Air Force Buy-to-Fly Ratio in Aircraft Manufacturing

A 5 kg flying titanium part requires machining away 95% of the starting stock — 500 kg of chips per part times 100 parts = 10,000 kg. Used to motivate why additive is cheaper for titanium aerospace brackets.

I passed the sign-in sheets. Most of you have attended enough classes that's not going to be an issue. This is from the Digital Alloys blog site. They've actually taken a Boeing part, and Boeing has done this analysis. If this is to make a five kilogram titanium part — remember buy-to-fly ratio — five kilograms is probably going to fit within the size of a soccer ball. And making a hundred parts, not just one part. You're not making one-of-a-kind unless you're a spacecraft.

32:1 historical buy-to-fly ratios down to <10:1 driven by 1980s near-net-shape manufacturing program. Cost multipliers on aluminum plate ($10/lb → $100/lb fly weight) and nickel superalloys ($100/lb → $1000/lb fly weight).

And this way you're going to machine away ninety percent of the weight out of this plate. Aircraft are weight-critical. You start with the thick plate and you're going to machine away ninety percent of the weight. The Air Force calls it the buy-to-fly ratio. The Air Force is the big purchaser of aircraft components. You talk about the pounds of metal purchased as a plate or a big forging that you're going to machine down into a rotor disc or a wing — and you could be paying five or ten bucks a pound for this aluminum plate, and you're going to machine away ninety percent of it.

Adapted here to automotive sheet metal: 2:1 buy-to-fly, half lost as scrap from window-cutout punching.

I could have predicted that. In "Defense and Industry Materials for the 21st Century," my little four- or five-page thing on material selection — the relative price of steel to aluminum is about two to one. So, duh. It's not hard to estimate these things. You say, well gee, it's only $500 more to buy that car. The problem that she didn't recognize in her thesis: she assumed that you could resistance spot weld aluminum just like you do steel. Remember I said it takes 3,000 spot welds in the average automobile, because you need 2,000 good ones. There are 50 to 100 billion spot welds made for automobile production in the world. If you build 30 million cars, and you've got 3,000 spot welds, that's 90 billion spot welds. Maybe it's a trillion spot welds. I had estimated that the spot welding in the average automobile was about a nickel per weld. So a nickel per weld times 3,000 welds — that's $150 for welding. The welding cost was equal to the material cost. If you go to that little handbook I gave you, the material cost of a car is about 10% of the cost of the car, the joining cost is about 10%, and the stamping and other costs may be 10%. How much of the stamping of the metal you buy — what's the buy-to-fly ratio for sheet metal of an automobile? It's about two to one. Half of it goes into scrap, because when you punch out those holes for the windows, that's scrap right out of that sheet metal.

Air Force framing: 11-lb finished part from 210-lb cold die forging (BTF ~20) vs. 72-lb hot die forging. At $100/lb alloy and BTF 20, you're paying $2,000/lb for flying metal before processing. Justifies near-net-shape investment.

You have to go through a lot of steps and a lot of different shapes before you get to the finished part. To give you an idea of what you might have to go through, the Air Force likes to call this the buy-to-fly ratio: how many pounds of metal do you have to purchase to make a forging that you can then sell as a part? Here's one where the finished part — part of one of these discs — weighs eleven pounds. If you do a cold die forging, it starts out at 210 pounds. That's a buy-to-fly ratio of almost twenty. Here's the hot die forging — if you do it hot, it's only seventy-two pounds, so that's obviously better. When you realize we're talking about alloys that may cost $100 a pound, and you've got a buy-to-fly ratio of twenty, you're now paying $2,000 a pound for the metal that you actually fly, without even the processing that's involved. So there's lots of money to be made in what's called near-net-shape processing.

The Air Force concept of buy-to-fly ratio (mass purchased ÷ mass that flies) introduced as a productivity metric. Tom cites 33:1 hypothetical, 20:1 Boeing wings, 100:1 some engine parts. Sets up the near-net-shape pivot to Chaparral later.

When we start looking at things like this, we get to something I haven't discussed in this class, but it's probably on some of the other videotapes if you take the welding and joining — something the Air Force calls the buy-to-fly ratio. Carl, what's the buy-to-fly ratio? In the manufacturing side of the Air Force they actually used to use it a lot in the '80s and '90s. It's how many pounds of metal you buy in ingot form or billet form, and in order to fly something, how much does the part weigh. So if you need to start with a billet of material that weighs 100 pounds, but the part that flies weighs 3 pounds, you have a buy-to-fly ratio of 33. That's not good when you're paying $100 to $200 a pound for the material. That means 97% of your cost — and let's face it, if you're talking about a 100-pound ingot or billet at $200 a pound, you're talking $20,000 for the part, and you're going to fly about $600 of that $20,000 of material. The rest of it goes into machining chips. You can recycle those at twenty cents on the dollar, depending — if it's a valuable metal you might get twenty cents on the dollar for it. So you've lost ten or fifteen thousand of value, just because these things have to be made in big heavy bulk and then machined away to much less.

The central economic argument for aerospace additive manufacturing. Titanium 6-4 T-bracket as worked example: 17:1 buy-to-fly via CNC machining means 95.4% of titanium becomes chips. Why the Air Force spent billions on near-net-shape manufacturing in the 80s–90s.

Now if you look at the wasted material — this is actually the final part made out of titanium, this is in his article, some sort of bracket. They found they can start with a plate of the same material, build up some things on top, and then start to machine them away and end up with this part. You say, that's a lot of work — except if you look at what's called the buy-to-fly ratio. If we're talking about aerospace parts, this is a very important parameter. The Air Force has been calling this the buy-to-fly ratio. They're claiming Joule printing, their process, is flat depending on the buy-to-fly ratio. That's not really true, but they're going to present it the best way. If you talk about hogging the whole thing out by CNC machining, the buy-to-fly ratio raises the price to three thousand dollars a kilogram at thirty-to-one buy-to-fly ratio. There are a lot of parts in an aircraft, particularly military aircraft, that are thirty-two to one. That's why the Air Force spent billions of dollars in the 80s and 90s on net-shape manufacturing processes, which didn't include additive manufacturing.

Referenced as the Air Force metric for plate-stock-versus-flying-weight in aircraft. Used to explain the 50% price uplift from scrap loss in plate fabrication.

So they decided they wanted to go to HSLA-60 for a lot of the lower-strength steel components. Newport News estimated they could save 2,000 tons on the weight of the next-class carrier if they had an HSLA-60 available. David Taylor with the steel companies developed an HSLA-60, which wasn't too hard to do — there were commercial versions, it was good dual-use technology. They said, well, we need to do a study to find out, if we go to this change, what's it actually going to cost as fabricated. My rule of thumb is, if the material costs you a dollar a pound, it will cost you ten dollars a pound as fabricated. Let's face it, you start out with these rectangular plates and you cut away thirty percent of them and send them back to the steel mill for scrap. That just increased your price by fifty percent per pound of what you buy. The Air Force calls it a buy-to-fly ratio — how many pounds do you buy versus how many actually fly in the airplane.