Author Topic: How much 'cross range' do grid fins provide?  (Read 6795 times)

Offline JamesH

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How much 'cross range' do grid fins provide?
« on: 01/05/2016 04:22 pm »
Anyone have any idea how much horizontal distance the grid fins add to the F9 descent? ie if it was coming down vertically from apogee (yes, I know it doesn't do that), how much lateral movement could the fins provide before landing?

According to some  SpaceX data, they can tilt the stage 20degs, but once tilted that imparts an acceleration to the stage, so it's not as simple as 20 degrees from apogee. I think.

Offline Lars-J

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Re: How much 'cross range' do grid fins provide?
« Reply #1 on: 01/05/2016 04:27 pm »
Anyone have any idea how much horizontal distance the grid fins add to the F9 descent? ie if it was coming down vertically from apogee (yes, I know it doesn't do that), how much lateral movement could the fins provide before landing?

Nobody outside SpaceX knows this... But before they added grid fins, the accuracy of the targeted sea landing spot was on the order of a mile or more. So it seems like grid fins provide at least a mile of lateral adjustment of the landing point on a normal trajectory.

According to some  SpaceX data, they can tilt the stage 20degs, but once tilted that imparts an acceleration to the stage, so it's not as simple as 20 degrees from apogee. I think.

Right. It may just be that tilting the stage beyond 20 degrees can cause it to break up from aerodynamic forces, or that the grid fins can't push the stage further than 20 degrees off its axis.
« Last Edit: 01/05/2016 04:32 pm by Lars-J »

Offline macpacheco

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Re: How much 'cross range' do grid fins provide?
« Reply #2 on: 01/06/2016 07:26 am »
Bare minimum a substantial % of total altitude.
I would think 1/3 of altitude should be a sure thing.
Its possible its as much as 1:1 or even more.

Just as a reference, Skydivers can track glide 1:1 very easily, the pros can exceed 2:1 (that's without the winged suits).
An important question to even try to use this logic is how high grid finds have full authority.
Even if a 20 degree deflection is possible, it probably isn't at hypersonic speeds (might scorch the stage lateral skin too seriously).

The important aspect is once lateral movement is established, the fins job is much easier to sustain that motion.
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Offline Lars-J

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Re: How much 'cross range' do grid fins provide?
« Reply #3 on: 01/06/2016 07:31 am »
Bare minimum a substantial % of total altitude.
I would think 1/3 of altitude should be a sure thing.
Its possible its as much as 1:1 or even more.

Just as a reference, Skydivers can track glide 1:1 very easily, the pros can exceed 2:1 (that's without the winged suits).
An important question to even try to use this logic is how high grid finds have full authority.
Even if a 20 degree deflection is possible, it probably isn't at hypersonic speeds (might scorch the stage lateral skin too seriously).

The important aspect is once lateral movement is established, the fins job is much easier to sustain that motion.

Not. even. close. They are just steering fins at the tail like an arrow. Not wings.

Offline guckyfan

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Re: How much 'cross range' do grid fins provide?
« Reply #4 on: 01/06/2016 07:55 am »
Bare minimum a substantial % of total altitude.
I would think 1/3 of altitude should be a sure thing.
Its possible its as much as 1:1 or even more.

Most of the altitude is vacuum. Do you mean altitude at the end of the reentry burn? Maybe half of that at most, probably less IMO.

Offline JamesH

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Re: How much 'cross range' do grid fins provide?
« Reply #5 on: 01/06/2016 10:47 am »
Bare minimum a substantial % of total altitude.
I would think 1/3 of altitude should be a sure thing.
Its possible its as much as 1:1 or even more.

Just as a reference, Skydivers can track glide 1:1 very easily, the pros can exceed 2:1 (that's without the winged suits).
An important question to even try to use this logic is how high grid finds have full authority.
Even if a 20 degree deflection is possible, it probably isn't at hypersonic speeds (might scorch the stage lateral skin too seriously).

The important aspect is once lateral movement is established, the fins job is much easier to sustain that motion.

Not. even. close. They are just steering fins at the tail like an arrow. Not wings.

The whole stage can act as an aerosurface once is tilted. The grid fins do the orientation, the stage itself is the 'wing'. Not an efficient one, certainly. Hence my original question.

Offline meekGee

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Re: How much 'cross range' do grid fins provide?
« Reply #6 on: 01/06/2016 02:33 pm »
Bare minimum a substantial % of total altitude.
I would think 1/3 of altitude should be a sure thing.
Its possible its as much as 1:1 or even more.

Just as a reference, Skydivers can track glide 1:1 very easily, the pros can exceed 2:1 (that's without the winged suits).
An important question to even try to use this logic is how high grid finds have full authority.
Even if a 20 degree deflection is possible, it probably isn't at hypersonic speeds (might scorch the stage lateral skin too seriously).

The important aspect is once lateral movement is established, the fins job is much easier to sustain that motion.

Not. even. close. They are just steering fins at the tail like an arrow. Not wings.
They act like trim surfaces.

Since they are at the far end, they only need to tilt the stage.   Then the entire stage acts like the steering surface.

But all this is also a function of air density.  If 1:1 is possible, it's only possible in the last couple of miles.

But you also get "cross range" by tweaking the post-separation burn.  If they coast for 1000 seconds, then a 100 m/s push is worth 100 km.
« Last Edit: 01/06/2016 02:37 pm by meekGee »
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Offline OxCartMark

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Re: How much 'cross range' do grid fins provide?
« Reply #7 on: 01/19/2016 02:28 am »
As a start on guessing the L/D of the returning stage look at the diagrams that SpaceX has provided of the trajectory for the whole flight.  IIRC, there are two of them out there that show the slope of the line changing (more toward shore) once it enters the atmosphere vs. the ballistic trajectory above the atmosphere.  Perhaps this line is accurate, perhaps the vertical and horizontal scales need to be re-scaled to get the actual slope of that line.  Then what does that line represent, average ability of the stage to create L/D or something near maximum (assuming if it can't get near maximum performance it will fall harmlessly into the sea rather than on land).  Or maybe that line is just an artist's impression.  Ahh, nevermind this method, just a thought experiment.

