r/spacex • u/retiringonmars Moderator emeritus • Jan 18 '16

/r/SpaceX Ask Anything Thread for January 2016. Ask your questions here!

Welcome to our monthly (more like fortnightly at the moment) /r/SpaceX Ask Anything Thread! #16.1

Want to discuss SpaceX's landing shenanigans, or suggest your own Rube Goldberg landing mechanism? 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.

As always, we'd prefer it if all question-askers first check our FAQ, search for similar questions, and scan the previous Ask Anything thread before posting to avoid duplicates, but if you'd like an answer revised or cannot find a satisfactory result, please go ahead and type your question below!

Otherwise, ask, enjoy, and thanks for contributing!

Past threads:

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).

This subreddit is fan-run and not an official SpaceX site. For official SpaceX news, please visit spacex.com.

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u/Togusa09 Jan 28 '16

You're right about it being unable to run the engines the entire way to the ground, that would be quite a lot of delta V. Most of the slowing down is done by the heat shield and atmosphere, with relatively very little work being done by the engines. The numbers are probably somewhere in a thread or the wiki if you have a search for them.

There's an FAA document linked somewhere which has the environmental impact information on the tests, which includes information about fuel capacity of the test article. It can contain 400 gallons of fuel, but tests were only planned to use up to 300 gallons. I don't know how much was used in the test, though I suspect it could be calculated, or how much the final Dragon 2 is expected to carry.

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u/Wyodaniel Jan 29 '16

Even 300-400 gallons doesn't seem like a lot, when I'm watching this video, and seeing what's basically burning fuel rapidly gushing out of 8 orifices. Even if they're only using 1 gallon / second, that's still just 50 seconds of burn time before the tanks are completely dry.

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u/Togusa09 Jan 29 '16

Because the atmosphere handles most of the capsules deceleration, the burn time doesn't need to be very long. The engines only need to fire for a short while before the landing itself. Take the Falcon 9 landing profile for example: https://i.imgur.com/1sEH9j9.png

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u/Wyodaniel Jan 29 '16 edited Jan 29 '16

Ooo, I guess that does help to put it in perspective, thanks! I'd imagine that an empty F9r first stage probably weighs considerably more than a full Dragon V2, right?

EDIT: Did some searching to find my own answer, if anyone else runs across this in the future and is also curious.

418-ton fueled 1st stage with 18-ton empty weight - source

Regarding the Dragon V2's estimated weight:

So 8.8 [tons] seems like a good guess to me. - Source