Author Topic: Landing rockets and the wind  (Read 50388 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.
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Online 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. 
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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 »

Offline 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.

Offline 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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Online 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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Offline 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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Online 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 »

Offline 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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Online 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.

Online 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.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #20 on: 12/31/2015 08:25 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.

Very, but also least draggy in that circumstance.

Offline Lars-J

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Re: Landing rockets and the wind
« Reply #21 on: 12/31/2015 04:57 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.



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.

So what the heck is the point of your argument? That a stage should have a wing cross section, and rotate quickly to face whatever gusts happen?? C'mon.

Offline Rocket Science

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Re: Landing rockets and the wind
« Reply #22 on: 12/31/2015 05:19 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).
If that is you knowledge base, you need to take one of the physics courses that I teach and as a pilot who started flying 30 years ago I will keep the controls firmly in my grasp away from you... Sorry...
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Offline Rocket Science

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Re: Landing rockets and the wind
« Reply #23 on: 12/31/2015 05:20 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?
Great question and welcome to NSF! :)
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Offline vulture4

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Re: Landing rockets and the wind
« Reply #24 on: 12/31/2015 05:23 pm »
The effect of crosswind on a booster vertical landing depends on the stage's maneuvering capabilities. A sudden gust could move the vehicle laterally or cause a sudden tilt. But the extraordinary decelleration of the booster stage as it approaches the pad exposes it to ground-level winds for only a second or two, so the gas jet thrusters on the booster would be very unlikely to run out of propellant. At worst they might need greater maximum thrust.

For Shuttle crosswinds over 20 knots were considered unacceptable, and this ocurred (IIRC) in roughly 5% of scheduled launch or landing attempts. SX would have to decide what risk they would accept, but the landing occurs only 10 minutes afte launch and wind prediction over this interval is quite accurate. If crosswinds become a frequent problem SX has the option of building a windbreak around the landing pad perimeter slightly taller than the booster stage, which would result in some turbulence at the top of the barrier but virtually eliminate crosswinds below it.

« Last Edit: 12/31/2015 05:29 pm by vulture4 »

Offline Rocket Science

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Re: Landing rockets and the wind
« Reply #25 on: 12/31/2015 05:43 pm »
Lets remember that the booster is dealing with not just max constant winds, but peak gusts and wind shear as well through descent until touchdown and engine stop.
« Last Edit: 12/31/2015 05:45 pm by Rocket Science »
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Online Lee Jay

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Re: Landing rockets and the wind
« Reply #26 on: 01/02/2016 05:13 pm »
So what the heck is the point of your argument? That a stage should have a wing cross section, and rotate quickly to face whatever gusts happen?? C'mon.

My point is, "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." is a factually inaccurate statement.  There are shapes that are quite symmetrical that can generate less reaction force than a smooth cylinder.  For example, a cylinder with strakes or VGs can have less drag than a smooth cylinder.

Offline Rocket Science

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Re: Landing rockets and the wind
« Reply #27 on: 01/02/2016 05:39 pm »
We should also consider that the rocket is not a perfect cylinder, it has protuberances such as the grid-fins, landing gear etc. in a crosswind situation those protuberances can create airflow stagnation points which will result in an off axis vector force (sideways) that the flight control system must deal with…

http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/poten/node38.html
« Last Edit: 01/02/2016 05:40 pm by Rocket Science »
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Offline georgegassaway

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Re: Landing rockets and the wind
« Reply #28 on: 01/02/2016 08:20 pm »
The video below of a Grasshopper "Divert" test is a good demonstration of flying in wind. For one, how it can do some horizontal maneuvering then hold steady in one spot (hover) despite a crosswind (note some non-propulsiive flame blown to the side by the wind).  I WILL NOTE of course that Grasshopper was unique in that it could hover, the Falcon boosters as used for operational orbital launches cannot hover, but this McGregor flight seemed the best demo of flying and landing in wind.

But anyway, you see it flying and maneuvering in some significant wind, come down and land.   And after the engine shuts off, you see how much wind there was by the smoke/dust blowing pretty swiftly from left to right.



For the successful landing, IIRC someone said the landing error was about 6 feet (2 meters)?  Has anyone worked out the landing position in relation to the wind direction? 

It would be interesting to know if the miss error was "downwind" of the middle of the X.  That would imply it was coming right down onto the center of the X then as it got slower and slower the wind pushed it to the side a bit.  That would be sort of expected since to hold into the wind and to drift,  it would need to tilt a little bit, but if it was tilted a little when it touched down, then that would add unwanted extra stress on one or two landing legs and possibly begin a physical wobbling action (depends somewhat on the degree of shock absorption and the moment of inertia.  Certainly the leg span is so wide and the vehicle so tail heavy that there seems to be little chance of falling over unless something extreme happened ). So, it would probably need to get vertical just before touching down, allowing the wind to push it laterally a bit. But not very far since it would only be a few seconds and inertia would prevent the wind from accelerating it laterally to the same speed as the wind.

The complicating issue to that is the desire to have as close to zero horizontal velocity as possible.  So there has to be some tradeoff, either accept  a bit of horizontal drift due to wind when it gets vertical for touchdown, or keep it tilted a little bit into the wind for zero horizontal drift, so the structure can handle the fact that one or two legs will touch down before the other legs and there will be some acceptable degree of settling-in rocking.