As far as has been mentioned previously, yes, its not the grid fins that get it done but rather the "body surfing" of the whole angled vehicle that generates aerodynamic ~lifteyness~.

I'd also like to point out that there is a video out there which we've all seen from a year or two ago which shows either the first or second semi-successful F9 return to water which shows the stage falling through the atmosphere, its taken from an airplane.  That was before grid fins were involved or known to us.  The stage is not vertical but rather ~15 degrees off vertical and stable in that attitude (though nitrogen thrusting could have been involved).  It wouldn't take much of a logical jump to think that air was already translating it sideways and that by adding grid fins they would / were able to direct the vector of that sidewaysness.

The word that nobody seems to want to acknowledge in this is "smart bomb".  OK, two words.  That's what we really have here and the technology being used or adapted.  A smart bomb starts from a high position at very high horizontal speed and finds its way down to a target on the ground using fins, sometimes grid fins.  The only difference here is that with the F9 the payload is going the opposite direction.

Offline Dante80

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Re: How much 'cross range' do grid fins provide?
« Reply #8 on: 01/19/2016 02:36 am »
It's a bit more complicated than an aerodynamic bomb being steered to the target. This is a blunt shaped 50m long grain silo coming down at ludicrous speed.

The grid fins are doing a lot of work, just to keep the thing steady.



For reference, an SRB recovery attempt (granted, in that example the CoM is higher up, but the shape is similar).

« Last Edit: 01/19/2016 02:47 am by Dante80 »

Offline OxCartMark

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Re: How much 'cross range' do grid fins provide?
« Reply #9 on: 01/19/2016 01:30 pm »
The grid fins are doing a lot of work, just to keep the thing steady.

Objection!  As evidence I provide this video of the July 2014 F9 launch of 6 Orbcomm satellites which had no grid fins doing not a lot of work.  It was just as rock steady and has approximately the same angle off vertical and presumably approximately the same L/D but just no steering.  Its rock solid in that orientation with either no active input other than the piddly urgings of the N2 jets.


For reference, an SRB recovery attempt (granted, in that example the CoM is higher up, but the shape is similar).

Objection!  Not relevant your honor!

That SRB thing is a heavy walled cylinder of nearly constant wall thickness along its length.  Its COM is not only higher up but nearly in the center of its length, width, height giving it aerodynamic ambiguity.  F9 is much different, it has a very concentrated mass at the end and a fluid mass which can be offcenter to the long axis which would give it that lean.

-------------------

I've now talked myself into thinking I've backed into the answer to the question of this thread.  But only if I restate the question to be "How much 'cross range' does a returning F9 have".  The difference being to acknowledge that (more or less) the cross range comes from the F9's body surfing at an angle through the atmosphere with the grid fins being used mostly as a rudder and to prevent spinning on the long axis of the cylinder (which happened on the first ocean landing that we were shown a picture of (the one that hit hard due to centerfuging fuel)).

That cross range is 10km from the ballistic impact point.  My evidence for this is Elon's tweet from the time that the grid fins were added: "Previous accuracy of the successful water landings was "on the order of 10km"  "

Offline meekGee

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Re: How much 'cross range' do grid fins provide?
« Reply #10 on: 01/19/2016 01:33 pm »
In the first vid, IIRC, the fins were out of commission in the last 30 seconds.

The second vid, my god, it is the most ungraceful bit of spaceflight in existence, including failed missions...

Both are breathtaking though - thanks.
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Offline bjornl

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Re: How much 'cross range' do grid fins provide?
« Reply #11 on: 01/19/2016 02:19 pm »
No, that was CRS-5. On CRS-6, a throttle valve had 'stiction' problems, but the grid fins operated all the way down.

Offline notsorandom

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Re: How much 'cross range' do grid fins provide?
« Reply #12 on: 01/19/2016 02:36 pm »
It's a bit more complicated than an aerodynamic bomb being steered to the target. This is a blunt shaped 50m long grain silo coming down at ludicrous speed.

The grid fins are doing a lot of work, just to keep the thing steady.



For reference, an SRB recovery attempt (granted, in that example the CoM is higher up, but the shape is similar).


The Ares 1-X contained 4 booster tumble motors which intentionally set the first stage spinning after separation.

Offline gin455res

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Re: How much 'cross range' do grid fins provide?
« Reply #13 on: 07/23/2018 09:10 pm »
What would happen if the first stage immediately did a burn after MECO directly upwards (with no gravity losses) to reduce the vertical component of velocity. (i.e towards the earth)


I was looking at the bulgaria-sat launch


The velocity was 7700 km/hr at the apogee, so that is the x-component
The velocity at   MECO was 8500km/hr (67km altitude)


Pythagoras gives a y-component of 3600km/hr at MECO and 25deg angle to the horizontal.


The velocity at re-entry burn (does this have gravity? losses, but air-resistance? gains) was 8600km/hr giving 3800km/hr and 26.5deg respectively.


after re-entry the velocity was down to 6600km/hr




My question is, if the first stage could reduce the upward velocity by 2000km/s then the stage would be travelling about 7860km/hr and at 11.75 degs to the horizontal as it passed the 67km altitude on the way down.


How would a first stage behave in this scenario and could it do a bit of hypersonic body surfing?
« Last Edit: 07/24/2018 04:46 pm by gin455res »

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