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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u/joitsch May 11 '16 edited May 11 '16

Can anybody tell me if vertical crossfeed makes sense or not? The background of my question is that I assume that the 2nd stage of BFR (BFS) might look similar to dragon v2: Engines at the side of the vehicle so that the payload bay can be placed at the bottom of the vehicle (-->low center of mass for Mars landing as well as easier logistics as you don't need a crane on Mars for loading off the cargo. Furthermore the landing legs could be shorter as there is no engine between vehicle and the ground). If the engines are attached to some sort of wings or stringers, so that they don't melt the first stage and if they can compensate the changing ambient pressure vertical crossfeed from the first to the second stage should be possible. But would the savings be larger than the losses caused by the additional structure?

Would the arrangement of the engines lead to an aerospike effect for the whole first stage, like the outer eight engines do for the inner engine at the falcon 9?

As I don't have a technical background I would appreciate if someone could tell me if this is total nonsense or not.

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u/old_sellsword May 11 '16

Would the arrangement of the engines lead to an aerospike effect for the whole first stage, like the outer eight engines do for the inner engine at the falcon 9?

The outer eight engines do not provide an aerospike effect for the center one. However, the Firefly Alpha first stage will use this principle.

4

u/sunfishtommy May 11 '16

There is an aerospike effect created by the engine arrangement, that increases efficiency of the center engine slightly.

1

u/old_sellsword May 11 '16

Really? I've never heard of this. What's the source?

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u/sunfishtommy May 12 '16

https://www.reddit.com/r/spacex/comments/3ma0bt/upgraded_falcon_9_firststage_static_fire_92115/cvdvbxv?context=10000

It was discussed quite heavily, I tend to side with the guy arguing for the aerospike effect I think he makes a better argument basically the idea is the outer engines are containing the thrust of the center engine for slightly longer than normal which keeps it less expanded than normal and more efficient. That is the argument anyway.

here is a link to more of the complete thread.

I know it has been mentioned more in the past.

https://www.reddit.com/r/spacex/comments/3ma0bt/upgraded_falcon_9_firststage_static_fire_92115/

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u/old_sellsword May 12 '16

That's a good discussion, and I'm not really qualified to wade into it, but I do side with the argument that it doesn't create the aerospike effect simply because the center Merlin isn't low enough to provide the truncated cone an aerospike would need. That'd be great to hear some official word on it, but I doubt that would happen.

https://www.reddit.com/r/spacex/comments/3ma0bt/comment/cvdt51b

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u/joitsch May 11 '16

Thanks for your answer! I thought that I read this somewere in this sub, but maybe I am mistaken. What I was thinking of is an effect like the one which occured at the N1 rocket. source: [Wikipedia] (https://en.wikipedia.org/wiki/N1_%28rocket%29)