But wow, a possible landing in 49 mph wind? Hope they never really have to find that out.  Certainly not an issue for RTLS landings since the launch limit is under 20 mph. But a theoretical scenario for an ASDS landing, if the ocean swells were within limits in such high wind (does not seem likely, though possible)

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Offline Roy_H

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Re: Landing rockets and the wind
« Reply #29 on: 01/04/2016 02:09 am »
Up thread it clearly says that the LANDING criteria is <20 knots (this information from SpaceX). Then someone suggests without anything to back them up that it is 50mph and all of a sudden everybody takes this as gospel.

Second they start justifying the difference from take-off to landing by saying the speed of take-off is much slower than landing. Ok this has some merit, but the main factor is inertial mass. The fully loaded rocket has many times the mass of the landing stage and less than twice the area. With high mass gusts will not be a problem, only average wind speed. The rocket engines are gimballed and apparently can easily handle the 20kt max take-off criteria. Far more difficult for the light weight landing stage.

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Offline tleski

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Re: Landing rockets and the wind
« Reply #30 on: 01/04/2016 02:51 am »
Landing Weather Looks good

See the post above for the SpaceX's "Landing Commit Criteria" in ORBCOMM-2 update thread. It clearly shows that that wind speed limit for landing is 50mph.
It is funny but another screen capture here:

http://forum.nasaspaceflight.com/index.php?topic=37739.msg1395299#msg1395299

entitled: "Launch Commit Criteria" shows item no. 3 - first stage landing (<10ft waves <20 kt winds).

It seems to me that the <10ft waves <20 kt winds criteria refer to landing on an ASDS (barging) and <50 mph winds to RTLS type landing. Any better explanation for this discrepancy? It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.


Offline docmordrid

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Re: Landing rockets and the wind
« Reply #31 on: 01/04/2016 08:21 am »
Up thread it clearly says that the LANDING criteria is <20 knots (this information from SpaceX). Then someone suggests without anything to back them up that it is 50mph and all of a sudden everybody takes this as gospel.
>

Maybe because this Landing Commit Criteria was posted in the ORBCOMM 2 launch updates thread. Post #105.
« Last Edit: 01/04/2016 08:29 am by docmordrid »
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Offline 1

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Re: Landing rockets and the wind
« Reply #32 on: 01/05/2016 12:07 am »

The complicating issue to that is the desire to have as close to zero horizontal velocity as possible.  So there has to be some tradeoff, either accept  a bit of horizontal drift due to wind when it gets vertical for touchdown, or keep it tilted a little bit into the wind for zero horizontal drift, so the structure can handle the fact that one or two legs will touch down before the other legs and there will be some acceptable degree of settling-in rocking.


Logically (for approximate values of logic), would this not all be handled the exact same way as the rest of the hoverslam software? Gusty winds are unpredictable by nature, but steady state winds should be tolerable to some degree. Tilt the rocket away from the wind on approach, and do a quick burst of the upper thrusters so the rocket rotates into the wind right as it approaches the ground. Do it right, and the wind zeros out horizontal and angular velocity in addition to vertical right as the rocket touches ground. No horizontal drift or rocking necessarily needed.

Offline Roy_H

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Re: Landing rockets and the wind
« Reply #33 on: 01/05/2016 12:12 am »
Landing Weather Looks good

See the post above for the SpaceX's "Landing Commit Criteria" in ORBCOMM-2 update thread. It clearly shows that that wind speed limit for landing is 50mph.
It is funny but another screen capture here:

http://forum.nasaspaceflight.com/index.php?topic=37739.msg1395299#msg1395299

entitled: "Launch Commit Criteria" shows item no. 3 - first stage landing (<10ft waves <20 kt winds).

It seems to me that the <10ft waves <20 kt winds criteria refer to landing on an ASDS (barging) and <50 mph winds to RTLS type landing. Any better explanation for this discrepancy? It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.

I stand corrected, I didn't realize where the 50mph figure came from. Thanks.
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Offline Paul451

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Re: Landing rockets and the wind
« Reply #34 on: 01/06/2016 10:59 pm »
It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.

Or because the landing pad itself isn't moving around, no matter how fast the wind.

The tighter limits on ASDS landing may be due to the limits of ASDS itself being able to stay stationary and level.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #35 on: 01/07/2016 12:37 am »
It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.

Or because the landing pad itself isn't moving around, no matter how fast the wind.

The tighter limits on ASDS landing may be due to the limits of ASDS itself being able to stay stationary and level.

I think it's fairly certain that the latest success is largely due to the landing pad not moving around.

They've demonstrated several times now that the extent of pad outside of the yellow circle doesn't have much do with it, aside from making recovery operations easier.
 
« Last Edit: 01/07/2016 12:38 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Online meekGee

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Re: Landing rockets and the wind
« Reply #36 on: 01/07/2016 12:52 am »



I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #37 on: 01/07/2016 01:10 am »



I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...

The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking)..  but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.

I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. :)

With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Online meekGee

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Re: Landing rockets and the wind
« Reply #38 on: 01/07/2016 01:14 am »



I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...

The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking)..  but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.

