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Ignoring all other structural loads besides the lengthwise compression of the rocket.

This is useful to know when theorizing about whether high altitude launches (e.g. from airplanes) could significantly reduce the load that the rocket structure needs to support, and thereby save some weight.

I noticed that SpaceShipOne's White Knight has less than a third of the wingspan of an A380, so a much larger version of it might be used for launching small satellites cheaply.

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  • $\begingroup$ A much larger version of SpaceShipOne's White Knight will be still a plane, but reaching a low orbit requires to go much higher (400 km instead of about 30 km) and much faster (about 8 km/s instead of about 1 km/s or less). 8 times the speed means 64 times of the kinetic energy. But a rocket needs a lot of fuel only to accelerate the fuel and not the payload. A rocket is needed anyway to go were a plane could not go. $\endgroup$
    – Uwe
    Commented Sep 26, 2018 at 11:39
  • $\begingroup$ Related and still needing a canonical answer: Upper stage structural loads on ascent? $\endgroup$
    – Russell Borogove
    Commented Sep 26, 2018 at 16:43

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Edit: original version of this answer was wrong at least three different ways. Answer quoted here gives drag as around 460 thousand pounds. With a total upper stage weight around 262 thousand pounds and 2Gs acceleration at that point suggests the second stage loads from the upper stages are very approximately 50/50 between the two load sources, assuming most drag is on third and higher stages or the fairing at the top of that stage. More reading suggests fins on first stage added a fair bit of stabilising drag so this is wrong but hopefully not as wrong as my first answer.

Not part of your question but The air launch tag and

and an existing air launch system may help frame more questions.

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    $\begingroup$ Thrust of the Saturn V first stage is about 8 million pounds, not forty thousand. It is most decidedly still accelerating at max Q; it’s just balanced by air density falling off very rapidly with altitude. $\endgroup$
    – Russell Borogove
    Commented Sep 26, 2018 at 11:18
  • $\begingroup$ The diameter of the third stage is 21.7 feet, the diameter of the first and second stage is 33 feet. Area is 2.3 times bigger, so drag to the third stage and payload may be smaller than the drag to the first and second stage together. Length of first and second stage is 60 % of the total length of a Saturn V with payload. So drag to first and second stage may be bigger than 50 % of total drag. Not to forget the drag of the engine nozzles of the first stage protuding the tanks diameter. $\endgroup$
    – Uwe
    Commented Sep 26, 2018 at 13:38

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