Inspired by the discussion about the Air India crash on this board, but seeking more general answers:
Modern jet engines on commercial airlines have huge diameters, but the turbines inside probably rotate slower than older designs. Engines on fighter craft seem comparably slender, but may spin at really high speeds.
How much rotational energy is stored inside various jet engine designs at maximum continuous operating parameters? Order of magnitude answers are fine.
Interesting question.
Most modern jet engines, fighter or airliner, consist of two independently rotating components, called “spools”. The inner or #2 spool has the main compressor and initial turbines. It’s smaller in diameter and rotates faster. The outer #1 spool drives the fan on the front by a secondary turbine(s) at the rear. It’s larger in diameter and turns more slowly. The rest of the engine is stationary net of things like the impellers in pumps.
Looking at the wiki for the CFM-56 which powers 737s, many A320s, and a few other airplanes we learn the #2 high speed/high pressure spool turns ~15,000 RPM at ful thrust. The #1 low speed low pressure spool turns at ~5000 RPM at full thrust. The fan is ~60"/155cm diameter and the whole engine weighs ~5300#/~2400kg.
A very rough rule of thumb would be to divide the total weight into equal 3rds for fixed parts, #1 spool, and #2 spool. So you’d have very roughly 800kg turning at 5K RPM and another 800kg at 15K RPM.
An obvious problem is that “rotational energy” (formally angular momentum) depends on the mass distribution by radius. The spools consist of a very dense shaft festooned with lighter blades with gaps between them. The 1 spool has a greater percentage of its mass farther from teh center than does the #2 spool. Without more dimensioned engineering drawings it’d be hard to make deeper calcs. But at least now we have the ideas and rough data.
I was not able to readily obtain similar figures for the GEnx which powers the 787 as a representative wide body engine.
For the GE90-110 that powers the current generation 777s, the spool RPMS are ~2300 & ~9300 RPM, the fan diameter is 128"/330cm and the engine total weight is ~19,000#/~9,000kg
To complicate matters even further, the Rolls Royce Trent series engines have three spools.
And we are now seeing some enthusiasm for geared turbines, as another way of driving ever larger fan discs. Currently from Pratt and Whitney (PW1000G) , and with rumblings from Rolls Royce for a future product (Ultrafan). The PW1000G has a 3:1 gearbox from the low pressure spool to the fan. There is a frightening amount of power going through that gearbox.
As each generation of engine appears with higher and higher bypass ratios this is somewhat inevitable.
Another way of gauging the energy: do an image search for “uncontained engine failure”.
Airliner engines are meant to fail in a way that does not produce flying metal. All material is supposed to stay within the engine housing and cowling, but on rare occasions it does not. And then you see just how much damage a rapidly spinning hunk of metal can do.
To clarify: since it’s “inner”, its shaft has to surround the shaft of the “outer” spool.
Do the spools on three-shaft engines all turn the same way, nowadays anyway? Didn’t one counter-rotate on one or two engines in the past?
Sloppy terminology on my part.
Think along the fore/aft axis = engine centerline. From that POV the #2 spool is “inner” in the sense it’s forward (compressor) section is closer to the central combustion section and its aft (turbine) section is also closer to the combustors. The #1 spool’s fan & turbine are “outer” = farther away along the engine centerline from the combustors.
You’re of course right that the #1 spool shaft is right on the engine centerline and the #2 spool shaft is a hollow tube that runs concentrically around the outside of the #1 shaft. And only along part of the #1 shaft’s total length.
If I take the power of an engine, and the time to spool up, I get 50KW * 10 = 500KJ
From what I’ve read, that would be accurate to 2 orders of magnitude – some where between 50MJ and 5KJ… 