I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. :)
I think that's a stretch...  Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.

(Some may recognize the aviation scenario)

So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"...  Meh.  It was the presence of hydraulic fluid....
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #39 on: 01/07/2016 01:28 am »

I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...

The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking)..  but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.

I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. :)
I think that's a stretch...  Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.

(Some may recognize the aviation scenario)

So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"...  Meh.  It was the presence of hydraulic fluid....

Sure.  That they nailed this landing was an impressive feat and I've said as much elsewhere.  At each landing attempt we've seen them fix (change) more than one parameter - but ruling out an obvious one (eg. the lack of hydraulic fluid) sure helps for the next time around.

Equally, I think it's a stretch to expect the ability to successfully land at LZ-1 means they could successfully land on an ASDS - just like a successful landing at LAX does not mean you could land on an aircraft carrier..   
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Online meekGee

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Re: Landing rockets and the wind
« Reply #40 on: 01/07/2016 01:32 am »

I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...

The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking)..  but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.

I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. :)
I think that's a stretch...  Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.

(Some may recognize the aviation scenario)

So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"...  Meh.  It was the presence of hydraulic fluid....

Sure.  That they nailed this landing was an impressive feat and I've said as much elsewhere.  At each landing attempt we've seen them fix (change) more than one parameter - but ruling out an obvious one (eg. the lack of hydraulic fluid) sure helps for the next time around.

Equally, I think it's a stretch to expect the ability to successfully land at LZ-1 means they could successfully land on an ASDS - just like a successful landing at LAX does not mean you could land on an aircraft carrier..
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

And the sad part is that I actually do have other stuff I need to do tonight...
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #41 on: 01/07/2016 01:48 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough.  For that you have my sincerest apologies and I shall try to be more vague next time.  8)

For the record, I meant to imply that, IMHO, the land not moving played a large part in the success of this latest landing.. the largest part being the work of the guys/gals at SpaceX.

And the sad part is that I actually do have other stuff I need to do tonight...

As do I this day.  Enjoy!
 
« Last Edit: 01/07/2016 02:39 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Lars-J

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Re: Landing rockets and the wind
« Reply #42 on: 01/07/2016 01:52 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you.  For that you have my sincerest apologies: I shall try to be more vague next time.  8)

Are you completely oblivious to the "largely" in your sentence, and what it means? Largely, meaning: to a great extent; on the whole; mostly. Do you not understand the reaction that gets, when you come up with a brand new explanation that according to you, you are fairly certain is the primary factor in the failure?

Offline CameronD

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Re: Landing rockets and the wind
« Reply #43 on: 01/07/2016 02:29 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you.  For that you have my sincerest apologies: I shall try to be more vague next time.  8)

Are you completely oblivious to the "largely" in your sentence, and what it means? Largely, meaning: to a great extent; on the whole; mostly. Do you not understand the reaction that gets, when you come up with a brand new explanation that according to you, you are fairly certain is the primary factor in the failure?

No, not oblivious.  It is my personal opinion (the "I think.." in my sentence) based upon videos posted here and personal experience of the difficulties working with floating platforms.

A brand new explanation?  No, not on the ASDS Thread it isn't.  Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.  Whether or not it is indeed a problem for SpX will, I suppose, be known to us all following the Jason-3 launch.

(As a small aside, it isn't only floating platforms that move around: Problems at one customers' site were only resolved once we discovered the ground the place was built on was moving up and down with the tide!)
« Last Edit: 01/07/2016 02:33 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Online meekGee

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Re: Landing rockets and the wind
« Reply #44 on: 01/07/2016 03:14 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.



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Offline CameronD

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Re: Landing rockets and the wind
« Reply #45 on: 01/07/2016 03:43 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.

Fair enough..  I do have one question though that maybe someone here knows the answer to:

What maximum angle-of-tilt of the landing platform is the F9 landing system/landing legs designed to tolerate??
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Online meekGee

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Re: Landing rockets and the wind
« Reply #46 on: 01/07/2016 04:22 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.

Fair enough..  I do have one question though that maybe someone here knows the answer to:

What maximum angle-of-tilt of the landing platform is the F9 landing system/landing legs designed to tolerate??
Who knows, but my guess is that the engine throttle-down timing is designed to transfer weight from the engine bells to the legs gradually, sort of a hand-over.

This means that when one leg hits first, it's not the end of the world.

(This is assuming the rocket is not actively matching tilt...  They can have an altimeter on each leg, and the period of barge rocking is longer than the touchdown time)
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Online abaddon

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Re: Landing rockets and the wind
« Reply #47 on: 01/07/2016 03:37 pm »
Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.
Er, "other information", like SpaceX stating that the most recent barge landing failed due to valve stiction, and that this has been fixed for subsequent attempts?

Not trying to say a barge landing is a slam dunk but this is about as obvious a factor as it gets.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #48 on: 01/07/2016 04:14 pm »
Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.
Er, "other information", like SpaceX stating that the most recent barge landing failed due to valve stiction, and that this has been fixed for subsequent attempts?

Not trying to say a barge landing is a slam dunk but this is about as obvious a factor as it gets.

Indeed. The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.


Offline bstrong

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Re: Landing rockets and the wind
« Reply #49 on: 01/07/2016 04:41 pm »
The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.

