Author Topic: Question about Grid Fins ...  (Read 335 times)

Offline bobdude11

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Question about Grid Fins ...
« on: September 07, 2017, 11:11:55 PM »
Watching yet another successful SpaceX launch and 1st stage recovery (just fascinates me how they are able to do that ...) and I was curious about grid fins.

Am I misunderstanding this: The grid fins provide less direct aerodynamic braking due to air able to more freely pass through while providing more roll, pitch, and yaw control due to the grid having multiple small airfoil surfaces?

I feel that this is much too oversimplified and that I am just not correctly comprehending how these work. I visualize them as many little 'wings' supporting each other in the box frame and this gives them the ability to provide the control aspects while allowing the air to flow around and through them so that they can still offer some aerodynamic braking while reducing the pressure loads that a more solid paddle type fin would create.

When I read this article (https://en.wikipedia.org/wiki/Grid_fin) it seemed to bear out some of my thinking, but since I am not even close to being any kind of aerospace engineer I am not knowledgeable enough to know if the article is accurate. As it matches some of my thinking, I feel it warrants coming to the experts for verification.

I would also appreciate any guidance on other good sites (I use NASA as one) for information along these lines so I don't have to bug you gentle folk so much with these simplistic questions.

Apologies for the diatribe ... I am struggling to ask it without sounding like a complete dummy (I do have pieces missing) :)

I am trying to explain how these work to my grandson, yet feel like I am not accurately doing it.

Any assistance is appreciated.
Robert Clark - InfoSec Analyst for Aviall, a Boeing Company
CISSP, MISM, MCSE and some other alphabet certifications.
I am moving to Theory ... everything works in Theory

Offline gwiz

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Re: Question about Grid Fins ...
« Reply #1 on: September 08, 2017, 05:26:11 AM »
The Wiki article seems pretty accurate to me.  These are control devices, intended to point the vehicle in the right direction, rather than drag brakes to slow it down.  The advantage over conventional fins comes from ease of stowage against the side of the vehicle when not in use.
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Offline smartcooky

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Re: Question about Grid Fins ...
« Reply #2 on: September 08, 2017, 05:30:02 AM »
They really are making this look routine....

https://www.youtube.com/watch?v=9M6Zvi-fFv4

Grid fins are nothing new. They have been used on missiles and LGBs since the 1980's.

I am also not an aerospace engineer (in the RNZAF I was an aircraft engineer specialising in Avionics) but my understanding is that Falcon 9's grid fins are designed to cause turbulent* airflow at hypersonic speeds. They control the direction of airflow during reentry, allowing pitch, roll and yaw amplitudes of up to 20° to correctly orient Stage 1 for a landing at LZ1  or one of the downrange ocean platforms. Although they will be offering some small airbraking effect, it will not be significant. The broad end of 9 Merlin engine bells at the business end of Stage 1 as it plummets to the Earth is going to be offering far more airbraking than the grid fins. If those on Falcon 9 are anything like the ones I have seen on missiles, they are simply a gridwork of planar surfaces.

*: AIUI turbulent airflow is considered better in this application because it is predictable  If solid, aerodynamic fins were to be used, the airflow would be laminar, but could switch to being turbulent unpredictably. This means that grid fins do not stall the way aerodynamic fins might, and they do not suffer the sharp break in loading exhibited by aerodynamic fins when used at high angles in excess of 45°. They are also less "aggressive" (for want of a better term), because they exhibit low "hinge moments" at a wide range of angles of attack. The hinge moment basically the force required to move a control surface. It varies with angle of attack and control surface deflection angle. Unlike grid fin, aerodynamic fins can cause sudden airflow changes at high angles of attack making them difficult to control and requiring much more precise actuator control inputs.

As I said, I'm not an aerospace engineer, but this is my understanding of how they work. If I'm wrong, I'm sure Jay or one of the others  will put me right
« Last Edit: September 08, 2017, 06:22:24 AM by smartcooky »
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Offline Zakalwe

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Re: Question about Grid Fins ...
« Reply #3 on: September 08, 2017, 01:33:10 PM »
They really are making this look routine....

Everyone said it was impossible until someone came along and started doing it.

SpaceX has now made more launches this year than Russia. That's a private firm with more launches than a whole country. Think about that for a minute.


This graphic shows just how dominant SpaceX is. In less than 5 years they have gone from nowhere to dominating the global market. You can say a lot of negatives about Musk, but he sure knows how to disrupt a market.
"Quod gratis asseritur, gratis negatur"
“That which can be asserted without evidence, can be dismissed without evidence.” – Christopher Hitchens.

Offline bobdude11

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Re: Question about Grid Fins ...
« Reply #4 on: September 08, 2017, 09:44:08 PM »
Thank you for all of the excellent responses. I was not aware of the concept of controlled turbulent air, but then, I have not studied aerospace sciences ( I chose the IT industry instead) so much of this is new to me.

I am a fan of aircraft, all things space, and flying in general (I do fly simulators ... so practically a pilot, right? :D) and have a basic understanding of aerodynamic forces when it pertains to lift, thrust, drag, etc.

Now I have a whole new path to explore and new things to learn.

Zakalwe, thanks for the graph. I wasn't aware of how well SpaceX is doing!

I do have one follow on question: Can ya'll point me to some other good resources for this kind of detailed information?

Once again, thank you all for being patient with me.
Robert Clark - InfoSec Analyst for Aviall, a Boeing Company
CISSP, MISM, MCSE and some other alphabet certifications.
I am moving to Theory ... everything works in Theory

Offline Dalhousie

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Re: Question about Grid Fins ...
« Reply #5 on: October 10, 2017, 12:49:39 AM »
They really are making this look routine....

