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#240
by
RDMM2081
on 23 Jun, 2017 20:58
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I've been meaning to ask, and this seems like a good opportunity with your excellent gif of the action (thank you!) but what exactly (or approximately) is that structure seemingly made out of the tubular metal(?). I cant even wrap my head around which stage it is on, I kind of think it is on stage 1, and lives in the interstage, but can anyone help clarify or help me get my bearings? Thanks!
The tubular structure supports a pusher rod that helps the stages separate cleanly. It is mounted on the top dome of the first stage, and is basically three tubes in tripod arrangement supporting a vertical pusher mechanism.
The graphic below shows the pusher both in its stowed and extended positions in relation to the second stage engine.
Wow, ask and ye shall receive! Thank you, that is what I was hoping to understand, I simply couldn't make sense of it in the grand scheme of the rocket configuration without that picture and description. It is a very interesting mechanism, and it seems to me a great example of SpaceX demonstrating their devotion to the KISS methodology!
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#241
by
cscott
on 23 Jun, 2017 21:21
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Yes, the best way to ensure the delicate Mvac engine nozzle edges don't hit the interstage is to grab the top stage by the nozzle and push it straight out.
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#242
by
JAFO
on 23 Jun, 2017 21:28
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
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#243
by
Kabloona
on 23 Jun, 2017 21:31
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the thrust chamber.
If you watch the gif again, you can see some sort of "fist" at the end of the pusher that's a bit bigger than the throat diameter, so it can't fit through the throat and get inside the thrust chamber.
So it's like pushing into a large funnel with your fist.
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#244
by
MrHollifield
on 23 Jun, 2017 21:35
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It did only appear like it was hovering above water before the transmission cut briefly, but keep in mind that the stage is coming in at an angle (see other drone ships landings shot from a distance), and a 3 engine landing burn will reach a bit further and impinge the ocean more. 
looks like there is a soot trail clear from one side of the asds to the other. will be interesting to see the non-live video release
I noticed that the soot trail runs from the edge of the ship inwards. I didn't think it ran from both sides but just from the right.
this is what i see
Dark marks on the right I think are shadow of the upper part of the booster and grid fins. It did leave a trail across the deck though.
https://gfycat.com/ThinSphericalAbalone
Thanks, Helodriver, for the GIF. The more I look at it, the more I think the darkness forward of the booster is just cloud shadow. From the shadow on the leg closest the camera, it is apparent the sun is coming from that direction. That means any shadow to the right of the stage are from off the ASDS, i.e. clouds. Also, the darkness behind and to the right of the stage continues into the water, again appearing to be cloud.
And looking at the other side of the stage, the deck features visible in the "before" frame are still there in the "after" frame, just dimmed. If 3 M1D exhausts had been dragged across the deck, I think they would have left more of a footprint on the deck.
Can't wait to see the onboard video to know what really happened
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#245
by
Martin.cz
on 23 Jun, 2017 21:37
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
In any case a very effective and elegant engineering solution! (IMHO)
That reminds me - are there any other multi-stage rockets using a similar up-the-nozzle-throat pushers during stage separation or is the Falcon 9 FT a on-of-a-kind in this regard ?
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#246
by
cscott
on 23 Jun, 2017 21:39
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Remember that it's normal for the drone ship cameras to cut out just before touchdown because the satellite link is all that SpaceX can afford.
...because no amount of money can buy you a satellite uplink from the middle of the Atlantic ocean through an ionized exhaust plume as an 18-ton 165-ft tall rocket lands on top of you.
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#247
by
Kabloona
on 23 Jun, 2017 21:44
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
In any case a very effective and elegant engineering solution! (IMHO)
That reminds me - are there any other multi-stage rockets using a similar up-the-nozzle-throat pushers during stage separation or is the Falcon 9 FT a on-of-a-kind in this regard ?