Shotwell does not share this opinion:

Quote
"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."

http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/

Edit to clarify: I'm disagreeing that the rocking is negligible. Not saying it was the cause of previous failures.
« Last Edit: 01/07/2016 04:43 pm by bstrong »

Offline mvpel

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Re: Landing rockets and the wind
« Reply #50 on: 01/07/2016 06:22 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Some people expressed surprise that it landed dead center on the pad - I was like "well, it's a robot, where else would it land?"
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline CameronD

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Re: Landing rockets and the wind
« Reply #51 on: 01/07/2016 10:19 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Maybe processors are fast, but the real world is still analog - and that fast processor and software is totally reliant on sensors to know what is (and isn't) going on out there.  Garbage In, Garbage Out. ;)

To explain:  In my mind, it is a matter of prediction - but you can't predict something happening and make allowance for it if you can't sense it sufficiently within bounds of accuracy and time.  Of course the usual way to work out what you do and don't need to sense is by simulation and testing and often what you thought might be a problem (eg. wind gusts) can be overcome by other means than sensing (eg. "A gust? We're going fast enough to not worry about it") within the same bounds thus sacrificing some amount of accuracy for reduced complexity.

What is less straight-forward to me is how they plan to predict the angle of the surface they're landing on.  MeekGee suggested maybe an altimeter/range-finder on each leg?  That sounds feasible to me - although that would presumably only come into play in the final meters before touchdown.  Their commit criteria do specify the max sea-state they'll allow a landing, but that wouldn't cater for a rogue wave at the wrong time (certainly possible on the open ocean) tilting the platform outside acceptable limits at the last second.. and what do you do then?  They can't just hover (or can they?), so presumably they have safety margins in place to cover that scenario.

They could also measure instantaneous 3-axis platform tilt and use that to issue an earlier abort-to-water-landing if they find they're getting close to the edge of their safety margins in the minutes before landing... but to know whether or not that's feasible (or even necessary) you'd need to know the limits of the F9 landing leg design. Perhaps they are doing both??

Anyways, I'd be interested to know what others here think - and will be very interested in the landing video for Jason-3. 


EDIT:  I suspect I'm overthinking this.  It's little different to land the Apollo LEM on the Moon - just fly until one leg is in contact and throttle down.  Everything else should then sort itself out...
 
« Last Edit: 01/07/2016 11:01 pm by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline CJ

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Re: Landing rockets and the wind
« Reply #52 on: 01/07/2016 11:02 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Maybe processors are fast, but the real world is still analog - and that fast processor and software is totally reliant on sensors to know what is (and isn't) going on out there.  Garbage In, Garbage Out. ;)

To explain:  In my mind, it is a matter of prediction - but you can't predict something happening and make allowance for it if you can't sense it sufficiently within bounds of accuracy and time.  Of course the usual way to work out what you do and don't need to sense is by simulation and testing and often what you thought might be a problem (eg. wind gusts) can be overcome by other means than sensing (eg. "A gust? We're going fast enough to not worry about it") within the same bounds thus sacrificing some amount of accuracy for reduced complexity.

What is less straight-forward to me is how they plan to predict the angle of the surface they're landing on.  MeekGee suggested maybe an altimeter/range-finder on each leg?  That sounds feasible to me - although that would presumably only come into play in the final meters before touchdown.  Their commit criteria do specify the max sea-state they'll allow a landing, but that wouldn't cater for a rogue wave at the wrong time (certainly possible on the open ocean) tilting the platform outside acceptable limits at the last second.. and what do you do then?  They can't just hover (or can they?), so presumably they have safety margins in place to cover that scenario.

They could also measure instantaneous 3-axis platform tilt and use that to issue an earlier abort-to-water-landing if they find they're getting close to the edge of their safety margins in the minutes before landing... but to know whether or not that's feasible (or even necessary) you'd need to know the limits of the F9 landing leg design. Perhaps they are doing both??

Anyways, I'd be interested to know what others here think - and will be very interested in the landing video for Jason-3. 
 

They definitely can't hover, so that's out as an option.

Ships at sea almost always roll by varying amounts (in both magnitude and period) even in a steady sea state, because the base swells themselves vary a bit, as does their period. So, I think you're right, a greater than expected roll can occur at any time. It would also be a greater factor if the downhill side at landing was also the downwind side of the F9. They can't time the landing, but IMHO it's plausible that they might partially compensate for wind by changing the paramiters of the GN2 thrusters on the interstage, to have one firing during touchdown to get ahead of any tilt. My guess is that, at most, this is a future possibility; right now  they are focused on getting a baseline landing right. If an anemometer and wind direction gauge is added to the ASDS at some point, this would be my guess as to the reason (but it would require an uplink). 

My guess; they aren't going to build in an abort mode for this issue; too great a chance of losing a recoverable F9 when weighed against the $cost of damage to the ASDS.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #53 on: 01/08/2016 03:20 am »
They definitely can't hover, so that's out as an option.

