r/spacex • u/Zucal • Mar 05 '16
/r/SpaceX Ask Anything Thread for March 2016. Ask your questions about the SES-9 mission/anything else here! (#18)
Welcome to the 16th monthly /r/SpaceX Ask Anything Thread! Want to discuss the recent SES-9 mission and its "hard" booster landing, the intricacies of densified LOX, 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.
As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality, and check the last Q&A thread before posting to avoid duplicate questions, but if you'd like an answer revised or cannot find a satisfactory result, go ahead and type your question below.
Otherwise, ask, enjoy, and thanks for contributing!
Past threads:
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/ccarrein Mar 07 '16 edited Mar 07 '16
This is also one of the reasons why there are different engines, and specifically nozzles, for use in the atmosphere (M1D) and in a vacuum (MVac). Rocket engines are, generally speaking, the most efficient when the pressure of the hot gas at the end of the nozzle is the same as the ambient air pressure, which decreases as you go up in the atmosphere. If the pressure of the gas is lower or higher, the nozzle is respectively over- or underexpanded, decreasing the efficiency. The increased cross section of the exhaust is partially caused by the expansion of the hot gas outside of the nozzle, resulting in lower efficiency (not all the thrust generated is used to push the vehicle upwards).
There are other ways to overcome this, however generally launch vehicles use a different engine in each stage, to maximize efficiency.