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Jordan Taylor Profile picture
@Jordan_W_Taylor
Jul 27, 2024 • 17 tweets • 7 min read • Read on X
Scrolly
Aircraft are thirsty and burn lots of fuel, right?

Wrong.

The average fuel efficiency of air travel today is about 67 mpg per passenger. That makes it more fuel efficient than your drive to work. The best hit 100mpg/passenger.

How did aviation manage it?

A thread. Image
It wasn't always so. The venerable 707, doyenne of the 60s jet set, was more than twice as wasteful: Its fuel consumption per hour was 50% greater than a modern 787-8, even though the 787 is 50% heavier, flies 50% further and carries a hundred more passengers. 

How? Image
Bypass ratio.

It's easier to accelerate a large volume of air gently than a small volume of air fast…

The bypass ratio is the ratio between the air mass flow through the fan and the flow through the engine core. This number has been going up & up…
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Lightweight materials.

Aluminium is light, but Titanium has a higher strength:weight ratio, and composite fibre reinforced plastics (CFRP) higher still. The proportion of aircraft weight given to super lightweight materials has only grown. Image
That has run hand-in-glove with massive advances in composite manufacturing, rendering it cheaper and more accessible than ever.
Compression ratio.

Air must be compressed before it's combusted. A high compression ratio aids combustion and energy extraction in the turbine, so higher is better… to a point.

Air resists compression so improving this is a battle of balancing the gains against the losses.
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Turbine entry temperature.

All being equal, higher turbine inlet temperatures are better for thermal efficiency. These are limited primarily by structural factors and cooling, as well as toxic Nitrogen Oxide formation. Image
One early development was improving turbine blade high temperature creep resistance by moving from an equiaxed grain structure through columnar grains and finally to single crystal turbine blades.
Another suite of improvements to turbine inlet temperatures came from active film cooling and the development of specialist thermal barrier coatings.
Aerodynamics.

A thousand subtle improvements, from computer modelled optimization of fan geometry, through flexible high aspect ratio wings (aided by materials advances), low drag trim, wingtip fences for induced drag reduction.

It all adds up.
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Here's a deeper focus on wingtip sails/ winglets, which were absent in the early jet age but almost ubiquitous now as we optimize for efficiency with tightly bound span limits, and evolving into ever more organic forms.
Automation.

The demise of the flight engineer signalled a fundamental change: Digital flight control systems and their propulsive equivalents (FADEC) can micromanage spoilers, fuel mixing, stator vane position, bleed air etc to achieve efficiencies unattainable manually. Image
The all electric aircraft.

Most notable in the 787 with it's oversized electrical generation capacity, many sub-systems previously powered by hydraulics or compressed air bleeds are going full electric for reasons of efficiency and minimising weight.
Twin engines, extended range.

It's better structurally, aerodynamically and propulsively to have two big engines instead of four little ones. The certification of efficient twin engine designs for extended range operation over water was a major step. Image
Geared fans.

In a modern turbofan engine the fans do most of the work, turned by a shaft connecting the low pressure turbine… but the turbine’s most efficient RPM is a lot higher than that of the fan.

So what did Pratt & Whitney do? Added a gearbox!
Blisks!

Bladed discs: Because a few big components can be much lighter than lots of assembled components.

It's tricky though.
This is an incomplete list, and you can add hub airports, variable ticket pricing, seating design and a bundle of other stuff too… but it's all a tribute to the power of incremental change: Over time it moves mountains… and mountain-sized aircraft.

Let's never stop! Image

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More from @Jordan_W_Taylor

Jordan Taylor Profile picture
Aug 30
It's the greatest story never told: It's the story of how a frugal county in the North of England invented the modern world.

Put on a flat cap and call up the whippet, because this is a thread about my home county, and the inventions that came out of Yorkshire! Image
Image
Steel!

Benjamin Huntsman invented high homogeneity crucible steel in Sheffield in the 1740s, firing with coke to fully melt the steel and homogenise the carbon content.

This became used… everywhere, and supercharged the ongoing industrial revolution. Image
Steam trains.

Steam locomotion had been in development for some decades by 1812, but arguably the world's first commercially successful steam locomotive was Matthew Murray's Salamanca. To him, we owe speed. Image
Read 14 tweets
Jordan Taylor Profile picture
Aug 26
A liquid rocket boost stage needs to pump fuel and cryogenic oxidiser to the combustion chamber at a rate that beggars belief: The 33 engines on the boost stage of SpaceX's monstrous ‘Superheavy’ booster each chew through about 700 kg of propellant every second. Put all those engines together and the flow rate of liquid fuel & oxygen would be sufficient to empty an Olympic swimming pool in under 2 minutes, if you could find an Olympic swimming pool for cryogenic propellant.

Can you think of any conventional lightweight pump that can do this? Me neither. You need something special…

The turbopump comprises a typically-axial turbine powered by hot, pressurised gas flow that powers centrifugal compressor pumps that pump the colossal quantities of propellant required and pressurize it to, potentially, hundreds of standard atmospheres.

It's a handy, lightweight way to provide pumping power, but it does require that you have a source of hot, high-pressure gas to work with.