Ships at sea almost always roll by varying amounts (in both magnitude and period) even in a steady sea state, because the base swells themselves vary a bit, as does their period. So, I think you're right, a greater than expected roll can occur at any time. It would also be a greater factor if the downhill side at landing was also the downwind side of the F9. They can't time the landing, but IMHO it's plausible that they might partially compensate for wind by changing the paramiters of the GN2 thrusters on the interstage, to have one firing during touchdown to get ahead of any tilt. My guess is that, at most, this is a future possibility; right now  they are focused on getting a baseline landing right. If an anemometer and wind direction gauge is added to the ASDS at some point, this would be my guess as to the reason (but it would require an uplink). 

My guess; they aren't going to build in an abort mode for this issue; too great a chance of losing a recoverable F9 when weighed against the $cost of damage to the ASDS.

Good point.

1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

2. There was an anemometer/wind direction gauge fitted to JRtI - and presumably M'303 and M'304 also -  but that would be for current wind conditions only and would tell you nothing about sea-state or predicted sea-state.

I do agree with you though...  They'd be unlikely to abort a landing even if the sensors all indicated a high probability that the stage will end up in the ocean as soon as the engines shut down. Since there's no payload on board (valuable or otherwise) they've lost nothing trying.

« Last Edit: 01/08/2016 05:00 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline joek

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Re: Landing rockets and the wind
« Reply #54 on: 01/08/2016 04:49 am »
1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

Those GN2 thrusters were dealing with other more serious problems: ran out of hydraulic fluid on one try; stuck valve on the other try.  We have never seen an attempted ASDS landing where everything performed nominally.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #55 on: 01/08/2016 05:02 am »
1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

Those GN2 thrusters were dealing with other more serious problems: ran out of hydraulic fluid on one try; stuck valve on the other try.  We have never seen an attempted ASDS landing where everything performed nominally.

True.  I've edited my post above accordingly. :)

With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #56 on: 01/08/2016 09:20 am »
The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.

Shotwell does not share this opinion:

Quote
"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."

http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/

Edit to clarify: I'm disagreeing that the rocking is negligible. Not saying it was the cause of previous failures.

Negligible compared with what would actually cause a problem to the landing. So relative not absolute. But whether that is the case remains to be seen.

Offline mvpel

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Re: Landing rockets and the wind
« Reply #57 on: 01/11/2016 02:25 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #58 on: 01/11/2016 02:58 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #59 on: 01/11/2016 10:57 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 

It won't 'bob'. It has too much mass. It will roll with long wavelength waves, but short wavelengths should be OK. But of course, even an aircraft carrier will bounce around if the waves are big enough.

Offline Doesitfloat

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Re: Landing rockets and the wind
« Reply #60 on: 01/11/2016 01:24 pm »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 
The load lines tell a different story.

When a ship carries more water ballast mass  than it's light ship weight. (Mass of Steel)
One can not use the term "lightly ballasted."
 
What is the definition of "bob"
We calculate ship motions in terms of roll, pitch and heave.  Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.





Offline CameronD

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Re: Landing rockets and the wind
« Reply #61 on: 01/11/2016 09:17 pm »
The load lines tell a different story.

When a ship carries more water ballast mass  than it's light ship weight. (Mass of Steel)
One can not use the term "lightly ballasted."

You have evidence of that?!?  What's the "water ballast mass" and the "light ship weight" of the ASDS??  ???
 
What is the definition of "bob"
We calculate ship motions in terms of roll, pitch and heave.  Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.

My apologies for using a nautical term I thought most here would understand.

If you'd prefer to substitute "roll, pitch and heave" where I wrote "bob", I'm happy for you to do that.

With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline JasonAW3

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Re: Landing rockets and the wind
« Reply #62 on: 01/11/2016 09:26 pm »
The load lines tell a different story.

When a ship carries more water ballast mass  than it's light ship weight. (Mass of Steel)
One can not use the term "lightly ballasted."

You have evidence of that?!?  What's the "water ballast mass" and the "light ship weight" of the ASDS??  ???
 
What is the definition of "bob"
We calculate ship motions in terms of roll, pitch and heave.  Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.

My apologies for using a nautical term I thought most here would understand.

If you'd prefer to substitute "roll, pitch and heave" where I wrote "bob", I'm happy for you to do that.

While this may seem a bit off topic, I think using a specialized SWATH Hull designed structure would greatly reduce the pitch and roll issues that they would have landing the stage.  (Ex Navy.  Had some experience with this subject).
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #63 on: 01/11/2016 09:31 pm »
While this may seem a bit off topic, I think using a specialized SWATH Hull designed structure would greatly reduce the pitch and roll issues that they would have landing the stage.  (Ex Navy.  Had some experience with this subject).

Agreed.  We had a good discussion about that a couple of years back:

https://forum.nasaspaceflight.com/index.php?topic=35244.msg1279594#msg1279594

« Last Edit: 01/11/2016 10:35 pm by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #64 on: 01/17/2016 09:19 pm »
Congratulations to SpaceX on the successful launch of Jason-3!

Barge needs much better stabilization in any sea state to keep landing area from moving. Barge moving up and down messes up with S1's sensors causing a hard landing. Landing would most likely have been successful if the barge was as stable as land.

Looks like I was right after all...  :(
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline mme

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Re: Landing rockets and the wind
« Reply #65 on: 01/17/2016 09:32 pm »
Congratulations to SpaceX on the successful launch of Jason-3!

