Author Topic: Grasshopper Discussion (including Grasshopper 2, aka F9R-Dev1) Thread 2  (Read 79145 times)

Offline mme

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- Are these impressions of differences accurate, or are they just artifacts of different points of view from the videos?
- If accurate, the test landings seem to be much slower and more under control than the barge landings.  Were the actual landings faster due to running out of hydraulic fluid and sticky valves?
- If the dynamics of a real landing are actually different, what exactly did SpaceX learn or intend to learn from Grasshopper?
I think the way to look at it is trying to walk (short hops with a ballasted rocket) before you try to run (decelerating from terminal velocity, deploying landing legs and landing a completely empty rocket).  It's not that it's radically different, it's that you're increasing the difficulty.  These landing attempts are experiments, SpaceX does not claim to have worked out all the details yet.  They are just confident that they will sometime in the next few attempts.

I also think people are extrapolating an awful lot from one data point.  CRS-6 was the first time a stage decelerating from terminal velocity has been close enough to the barge to have any shot at landing.  And it was suffering a lag induced oscillation.  (Basically CRS-5 had no chance and had to make an extreme divert to "hit" the barge so I think it's irrelevant as far as the final landing sequence goes.)

Regarding the Grasshopper/F9R-Dev 1 rockets, I'd be curious how heavily they were ballasted.  I would think that the more heavy the rocket is, the more stable it would be.  But it may also be that they just did not suffer the same amount of "stiction" (assuming that is the root cause of the oscillation.)  The Grasshopper/F9R-Dev 1 rockets may have made it look easy, but don't forget that a plugged sensor resulted in F9R-Dev 1 self-destructing.  Redundant systems and hold downs on F9Rs should prevent that type of RUD.  But I think that the "stiction" issue can be viewed similarly.  It's a variable that was not adequately accounted for during an experiment.

Wash, rinse, repeat.
Space is not Highlander.  There can, and will, be more than one.

Offline Mader Levap

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- If the dynamics of a real landing are actually different, what exactly did SpaceX learn or intend to learn from Grasshopper?
I think without Grasshopper they would have even more problems with landings than they have now.
Be successful.  Then tell the haters to (BLEEP) off. - deruch
...and if you have failure, tell it anyway.

Offline guckyfan

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I guess they stopped the Falcon 9 dev2 tests in New Mexico when they realized they would probably lose it on first try. It's cheaper to try landing on the ASDS with flight stages.

Offline cscott

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I guess they stopped the Falcon 9 dev2 tests in New Mexico when they realized they would probably lose it on first try. It's cheaper to try landing on the ASDS with flight stages.
Well, I think you can certainly design a much more aggressive test schedule when you don't have to worry about damaging the test article.  And from a logistics standpoint, "test every month" (or so) is probably much easier to plan for than having to account for the possibility that the test article will be lost and you'll have to stand down for an extended period while a new one is built.

It's a tradeoff, and SpaceX in typical fashion has actively explored both sides of it.

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