Author Topic: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video  (Read 134759 times)

Offline Robotbeat

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #220 on: 07/28/2014 01:50 AM »
A long enough reentry burn is the same as a reentry burn... There's not a sharp line, there. :)
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Offline deruch

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #221 on: 07/28/2014 03:59 AM »
A long enough reentry burn is the same as a reentry burn... There's not a sharp line, there. :)

I'm guessing one of those "reentry"s is supposed to be something else.  I think you're saying either [1] that a long boost-back burn is the same as a re-entry burn or [2] that a long re-entry burn is the same as the landing burn.  As my post was related to the boost-back bit, that's the way I'm leaning.

[1] In theory, the boost-back burn could continue through the stage's interaction with the upper atmosphere and thus accomplish both the boost-back and the re-entry slowing burn, thereby eliminating the need for an additional relight of engines on an RTLS flight.  I'm perfectly willing to accept that there isn't necessarily a sharp dividing line between the two.  However, according to my understanding of the practice of this particular landing, the booster stage flew a purely ballistic trajectory that did not include what would colloquially be considered a "boost-back towards the launch site".  i.e. Its two engine relights were accomplished with the goals of A) surviving re-entry and B) achieving a soft landing.

[2] I don't think this is what you meant, but if it is you're wrong.  The re-entry burn uses 3 engines.  The landing only uses 1.  I'm pretty sure that the stage isn't continuously firing all the way from re-entry to touchdown (even if it's just the center engine, with the two side engines shut off after sufficient slow down).  Plus, SpaceX explicitly said there were two relightings of engines.  I pointed out above why I don't believe a distinct boost-back burn was one of those relightings.
« Last Edit: 07/28/2014 04:00 AM by deruch »
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Offline Robotbeat

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #222 on: 07/28/2014 04:19 AM »
Neither, sorry for the typo!. A long reentry burn is like a boost-back burn. (On the way back, entry velocity is much lower.)
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Offline GregA

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #223 on: 07/28/2014 04:40 AM »
Great vid!  The only minor critique I have is that we're pretty sure that there wasn't a boost-back, just re-entry and landing burns.  So, if I was making this video (I'm in no way skilled enough to make one anywhere near as good as your is though), I don't think I would show one even in animation.  But this is a fairly minor technical point.  Great job!
Thanks Deruch :)

I also though about that... as well as the animation of the rocket descending and legs opening while still at quite an altitude - when it seems that the actual attempts have opened much lower(?). I think reusability plans have evolved in the last 2 years (the animation still has the square layout of engines too).

I wasn't trying to emulate the 2 'land'ings so much as give a good story of the overall process we're seeing developed. It was partly a "side by side" of the 2 successful attempts, and partly an upgrade to the animation via as much real footage as possible - because both of the supplied videos have weaknesses. They're not clear, and too easy for the casual viewer to misinterpret the landing videos as having the engines ignite only at the last second. In the side-by-side I wanted to show the common leg movements, matching flames, and the similar flame activity in the F9R when near real ground (with a linking wide shot for some perspective).

But yeah... the purist in me wants to keep it entirely consistent with the OG2!

Offline guidanceisgo

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #224 on: 07/28/2014 05:15 AM »
Anyone know if they are going to do a hover / translation on any of the upcoming flights. (ie, hover then start translating towards the coast.)  It seems like that would be a logical step in the test plan.  I suppose they would have to do something like this prior to coming onshore to land at the Cape since a boost IIP on the land would be scary.  It would be really cool to see!

Offline deruch

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #225 on: 07/28/2014 05:26 AM »
Great vid!  The only minor critique I have is that we're pretty sure that there wasn't a boost-back, just re-entry and landing burns.  So, if I was making this video (I'm in no way skilled enough to make one anywhere near as good as your is though), I don't think I would show one even in animation.  But this is a fairly minor technical point.  Great job!
Thanks Deruch :)

I also though about that... as well as the animation of the rocket descending and legs opening while still at quite an altitude - when it seems that the actual attempts have opened much lower(?). I think reusability plans have evolved in the last 2 years (the animation still has the square layout of engines too).

I wasn't trying to emulate the 2 'land'ings so much as give a good story of the overall process we're seeing developed. It was partly a "side by side" of the 2 successful attempts, and partly an upgrade to the animation via as much real footage as possible - because both of the supplied videos have weaknesses. They're not clear, and too easy for the casual viewer to misinterpret the landing videos as having the engines ignite only at the last second. In the side-by-side I wanted to show the common leg movements, matching flames, and the similar flame activity in the F9R when near real ground (with a linking wide shot for some perspective).