The throat is typically the strongest part of the nozzle, because it experiences the highest pressure (=chamber pressure) in the exhaust stream. Pressure then drops downstream of the nozzle as the exhaust expands and accelerates. So the nozzle throat has to be beefy, and as long as you cushion the push, so to speak, it's a good place to push on.
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#248
by
TrevorMonty
on 23 Jun, 2017 21:45
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Was this the first successful 3-engine landing?
- Ed Kyle
JCSat 14 used a 3-1 landing burn while Thaicom 8 used a 1-3-1 landing burn, so both can technically count as the first successful 3-engine landings.
3 all the way to the deck? That's some fine timing to be sure.
If this booster fly's again that will be the true measure of success.
Even if stage doesn't fly again alot parts from it will. Still significant savings to had in recovering these hot entry stages.
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#249
by
SoulWager
on 23 Jun, 2017 21:47
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Remember that it's normal for the drone ship cameras to cut out just before touchdown because the satellite link is all that SpaceX can afford.
...because no amount of money can buy you a satellite uplink from the middle of the Atlantic ocean through an ionized exhaust plume as an 18-ton 165-ft tall rocket lands on top of you.
I'd disagree with that. You can build a buoy with your uplink antenna on it, and run a cable back to the ASDS. Get your antenna far enough from the rocket that it doesn't lose signal. It's certainly within the means of SpaceX, just not worth the expense or the recovery team's labor deploying and retrieving it all the time.
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#250
by
Kabloona
on 23 Jun, 2017 21:49
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Remember that it's normal for the drone ship cameras to cut out just before touchdown because the satellite link is all that SpaceX can afford.
...because no amount of money can buy you a satellite uplink from the middle of the Atlantic ocean through an ionized exhaust plume as an 18-ton 165-ft tall rocket lands on top of you.
Isn't it actually the vibration that causes the dish to lose signal lock? IIRC that's what one of the SX'ers said somewhere, and we always see the ASDS camera start to vibrate just before LOS.
https://forum.nasaspaceflight.com/index.php?topic=34077.msg1499726#msg1499726Not disagreeing with your point that it's hard, though. ;-)
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#251
by
cscott
on 23 Jun, 2017 22:01
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Remember that it's normal for the drone ship cameras to cut out just before touchdown because the satellite link is all that SpaceX can afford.
...because no amount of money can buy you a satellite uplink from the middle of the Atlantic ocean through an ionized exhaust plume as an 18-ton 165-ft tall rocket lands on top of you.
Isn't it actually the vibration that causes the dish to lose signal lock? IIRC that's what one of the SX'ers said somewhere, and we always see the ASDS camera start to vibrate just before LOS.
https://forum.nasaspaceflight.com/index.php?topic=34077.msg1499726#msg1499726
Right, I shouldn't have said "no amount of money". Absolutes get you in trouble.
It is the vibration that is the main issue AIUI. I'm sure enough money *could* solve the problem... ie moor another ASDS a mile off, and stream from that... but it's not worth the $$$.
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#252
by
rpapo
on 23 Jun, 2017 22:09
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...but it's not worth the $$$.
Especially when it's only for our convenience and viewing pleasure. SpaceX has good recordings, but they simply aren't available until afterwards.
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#253
by
sanman
on 23 Jun, 2017 22:13
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It is the vibration that is the main issue AIUI. I'm sure enough money *could* solve the problem... ie moor another ASDS a mile off, and stream from that... but it's not worth the $$$.
Why use another ASDS - why not just have a trailing buoy with an antenna on it, or something?
Can't be that expensive.
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#254
by
wannamoonbase
on 23 Jun, 2017 22:23
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Congratulations SpaceX on another successful mission.
Do I really have to wait another 2 whole days until the next launch attempt?
