1.3 Where the Profile Comes From
about 6 minutes · 2 figures, 1 interactive · quiz of 5 · Slides
After this section you can: say where a test profile's numbers came from, name what was traded to get them, and write the one sentence the report needs if the source cannot be found.
Every profile is one of five things, and you should be able to say which one you are looking at.
A random profile is written as an ASD (acceleration spectral density, the standards' word for the autospectrum of an acceleration, in g squared per hertz; often called PSD on the test floor). Its overall level, the grms, is the square root of the area under it (2.4).
Measured. Sensors on the real article in the real step. This is the best case and the rarest.
Enveloped. A smooth line drawn over measured data so that one test covers many conditions: several trips, several aircraft stations, several vehicles. An envelope over flight data is what many programs call the maximum expected environment; others say maximum predicted environment (MPE) or maximum expected flight level, for the same idea. What the lab receives is a breakpoint table: the frequency and level at each corner of the profile, and the slope between corners in dB per octave (an octave is a doubling of frequency, 100 to 200 Hz or 1,000 to 2,000 Hz). Every envelope over-tests somewhere. That is the trade, and it is made on purpose, because one test is cheaper than ten. Look at an envelope and find where the margin between the line and the data is thick and where it is thin. The thin bands are where the test is honest and the thick bands are where it over-tests for the sake of one test instead of ten. The breakpoint table alone does not show the gap; ask whoever derived the envelope for the plot of the line over the data it came from.
Tailored. A standard's default profile (1.4 names the standards that carry them) adjusted to the platform and the step. The default curves in the standards are for when no data exists, and their text states it. A default curve with a program name typed on top of it is not tailoring. Real tailoring shows its work: the default's breakpoints moved to the frequencies measured on this platform, the levels set from data taken at this mounting location, and the reason for each change written down.
Inherited. The last program's profile, or the one the customer has always used. This is the most common source and the least examined. "Heritage" usually means the reasoning has been lost, not that it was bad. It may have been excellent reasoning, for a different vehicle. A profile taken from a launch vehicle's user's guide is inherited too, as far as the lab can see: behind it is the launch provider's envelope or prediction, and which one it is goes to the customer as a question.
Predicted. No flight exists yet, so the environment comes from a model of the article driven by what a similar vehicle does to its payload. Every prediction carries an assumption the curve no longer shows, for example that this article loads its mounting the way the similar vehicle's payload did, and it is the profile most likely to change after the first flight.
Two things ride on top of any source. Margin is added on purpose, and it should be visible as its own step in the derivation rather than baked into the curve. Duration is scaled (time compression, by fatigue equivalence): a long exposure at a low level becomes a shorter, hotter test, and that exchange is never free, because it moves the test toward a level the article never sees in service. That is enough to ask about here; the derivation itself, the enveloping, the margin step and the duration arithmetic, is Volume 2's subject.
Decide. Ask where the numbers came from before the test is scheduled. If the answer is a document number and nothing else, find what is behind it, or write down that it could not be found. The report needs that sentence either way, because the reviewer's first question will be the same one.
1. The step
One step of the sequence from Figure 1.2: carriage, tonal on broadband, hours at a time.
2. What the sensor saw
One record of many, half a second of it, level normalized.
3. Sorted by frequency
The same signal as an ASD (acceleration spectral density, the standards' word; often called PSD on the test floor), with two sibling records (dashed, dotted) behind it. The step from the record to this curve is the autospectrum, and how the controller averages it is Volume 2's subject.
4. Enveloped into a spec
Drag the amber breakpoints. Shaded: where the envelope sits above every record, the over-test.
| breakpoint | level |
|---|
Common Questions
If the damage is held equal, why is time compression not free?
Because equal damage on paper is not the same exposure. The exchange holds the fatigue damage equal, but the shorter test runs at a level the article never sees in service, and that level can find failures the real exposure would not. The arithmetic of the exchange is Volume 2's subject.
How far do I chase a heritage profile before I write that the source could not be found?
The deadline is the schedule: the search is done before the test is scheduled, not after it. What you are looking for is the reasoning, not an older document number, so the search ends at the program or the person who derived the profile, or at the point where nobody can say. If what you find shows the profile was derived for a different vehicle, that is a question for the customer before the run, not a reason to run it as is.
Is finding the source the lab's job or the customer's?
The profile comes with the requirement, so the answer sits with whoever wrote it, but the lab is the one that has to ask, because the reviewer who reads the report will ask the same question first. The report is the lab's argument that the result proves the requirement (1.5, The report), so whatever the search turns up, or fails to, goes into it.
Our profile comes from the launch vehicle's user's guide. Do I still have to chase the source?
Yes, though the chase is short. To the lab that profile is inherited: behind it is the launch provider's envelope or prediction, written for the vehicle's payloads in general and not for this article's mounting. Ask the customer which it is and what margin was added on top, and write the answer in the log header; if it cannot be found, the report states that in one sentence (1.3, Decide).
Quiz: Try It
Open book. 5 questions drawn from this section's pool. 4 correct passes the quiz. Nothing is recorded here; in the course your passing attempt opens the next section.