But yeah... the purist in me wants to keep it entirely consistent with the OG2!
I haven't been paying much attention to it lately, but in the CRS-3 video interpretation thread, user sittingduck is working on creating an accurate animation of the CRS-3 booster landing.  You might be interested in the final product.  I'm not sure how far along it is though.
« Last Edit: 07/28/2014 05:30 AM by deruch »
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Offline TrevorMonty

Assume video.

Offline rpapo

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #227 on: 07/28/2014 10:16 AM »
Anyone know if they are going to do a hover / translation on any of the upcoming flights. (ie, hover then start translating towards the coast.)  It seems like that would be a logical step in the test plan.  I suppose they would have to do something like this prior to coming onshore to land at the Cape since a boost IIP on the land would be scary.  It would be really cool to see!
I agree it would be cool to see, but hover isn't going to happen on any real flight, because to do that they would have to have enough fuel left over in the rocket to where the T/W ratio is one or less when throttled down to 60%.  The only reason the Grasshopper and F9R-Dev1 rockets have been able to hover is because they are ballasted with much more fuel than would be present on a real return from launch.

Translation may occur, to a small extent, during the landing burn.  Grid fins should have the effect of minimizing this.
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Offline Dudely

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #228 on: 07/28/2014 11:36 AM »
Anyone know if they are going to do a hover / translation on any of the upcoming flights. (ie, hover then start translating towards the coast.)  It seems like that would be a logical step in the test plan.  I suppose they would have to do something like this prior to coming onshore to land at the Cape since a boost IIP on the land would be scary.  It would be really cool to see!
I agree it would be cool to see, but hover isn't going to happen on any real flight, because to do that they would have to have enough fuel left over in the rocket to where the T/W ratio is one or less when throttled down to 60%.  The only reason the Grasshopper and F9R-Dev1 rockets have been able to hover is because they are ballasted with much more fuel than would be present on a real return from launch.

Translation may occur, to a small extent, during the landing burn.  Grid fins should have the effect of minimizing this.

I disagree. I thought that was why they were choosing a pad next to the ocean- the return will target a patch of water 200-400 meters offshore and the rocket will use the landing burn to translate to the pad.

Otherwise they risk damage to the pad. Bare concrete pads still cost a significant amount of money.
« Last Edit: 07/28/2014 11:38 AM by Dudely »

Offline rpapo

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #229 on: 07/28/2014 11:51 AM »
I disagree. I thought that was why they were choosing a pad next to the ocean- the return will target a patch of water 200-400 meters offshore and the rocket will use the landing burn to translate to the pad.

Otherwise they risk damage to the pad. Bare concrete pads still cost a significant amount of money.
I didn't say translation would not occur, only that it would be minimal.  There's only so much translation you can do when your entire burn lasts only about 24 seconds.  And there will be no hovering . . . unless Mueller and his staff manage pull a rabbit out of a hat and suddenly discover how to get a Merlin 1D to throttle down more than 50%.

Yet another rabbit, that is.
« Last Edit: 07/28/2014 11:53 AM by rpapo »
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Offline cscott

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #230 on: 07/28/2014 02:23 PM »
Anyone know if they are going to do a hover / translation on any of the upcoming flights. (ie, hover then start translating towards the coast.)  It seems like that would be a logical step in the test plan.  I suppose they would have to do something like this prior to coming onshore to land at the Cape since a boost IIP on the land would be scary.  It would be really cool to see!

The falcon is incapable of hovering.  Thrust/weight is always greater than 1 for the rocket (even with only one engine firing -- the rocket is very light when almost all the fuel has been burned).  It lands with a "hoverslam" approach, where it reaches zero velocity precisely at zero altitude.  Any significant translation occurs during the "boostback" and/or "retro burn".

The question of whether/how SpaceX is going to do a "boost back" has been oft asked in these forums.  Nobody knows yet.  But Flights 15 and 16 will involve a "solid" landing, which is likely to involve some amount of boost back.

Offline Elmar Moelzer

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #231 on: 07/28/2014 03:09 PM »
unless Mueller and his staff manage pull a rabbit out of a hat and suddenly discover how to get a Merlin 1D to throttle down more than 50%.
Yet another rabbit, that is.
Wasn't that confirmed already at some point? I distinctly remember a quote from Musk about the surprisingly large throttle range of the Merlin 1D (anyone here who can dig it up?).

Offline cscott

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #232 on: 07/28/2014 03:12 PM »
Wasn't that confirmed already at some point? I distinctly remember a quote from Musk about the surprisingly large throttle range of the Merlin 1D (anyone here who can dig it up?).