Everyone said it was impossible until someone came along and started doing it.

SpaceX has now made more launches this year than Russia. That's a private firm with more launches than a whole country. Think about that for a minute.


This graphic shows just how dominant SpaceX is. In less than 5 years they have gone from nowhere to dominating the global market. You can say a lot of negatives about Musk, but he sure knows how to disrupt a market.

SpaceX are basically bankrolled by the US government, so it's not surprising they are doing well.

Offline smartcooky

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Re: Question about Grid Fins ...
« Reply #6 on: October 10, 2017, 03:43:18 PM »
They really are making this look routine....

Everyone said it was impossible until someone came along and started doing it.

SpaceX has now made more launches this year than Russia. That's a private firm with more launches than a whole country. Think about that for a minute.


This graphic shows just how dominant SpaceX is. In less than 5 years they have gone from nowhere to dominating the global market. You can say a lot of negatives about Musk, but he sure knows how to disrupt a market.

SpaceX are basically bankrolled by the US government, so it's not surprising they are doing well.

I'm quite sure that Iridium, Echostar, Koreasat, SES, Formosat, Intelsat, Inmarsat, Bulgariasat, Sky Perfect JSAT, EutelSat, Thales Alenia Space, Orbcomm, Thaicom , MDA Corp, and ATSB are not US Goverment.

I'm also sure that the US Government would rather pay a US company to launch OTV and resupply the ISS than the Russian government.
« Last Edit: October 10, 2017, 04:19:20 PM by smartcooky »
► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
►"Conspiracism is a shortcut to the illusion of erudition

Offline Dalhousie

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Re: Question about Grid Fins ...
« Reply #7 on: October 10, 2017, 07:23:03 PM »
They really are making this look routine....

Everyone said it was impossible until someone came along and started doing it.

SpaceX has now made more launches this year than Russia. That's a private firm with more launches than a whole country. Think about that for a minute.


This graphic shows just how dominant SpaceX is. In less than 5 years they have gone from nowhere to dominating the global market. You can say a lot of negatives about Musk, but he sure knows how to disrupt a market.

SpaceX are basically bankrolled by the US government, so it's not surprising they are doing well.

I'm quite sure that Iridium, Echostar, Koreasat, SES, Formosat, Intelsat, Inmarsat, Bulgariasat, Sky Perfect JSAT, EutelSat, Thales Alenia Space, Orbcomm, Thaicom , MDA Corp, and ATSB are not US Goverment.

I'm also sure that the US Government would rather pay a US company to launch OTV and resupply the ISS than the Russian government.

The US government has underwritten SpaceX development of Dragon and F9 and paid for their services to the tune of nine billion and and counting.

As for Russian resupply of the ISS, it's an international facility and resupply missions are what the Russians are very good at.

Offline Glom

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Re: Question about Grid Fins ...
« Reply #8 on: October 10, 2017, 07:43:12 PM »
Launch aid! Launch aid! They need to slap 300% tariffs on them.

Offline BazBear

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Re: Question about Grid Fins ...
« Reply #9 on: October 10, 2017, 08:56:13 PM »
It looks like $9 billion well spent to me. Space is expensive, and NASA's record on containing costs when they're riding herd on a launch vehicle development program hasn't been very good. The Russians do a very fine job, but the US needs it's own ability to get stuff to the ISS (or wherever) at a reasonable long term cost.
"It's true you know. In space, no one can hear you scream like a little girl." - Mark Watney, protagonist of The Martian by Andy Weir

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Offline smartcooky

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Re: Question about Grid Fins ...
« Reply #10 on: October 11, 2017, 12:06:19 AM »
It looks like $9 billion well spent to me. Space is expensive, and NASA's record on containing costs when they're riding herd on a launch vehicle development program hasn't been very good. The Russians do a very fine job, but the US needs it's own ability to get stuff to the ISS (or wherever) at a reasonable long term cost.

Yep. They can spend $9 billion paying SpaceX to develop a launch system that they can use, and that will launch 20 times a year, or they can spend $18bn developing their own launch system that will only launch 3-4 times a year and at twice the cost per launch of SpaceX. Hmmm... Now that is a tough choice isn't it?.

Add to that, anything that NASA does is subject to the biennial and quadrennial whims of politicians and pork-exponents. We've already seen what a pig's breakfast the last launch system was.

Just as an aside, IMO we are about 40 years behind where we could have been. Had NASA not got sidetracked into the costly dead end that was the Space Shuttle, we would now likely have a permanent manned presence on the Moon, and would have already landed people on Mars. Honestly, I think they could have been better off keeping on with the Apollo programme to Apollo 22 as originally, planned and continued developing the Saturn family of rockets. I would have loved to have seen how far they could have pushed the development of the F1 engine.
► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
►"Conspiracism is a shortcut to the illusion of erudition

Offline Zakalwe

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Re: Question about Grid Fins ...
« Reply #11 on: October 11, 2017, 03:57:55 AM »
It looks like $9 billion well spent to me. Space is expensive, and NASA's record on containing costs when they're riding herd on a launch vehicle development program hasn't been very good. The Russians do a very fine job, but the US needs it's own ability to get stuff to the ISS (or wherever) at a reasonable long term cost.

Absolutely.
NASA spent just under 1/4 $Billion building a tower for the Ares rocket that was never used. There's been nearly 1/2 $Billion spent on modifying the Mobile Launch Platform that is now only useful for a single SLS launch. Following launches will need further expensive modifications.
 
"Quod gratis asseritur, gratis negatur"
“That which can be asserted without evidence, can be dismissed without evidence.” – Christopher Hitchens.