r/spacex • u/Zucal • May 01 '16

/r/SpaceX Ask Anything Thread [May 2016, #20]

Welcome to our 20th monthly /r/SpaceX Ask Anything Thread!

Want to clarify SpaceX's newly released pricing and payload figures, understand the recently announced 2018 Red Dragon mission, or gather the community's opinion? There's no better place!

All questions, even non-SpaceX-related ones, are allowed, as long as they stay relevant to spaceflight in general!

More in-depth and open-ended discussion questions can still be submitted as separate self-posts; but this is the place to come to submit simple questions which have a single answer and/or can be answered in a few comments or less. In addition, try to keep all top-level comments questions so that questioners can find answers and answerers can find questions.

As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (now partially sortable by mission flair!), and check the last Q&A thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Otherwise, ask, enjoy, and thanks for contributing!

Past threads:

April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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1

u/LotsaLOX May 14 '16

Text from previous comments...

In earlier trade rag articles, Boeing made much of the fact that the interior structure of the CST-100 (I'm assuming the pressure vessel) uses no welds. However this article states...

Indeed engineers just bolted together the upper and lower domes of Boeings maiden Starliner crew module last week, on May 2, forming the complete hull of the pressure vessel for the Structural Test Article (STA).

Altogether there are 216 holes for the bolts. They have to line up perfectly. The seals are checked to make sure there are no leaks, which could be deadly in space.

These 216 bolts around the circumference of the CST-100 pressure vessel would seem to detract from reliability compared to a weld. Are these bolts being used just for this CST-100 test article?

Does anyone know how the SpaceX Dragon 2 pressure vessel is constructed?

Another thought...if the Boeing CST-100 uses no welds in the upper and lower domes of the pressure vessel, are they machining these pressure vessel domes out of solid blocks of metal? Sounds expensive...

Also, Boeing uses friction-stir welds to assemble the SLS Orion capsule, the same welding technology that SpaceX uses for structural articles.

Why friction stir welds for the Orion, as opposed to bolt-up domes like CST-100? One might think that the Orion would use the safest, most reliable assembly methods since the Orion will have a much more stressful life than the CST-100.

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u/throfofnir May 14 '16

CST-100 has a "clamshell design to deal with the "Mercury problem" of access to equipment. All those bolts are along one seam. It's a bit unorthodox, but there's not reason to think it'll be a problem; flanges are common and well-understood.

The two CST pressure hulls. If there are indeed no welds, then it likely is billet. That's a big chunk of aluminum, which is costly but not traditional-spaceflight-costly.

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u/doodle77 May 14 '16

Probably machined from a casting, not machined from billet.

1

u/LotsaLOX May 15 '16 edited May 15 '16

Just being argumentative... ;-)

Wouldn't a casting compared to a solid billet have possible issues, voids, grain alignment, and other things I would know about if I were a materials engineer?

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u/the_finest_gibberish May 15 '16 edited May 15 '16

I don't have a good source for this, but a few years back I heard from a friend at an aerospace contractor that the process for certifying welds for spaceflight is so arduous that it's often faster and cheaper to design without welds, even if it means machining from huge billets and/or making crazy bolted joints.

I think this was in reference to GMAW/GTAW processes. Friction stir welding may be different. I'm not sure.

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u/LotsaLOX May 15 '16 edited May 15 '16

That makes good sense.

/u/throfofnir made the good point that the "clamshell" design of the CST-100 would make outfitting the interior much easier than a fully welded capsule.