Landing rockets and the wind

[CJ]

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. 

[SoulWager]

I'd estimate surface winds at 25mph, based on the cloud of dust blowing away after touchdown in this video:

[CameronD]

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!!

[abaddon]

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:

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.

[dorkmo]

got my copy of Lars' book today lulz

[rocketmaniac000]

This is a fantastic questions and I've enjoyed reading through this thread. Thanks you!

[dondar]

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:

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.

[JamesH65]

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:

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.

[abaddon]

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.

[r8ix]

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.

[1]

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.}

[alugobi]

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.

[edzieba]

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.

[dondar]

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:

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.