Rolls-Royce M250 BLISK (bladed disk) for helicopters

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.