Hacker News Reader

Comment by danbruc | Hacker News Reader

From looking at their website [1], it looks to me like they are just using a jet turbine and routing the air through pipes to those ring-shaped outlets.

[1] https://jetoptera.com/technology/

View on HN

Thanks for the link. I have a question. They say:

"The energy required to propel our smallest aircraft at 200 mph for 30 minutes is 100 kWh. This energy can be provided by 26 kg of jet fuel or 600 kg of modern Li-ion batteries."

Is really "26 kg of jet fuel" comparable to "600 kg of modern Li-ion batteries"?

Thanks!

I do wish in those kind of comparison's they would make them equivalent: as in "26kg of jet fuel + XXXkg of turbine" vs "600kg of modern Li-on batteries + XXXkg of electric motor" since for a flying vehicle the all-up weight is the significant number.

According to Wikipedia [1] jet fuel has 43 MJ/kg and lithium ion batteries with silicon nanowire anode have 1.566 MJ/kg. With that you get get 714 kg of batteries for 26 kg of jet fuel. The Tesla Model S 85 kWh battery has 0.57 MW/kg [2] which gives 1961 kg. Looking only at the energy density, the numbers from the article seem quite optimistic for the battery weight, but it is not clear what they actually calculated and I guess their numbers might contain factors accounting for efficiency. Do they account for the difference in weight, of the energy source and energy conversion mechanism, 26 kg vs 600 kg is nothing were I would expect similar performance?

[1] https://en.wikipedia.org/wiki/Energy_density

[2] https://en.wikipedia.org/wiki/Tesla_Model_S#Battery

>The Tesla Model S 85 kWh battery has 0.57 MW/kg [2]

Note that your source is from 2015. The Model S battery of today has a higher capacity (100 kWh) and has undergone constant lightweighting improvements.

This article [1] says that it is now 0.65 MW/kg which is a 15 % improvement.

[1] https://www.teslaoracle.com/2022/02/18/model-s-plaid-battery...

Electric->Mechanical is much more efficient than Fuel->Mechanical so that is probably where the difference is coming from.

1 gallon of regular gasoline is 33kWh and a gallon weighs 6lbs. For a 33kWh battery pack comprised of 18650 Samsung 35E cells, you’re probably looking at around 300lbs for just the cells alone.

Irregardless, it doesn’t matter because an electric motor will be far more efficient using the energy stored than a fueled engine (be it gas turbine or ICE). In automobiles for example, 80% of the energy from the gasoline is wasted.

The efficiency of gasoline and Diesel engines is a lot better, over 30% and even over 40% for Diesel. I think the best gasoline engines get close to 40%.

Sure, the best diesel engines get 40-45% and some gasoline engines approach 40%, but that is definitely not the norm. The average is 20% for gasoline (hence my prior figure) and 30% for diesel, which is a far cry from the average electric motor’s efficiency of around 90% in automobiles.

Those are all roughly true numbers but you forgot charge efficiency: energy extracted out of battery / energy input into battery. That's also around 90% (assuming higher voltage charging, not 120v AC which would be closer to 60%). That would put total efficiency of battery electric closer to 80%.

But not necessarily more efficient at thrust.. a turbine is mixing fuel with oxygen from the env to both spin the turbine but also vents the exhaust and gets thrust that way. So it doesn't carry smth like 3/4 of its reaction mass. I know that turbofans duct in most of their flow, so not sure how this nets out on a large-scale device.

Reply on news.ycombinator.com