Rolls Royce M250 BLISK (bladed disk) for helicopters

Reference example of integral blade-disc construction for weight savings on helicopter engines.

I should have looked at the blisk on the web. The web's getting better and better — I did this morning. Here's a nice graphic of a turbine disk that is not a blisk. Here's your root, where the vane has to attach to the big heavy flange. With a blisk you can make the whole thing integral, which they do for the M250 engines of Rolls-Royce. The integral version is a lot lighter than this great big mass right here. So you'd like to do that because there's collateral weight savings.

The only commercial-production blisk Tom knows — Rolls-Royce M250, used on helicopters and small aircraft. Cast, not welded. Probably the highest-production jet engine in the world.

There's only one engine I know of in commercial production that uses a blisk, and it's a cast blisk. I wish I could get one — it's about this big around. It's made by Rolls-Royce, it's the M250 engine, used on helicopters and small aircraft. It's probably the highest-production jet aircraft engine in the world. But it's a small helicopter or a small plane like a Lear jet. I don't know if it's on a Lear, but that's one example of blisk, and it's a cast part.

Cited as the only high-volume cast blisk engine in production (~50,000 units, 30+ years). Used to introduce the collateral-weight-savings argument.

So a pound saved is worth a lot of money, but it depends on where it's saved. If you save it in the disc — the disc has a great big flange on the outside, and the further out it is the worse it is, because it's spinning and centrifugal force means there's a lot of stress. If you have to have a big heavy flange to make the mechanical attachment — people would love to weld the blade directly to the disc, and then you don't need this heavy flange and heavy root section. Plus, when you're going at those speeds, you don't want any vibration or flutter, so they machine those very precisely, like two-millionths-of-an-inch tolerance. They have to broach them — if you know what broaching is, you basically use a knife of a known shape and size and just cut right through, so it's absolutely identical. It's the most expensive part of the blade-making process, making that root section strong. If you could weld the blade to the disc, you can get rid of all this extra weight for the mechanical attachment. That's called a bladed disc, or a blisk. There's only one high-volume engine I know of, it's been around for over thirty years, the Rolls-Royce M250 — used to be Detroit Diesel Allison, but Rolls-Royce owns Detroit Diesel Allison now in Indianapolis. It makes a cast blisk. The disc is actually cast with the blades. I'd like to get one. They've made about 50,000 of these engines. It's only about this big, and the blades are integrally cast to the disc. It's the only blisk that I know of in high-volume production. They'd love to make the great big disc as a blisk, because they could save twenty pounds on a disc. If you save twenty pounds on the disc, and if you have ten discs in an engine, you save two hundred pounds in the engine. If you save two hundred pounds out there on that little diving board we call the wing, you save two thousand pounds on the airframe. So the Air Force is very interested in this, because two thousand pounds of weight savings could be either further range or more payload. You've got the tradeoff: carry more fuel for greater range, or more payload to drop on the enemy.

The only commercial BLISK Tom knows of — Detroit Diesel Allison (now Rolls Royce) M250, ~10,000 made. Cast as one unit, superalloy. Used to anchor the weight-savings-cascade argument.

One of these is called a bladed disk, or a BLISK. You'd love to be able to take the turbine disk, which right now has this complex geometry to join the turbine blade to the disk, and you have this heavy structure right in here. If you could weld these things together or cast them together, you could reduce a lot of weight. And that's exactly what has been done in some cases. The only commercial BLISK that I know of happens to be on the Detroit Diesel Allison, now Rolls Royce, M250 engine, which they've made something like 10,000 of. They go in helicopters and other things. They cast the blades to the superalloy, all cast as one unit. It's a small turbine engine for helicopters and small planes, but not for great big commercial planes. People can't cast with the quality they need in the big disk. People are trying welding. The Air Force has spent tens, if not hundreds, of millions of dollars trying to figure out how to weld blades to disk, because there's tremendous value.

Used to illustrate sprung-vs-unsprung-weight logic and to set up the Air Force friction-welding ambition. Cast blisk eliminates the Christmas-tree root structure that consumes 60-70% of rotating-assembly weight.

One of the best examples I have is when I talk about welding, and friction welding, I talk about the blisk. A blisk is a bladed disc. They've built about thirty or forty thousand Rolls-Royce M250 engines that go in helicopters and small aircraft like Cessnas. It's a small turbofan engine, and it has a cast blisk about this big. It's a single blisk of nickel-based superalloy, and the blades are cast directly onto the disc. You don't have all the weight that you have in a typical turbine blade in a bigger engine, where you have to join the blade — which is a simple little airfoil — to the disc. Blades and vanes are airfoils generally; blades rotate and stators don't.

The only production aircraft engine Tom knows with cast integral disc-and-blade — M250 Detroit Diesel Allison (now Rolls Royce). 30-year-old design, all-cast technology, 10–20K units, in helicopters and small business jets.

The Air Force would love to make a blisk and they still have big programs to do it. The only aircraft that I know that actually has a cast integral disc and blade — and I'd like to get one — is an M250 Detroit Diesel Allison, which is now Rolls Royce. They make like 10,000 or 20,000 of these engines; they go in helicopters and small business jets. Very high volume engine. It's a cast technology, everything is cast. It's not as fancy as these single-crystal blades. It's a 30-year-old engine, but they do use blisk technology.

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