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Comment by joshfraser | Hacker News Reader

Flying at higher altitudes reduces drag and increases efficiency due to the thinner air. Ideally, we'd be flying planes a lot higher than we do today. The problem is combustion engines need oxygen and the higher you go, the less oxygen you have available. Electric planes don't have that problem which could be the secret to faster global travel.

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Turbos go a long ways to overcoming the thinner air problem with ICE engines. There is another, arguably bigger reason flying above 10k feet is uncommon—oxygen requirements. Few small planes are pressurized and O2 systems are expensive extras which require additional maintenance and filling. Systems which service passengers as well even more-so.

I flew a Cessna 206U for a few years and it was fine up to 15k feet or more, but I'd almost always skim just below 10,500 so I didn't have to bring along supplemental oxygen. I suspect that's where most of these guys will live, in the 7,000-10,000 foot range. It's much faster then flying near sea level but without the problems with O2.

Why can’t planes just pressurize the outside air for breathing? No need to bring your own o2?

Big planes do this. In a little plane, that's expensive and heavy and you still need to have a backup system in case it fails.

Because for a small aircraft, the weight and energy requirements of a compressor is greater than the weight of a small tank full of oxygen.

And the increased structural and sealing requirements of being several PSI above ambient. That will add cost, complexity and weight.

Most planes use turbine bleed air to pressurize the cabin. This of course implies that the plane has a turbine and a cabin that can hold positive pressure. They are usually $500/h+ planes.

Unless you're thinking supersonic travel, the "coffin corner" (difference between stall speed and Mach 1 (or Vne)) also narrows as you get higher. In an extreme case, the U-2 cruises at 70,000 ft, and at that altitude the stall speed is about 10 knots below the maximum speed.

Flying at higher altitudes in lower density air also reduces lift, though [1]. A plane does get extra benefit from flying higher as drag on the fuselage is reduced, but there is a limit — the plane has to go faster to make up for the reduction in lift due to the reduction in air density. An electric plane ends up being able to fly a little higher, but not by much. See these notebooks for details:

https://github.com/gusgordon/atmosat/blob/master/atmosat.ipy...

https://github.com/gusgordon/electric_jet/blob/master/plane....

[1]: https://en.wikipedia.org/wiki/Lift_(force)#Lift_coefficient

This is not correct. Here are the formulae:

Lift = weight = 0.5 * \rho * v^2 * S * C_L

Power = drag * speed = 0.5 * \rho * v^3 * S * C_D

where \rho is the air density. A good assumption for cruise is (C_L/C_D)_max. If you do the math, you realize that for a constant weight, the power output must be proportional to:

Power \propto \rho^{-0.5}

For example, to fly a plane at 10km above sea level you need twice as much power than sea level. This is totally independent of your propulsion system.

Flying at higher altitudes also increases chance of developing cancer due to cosmic radiation.

https://www.ncbi.nlm.nih.gov/pubmed/12862322 https://www.cdc.gov/niosh/topics/aircrew/cosmicionizingradia... https://www.cdc.gov/nceh/radiation/air_travel.html

Couldn't this be curbed by better radiation shielding on the plane itself?

Not necessarily. Photons at Compton-dominant energies (200-5000 keV) can’t be effectively shielded because every material (including, eg, lead) absorbs them about the same. Also, shielding is inevitably heavy.

The first study:

> METHODS: A cohort of 10,051 male and 160 female airline pilots [...] was followed for cancer incidence [...].

> RESULTS: Among male pilots, there were 466 cases of cancer diagnosed vs. 456 expected.

So, a heightened risk of (466-456)/10051 = 0,001% if I read this correctly.

Or am I misunderstanding something here? I guess so, because this seems close to negligible.

You forgot to multiply by 100 when you went to %, so it's actually 0.1% higher. Still pretty low.

ah, thanks, silly mistake! Anyway, still a surprisingly low risk. Seems within the margin of error to me.

The SR-71 and U-2 flew plenty high and had enough oxygen. When you’re high enough that there’s no oxygen, you have no atmosphere, and then your wings and turbines don’t do anything useful. That means it’s rocket time.

Also, depressurization happens, and the risks and complexity of workarounds increase with altitude.

> The SR-71 and U-2 flew plenty high and had enough oxygen

Actually none those two planes had fully pressurized cockpits. I guess they were pressurized at 26k feet altitude pressure. That's why the pilots were literally wearing spacesuits.

https://images.app.goo.gl/KjqGARoyiddhDxAE6

GP was referencing oxygen in regards to combustion, not the cockpit.

The space suits weren't because of the cockpit altitude (when the cockpit was pressurized), they were because the crew were operating well above the Armstrong Limit and no amount of oxygen could keep them awake in the event of depressurization without the suit.

It's not just that they need oxygen. Cold air is good for thermal efficiency. Even if the heat source were nuclear (as in https://en.wikipedia.org/wiki/Project_Pluto engines) there would be a problem with the thermal efficiency. (see https://en.wikipedia.org/wiki/Thermodynamic_cycle and https://en.wikipedia.org/wiki/Carnot_cycle for why) Air temperature rises from about 217 Kelvin to about 270 Kelvin as you climb up the stratosphere. The temperature goes back down again as you get up to around 50 km (80 miles) but that is really high altitude.

There is an upside though. Subsonic aircraft can go faster as the temperature rises because the speed of sound increases. It goes from about 295 m/s to 329 m/s. The extra 11% is nice.

Graph of the air by altitude: https://upload.wikimedia.org/wikipedia/commons/9/9d/Comparis...

The majority of air passed through modern jets is bypass air. I don't think the availability of oxygen is the clincher.

Things like refueling speed, the weight of the batteries themselves (which don't burn up as you consume them like combustibles, and the overall better energy density of combustibles probably make the whole thing more attractive at today's tech.

> Electric planes don't have that problem which could be the secret to faster global travel.

However, to make it feasible for commercial air travel, you need much faster charging times, because airlines want to have as short of a turnaround as possible for their planes.

But I can totally see how electric planes would work for hobby piloting in the first place.

They could also make the battery swappable. That is more of a logistic problem, but perhaps easier.

Also doable. Either way, don't underestimate the simplicity and energy transfer rate of a fuel truck.

On short routes, passenger loading time dominates. Electric could work for going between islands for Hawaii, Isle of Mann, New Zealand, and Japan. It could connect Martha's Vineyard or Long Island with various nearby places.

It's a ferry replacement.

Is the speed of sound barrier higher too?

The speed of sound decreases as altitude goes up, unfortunately.

Sometimes. Speed of sound decreases with temperature. At any flight level above 300 speed of sound stays the same and the increases with altitude.

Good to know, thanks for the correction.

No, it's lower, speed of sound is 38.94 * sqrt(temperature) with the speed in knots and the temperature in Kelvin.

Every airliner notices this when climbing, you get closer to the Mmo speed (maximum Mach number for operating) while at the same time the IAS decreases and you get closer to a stall due to decreasing airdensity. At some point you cannot go faster due to maximum Mach while you also cannot go slower due to minimum IAS to avoid a stall. At that point you cannot climb higher even if your engines have the power.

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