing dyanamics for the aircraft itself simpler. On whole, though, I'm questioning the usefulness of this, particularly given coplexities, a likely low airspeed, and competition with ground-based alternatives (high-speed rail, Chunnel) which would bypass the power storage requirements entirely, and would likely operate at equivalent or greater speeds, direct to city centres.
- J-dawg 10y agoThanks for the comprehensive reply. I find it interesting to consider ideas like this, even if they ultimately turn out to be unworkable.
- Gravityloss 10y agoMore likely to be a tow line (like are used already for gliders) or a tow aircraft?
- nradov 10y agoYou've got to be kidding. The civil aviation authorities would never allow airlines to jettison solid objects over populated areas for safety reasons.
- J-dawg 10y agoWhat if the jettisoned objects were autonomous aircraft that could land accurately at a specified location?
- srssays 10y agoA better idea would be to dig oil out of the ground, process it into kerosene, and use that to power jet turbines on the aircraft.
- deleted 10y ago[deleted]
- J-dawg 10y agoAt some point we're going to need to stop doing that, hence the need for alternative ideas.
- srssays 10y agoSure. How about LNG (liquid methane)? We have enough natural gas to last quite a while. In the far future, hopefully we will have mastered producing kerosene using algae.
- tankenmate 10y agoLNG tanks are far from lightweight; to keep LNG at room temperature is about 320 atmospheres of pressure. Liquid propane on the other hand is about 8 atmospheres. Mind you at altitude it's around -50 ~ -60 C, but that will only reduce the pressure requirements to approx 100 atm. Also, LNG has about 60% of the energy density of kerosene. SO to use LNG you'd be taking a double whammy, lower density and heavier tanks.
- dredmorbius 10y agoI'll give you credit for out-of-the-box^Wairframe thinking there....
- jpm_sd 10y agoThat's called a "bomber".
- blutack 10y agoElectric motors would need to be orders of magnitude more efficient than turbofans (which they aren't). You also need to compare the overall efficiency developed as thrust, not just thermal efficiency. There's also the issue of the mass of batteries not going away during the flight, unlike fuel, which will require maximum landing weight to equal maximum takeoff weight - this will impose a structural penalty versus IC aircraft.
- cjrp 10y ago> There's also the issue of the mass of batteries not going away during the flight Excellent point; currently as fuel is burned off, planes can climb to higher, more efficient altitudes (known as a step climb). What's the performance penalty for not being able to do this as your batteries aren't getting lighter as the flight progresses.
- neolefty 10y agoMaximum efficiency is ... 100%. So if a turbofan is 30% efficient, then the best you can do is about 3x.
- ericd 10y agoGood point, but something to keep in mind is that the efficiency of the motors in turning stored energy into motion is only one part of the efficiency of the aircraft. If a plane is able to fly much higher due to the engines no longer being air breathing, that would also change the equation, as drag would be much lower with the thinner atmosphere.
- Gravityloss 10y agoAlso you can tune the location and amount of propulsors, and have a high efficiency rotation rate with direct drive electric motors. The current turbofan engines are close to the size limits in relative fan size and bypass ratio without a gearbox, which adds cost and mass. (Only one manufacturer is in the process of introducing a geared turbofan.) Because of no high speed jet exhaust and large area thrusters, these aircraft have the potential to be able to take off really steeply and to be really quiet.