Barge needs much better stabilization in any sea state to keep landing area from moving. Barge moving up and down messes up with S1's sensors causing a hard landing. Landing would most likely have been successful if the barge was as stable as land.

Looks like I was right after all...  :(

According to Musk the issue is that a leg did not latch.  Obviously sea state can be an issue but there is no evidence that it was the issue with this landing.

https://twitter.com/elonmusk/status/688814634489413632
Quote
Definitely harder to land on a ship. Similar to an aircraft carrier vs land: much smaller target area, that's also translating & rotating.

https://twitter.com/elonmusk/status/688816554306191360
Quote
However, that was not what prevented it being good. Touchdown speed was ok, but a leg lockout didn't latch, so it tipped over after landing.

https://twitter.com/Uncle_Gus/status/688816725106667520
Quote
@elonmusk Does that mean the same thing would have happened on land?

https://twitter.com/elonmusk/status/688816810754359297
Quote
@Uncle_Gus probably

They'll fix the latch and try again and then we'll get real data on how sea state affects landing attempts.
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Offline jg

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Re: Landing rockets and the wind
« Reply #66 on: 01/17/2016 09:34 pm »
Congratulations to SpaceX on the successful launch of Jason-3!

Barge needs much better stabilization in any sea state to keep landing area from moving. Barge moving up and down messes up with S1's sensors causing a hard landing. Landing would most likely have been successful if the barge was as stable as land.

Looks like I was right after all...  :(

Later tweets indicate the landing was soft enough, with the problem being that the collet locking one of the legs failed to (fully?) activate, resulting in the rocket toppling over.  Elon indicated the problem would likely have occurred in a landing on land attemp..

Will be a fun video to watch, when they release it.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #67 on: 01/17/2016 09:54 pm »
However, that was not what prevented it being good. Touchdown speed was ok, but a leg lockout didn't latch, so it tipped over after landing.

They'll fix the latch and try again and then we'll get real data on how sea state affects landing attempts.

Later tweets indicate the landing was soft enough, with the problem being that the collet locking one of the legs failed to (fully?) activate, resulting in the rocket toppling over.  Elon indicated the problem would likely have occurred in a landing on land attemp..

Will be a fun video to watch, when they release it.

I know that's what's been said, but as an engineer with a few years experience working with floating platforms (amongst other things) I'm not sure that's the full picture.  ISTM they didn't deploy the legs early enough to account for the swell ..and IMHO they probably would have landed on land quite okay - but without detailed analysis it's safer to agree that they wouldn't have.

If what I think happened, had the latch not failed the entire stage may (worst case) have toppled into the sea.

Yep, the video sure will be fun to watch.  Who's bringing the popcorn? :)

With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Kabloona

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Re: Landing rockets and the wind
« Reply #68 on: 01/23/2016 03:29 am »
Speaking of landing in the wind, this is an interesting tidbit from the Blue Origin press release after their second hop of New Shepard:

Quote
made several software improvements, including a noteworthy one. Rather than the vehicle translating to land at the exact center of the pad, it now initially targets the center, but then sets down at a position of convenience on the pad, prioritizing vehicle attitude ahead of precise lateral positioning. It’s like a pilot lining up a plane with the centerline of the runway. If the plane is a few feet off center as you get close, you don’t swerve at the last minute to ensure hitting the exact mid-point. You just land a few feet left or right of the centerline. Our Monte Carlo sims of New Shepard landings show this new strategy increases margins, improving the vehicle’s ability to reject disturbances created by low-altitude winds.
« Last Edit: 01/23/2016 03:30 am by Kabloona »

Offline obi-wan

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Re: Landing rockets and the wind
« Reply #69 on: 01/23/2016 03:47 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 
The load lines tell a different story.

When a ship carries more water ballast mass  than it's light ship weight. (Mass of Steel)
One can not use the term "lightly ballasted."
 
What is the definition of "bob"
We calculate ship motions in terms of roll, pitch and heave.  Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.

Actually, roll, pitch, and yaw are the terms for angular motion about the center of gravity for both nautical and aerospace vehicles. Defining the X axis as forward, Z axis is up, and (for a right-hand coordinate frame) Y to the left/port side, what the aerospace engineer would refer to as X, Y, and Z translation in the nautical world is surge, sway, and heave, respectively.

Offline mvpel

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Re: Landing rockets and the wind
« Reply #70 on: 01/23/2016 04:52 am »
I know that's what's been said, but as an engineer with a few years experience working with floating platforms (amongst other things) I'm not sure that's the full picture.  ISTM they didn't deploy the legs early enough to account for the swell ..and IMHO they probably would have landed on land quite okay - but without detailed analysis it's safer to agree that they wouldn't have.

If what I think happened, had the latch not failed the entire stage may (worst case) have toppled into the sea.

Take another look at the video and observe the position of the sun. Put your mouse cursor over it, even. Observe that it does not shift one iota for the entire 45 second duration right through the fall and explosion. There was no swell during the phase of the landing that actually matters. The stage would have fallen over on land as well as a result of the unseated collet. And the center of gravity of the stage is very low, so it takes an estimated 23 degrees of tilt to shift it past the tip of the legs when properly locked out. It would not have toppled into the sea.



