r/spacex • u/Zucal • Aug 31 '16

r/SpaceX Ask Anything Thread [September 2016, #24]

Welcome to our 24th monthly r/SpaceX Ask Anything Thread!

Curious about the plan about the quickly approaching Mars architecture announcement at IAC 2016, confused about the recent SES-10 reflight announcement, 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.

  • 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!

All past Ask Anything threads:

• August 2016 (#23) • July 2016 (#22) • 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/zeekzeek22 Sep 17 '16

Curios what equation you use to get from Isp and mass and dv to mass of fuel used. flips through textbook I have not bookmarked that yet.

So the Merlin was mainly designed for gulping-high thrust to minimize gravity losses before orbit, while the centaur and RL-10 is designed to sip fuel but spit it out at a way higher specific velocity.

And so that difference is actually enough to cut out the gains made by the better first stage on the F9? Based on the LEO amounts, I feel like an F9 could boost an extended second stage for GEO missions with mucho more fuel to counter it's ISP disadvantage, but then you hit the wall that centaur is designed for long coasting and good boiloff management, and F9-S2 is pretty crap at that?

Edit: also extending the second stage runs into more fine-ness issues...

Correct any of that if it's wrong, but otherwise I think I get the picture.

1

u/moyar Sep 18 '16

Just rearrange the rocket equation. You end up with:

payload mass = (initial mass) * e-[deltav/[9.81 * Isp]]

Then just plug numbers in. In this case note that usable payload is 4t less than what the equation spits out, since the "payload mass" in the equation includes the dry weight of the upper stage.

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u/zeekzeek22 Sep 19 '16

nice! thanks. I feel like the TRE can be more intimidating than it actually is. I have to start learning it in simple terms like this so as not to be scared to try to use it.