Hacker News Reader

Comment by ssuds | Hacker News Reader

In a past life I worked on a rocket engine turbopump that spun at 160,000 RPM. It’s crazy to visualize how fast that is. The stresses in the metal are so high that we needed speciality alloys made exclusively for us. The parts are literally ripping themselves apart.

View on HN

High speed turbo machinery is certainly an odd area -- like you said, its on the verge of ripping itself apart. The Leybold turbo molecular vacuum pump on my bench at home is rated to spin at 60,000 RPM. In the old days of metal fabrication you'd have two specialty precision machines to locate, drill and bore holes -- the jig bore and jig grinder. Once you bored a hole, if the part was heat treated there was almost certainly a need to touch up the bore from slight deformation so a jig grinder, a jig bore base with a 100,000+ RPM pneumatic grinder, is used to perform the final precision machining.

> like you said, its on the verge of ripping itself apart

They didn't say that - they said "the (exclusive speciality alloy) parts are literally ripping themselves apart". That sounds like a seriously flawed design to me

It's just hyperbole.

When parts spin that fast centrifugal force becomes a serious problem. Everything is a spring, nothing is actually 100% solid. Anything you make that spins that fast will at minimum grow larger in the direction of centrifugal force and shrink in perpendicular axis. If you don't account for that stretching and shrinking things like turbine blades will end up scraping the walls of the enclosure at speed.

Even minor variations in weight get multiplied and create exponentially higher stresses on the part. If you don't have the right alloy with the right hardening/annealing parts will rip themselves apart from attempting to support their own weight against the centrifugal force.

There's a reason modern high-bypass turbofan jet engines use monocrystaline blades because growing a single crystal of such size is the opposite of easy or inexpensive. Plus in the core you are dealing with so much heat no alloy exists that can withstand it for long so you have to machine cooling channels inside the blades (which have complex curves) to keep them from getting soft and flying apart.

Yup, you nailed it. This is exactly what I meant when I said “tearing themselves apart”

So not literally ripping themselves apart then

Years back I was an intern in a bio lab whose ultracentrifuge achieved spin rates similar to a turbopump. On its titanium head was a bar code used to regulate the speed via a beam of light. Unfortunately the manufacturer issued some with the wrong bar code, causing them to overspin and, at one lab, undergo "rapid unexpected disassembly", sending chunks of flying titanium through concrete walls. Fortunately it was at night and no-one was hurt, but the salespeople travelled out to every customer site the next day to implement the product recall.

I did a stint back in the day at the IBM Cottle road site sputtering hard disks. All the tools had roughing pumps that would bring the tool to near vacuum before the turbo pumps would kick in as anything even remotely close to atmosphere would just rip the blades apart. Then the older huge tool, the Ulvac had a cryo-pump which was just a big chamber that would cool a sponge to 20 kelvin and molecules would just come in contact and slow down...

What diameter was the part that spun at that speed? Also do you have some idea of the maximum forces exerted on it? It might not be much higher (as in, less than on order of magnitude) than the peak forces exerted on some parts of a reciprocating engine. And the turbine doesn't have to constantly switch between tension and compression, which I think should help a lot when designing a part that moves quickly.

Tiny! Like <4”. It would be near impossible to design something spinning so fast at a larger diameter

A rifle round fired at 3000 feet per second from a 1:7 twist barrel turns a bit more than 300,000 RPM.

Turbos you find in cars like the Civic operate between 180000 - 200000 RPM. But working at a rocket engine manufacture is more interesting.

I bet the size difference plays a role, and a car turbo that spins to 200k rpm will definetly be smaller than a turbopump for rocket engine.

Yes it does, it matters a lot. With smaller dimensions, tip speeds remain manageable. As soon as parts of the thing get supersonic, everything gets much more complicated.

I'm going to assume the rocket engine turbo has a hell of a lot more mass which makes it a far more difficult problem.

Reply on news.ycombinator.com