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#255
by
virnin
on 23 Jun, 2017 22:28
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
From re-watching the GIF playing above, I'm thinking the push was NOT straight-up or perhaps not evenly placed. The US was definitely rotating before the MVAC fired up and it took a few more seconds after that for the stage to straighten up. If it hadn't rotated back to thrusting straight in-line with the camera view, I would write it off to rotation of the 1st stage.
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#256
by
Lars-J
on 23 Jun, 2017 22:47
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
From re-watching the GIF playing above, I'm thinking the push was NOT straight-up or perhaps not evenly placed. The US was definitely rotating before the MVAC fired up and it took a few more seconds after that for the stage to straighten up. If it hadn't rotated back to thrusting straight in-line with the camera view, I would write it off to rotation of the 1st stage.
No, it is still likely caused by a very slight rotation of the first stage (and entire stack) after MECO. The 2nd stage simply compensates once the MVac ignites.
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#257
by
Kabloona
on 23 Jun, 2017 23:01
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
From re-watching the GIF playing above, I'm thinking the push was NOT straight-up or perhaps not evenly placed. The US was definitely rotating before the MVAC fired up and it took a few more seconds after that for the stage to straighten up. If it hadn't rotated back to thrusting straight in-line with the camera view, I would write it off to rotation of the 1st stage.
No separation event is perfect, they all have some slight asymmetry. Every separation system/event is analyzed during design for "tip-off rates", ie pitch/yaw induced by the separation event. What you see is a small "tip-off rate" of either the first or second stage, or both since the pusher acts equally on both stages.
So, even though the system is designed to push along the centerline, it's never going to result in a perfect zero-rate tip-off ; there's always some small residual pitch/yaw effect that gets nulled by the GNC system when the main engine starts up.*
(*My favorite bad S1/S2 separation story is from an early Pegasus flight where the shaped detonating cord failed to completely sever the interstage. So the first stage was still hanging onto the second stage by several strands of carbon fiber, and it swung the whole stack around so that stage 2 was facing almost 180 degrees backwards when it fired, breaking free of stage 1, and vectoring itself 180 degrees back on course. IIRC, that was how Antonio Elias told the story. By comparison, today's F9 separation was picture perfect.)
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#258
by
Roy_H
on 23 Jun, 2017 23:08
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Forgive my shallow understanding of the engine, but what does the rod push against? Isn't the injector plate up there somewhere, or is there some kind of ring before the top of the nozzle it pushes against?
It pushes against the nozzle throat, which is the narrowest convergence of the nozzle. The injector is farther up inside the engine.
I guess given the working pressures and temperatures a gentle evenly placed straight-up push should not be an issue, right ?
In any case a very effective and elegant engineering solution! (IMHO)
That reminds me - are there any other multi-stage rockets using a similar up-the-nozzle-throat pushers during stage separation or is the Falcon 9 FT a on-of-a-kind in this regard ?
AFAIK this is unique to SpaceX. This is a very clever solution to a problem unique to SpaceX. Most separations just drift away, and because saving the booster is not an issue, the second stage can fire as soon as it is clear and they don't care if the first stage gets burnt or damaged. SpaceX wants a good distance before the second stage fires and they also want to get this distance very quickly so they can rotate the first stage for the boost back burn. The pusher imparts some momentum to the second stage while slowing down the first stage a little. I know this is small potatoes, but I wonder if that little extra acceleration to the second stage is enough to compensate for the extra fuel used to carry the mass of the pusher mechanism.
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#259
by
starsilk
on 23 Jun, 2017 23:15
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It is the vibration that is the main issue AIUI. I'm sure enough money *could* solve the problem... ie moor another ASDS a mile off, and stream from that... but it's not worth the $$$.
Why use another ASDS - why not just have a trailing buoy with an antenna on it, or something?
Can't be that expensive.
because the ASDS is stationary while waiting for the landing. if it was 'under way' sailing at a fixed rate (as BO plan to do) it would work... but stationary, the buoy would just sit there, right next to the ASDS, or have to be deployed well before landing and hope it doesn't drift too close.
