Author Topic: Landing rockets and the wind  (Read 25497 times)

Offline El Commediante

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Landing rockets and the wind
« on: 12/27/2015 12:03 PM »
Hi!

I signed up to learn what is the influence of side wind on a rocket in the last seconds of landing. I thought it might be a problem, considering relatively big side surface of the rocket, high air pressure at the sea level or sudden changes in wind speed. Is there a wind speed at which rocket landing is impossible or extremely risky?

Offline Kabloona

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Re: Landing rockets and the wind
« Reply #1 on: 12/27/2015 06:05 PM »
Yes, winds will affect the rocket on landing. SpaceX has probably run landing simulations with various wind speeds in order to determine how much wind the first stage will be able to cope with. For CRS-7 they set a landing criterion of wind speed <20 knots, which you can see in a screen capture in the link below.

http://forum.nasaspaceflight.com/index.php?topic=37739.msg1395299#msg1395299
« Last Edit: 12/27/2015 06:08 PM by Kabloona »

Offline pippin

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Re: Landing rockets and the wind
« Reply #2 on: 12/27/2015 06:37 PM »
Will probably be similarly to launch but different :)
During launch the LV rises much slower, yet it's much more heavy.
When returning it's much lighter but also much faster.

Offline AJW

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Re: Landing rockets and the wind
« Reply #3 on: 12/27/2015 06:55 PM »
The fluctuation in wind speed (gusts) can be as critical as the wind speed itself.   If the wind speed is constant, it is much easier to compensate for than when the wind speed is changing dramatically and rapidly.   When looking at NOAA charts, gusts are plotted separately from wind speed starting at 15knots since the difference between the two is what causes the real challenges.  Using average wind speed over time can help, but the difference between 20knot constant, and 15knot winds with gusts up to 30knots is quite significant.

Offline meekGee

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Re: Landing rockets and the wind
« Reply #4 on: 12/27/2015 07:54 PM »
Aren't gusts more of a surface phenomena?

If the stage is coming down at +0.7g, then the last 100' get covered in less than 3 seconds.

Even if the stage did not correct for it, how much influence can the gust have over Falcon in 3 seconds?
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Offline KelvinZero

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Re: Landing rockets and the wind
« Reply #5 on: 12/27/2015 08:01 PM »
Someone claimed that in an earlier thread: Fast hoverslam landings can actually be easier for computers because if your landing fits in a small enough time interval chaotic gusts just look like prevailing winds.

Offline AJW

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Re: Landing rockets and the wind
« Reply #6 on: 12/27/2015 11:53 PM »
Wind gusts have a number of causes including those changes in air currents most airplane passengers know as turbulence, so this is not just a ground effect.  Speed of descent may lessening the impact though wind speed on the ground is often significantly lower than at higher altitudes due to friction.

I don't think there is a simple answer to the original post's question. 

Offline Burninate

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Re: Landing rockets and the wind
« Reply #7 on: 12/28/2015 12:13 AM »
Is there a wind speed at which rocket landing is impossible or extremely risky?

Inevitably, yes.  A hoverslam is not a steady-state solution to the control problem of landing a spacecraft, but an instantaneous one, and any degree of horizontal winds means that a whole other pair of variables - horizontal velocity - need to be zeroed out.  If you had a perfect deep throttle capability, a steady-state landing solution would be possible in zero wind, but even then there aren't really enough degrees of freedom to solve the equation with significant ground windspeed reliably - your means of control are too highly coupled.

Here I argue that some SuperDracos mounted on the other side of the rocket would give valuable control authority that could substantially raise whatever the windspeed threshold turns out to be:

http://forum.nasaspaceflight.com/index.php?topic=36546.msg1318143#msg1318143

We don't have any evidence of what the threshold actually is, other than the public tests we've seen that successfully achieved landings.

For SpaceX at present, just demonstrating reusability is a coup;  It seems it's possible to land a stage without this control authority, in the winds present on good-weather days.  It's only when they're using landing regularly for frequent launches, and especially when Falcon Heavy is landing stages in two locations at once, that edge conditions like wind become bottlenecks on fleet function.

« Last Edit: 12/28/2015 12:26 AM by Burninate »

Online Lars-J

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Re: Landing rockets and the wind
« Reply #8 on: 12/28/2015 01:46 AM »

[quote author=El Commediante
For SpaceX at present, just demonstrating reusability is a coup;  It seems it's possible to land a stage without this control authority, in the winds present on good-weather days.  It's only when they're using landing regularly for frequent launches, and especially when Falcon Heavy is landing stages in two locations at once, that edge conditions like wind become bottlenecks on fleet function.

I would assume that landing weather limits are not that different from launch weather limits, so In the case of RTLS the bottleneck won't be as significant as you think.

Offline cscott

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Re: Landing rockets and the wind
« Reply #9 on: 12/29/2015 04:43 PM »
I believe the landing wind limited for the orbcomm flight was 50mph.

