When an engineer starts out to build something, one of the first questions to be answered is how much load must it carry in normal service? The next question is similar: hom much load must it carry at maximum? An engineer can study those questions deeply or very superficially, but having a credible answer is a vital step in at the start of a design process.
Here is an example. If you design and build a step ladder which just barely holds your weight without breaking, what will happens after the holidays when your weight may be somewhat more than it was before you eat Aunt Martha’s Christmas dinner? You really don’t want to throw out your stepladder in January and build a new one do you? Obviously you would should build a stepladder that can hold just a little bit more. Don’t forget what might happen if you loan your stepladder to your coach-potato neighbor who weighs a lot more than you do? Can you say lawsuit?
So how do you determine what your stepladder should hold? Do you find out who is the heaviest person in the world and make sure it will hold that person? Probably not. Better, pick a reasonable number that covers, say, 95% of all folks, design the ladder to that limit and put a safety sticker on the side listing the weight limit. Yep, that is how most things are constructed.
But that is not all. Once you determine normal or even the maximum load it is a wise and good practice to include a “factor of safety”. That means that you build your stepladder stronger than it needs to be. This helps with the idiots that don’t read the safety sticker; it also helps protect for some wear and tear, and it also can protect if the actual construction of your stepladder falls somewhat short of what you intended. So you might build your stepladder with a FS of 2. That would cover 95% of all folks with plenty of margin for foolish people that try to accompany their friend climbing the ladder; or when your ladder has been in service for 25 years (like mine), or when your carpenter buddy builds the stepladder with 1/4” screws rather than ½” screws like you told him to.
Factors of safety are not pre-ordained. They have been developed over the years through experience and unfortunately through failures. Some factors of safety are codified in law, some are determined by professional societies and their publications, and some are simply by guess and by golly. Engineering is not always as precise as laypeople think.
“ . . . the choice of an appropriate factor of safety is one of the most important decisions the designer must make. Since the penalty for choosing too small a factor of safety is obvious, the tendency is to make sure that the design is safe by using an arbitrarily large value and overdesigning the part. (Using an extra-large factor of safety to avoid more exacting calculations or developmental testing might well be considered a case of “underdesigning” rather than “overdesigning.”) In many instances, where only one or very few parts are to be made, overdesigning may well prove to be the most economical as well as the safest solution. For large-scale production, however, the increased material and manufacturing costs associated with overdesigned parts result in a favorable competitive position for the manufacturer who can design and build machines that are sufficiently strong but not too strong.
Standards for factors of safety are all over the place. Most famously, the standard factor of safety for the cables in elevators is 11. So you could, if space allowed, pack eleven times as many people into an elevator as the placard says and possibly survive the ride. For many applications, 4 is considered to be a good number. In the shuttle program the standard factor of safety for all the ground equipment and tools is 4.
When I was the Program Manager for the Space Shuttle, there were a number of times when a new engineering study would show that some tool either could be exposed to a higher maximum load than was previously thought, or that the original calculations were off by a small factor, or for some reason the tool could not meet the FS of 4. In those circumstances, the program manager – with the concurrence of the safety officers – could allow the use of the tool temporarily – with special restrictions – until a new tool could be designed and built. These “waivers” were always considered to be temporary and associated with special safety precautions so that work could go forward until the standard could once again be met with a new tool.
In the aircraft industry, a factor of safety standard is 1.5. Think about that when you get on a commercial airliner some time. The slim factor of safety represents the importance of weight in aviation. It also means that much more time, engineering analysis, and testing has gone into the determination of maximum load and the properties of the parts on the plane.
For some reason, lost in time, the standard FS for human space flight is 1.4, just slightly less than that for aviation. That extra 0.1 on the FS costs a huge amount of engineering work, but pays dividends in weight savings. This FS is codified in the NASA Human Ratings Requirements for Space Systems, NPR 8705.2. Well, actually, that requirements document only references the detailed engineering design requirements where the 1.4 FS lives.
Expendable launch vehicles are generally built to even lower factors of safety: 1.25 being commonplace and 1.1 also used at times. These lower factors of safety are a recognition of the additional risk that is allowed for cargo but not humans and the extreme importance of light weight.
