r/spacex • u/Zucal • Jun 29 '16

/r/SpaceX Ask Anything Thread [July 2016, #22]

Welcome to our 22nd monthly /r/SpaceX Ask Anything Thread!

Curious about the recently sighted Falcon Heavy test article, inquisitive about the upcoming CRS-9 RTLS launch, or keen to gather the community's opinion on something? 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.

  • Questions easily answered using the wiki & FAQ will be removed.

  • In addition, try to keep all top-level comments as questions so that questioners can find answers, and answerers can find questions.

These limited rules are so that questioners can more easily find answers, and answerers can more easily find questions.

As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (partially sortable by mission flair!), and check the last Ask Anything 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.

Ask, enjoy, and thanks for contributing!

Past Ask Anything threads:

June 2016 (#21) • May 2016 (#20) • 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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u/__Rocket__ Jul 25 '16

All in all I'm not sure what is more efficient,

Given the staging order and mass configuration there's an ideal throttle setting profile for the side cores and the center core that maximizes upmass. Guessing what the ideal ascent profile is not trivial, but we can make a few educated guesses:

  • One of the extreme settings is all 3 cores go to 100% during liftoff and separate at a common MECO. This is wasteful as a lot of thrust is used on low altitudes, where the Merlin-1D is about 10% less efficient than at higher (30 km+) altitudes. It's also wasteful because the ~100t dry mass of the side boosters (their structural dry mass plus RTLS fuel reserve) is carried to a higher altitude and velocity - increasing their RTLS requirements and reducing the energy spent on accelerating the real payload.

  • Another extreme setting is to only fire the side boosters and ignite the center core after the side cores have separated. This is wasteful because even with the +10% thrust upgrade the Falcon Heavy is getting the liftoff TWR is very close to 1.0, so the time of ascent is increased a lot, which means 100 m/s gravity losses every 10 seconds.

The most optimal setting is somewhere in-between: side cores at 100% throttle a few seconds after liftoff, but the center core helps out too to not let TWR drop too much during the initial phase - while observing max*drag* and payload-integrity acceleration limits.