18
$\begingroup$

Take a look at the case of new engines such as the F-135:

F-135 engine with case

or the F-119:

F-119 engine with case

Why do they have a case with a triangular tiling pattern? Is this a new technology? A new material?

$\endgroup$
35
$\begingroup$

This type of structure is called isogrid structure. Solid walls are usually very heavy and therefore engineers use more weight-efficient structures such as sandwich structures or stiffened structures with stringers and frames. A downside is the huge manufacturing cost compared to other options.

As you can see from NASA Report CR-124075 "Isogrid Design Handbook", which was written 50 years ago, it is not a new type of structure.

| improve this answer | |
$\endgroup$
  • 6
    $\begingroup$ Damn you, you remembered the isogrid, totally escaped my stress-demented mind 😃 I think in the case of these jet engines, the manufacturing costs are not huge if we think about the viable options that fullfill the design goals: there might be none, actually. $\endgroup$ – Jpe61 Nov 22 at 17:23
  • $\begingroup$ I think we can go further back than NASA - how about the Vickers Wellington, for example? $\endgroup$ – Mike Brockington Nov 23 at 19:56
  • 2
    $\begingroup$ @MikeBrockington That was a geodetic airframe, like the airship S.L.1. I don't consider them to be the equivalent structures because isogrid structures consist of a load-carrying skin with stiffeners forming triangles, while geodetic ones are a frame structure with many rectangles and mostly a non carrying skin like fabric. $\endgroup$ – Gypaets Nov 23 at 23:51
  • 1
    $\begingroup$ The isogrid principle appears identical to the geodesic or geodetic construction of the R100 airship, Wellington bomber and Buckminster Fuller's domes. Is there any definitive difference? $\endgroup$ – Guy Inchbald Nov 24 at 10:00
  • 2
    $\begingroup$ @GuyInchbald They're optically very similar, but see my comment before. The skin of the three examples you mention is mostly fabric and cannot carry loads as the isogrid skin does. From a structural point of view one is a frame structure while the other is a stiffened shell. $\endgroup$ – Gypaets Nov 24 at 11:17
27
$\begingroup$

This is simply a way to achieve required rigidity/strength and vibration dampening with less material than in a solid, even thickness design.

Varying the weave pattern gives a relatively easy and a very efficient way to model and produce varying properties to different sections, without having to change the "base" structure thickness.

Successfully utilizing it does require advanced computing and manufacturing abilities, therefore it was not being used commonly in more ancient times.

| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ I wouldn't say that it was at all uncommon in older times (depending on your definition of "more ancient". Consider the common I-beam, for example, or structures build using tubing rather than solid rods, e.g. the tube and fabric structure of older airplanes. $\endgroup$ – jamesqf Nov 22 at 17:47
  • 4
    $\begingroup$ You need to get fairly ancient: the Schütte-Lanz SL-1 utilized it in 1909, and Vickers-Armstrong used geodetic airframes in the 1930s. The calculations are complicated, though, which is why use of the technique only became common recently. $\endgroup$ – Mark Nov 22 at 18:07
  • $\begingroup$ The structures you mention are somewhat different. The engine casings or shells in the pictures are are inseparable uniform solids, where as in the case of the airships and the V-A the skin and the grid were separate. Maybe glued or riveted together, dunno, but still separate "entities". $\endgroup$ – Jpe61 Nov 22 at 20:29
  • $\begingroup$ It might seem that the wimpy little ribs would not add much stiffness. What is invisible in the pictures is that the outside of each triangular facet is flat, but the inside is curved to form a continuous smooth circular cylinder. Therefore the middle of each facet is very thin, but the ribs surrounding the triangle prevent any impact damage from an internal failure in the engine from spreading across the whole casing. (In fact, I remember one "accident" where the company making a prototype casing actually cut right through some of the thinnest sections while attempting to machine it!) $\endgroup$ – alephzero Nov 23 at 3:01
  • $\begingroup$ Not sure what you're referring to regarding the required advanced computing capabilities, but note that the main reason for using equilateral triangles for the mesh (true "isogrid") is that it simplifies the design calculations a whole lot by making the properties of the whole lattice isotropic. A "waffle grid" (rectangular lattice) is much easier to manufacture but much harder to design. "advanced computing" actually enables more intricate and tailored lattice patterns than these two simple examples, so weird lattices have become a hot topic recently. $\endgroup$ – TooTea Nov 23 at 13:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.