It's even bigger after they added the wings.
« Last Edit: 01/23/2016 05:19 pm by mvpel »
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Offline Lar

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Re: Landing rockets and the wind
« Reply #71 on: 04/18/2016 05:01 pm »
Bump.

I think it's fairly clear at this point that wind did have some effect on the CRS-8 landing. :)
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Offline MattMason

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Re: Landing rockets and the wind
« Reply #72 on: 04/18/2016 08:17 pm »
Bump.

I think it's fairly clear at this point that wind did have some effect on the CRS-8 landing. :)

Yep. In the post-launch presser, Elon Musk noted that the F9 was fighting 50 MPH gusts, which perhaps explains the angle and the touchdown bounce.
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Offline kevinof

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Re: Landing rockets and the wind
« Reply #73 on: 04/18/2016 08:49 pm »
Not sure about that figure. I know it's what Musk said but as a life long ocean sailor that landing didn't look anything near 50 mph.

That's over 40 knots and at that wind you would expect a lot more blown wave tops and spray.  Looked more like 25/30 kts max to me.
« Last Edit: 04/18/2016 08:52 pm by kevinof »

Offline S.Paulissen

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Re: Landing rockets and the wind
« Reply #74 on: 04/18/2016 08:56 pm »
Not sure about that figure. I know it's what Musk said but as a life long ocean sailor that landing didn't look anything near 50 mph.

That's over 40 knots and at that wind you would expect a lot more blown wave tops and spray.  Looked more like 25/30 kts max to me.

Agreed, but winds at surface are much lower than gusts at altitude.
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Offline JFARNS

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Re: Landing rockets and the wind
« Reply #75 on: 04/18/2016 09:54 pm »
Not sure about that figure. I know it's what Musk said but as a life long ocean sailor that landing didn't look anything near 50 mph.

That's over 40 knots and at that wind you would expect a lot more blown wave tops and spray.  Looked more like 25/30 kts max to me.

Agreed, but winds at surface are much lower than gusts at altitude.

On the presser video, Musk said 50 mph winds "on the way down".

Online Lee Jay

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Re: Landing rockets and the wind
« Reply #76 on: 04/18/2016 10:09 pm »
As a rule of thumb, use the power law with exponent of 0.11 for vertical wind shear near the Earth's surface over water.

So, if the wind is 9m/s at 10 meters, at 100m it would be 9*(100/10)^0.11 = 11.6m/s.

Offline the_other_Doug

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Re: Landing rockets and the wind
« Reply #77 on: 04/19/2016 02:21 pm »
As a rule of thumb, use the power law with exponent of 0.11 for vertical wind shear near the Earth's surface over water.

So, if the wind is 9m/s at 10 meters, at 100m it would be 9*(100/10)^0.11 = 11.6m/s.

Also as a rule of thumb, take any external load factor quoted by Musk as affecting a rocket and divide by ten.  Of course, when speaking of schedules, take any timeframe quoted by Musk and multiply by a factor of ten... ;)
-Doug  (With my shield, not yet upon it)

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Re: Landing rockets and the wind
« Reply #78 on: 04/19/2016 05:41 pm »
The best way to reduce wind impact is to land with three engines...

These one-engine almost-hover landings are painfully slow to look at.  Like watching paint dry.

Someone give me some excitement!
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #79 on: 04/20/2016 12:48 am »
The best way to reduce wind impact is to land with three engines...

These one-engine almost-hover landings are painfully slow to look at.  Like watching paint dry.

Someone give me some excitement!

They're probably not a little concerned about the effect a 3-engine hover-slam might have on the deck... paint.
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline CJ

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Re: Landing rockets and the wind
« Reply #80 on: 04/20/2016 04:07 am »
The best way to reduce wind impact is to land with three engines...

These one-engine almost-hover landings are painfully slow to look at.  Like watching paint dry.

Someone give me some excitement!

They're probably not a little concerned about the effect a 3-engine hover-slam might have on the deck... paint.

For all we know, the paint on those ASDS deck plates was fine once they fished them out of the bilge.  :)

I think MeekGee has a point though; the 3-engine landing profile would be far more wind-tolerant than the single-engine burn. IMHO, we'll see this show up in the landing limits criteria the next time they try a 3-engine landing due to low margins; the wind speed redline should be higher. 

Offline SoulWager

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Re: Landing rockets and the wind
« Reply #81 on: 04/20/2016 08:01 am »
I'd estimate surface winds at 25mph, based on the cloud of dust blowing away after touchdown in this video:

Offline CameronD

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Re: Landing rockets and the wind
« Reply #82 on: 04/20/2016 11:47 pm »
I'd estimate surface winds at 25mph, based on the cloud of dust blowing away after touchdown in this video:

One thing is for certain, that ain't no dust cloud!!
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

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Re: Landing rockets and the wind
« Reply #83 on: 08/15/2016 09:02 pm »
Interesting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less.  The timestamp where it is discussed is starting at 13:00 in the below video.  The quote:
Quote
In the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn.  Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second.  That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time.  A one engine burn uses more fuel, but it's a softer landing and you get more control.  And that allows you to land in higher winds.

