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Was reading about the new vertical pad loading system SpaceX wants to build. Of interest was this quote

Known as a mobile service tower (MST), SpaceX has managed to avoid the need for the expensive, complex, and extremely unwieldy infrastructure for the first decade of Falcon 9 and Falcon Heavy launch operations. Instead, SpaceX has designed its launch vehicles around the concept of horizontal integration, meaning that its Falcon rockets can be entirely integrated and prepared for flight before going vertical for launch. This approach has ensured easy, cheap access to the entire rocket and payload up until the last few days of static fire and launch operations, lowering the cost of launch.

Beyond Russian spaceflight operations, SpaceX, and a handful of other companies around the world, nearly all other major launch providers and space agencies – including the United Launch Alliance (ULA), Arianespace, ISRO (India), and CNSA (China) – rely almost exclusively on vertical integration. With its new Pad 39A mobile tower, SpaceX will soon join that small club, giving it the ability to compete on completely even footing with ULA and others for lucrative military launch contracts.

The article never addresses why you would need vertical vs horizontal loading. The horizontal method seems to have been quite successful, so why would you need (or even want) vertical?

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    $\begingroup$ I know it’s not the actual answer, but if SpaceX was going to a 4-booster-plus-center-core Falcon Superheavy, vertical integration would be almost a necessity. $\endgroup$
    – Russell Borogove
    Commented Mar 6, 2020 at 17:29
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    $\begingroup$ @RussellBorogove Yeah, but in that case I'd expect the rocket itself to need some pad assembly as well $\endgroup$
    – Machavity
    Commented Mar 6, 2020 at 20:56
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    $\begingroup$ Because the military has always done it that way? $\endgroup$
    – Organic Marble
    Commented Mar 6, 2020 at 23:14
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    $\begingroup$ Both the quoted text and a bunch of answers refer to "vertical integration" rather than "vertical loading". Not to be confused with the way spacex operates: en.wikipedia.org/wiki/Vertical_integration $\endgroup$
    – bobsburner
    Commented Mar 9, 2020 at 10:14
  • $\begingroup$ When I first read this question, the phrasing "why would someone" made me think "why would a person," and I completely expected the answer to be, "Duh, so the crew doesn't have to get in and lie there for days before launch." $\endgroup$
    – Dan
    Commented Mar 9, 2020 at 10:55

4 Answers 4

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Payloads are attached to (expendable or vertical landing) vertical take off vehicles at the ends where they touch. Given satellites weigh several tonnes, and are several meters in length horizontal integration puts a lot of torque on that connection, and to lesser degrees on other parts of the spacecraft structure.

Reinforcing the spacecraft to withstand these loads requires a redesign effort and will incur extra weight - which means something will have to be sacrificed.

Some spacecraft are designed to be transported horizontally by the mounting structure, for these it isn't an issue.

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  • $\begingroup$ NB, I couldn't read the article, it just redirects me to eugdpr.org $\endgroup$
    – user20636
    Commented Mar 6, 2020 at 18:35
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    $\begingroup$ Hmmm, not true for STS, trunions $\endgroup$
    – user20636
    Commented Mar 6, 2020 at 20:49
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    $\begingroup$ almost all STS payloads were vertically integrated. Only a very few, and mostly early in the program, were horizontally integrated. One of the more famous of those was Spacelab. However, almost all of them sat in the payload processing facility horizontally. $\endgroup$
    – Organic Marble
    Commented Mar 6, 2020 at 23:22
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    $\begingroup$ I've narrowed my answer to exclude the shuttle as a different type of class of vehicle (it might be the only one in the class, but that doesn't mean it's a special case...) but the payload mounting system was designed to hold them in both vertical and horizontal (in case of abort) orientations @OrganicMarble . If one has launch orientation integration capability, it makes sense to use it, as one has greater control of the payload during orientation. I'm assuming some STS payloads were marmon clamped to upper stages, and they'd still have to be engineered to handle a hard horizontal touchdown $\endgroup$
    – user20636
    Commented Mar 7, 2020 at 7:28
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I didn't dig too hard for sources because this is probably a very minor expansion on top of the other answers, but WHICH payloads might require vertical integration? Ones with big stinking mirrors inside like spy satellites and space telescopes.

This reddit post lays out some rationale:

This is mostly telescopes like Hubble. They have a very delicate mirror that is designed to be as light as possible but still maintain its shape in freefall. They have extra strength in one direction so that they do not warp out of shape while they are on Earth. They will warp in the gravity but they are designed to warp back when they reach orbit. If they get lateral forces they can bend out of shape in a way that they can not return from and you will get a blurry image. The vibrations of launch is not enough to bend them out of shape but a constant force might do.

Seems legit.

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    $\begingroup$ It's not just big mirrors going to LEO, sportsball-field sized radio antennas going to GEO are the other major category of VI payload currently flying from the US on Delta IV Heavy rockets. $\endgroup$
    – Dan Is Fiddling By Firelight
    Commented Mar 9, 2020 at 21:19
  • $\begingroup$ Sounds right. Make that a reddit comment and I'll link it into the answer :) $\endgroup$
    – Erin Anne
    Commented Mar 10, 2020 at 2:16
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Read the whole article:

Most importantly, the tower would allow SpaceX technicians to crane certain US military payloads – encapsulated inside a Falcon payload fairing – onto the top of the rocket.

At the end of the day, that’s really the only reason SpaceX needs such a tower – certain customers (the US military and, to a lesser extent, NASA) have certain payloads that they either can’t or won’t tweak to allow for horizontal integration.

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    $\begingroup$ I read that, but it still doesn't address why you would want to do it that way, just that it's done that way $\endgroup$
    – Machavity
    Commented Mar 6, 2020 at 17:09
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    $\begingroup$ To put it another way, that tells us why SpaceX would want to build a contraption like this. It doesn't tell us why the military (or anyone else) would prefer vertical to horizontal. What are these tweaks needed for horizontal? $\endgroup$
    – Machavity
    Commented Mar 6, 2020 at 17:12
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    $\begingroup$ Horizontal would I expect need the item being launched to be strong enough to be tipped on its side indefinitely while the rocket was prepped. Could possibly be made lighter if it only had to withstand gravity vertically, vs vertically and horizontally. How long can you stand on your feet? Now how long can you hold a plank position? Mechanical things are the same. $\endgroup$
    – CrossRoads
    Commented Mar 6, 2020 at 17:26
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    $\begingroup$ That's starting to sound like an answer :) $\endgroup$
    – Machavity
    Commented Mar 6, 2020 at 17:27
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    $\begingroup$ Given that all the launchers commonly used by the military use vertical integration, there's little incentive for the military to redesign its payloads for horizontal integration. In other words, it's up to SpaceX whether they really want to launch government payloads or not. The government is fine with sticking to the launch providers they've been using for decades. $\endgroup$
    – TooTea
    Commented Mar 6, 2020 at 20:42
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Something of a guess but it sounds plausible:

Satellite propulsion systems that use surface tension devices to separate the pressurant gas from the propellant may have some orientation constraints to prevent gas bubbles being trapped in the surface tension device (aka propellant management device, PMD).

There are several types of PMD, some are rather more robust than others. Off the top of my head I can't readily imagine that orientation would be a problem if the tank was to be launched 90% or 95% full, but there may be more to it than that.

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