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Scramjet technology allows for propulsion once Mach4 is reached, without needing oxidizer thus reducing the weight up to 80% (as read in https://medium.com/@rsn/spacex-falcon-9-vs-isros-reusable-launch-vehicle-c52d4d56f87d#.5sjgv3mv1)

Why doesnt spaceX plan to use scramjets for propulsion ?

Would a combination of maglev mass-driver (to reach mach3) with scramjet (to continue to space) make sense ?

How much could that reduce the cost of rocket launching ?

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marked as duplicate by Nathan Tuggy, uhoh, Rory Alsop, Jan Doggen, GdD Feb 10 '17 at 8:52

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Your base assumption is wrong: scramjets needs oxidiser

However, they are designed to use atmospheric O2 as their oxidiser supply. Since the rocket needs to go to space, where there is no significant amount of oxygen, they cannot work up there.

Indeed, they could be used for the narrow part between insufficient speed and too high altitude.

This is probably too narrow to be useful, and would render the design much more complex, making it impractical.

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  • $\begingroup$ There is another limit for scramjet, not only insufficient speed and too high altitude, but also too high speed for scramjet operation. $\endgroup$ – Uwe Feb 10 '17 at 9:14
  • $\begingroup$ @Uwe Yes. But I preferred to ignore "engineering" issues for the scramjet itself. $\endgroup$ – Antzi Feb 10 '17 at 9:26

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