Author Topic: Landing rockets and the wind  (Read 50401 times)

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...
"The laws of physics are unforgiving"
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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.

Offline 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.

Offline 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.

Tags: SpaceX rockets wind 
 

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