Unlike launch, there's no nearby launch tower or lightning protection to be blown into.

Online macpacheco

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Re: Landing rockets and the wind
« Reply #10 on: 12/30/2015 07:22 AM »
I believe the landing wind limited for the orbcomm flight was 50mph.

Unlike launch, there's no nearby launch tower or lightning protection to be blown into.
What's the launch wind criteria ?
50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.
Even for ASDS 44 knot wind is a substantial margin.
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Offline meekGee

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Re: Landing rockets and the wind
« Reply #11 on: 12/30/2015 10:02 PM »

[quote author=El Commediante
For SpaceX at present, just demonstrating reusability is a coup;  It seems it's possible to land a stage without this control authority, in the winds present on good-weather days.  It's only when they're using landing regularly for frequent launches, and especially when Falcon Heavy is landing stages in two locations at once, that edge conditions like wind become bottlenecks on fleet function.

I would assume that landing weather limits are not that different from launch weather limits, so In the case of RTLS the bottleneck won't be as significant as you think.

IIRC, some weather limitations have to do with debris and propellant dispersal, no?

Also, they have to do with the long ionized tail that the outbound rocket leaves behind.

The engine of an inbound rocket re-lights at about 5000', and the "tail" is pointing the other way, and should be a lot more diffuse given that the rocket just plowed through it.

The incoming rocket is also lighter, but also doesn't have an umbilical tower right near by.

I don't know if these difference change the go-no-go conditions, but the two scenarios are not very similar.
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Offline Rocket Science

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Re: Landing rockets and the wind
« Reply #12 on: 12/30/2015 10:13 PM »
I believe the landing wind limited for the orbcomm flight was 50mph.

Unlike launch, there's no nearby launch tower or lightning protection to be blown into.
What's the launch wind criteria ?
50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.
Even for ASDS 44 knot wind is a substantial margin.
Launch: <20
Landing: <50
I've asked Chris if he could verify the landing number (we need gust limit as well).
That 50mph seems huge in terms of landing controlabity... If true, very impressive... IMO.
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Online macpacheco

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Re: Landing rockets and the wind
« Reply #13 on: 12/30/2015 11:19 PM »
What's the launch wind criteria ?
50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.
Even for ASDS 44 knot wind is a substantial margin.
Launch: <20
Landing: <50
I've asked Chris if he could verify the landing number (we need gust limit as well).
That 50mph seems huge in terms of landing controlabity... If true, very impressive... IMO.
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

And the real concern isn't controllability while in flight, but the period between the last second or two before touchdown until a few seconds after, aka the landing flare, and toppling risk (which can't happen while in flight). (Not a rocket engineer, but as a hobbie cat sailor, skydiver and private pilot I know a thing or two about the wind).
« Last Edit: 12/30/2015 11:22 PM by macpacheco »
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Offline Lee Jay

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Re: Landing rockets and the wind
« Reply #14 on: 12/30/2015 11:21 PM »
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

Not much could be further from the truth.

A cylinder like that has a drag coefficient of around 1.2!

Ever seen this drawing?  This is to scale and both have about the same drag.

« Last Edit: 12/30/2015 11:23 PM by Lee Jay »

Online macpacheco

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Re: Landing rockets and the wind
« Reply #15 on: 12/30/2015 11:23 PM »
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

Not much could be further from the truth.

A cylinder like that has a drag coefficient of around 1.2!
What better shape could be used ?
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Offline Lee Jay

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Re: Landing rockets and the wind
« Reply #16 on: 12/30/2015 11:27 PM »
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

Not much could be further from the truth.

A cylinder like that has a drag coefficient of around 1.2!
What better shape could be used ?

For a fuel tank?  Nothing.  That's why all rockets are basically cylindrical.  You just have to deal with the fact that they have to fly (up or down) through cross winds.

Offline Jim

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Re: Landing rockets and the wind
« Reply #17 on: 12/31/2015 01:51 AM »
The differences between launch and landing are the proximity of the launch pad hardware and also the large sail area of the fairing.

Offline CuddlyRocket

Re: Landing rockets and the wind
« Reply #18 on: 12/31/2015 02:43 AM »
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

Not much could be further from the truth.

A cylinder like that has a drag coefficient of around 1.2!

Ever seen this drawing?  This is to scale and both have about the same drag.



That's only true for the wind direction indicated. If the wind was 90 degrees to that, the results would be different! Real winds come from all directions and a cylinder is symmetric.

Offline Lee Jay

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Re: Landing rockets and the wind
« Reply #19 on: 12/31/2015 03:20 AM »
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.

Not much could be further from the truth.

A cylinder like that has a drag coefficient of around 1.2!

Ever seen this drawing?  This is to scale and both have about the same drag.



That's only true for the wind direction indicated. If the wind was 90 degrees to that, the results would be different! Real winds come from all directions and a cylinder is symmetric.

Symmetric and very, very draggy.

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