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Nikxice
Active Member
A couple of days ago Wikipedia stated, "The Saturn V is the tallest, heaviest, and most powerful (highest total impulse) rocket ever brought to operational status"..........Interesting, today it's revised to read, "As of 2021, the Saturn V remains the tallest, heaviest, and most powerful (highest total impulse) rocket ever brought to operational status." Once SpaceX addresses those last two words, the world is gonna change!
So... in Part 1 of the Everyday Astronaut Starbase video Elon says the grid fins don't fold flat during ascent.
The discussion was that it probably wasn't necessary as they aren't in the flow anyway.
That seems counter-intuitive to my non-rocket-scientist brain, especially when they've ben playing a game of fractional percentages to balance fuel burn to get as much payload to orbit, yet still have just enough fuel to land a booster. Elon talked in the same video about trying to get remaining fuel under a ton, which seems an awfully slim margin.
For the grid fins to effectively steer/help slow the rocket, they have to be in the flow when extended for the return. Even if the grid fins contribute 0.1% additional fuel burn due to drag, that seems like a lot.
Thoughts:
1) This is only not a big deal for the test article that they are gonna dump in the ocean anyway
2) The flow around the fairing during ascent really excludes the fins from producing any drag. On descent the flow is different and they are then capable of producing the drag needed
3) The margins for fuel on the Starship booster are different than for Falcon 9 so that it matters less
4) Other?
Also, these appear to not be titanium... again assuming because of it being a disposable test mule...
The discussion was that it probably wasn't necessary as they aren't in the flow anyway.
That seems counter-intuitive to my non-rocket-scientist brain, especially when they've ben playing a game of fractional percentages to balance fuel burn to get as much payload to orbit, yet still have just enough fuel to land a booster. Elon talked in the same video about trying to get remaining fuel under a ton, which seems an awfully slim margin.
For the grid fins to effectively steer/help slow the rocket, they have to be in the flow when extended for the return. Even if the grid fins contribute 0.1% additional fuel burn due to drag, that seems like a lot.
Thoughts:
1) This is only not a big deal for the test article that they are gonna dump in the ocean anyway
2) The flow around the fairing during ascent really excludes the fins from producing any drag. On descent the flow is different and they are then capable of producing the drag needed
3) The margins for fuel on the Starship booster are different than for Falcon 9 so that it matters less
4) Other?
Also, these appear to not be titanium... again assuming because of it being a disposable test mule...
The statement is a bit strange but perhaps they will just rotate the gridfins during launch so that they are at 90 degrees to the flow, thus greatly reducing the drag they produce and bringing it much closer to the drag that would occur if they were folded (difference in drag will be related to the gridfin aspect ratio). It will certainly be interesting to see the launch
Ah, interesting... rotating them 90 degrees would certainly reduce the cross-sectional area presented to the flow. Did he say that's what they were going to do and I just missed it?
Because as I recall he didn't suggest rotation as an alternative to folding, but rather that it wasn't necessary at all, because they "weren't in the flow".
I'll have to go re-watch it.
(also interesting to note they are electrically actuated on Starship, whereas on Falcon 9 they are known to be hydraulic. I also wonder that the complete degree of rotation they are capable of is...)
Because as I recall he didn't suggest rotation as an alternative to folding, but rather that it wasn't necessary at all, because they "weren't in the flow".
I'll have to go re-watch it.
(also interesting to note they are electrically actuated on Starship, whereas on Falcon 9 they are known to be hydraulic. I also wonder that the complete degree of rotation they are capable of is...)
Nikxice
Active Member
Not sure how much of a factor, but I do recall Elon saying somewhere in the Tim Dodd interview, the SH/Starship liftoff would be quite rapid. I'm thinking maybe this beast won't be spending too much time in the atmosphere during the ascent stage, so therefore won't be taking a big drag hit from the grid fins.
Yesterday Scott Manley went in-depth on Starship heat shield tiles. He also gave a nod to Tim Dodd for his 3 Part Q&A with Elon.
petit_bateau
Active Member
The use of model 3 motors has certainly been said before. So too has the comment re stainless.
Total effective surface area is likey less with them at 0 vs +/- 90. Definitely a smaller max contiguous area.
Fins are for attitude control on reentry, not so much deceleration.
Fins are for attitude control on reentry, not so much deceleration.
JRP3
Hyperactive Member
Looking at the grid fins you need to look at the cross sectional area presented to the wind when folded vs deployed. Folded they present the large flat area of the side plus probably turbulence in the grid openings, deployed they present the thin edges of the grids.
Understand that they are primarily control surfaces, but they absorb enough re-entry energy to heat up enough to need ablative paint on the surface of them from before they switched to titanium... and the IR shots of the new ones show them to get pretty toasty.
They're designed to choke at supersonic...right? Or did I totally miss something at some point in history? Maybe that far down the vehicle never goes supersonic?
Unrelated to that comment but related to the trade, the additional mass/complexity required to fold the fins [for ascent] and, I'd guess more probably, support the load of the stage on the landing catch could very well have ended up with "we'd just as soon not have them be flappy".
My understanding was that the spikes on the underside of the grid fins (hence on the leading edge during re-entry) were designed to operate in a compressible flow regime.
Comparing the Falcon 9 and SuperHeavy grid fins the 'mesh' seems much closer on the F9, more open on the SuperHeavy.
Agree that it will depend on the regime the grid fins see during ascent
There is a thing that has been bothering me for a long while.
I have watched a lot of the launches and landings in "real time" going back to grasshopper, which was filmed, so "surreal time"
during descent, when the first stage is screaming in around 8,000+ klicks, it does an entry burn for what ? 20 seconds or so, to initiate slow down to roughly somewhere around 5,000 klicks
,
the first stage _continues_ decelerating, like it is riding a bubble of hot compressed air to help bleed off speed (a virtual parachute?)
when it does the landing burn it has bled off most of the speed (and used the grid fins to steer)
Is this what is happening?
My understanding is that the landing burn decelerates it to the point that it can withstand the slam it's about to do in the denser part of the atmosphere as well as the combustion gasses are cooler than what the rocket normally would experience due to compression heating. After that it's that dense atmosphere that continues to slow the booster down.
Atmosphere keeps getting thicker and it starts above terminal velocity. So it continues decelerating.
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