Now, where would you find that in a rocket engine?

Indeed. In order to burn fuel, we must pump it. In order to pump it, we may have to burn some of it.

Um…Image
Image
The Gas Generator Cycle.

A small quantity of the pressurised fuel & oxidiser flows are tapped, brought to a small combustor, vaporised, ignited then expanded through a turbine that powers the fuel and oxygen compressor cycles.

Inevitably the gas generator can't run with a completely nominal fuel:oxy mix, as it would get so hot that it would melt the turbine blades, so typically a gas generator will trade off some efficiency and run fuel rich to power the turbopumps.

-Why not oxy rich? Because fuel has a higher specific heat at constant pressure (Cp) and so you need less mass flow through the gas generator if it's fuel rich than oxy rich, meaning more useful propellant goes to the main combustor & nozzle that moves the rocket.

So the upside of a gas generator cycle is relative simplicity and robustness, which is why it's used on the most reliable rocket motors around, the SpaceX Merlin. The downside is that you trade away efficiency by throwing away some of your propellant, meaning that the tyranny of the Tsiolkowsky rocket equation will kick you where the sun don't shine.Image
Image
Staged combustion attempts to address this, by taking either a fuel rich or oxy rich preburner, operating at a much higher flow volume than a standard gas generator, and routing the hot gases that leave the turbine straight to the combustion chamber so that they're not lost. This not only increases the average propellant exhaust velocity (since none of it is lost) and therefore efficiency, but also allows a lower average temperature in the preburner and turbine, since there's a higher volume throughput instead.

On the flipside you must deal with hugely increased engineering complexity, an increased potential for feedback control problems between the different parts of the engine, and also a much higher pressure preburner, since it will still need to deliver high working pressures to the combustion chamber after the losses of the turbine and injectors.

The Soviets got there first, and some of their genius manifested in the Russian RD180 oxy-rich staged combustion engine, which was bought by the Americans and used in Atlas rockets for many years. Its unique oxy-rich staged combustion cycle was efficient but not without drawbacks, as high temperature gaseous oxygen is brutal to exposed metal surfaces, demanding an enamel coating on many parts of the engine.

And it gets even more complex than that…Image
Read 5 tweets
Jordan Taylor Profile picture
Jul 18
Last month Rolls-Royce won the UK's small modular reactor competition to develop and build SMRs in the UK. It could be a new dawn for nuclear power.

But who else was in the competition, what was special about each design, and which is your favourite?

An SMR thread… Image
What's an SMR?

A small modular reactor is a way of beating the brutally high capital costs of building nuclear power: By simplifying assembly (modularity) and minimising subsystem size so almost all of it is factory built you harvest industrial learner effects and low costs. Image
Boiling water vs pressurised water reactors.

All designs in this list are either PWRs or BWRs, the most common reactor styles today. I've a thread on the basics if you need it, but otherwise on with the show!
Read 21 tweets
Jordan Taylor Profile picture
Jul 4
In April on a mountain in Chile the Vera Rubin observatory gathered first light, and this telescope will be world-changing! -Not because it can see the furthest… but because it can see the fastest!

The Vera Rubin telescope thread! The value of speed, and unique technology… Image
Who was Vera Rubin?

She first hypothesized the existence of dark matter, by observing that the rotation speed of the edge of the galaxy did not drop off with radius from the centre as much as it should. The search for dark matter, and other things, will drive this telescope… Image
Does it see a long way?

Yes, but it’s not optimized for that: The battle of the big mirrors is won by the Extremely Large Telescope which, yes, is meant to see a long way. Vera Rubin is not that big, but that doesn’t matter because it has a different and maybe better mission. Image
Read 22 tweets
Jordan Taylor Profile picture
Jun 20
Rotating detonation engines: Riding the shockwave!

A technology that could revolutionise aviation, powering engines with endlessly rotating supersonic shockwaves. It could bring us hypersonic flight, super high efficiency and more.

The detonation engine thread… Image
Almost all jet engines use deflagration based combustion, not detonation, but while fuel efficiency has been improving for decades, we're well into the phase of decreasing returns and need some game-changing technologies.

One is the rotating detonation engine (RDE). Image
To understand the appeal of RDEs, you need to know that there are two forms of combustion cycle: Constant pressure, where volume expands with temperature, and constant volume, where pressure goes up instead.

Most jet engines use constant pressure. RDEs use constant volume. Image
Read 21 tweets
Jordan Taylor Profile picture
Jun 6
As a new graduate I once had to sit down and draft an engine test program for a subsystem of a new model of Rolls-Royce aero engine. It was illuminating.

So here's a thread on some of the weirder things that this can involve: The jet engine testing thread! Image
Fan Blade Off!

Easily the most impressive test: A jet engine needs to be able to contain a loose fan blade. In the FBO test, either a full engine or a fan & casing rig in low vacuum is run to full speed, then a blade is pyrotechnically released.
Frozen.

The Manitoba GLACIER site in Northern Canada is home to Rolls-Royce's extreme temperature engine test beds. Not only must these machines be able to start in temperatures where oil turns to syrup, but in-flight ice management is crucial to safe flying. Image
Read 15 tweets

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