Wikipedia:Reference desk/Archives/Science/2019 August 6

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August 6

What power law or physics relation makes distributing required thrust capability into fewer jet engines better for fuel cost?

Sagittarian Milky Way (talk) 15:10, 6 August 2019 (UTC)

My guess would be thermodynamics and inertia. Having two engines vs. one with the same thrust as both smaller engines means roughly twice the number of moving parts which have to be spun up and kept spinning for the turbine to produce thrust. Overcoming those frictional losses takes energy. Also, it is generally more weight-efficient to make a turbine larger than it is to add another whole engine - reducing the weight of the aircraft improves fuel efficiency. NorthBySouthBaranof (talk) 15:14, 6 August 2019 (UTC)

From Chapter 14 and Chapter 15 of the Airplane Flying Handbook, one of the chief advantages of the turbine (turboprop and turbojet) is a high power per unit of weight and the high thermodynamic efficiency. If the airplane's designer can produce the total required power - for all phases of flight (including take-off and landing, and cruise flight) - using fewer engines, that's a reason to use fewer engines. To really understand all the trade-offs, though, you need to get in to the details of engine engineering; manufacturing and operational economics; reliability; normal- vs. abnormal- operations; and so on. Some great reading materials - full-length free books - are at the FAA's handbooks, textbooks, and manuals website, which is usually the first place I look before I jump into a more specific technical resource.

In this case, because you're asking about the engineering and design of jet aircraft, I'd head straight for Aerodynamics for Naval Aviators, which is a great introductory book for the up-and-coming jet student. Page 106 starts the section that's all about turbojet efficiency.

Nimur (talk) 17:04, 6 August 2019 (UTC)

Thanks, I will read that section. Sagittarian Milky Way (talk) 19:37, 6 August 2019 (UTC)

If you consider the whole plane, quite obviously auxiliary systems (fuel piping, command and control cables, attaching bolts, etc.) come a cost/weight. More systems means more complex thing, longer check lists, more cause of errors, ... This applies just as well to all systems, not just engines (fi, planes have a single big tail rather than a number of smaller).

If you consider the engine alone, I see a number of possible reason, all of them beyond my mastery

• Reynolds number: smaller engine means smaller Reynolds number, and I guess this is somehow detrimental for jets just like I know it is for ships
• Carnot efficiency: you want as high temperature as possible, and as few leakage of heat as possible. The bigger the system, the smaller the surface to volume ratio, the easier you can control heat where needed instead of losing it in cooling system to prevent overheating.
• design: bigger parts can easier be made to the required property (heat resistance, tensile strength, ...)

Gem fr (talk) 08:37, 7 August 2019 (UTC)

What's the most complex animal that is isotonic with seawater?

Are there even saltier animals in salt lakes? Why are ocean fish barely saltier than mammals? Or am I misremembering the salinity of reptiles or amphibians? Sagittarian Milky Way (talk) 19:29, 6 August 2019 (UTC)

There's something about how saltwater fish are capable of excreting salt. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:22, 6 August 2019 (UTC)

So even though they evolved in saltwater since the first cell their ancestors let their cells run less salty for some advantage at the expense of having to spend energy to constantly pump the salt out or die. What is that advantage? Sagittarian Milky Way (talk) 00:11, 7 August 2019 (UTC)

Membrane potentials are inextricably linked to being alive - they provide the rhythm of the heart, the pulse of neurons, the force that moves sugar into cells... many of the things that make a cell "alive". Maintaining salt gradients seems nearly universal in cells. Life and entropy are nearly polar opposites. — soupvector (talk) 01:01, 7 August 2019 (UTC)

Do you mean animals that maintain an isotonic extracellular fluid or intracellular fluid? If the former, the most crustaceans allow their extracellular osmolality to passively change. See:

Pequeux A.J.R., Gilles R. (1984) Control of the Extracellular Fluid Osmolality in Crustaceans. In: Pequeux A., Gilles R., Bolis L. (eds) Osmoregulation in Estuarine and Marine Animals. Lecture Notes on Coastal and Estuarine Studies, vol 9. Springer, Berlin, Heidelberg Klbrain (talk) 07:09, 7 August 2019 (UTC)