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In the textbook Spacecraft Systems Engineering 4th ed. page 14, table 2.1 ("Acceleration values for the Ariane 5 launch vehicle") provides data on the loading of the Ariane 5 spacecraft. One of the columns listed is "Additional line load" given in units of N/mm. However, the meaning of this field isn't actually described.

Table 2.1 shows the design loads that a satellite must achieve associated with various mission events for the Ariane 5 launch vehicle[...].

Table 2.1 of Spacecraft Systems Engineering

Table 2.1: Acceleration values for the Ariane 5 launch vehicle [3].

No other context is provided to explain or generalize these terms. The table is reproduced as best I can in MathJax, but differs non-critically from print copy.

What is the line load "additional" with respect to (some static load?), and more generally, how should it be interpreted?

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This NASA document, "Preliminary In-Flight Loads Analysis of In-Line Launch Vehicles Using the VLOADS 1.4 Program", has a nice answer for you:

Line loads are measures of load intensity which are derived from the stress equations for a monocoque shell with an applied axial force, pressure, and bending moment. Line loads, measured in pounds per inch, are calculated by dividing the longitudinal monocoque shell stress by a unit thickness. (page 5).

Don't be fooled by the units, btw: "pounds/inch" really means "pound-force per inch"; your book uses SI units. If you're capable of studying that book, you can do the conversion by yourself. Godspeed with studying !

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    $\begingroup$ This is super helpful and exactly what I needed! Thank you. Can I ask how you found this, so I can find similar reference documents in the future? $\endgroup$ – user8263 Aug 24 '19 at 20:05
  • $\begingroup$ @Aza You'll be surprised: I had a hunch of what line load was, but to be sure I googled it, hoping to find a NASA document - using a few moderately advanced search options of the engine, however. IIRC the search text was "line load" site:nasa.gov (the parentheses are there to force google to only look for exact occurrences, in that order, of the words). $\endgroup$ – Jan van Oort Aug 26 '19 at 4:06

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