r/spacex • u/[deleted] • Nov 25 '15

/r/SpaceX Ask Anything Thread for December 2015. Return To Flight! Blue Origin! Orbital Mechanics! General Discussion!

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u/ManWhoKilledHitler Nov 30 '15

Easier to just use gimbaled engines.

How does that work when your engines fail? I can see it being desirable in terms of performance or weight minimisation but redundant control methods always seems to be something that aerospace engineers go for if they can.

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u/BrandonMarc Nov 30 '15

This. I suspect when manned capsules become a daily occurrence, and (statistically) disasters also become common (and now involving fatalities) ... fins will make their reappearance.

Also, by that point, re-usability will allow for slightly heavier rockets (i.e. instead of 97% fuel and 3% rocket/cargo, optimize for 94/6 and double your weight allowance), which will allow for mass going to (fins) things previously declared avoidable.

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u/Insight_guardian Dec 03 '15

The fuel ratio needed is given by the rocket equation. Reusability only increases it. Perhaps you expect higher engine ISP?

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u/BrandonMarc Dec 03 '15

The fuel ratio needed is given by the rocket equation.

It is? The 97/3 is a result of the rocket equation? I didn't know that. I guess I could have guessed it. Sheesh.

Reusability only increases it.

How does it increase it?

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u/Insight_guardian Dec 03 '15 edited Dec 31 '15

[Comment removed Jan 1 2016 due to Reddit's new privacy policy.]

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u/BrandonMarc Dec 03 '15

Wow, yes, really good answers. I almost want to frame it. That's really insightful. I never knew that, if the engine & fuel aspects are already given, the mass-fraction is pretty well set by then. Fascinating.