« Last Edit: 08/15/2016 09:03 pm by abaddon »

Offline dorkmo

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Re: Landing rockets and the wind
« Reply #84 on: 06/27/2017 04:21 pm »
got my copy of Lars' book today lulz

Offline rocketmaniac000

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Re: Landing rockets and the wind
« Reply #85 on: 11/05/2021 10:02 pm »
This is a fantastic questions and I've enjoyed reading through this thread. Thanks you!

Offline dondar

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Re: Landing rockets and the wind
« Reply #86 on: 03/14/2022 09:13 pm »
Interesting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less.  The timestamp where it is discussed is starting at 13:00 in the below video.  The quote:
Quote
In the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn.  Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second.  That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time.  A one engine burn uses more fuel, but it's a softer landing and you get more control.  And that allows you to land in higher winds.


Wind provides translation (horizontal) speed and torque. Both are very undesirable (platform landing requires also precise landing trajectory, i.e. rocket should land not only vertically but vertically in the specific point).
So you want to compensate wind forces. By using significantly longer one engine burn they have better control i.e. more time to do exactly that.

Offline JamesH65

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Re: Landing rockets and the wind
« Reply #87 on: 03/17/2022 11:43 am »
Interesting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less.  The timestamp where it is discussed is starting at 13:00 in the below video.  The quote:
Quote
In the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn.  Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second.  That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time.  A one engine burn uses more fuel, but it's a softer landing and you get more control.  And that allows you to land in higher winds.


Wind provides translation (horizontal) speed and torque. Both are very undesirable (platform landing requires also precise landing trajectory, i.e. rocket should land not only vertically but vertically in the specific point).
So you want to compensate wind forces. By using significantly longer one engine burn they have better control i.e. more time to do exactly that.

Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...

So now I am conflicted.

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Re: Landing rockets and the wind
« Reply #88 on: 03/17/2022 02:55 pm »
Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...
Exactly.  It was some time ago (quite the necro-reply!) but it did run counter to the general thinking around here that was prevalent at the time, including my own.
« Last Edit: 03/17/2022 02:55 pm by abaddon »

Online r8ix

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Re: Landing rockets and the wind
« Reply #89 on: 03/17/2022 04:25 pm »
Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...
Exactly.  It was some time ago (quite the necro-reply!) but it did run counter to the general thinking around here that was prevalent at the time, including my own.
They're not mutually exclusive concepts.

Offline 1

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Re: Landing rockets and the wind
« Reply #90 on: 03/18/2022 12:15 am »
It's worth remembering that much of the original discussion about this was focused on 'hoverslam' vs 'hover in place and then slowwwwwly decend' rather than '1-engine hoverslam vs 3-engine hoverslam' (although it's true that this particular conversation in this particular thread does indeed discuss 1 engine vs 3). Much of our educated guessing, long before landing became routine, was centered around analysis of the feasibility of hoverslam at all.

Today, 100+ successful landings later it is unquestionably clear that hovering in place is not at all needed. By the same token, SpaceX has not bothered to develop a 3-engine hoverslam approach, so those best in the know clearly do not believe there's significant benefit to be had.

With 6 years of hindsight, I now wonder if a 3-engine hoverslam is indeed better from first principals, but may be unworkable on the Falcon 9. I seem to remember reading somewhere that the outboard engines do not have as much gimbal authority as the center*; and attempting a landing in one third the time but with less than 3x control vs a single engine could very conceivably reduce the landing tolerances; wind and otherwise.

*Someone please correct me if I'm wrong on this.

Offline alugobi

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Re: Landing rockets and the wind
« Reply #91 on: 03/18/2022 02:37 am »
I don't think that they fret about the rocket landing in the wind. It's the effect on the stability of the landed booster in high seas that appears to get launches postponed.  --Seas churned up by that wind.

Offline edzieba

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Re: Landing rockets and the wind
« Reply #92 on: 03/21/2022 01:01 pm »
Wind can still be an issue e.g. Starlink 4. In that case the limitation was that wind measurements have to be forward-predicted prior to launch rather than measure din real-time, so if the wrong values are provided from the start the booster is compensating for the wrong winds.

Offline dondar

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Re: Landing rockets and the wind
« Reply #93 on: 03/27/2022 07:21 pm »
Interesting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less.  The timestamp where it is discussed is starting at 13:00 in the below video.  The quote:
Quote
In the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn.  Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second.  That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time.  A one engine burn uses more fuel, but it's a softer landing and you get more control.  And that allows you to land in higher winds.


Wind provides translation (horizontal) speed and torque. Both are very undesirable (platform landing requires also precise landing trajectory, i.e. rocket should land not only vertically but vertically in the specific point).
So you want to compensate wind forces. By using significantly longer one engine burn they have better control i.e. more time to do exactly that.

Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...

So now I am conflicted.
this is the problem with the optimization of the real problems. You end always doing splits trying to sit between chairs.
People were talking about saving fuel. Hoverslam is the optimal way to "speed break" i.e. to cancel landing speed right before touching surface using max trust.

SpaceX got first the economics of flight right (they kept re-usability payload losses to the minimum). When they'd realized they have a tone+ of propellant over they'd started to look for the ways to utilize this extra when necessary/possible. I.e. they started to expend landing envelope to the higher winds, more energetic trajects etc.

Tags: SpaceX rockets wind 
 

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