It is common for people to talk about human rating expendable launch vehicles with a poor understanding of what that means. Among other things, it means that the structure carrying the vast loads which rockets endure would have to be significantly redesigned to be stronger than it currently is. In many cases, this is tantamount to starting over in the design of the vehicle.
So to the hoary old punch line: Would you want to put your life on the top of two million parts, each designed and manufactured by the lowest bidder?
So I was floored just a few months later when I first heard it: “you are not a real engineer”. I was just “an ops guy”.
In the NASA pantheon of heros, the highest accolade any employee can be granted is that they are a “real engineer”. Not even astronauts rate higher. The heart of the organization worships at the altar of engineering: accomplishment, precision, efficiency. What does it take to be a “real engineer”?
In the ethos and mythology of NASA, a real engineer is one who has several characteristics.
First, they must have a superb grasp of the physics of their subject, a complete an total knowledge of the details of their specialty. This almost goes without saying. No nincompoops allowed; no fuzzy thinkers who are vague on the basic concepts. A “real engineer” knows his arcane stuff forwards and backwards and from the middle out towards both ends and can recite it in his sleep. “Let me tell you about the inviscid terms of the Navier-Stokes equation . . . ” a real engineer might say.
Second, a “real engineer” must create something, taking it from original concept to working, functioning reality. No view graph engineers ever get the title “real engineer”. If it doesn’t fly or move or compute or generate power, or do some concrete something, you haven’t built something real and without building something real, you are never going to be a “real engineer.” And the thing has to work; if it flops, then you are merely a tinkerer, not a “real engineer”.
Thirdly, “real engineers” are mild mannered; never needing to raise their voices, not loquacious, not given to long and convoluted discussions. No, real engineers are soft spoken and terse; they are recognized by their brevity and the ability to concisely summarize a technical point in a way that admits to no further discussion.
NASA is full of “real engineers”.
So us poor ops guys, who never had a drafting table, who never went into the machine shop to hand blueprints to a tech, who never got to blow up anything on the test stand; we failed miserably on the standard of being a “real engineer”. We merely operated the stuff that the real engineers built.
Along the same lines . . . .
I have been privileged to watch the advanced concept boys at work. They are marvelous. Through the study of all past and current rockets they have developed a number of “empirical models” — rules of thumb if you will — that can help in the initial ideas about building spacecraft. If you want to lift a certain number of metric tonnes to low earth orbit, given a particular rocket type (solid, liquid, hypergolic, cryogenic, hydrogen, kerosene, etc.) the advanced concept boys can give you a variety of options based on known ratios of structure weight to propellant weight, burn out mass, etc. And they can give you a rough guess at the cost. And they can evaluate multiple options and compare them one to another in very short order.
In the summer of 2002, I got to participate in an exercise for about two months of possible design options of manned missions to Mars. The advanced concept boys generated a new heavy lift launch vehicle about every other day and could compare all the designs against each other on a number of figures of merit. Its heady work to invent new Saturn V class rockets in the computer lab. Taller, shorter, with solid boosters or not, using kerosene or hydrogen or whatever. One engine, two engines, five engines, twenty engines; two stages, three stages, four. Whew. At the end of two months the team had a great list of options and the pros and cons for every launcher. And I found out that the advanced concept boys have been studying this problem for 40 years! They have evaluated hundreds, thousands of various options. Then they refined their studies, re-examined the basis for their methodologies and started in again.
But you know what? Advanced concept folks, even with all their knowledge of engineering principles, they are not “real engineers”.
Real engineering starts after the viewgraphs stop. Real engineers take the concept — boiled down as it may have been from hundreds of starting options — they take the concept and start making it real. When you have to really design and build the rocket in its detailed glory; when you have to take the subsystems out to the test stand and start them up and see if they hang together; when you go from weight estimates to actual plans and find out what the gizmo really does weigh — that is real engineering.
That is where you find out if the concept really will work or not; what the real problems are and how to solve them.
When the rocket really flies you have proof positive of the real stuff of engineering — and whether you have it or not.
That is what real engineering is all about.