If I recall correctly, it was a twitter quote that was much debated.  IIRC he said "40% throttle" or some such, and readers were sharply divided over whether he meant "throttle down 40%" (aka, total thrust 60% of maximum) -- which would not be surprising -- or "total thrust of 40% of maximum", which would be surprising.

Evidence since then has not supported the surprising interpretation.

UPDATE: swapped 60% and 40% to make my recounting accurate.  Thanks to sublimemarsupial for digging up the original twitter exchange.
« Last Edit: 07/28/2014 07:07 PM by cscott »

Offline sublimemarsupial

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #233 on: 07/28/2014 03:19 PM »
Wasn't that confirmed already at some point? I distinctly remember a quote from Musk about the surprisingly large throttle range of the Merlin 1D (anyone here who can dig it up?).

If I recall correctly, it was a twitter quote that was much debated.  IIRC he said "60% throttle" or some such, and readers were sharply divided over whether he meant "throttle down 60%" (aka, total thrust 40% of maximum) -- which would be surprising -- or "total thrust of 60% of maximum", which would be less surprising.

Evidence since then has not supported the surprising interpretation.

Jon Goff asked how far the M1d could throttle, and Elon replied "~40%". What that meant was not clarified.

Here's a link to the tweets in question: https://twitter.com/elonmusk/status/462104679116050432

Offline Lars_J

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #234 on: 07/28/2014 04:13 PM »
The M1D can throttle down to at least 50% - at least the vacuum version, I have seen this in a NASA presentation. I assume the same holds for the 'plain' M1D.

So when "~40%" was mentioned, the only reasonable interpretation (IMO) is that they can now throttle it even deeper than previously published information.

Offline AJA

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #235 on: 07/31/2014 07:49 PM »
What would be acceptable vertical velocity limits for "hovering"? 0 +/- 5 m/s? Can the stage do that by pulsing the engines, or even by slewing the throttle up and down constantly? If the response time of the engine, and the rates are fast enough, then you wouldn't need the T/W to be ~= 1 at all. (Although, obviously that does help).


Basically, can they playing a game of flappy bird, with a Falcon?

Offline rpapo

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #236 on: 07/31/2014 10:39 PM »
I don't think you can pulse the Merlins like you can the Dracos.
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Offline Dudely

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #237 on: 08/01/2014 02:25 PM »
What would be acceptable vertical velocity limits for "hovering"? 0 +/- 5 m/s? Can the stage do that by pulsing the engines, or even by slewing the throttle up and down constantly? If the response time of the engine, and the rates are fast enough, then you wouldn't need the T/W to be ~= 1 at all. (Although, obviously that does help).


Basically, can they playing a game of flappy bird, with a Falcon?

No, because of instabilities with the prop in the engine. Basically the same reason they can't throttle down beyond a certain point- because the propellant has to mix and combust at a certain rate and if it doesn't you might get too little combustion for a brief moment, then a spike in pressure as the buildup of uncombusted, poorly mixed propellant finally combusts. And there goes your engine.

I think the dracos can pulse because their fuel is hypergolic.

Offline HMXHMX

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #238 on: 08/01/2014 05:59 PM »
What would be acceptable vertical velocity limits for "hovering"? 0 +/- 5 m/s? Can the stage do that by pulsing the engines, or even by slewing the throttle up and down constantly? If the response time of the engine, and the rates are fast enough, then you wouldn't need the T/W to be ~= 1 at all. (Although, obviously that does help).


Basically, can they playing a game of flappy bird, with a Falcon?

No, because of instabilities with the prop in the engine. Basically the same reason they can't throttle down beyond a certain point- because the propellant has to mix and combust at a certain rate and if it doesn't you might get too little combustion for a brief moment, then a spike in pressure as the buildup of uncombusted, poorly mixed propellant finally combusts. And there goes your engine.

I think the dracos can pulse because their fuel is hypergolic.

The primary issue with regard to depth of throttle is flow separation from the nozzle; this statement is true provided proper care has been taken in designing the injection orifices to permit the desired throttle setting.  The pintle injector has an advantage in this situation since it can be face-throttled as was done with the Lunar Module LMDE.  But I'm fairly sure SpaceX is not face throttling the Merlin 1D; they may be face-throttling the SuperDraco.  Propellant choice is not really a major factor in throttling.

Online abaddon

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Re: Falcon 9 v1.1 ORBCOMM - First Stage Ocean Landing Video
« Reply #239 on: 08/01/2014 06:24 PM »
I think the dracos can pulse because their fuel is hypergolic.

I thought it was because they were pressure fed... much easier to smoothly change the throttle than a pump-fed system.  Or maybe it is both...

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