Comment by nickff | Hacker News Reader

Comment by nickff | Hacker News Reader

Parent Full thread

>"Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel."

Turning chemical energy (low entropy energy) into electrical energy (low-medium entropy) into thermal energy (high entropy) to make mechanical energy (of intermediate) would be incredibly inefficient. Combustible fuels are much better than batteries at making things hot; I don't have the time to run the numbers right now, but what you are saying is incredibly wasteful. Please take a thermodynamics course.

Fans are commonly used on supersonic aircraft. Most modern fighter aircraft use medium or low bypass turbofans, which are using fans to go supersonic. Turbofans are less efficient than turbojets over Mach ~1.6-1.8 (or in reheat), but a ducted fan could probably do better.

You are correct that it's wasteful, if one joule of electric energy and one joule of chemical energy cost the same.

But if you are getting electric energy for free from solar panels, and you're getting chemical energy by pumping taxed dinosaur carbon into the air, then you might find that the electric energy could be more efficient.

You're absolutely correct that it's wildly inefficient by any metric now. But if clean electric energy became wildly abundant, that would change.

In computing, there are all kinds of wildly wasteful things (web browsers) that are the best way to do things because computation is extremely cheap. It's very interesting to think about what kinds of devices would make economic sense if electric energy had the same price collapse that computation has done.

The fact that we "burn electricity" in electric water heaters, stoves, and toasters rather than only using high-entropy energy sources like gas water heaters and gas stoves is an example where present-day electric energy supply chains have beaten out the theoretical advantages of only using high-entropy energy sources when it is possible to do so.

This is not about financial cost, it is about weight and complexity. It would be less costly and more efficient to separate water into hydrogen and oxygen (using solar power if you like), then use them to power a rocket engine.

Aircraft weight turns directly into fuel cost via induced-drag. If electric energy is wildly inexpensive, you can afford very heavy airframes.

See https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

There are two major problems with very heavy airframes:

1) Aircraft cost is proportional to empty weight.

2) Range is proportional to total stored energy / drag and drag is proportional to weight. If you have low specific energy, you cannot go far.

One more to add. Current commercial aircraft can jettison fuel to reduce the landing weight and minimize risk if the structure is compromised. Hard to see how this design would.

Jets also constantly lose weight while burning fuel. This results in airliners flying higher and more efficiently as the flight progresses. With batteries, you're kinda stuck with the weight.

And it's true that airliners take off with more weight than they are speced to land with. They have to dump fuel if landing before burning it off.

No reason I can think of why you can't drop empty batteries in an emergency situation. In fact, you probably want swapable batteries to begin with, don't want to waste time on the tarmac charging up.

One reason would be if they're built into the wing surfaces, as this article proposes. You probably don't want to jettison your wings.

> But if you are getting electric energy for free from solar panels

If you are getting electrical energy for free I have some WAY WAY better uses for it than an airplane.

Let's save hydrocarbon fuel for mobile installation (planes and cars), and use the solar power for fixed installations (houses, industry).

Maybe once you maxed out houses, we can come back to the mobile stuff.

Even if it's free, converting energy to thrust at say 1:0.95 mechanically you can go faster vs. 1:0.35 with a heat engine.

Reply on news.ycombinator.com