Comment by namirez | Hacker News Reader

Comment by namirez | Hacker News Reader

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Your numbers are not quite right.

> jet fuel's USEFUL specific energy is more like 10-15MJ/kg due to needing to be burned.

New turbofans have efficiencies close to 50%, so it's more like 20 MJ/kg. Also, you need to take into account the prop efficiency for electric planes. Beyond Mach 0.5, the efficiency degrades very rapidly. So it's not just electric motor vs jet engine.

> more glider-like L/D of 25-35 (the best gliders can do 70).

This is not limited to electric planes. The reason no one has done it so far is the aeroelastic divergence.

> improved modern materials means you can just have most of your take-off mass be battery, compensating for the poorer specific energy

Again, this can be done for any aircraft, not just the electric ones.

> as low as 1200km is the minimum range needed for transatlantic flights

Do you really believe there is a market for 1200km hops at Mach 0.5. We already have ATR 72 and Bombardier Q400. Why no one is using them for long haul flight?

>New turbofans have efficiencies close to 50%, so it's more like 20 MJ/kg.

Depends if we're talking LHV or HHV. Additionally, we're talking smaller jet fueled vehicles which rarely have access to state-of-the-art efficiency.

>Beyond Mach 0.5, the efficiency degrades very rapidly.

In this case, we're talking Mach 0.5 or less.

>this can be done for any aircraft, not just the electric ones.

Didn't imply otherwise. The highest performance jet plane can fly literally around the world. But this is far more performance than anyone needs in an aircraft. And for whatever reason (partially because of aging fleets, at least for very small commuter aircraft), generally there have been efficiencies left on the table still, just the same as conventional car makers left efficiencies on the table which Tesla and the like have taken advantage of.

> Do you really believe there is a market for 1200km hops at Mach 0.5.

For domestic flights, yes. For transatlantic, no, not a large market. Not as long as jet planes aren't paying for their climate externalities.

>We already have ATR 72 and Bombardier Q400. Why no one is using them for long haul flight?

Because those have the same maintenance requirements and use the same fuel as jets, which are just as efficient and faster.

I don't think we'll do many transatlantic electric passenger flights until the climate externalities are priced-in for the cost of air travel or until lithium-air or similar. But now we've moved the goal posts to long-haul and transoceanic flight. Electric flight is clearly possible and for domestic flights it'll likely be cost-competitive even without fully accounting for climate externalities, particularly for these 9 passenger FAR-23 electric aircraft competing with fuel-guzzling and maintenance-heavy 40 year old fleets.

> > improved modern materials means you can just have most of your take-off mass be battery, compensating for the poorer specific energy

> Again, this can be done for any aircraft, not just the electric ones.

Not only that: you burn the fuel, so for the second half of the flight you only need to carry half the fuel. But you have to carry the full battery mass (modulo E=mc^2) all the way.

True! Actually airplanes are loaded with just enough fuel to finish their mission safely. It's not like a 777 always taking off with maximum fuel.

Dumb idea: If the weight of the batteries is significant, then dump out the empty ones with parachutes partway through the flight?

Taking seriously the idea for a moment: you don't consume batteries one at a time, they are in various degrees of parallelism. Also the balance of the plane is affected by mass ejection.

Like this? https://www.youtube.com/watch?v=KoGIPk8Vrl0

Indeed, there are various ways to handle "staging" of aircraft, including parachuting batteries, electric towplanes, assisted groundlaunch, piggyback, etc. These may have niche uses, particularly for cargo flights, but I suspect that will only happen if 1) jet air travel pays for its full climate cost and 2) we stop making any progress in battery technology.

It'd be fun to try, though.

They do this for the 2nd stage of the Electron rocket, though the batteries are not recovered. Scott Manley video:

https://youtu.be/U5k1mlu6A7I?t=106

Well, that’s in part a point for the batteries, though. You spend a lot of energy to lift all that fuel that you just blow out with all its potential energy up in the air, whilst the batteries provide you with that very potential energy on the way down as well. Glider planes quiet commonly are filled with water to make them heavier so they can make more use of the potential energy they gain from the externally powered take off and so that they can bridge longer distances without lift generating upwind.

Actually, gliders with water ballasts have the same L/D full or empty. The difference is that they can fly faster when the tanks are full, but they don’t travel larger distances

Which brings me to the ask the following:

Would electric towplanes be doable for passenger aircraft? (Long or short haul, electric or otherwise).

If one of the downside of electric planes is the range, while fuel is cheap, making them cheaper to operate, and take-off uses a significant portion of the total energy, I could see very powerful electric towplanes as a way to reduce the amount of fuel needed for takeoff. This would:

* Allow to travel longer distances with the same amount of fuel (enabling electric airplanes to fly longer distances). Alternatively, allow planes to fill up with less fuel for the same distance.

* Improve fuel efficiency of big airliners, and their carbon footprint if you consider regular ones.

One last question: aircraft that fly shorter routes typically embark less fuel, but would that be doable with electric planes?

That's not a point for batteries. I cannot imagine a circumstance where, during cruise or even descent, you'd wish that you'd be carrying more mass (except possibly to have a higher AoA and thus higher V_a if you want to fly through turbulence faster).

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