Why not four pairs of SuperDracos in the F9 S1 interstage?

[QuantumG]

Well, this is probably off-topic and will get deleted, but for what it's worth, I mostly use a general-purpose programming language of my own design.

This explains so much.

[PreferToLurk]

This might get deleted, but I think it might help illustrate a basic point that many of the more layman visitors to this forum seem to misunderstand frequently.  Hovering seems like it would help because we are taught/trained from the first time our Mothers told us not to run in the house that slow = careful. This makes the fast hoverslam seem like an unnecessary risk that is only being taken because of the limitations of the merlin engine.  This is simply not true.  The power, accuracy, and speed of computer control algorithms is highly unintuitive.

So, without any more lecturing or rambling, I give you Quadrocopter Pole Acrobatics!:

www.youtube.com/embed/pp89tTDxXuI

Forum regulars may remember this clip being posted in one of the original grasshopper threads. 

[douglas100]

Yes, I'd forgotten how amazing it was. I think you've made your point!

[JamesH]

Indeed very impressive. I think my point about the hoverslam now veers more towards the unknowns rather than the computer control then. Quad copters in a room are extremely controllable - you can position them exactly (and they are hovering... and can vary t:w positively and negatively), there is no outside influences (apart from interactions between the craft).

A hoverslam has a more 'unknowns' - wind, engine start and throttle performance, speed and location determination etc.

However, having seen the new pictures, they are clearly almost solved problems!

[ChrisWilson68]

Indeed very impressive. I think my point about the hoverslam now veers more towards the unknowns rather than the computer control then. Quad copters in a room are extremely controllable - you can position them exactly (and they are hovering... and can vary t:w positively and negatively), there is no outside influences (apart from interactions between the craft).

A hoverslam has a more 'unknowns' - wind, engine start and throttle performance, speed and location determination etc.

However, having seen the new pictures, they are clearly almost solved problems!

I think you're missing an important part of how control systems work.

They don't have to understand your "unknowns".  They don't have to expect them, they don't have to model them.  All they have to do is notice that something is causing them to start to deviate from their intended path and apply control inputs to compensate.

As long as the control algorithm is built to work for the properties of the control inputs, it can be very robust and handle all sorts of unexpected outside forces with the same very simple algorithm.

[JamesH]

Indeed very impressive. I think my point about the hoverslam now veers more towards the unknowns rather than the computer control then. Quad copters in a room are extremely controllable - you can position them exactly (and they are hovering... and can vary t:w positively and negatively), there is no outside influences (apart from interactions between the craft).

A hoverslam has a more 'unknowns' - wind, engine start and throttle performance, speed and location determination etc.

However, having seen the new pictures, they are clearly almost solved problems!

I think you're missing an important part of how control systems work.

They don't have to understand your "unknowns".  They don't have to expect them, they don't have to model them.  All they have to do is notice that something is causing them to start to deviate from their intended path and apply control inputs to compensate.

As long as the control algorithm is built to work for the properties of the control inputs, it can be very robust and handle all sorts of unexpected outside forces with the same very simple algorithm.

Yes, I understand all that. I suppose what I'm most specifically thinking about is wind sheer at low levels. You have a lot less time to compensate for issues at that level. Also, the intentionally late ignition of the engine also means less time to compensate for problems that occur at that point. That's what was really making me wonder whether hoverslam or hovering is the better option.  Of course if the barge is sending telemetry to the rocket with the current wind conditions it can take that in to account some distance away, and out at sea the wind does stay fairly consistent. And making a very reliable engine goes some way to ensuring hoverslams are reliable.

[Jim]

Of course if the barge is sending telemetry to the rocket with the current wind conditions

it isn't

[Dudely]

The computers on the F9 are designed to respond two orders of magnitude faster than they need to.

The problem is not as difficult as it sounds. Before you encounter a large force you will encounter a smaller force. Compensating for the small force quickly enough avoids it from ever building up into a large force you cannot control.

I'm at work so I'm not going to do the math, but I don't think there is a wind speed on Earth that can force a stage to divert significantly in the amount of time it takes for the engine/fins to react to the sensor. (Don't quote me, but I think I read they got it down to about 4ms by using a custom Linux kernel)

[kevinof]

As far as we know the barge isn't sending any telemetry to the stage so all the stage can do is aim for a spot. It's like an autopilot/chart plotter on a boat - you tell the boat to aim for X but wind and tides cause drift and the boat keeps adjusting as it aims for the spot. The boat has no idea what's causing the drift but it keep correcting until it's on the lay line.

Indeed very impressive. I think my point about the hoverslam now veers more towards the unknowns rather than the computer control then. Quad copters in a room are extremely controllable - you can position them exactly (and they are hovering... and can vary t:w positively and negatively), there is no outside influences (apart from interactions between the craft).

A hoverslam has a more 'unknowns' - wind, engine start and throttle performance, speed and location determination etc.

However, having seen the new pictures, they are clearly almost solved problems!

I think you're missing an important part of how control systems work.

They don't have to understand your "unknowns".  They don't have to expect them, they don't have to model them.  All they have to do is notice that something is causing them to start to deviate from their intended path and apply control inputs to compensate.

As long as the control algorithm is built to work for the properties of the control inputs, it can be very robust and handle all sorts of unexpected outside forces with the same very simple algorithm.

Yes, I understand all that. I suppose what I'm most specifically thinking about is wind sheer at low levels. You have a lot less time to compensate for issues at that level. Also, the intentionally late ignition of the engine also means less time to compensate for problems that occur at that point. That's what was really making me wonder whether hoverslam or hovering is the better option.  Of course if the barge is sending telemetry to the rocket with the current wind conditions it can take that in to account some distance away, and out at sea the wind does stay fairly consistent. And making a very reliable engine goes some way to ensuring hoverslams are reliable.

[JamesH]

Of course if the barge is sending telemetry to the rocket with the current wind conditions

it isn't

Would doing that make it easier to land accurately, or as others have said, are local wind conditions fairly irrelevant to the landing, i.e. can be compensated for in the last few seconds of descent whatever may be going on?

[Jim]

Would doing that make it easier to land accurately, or as others have said, are local wind conditions fairly irrelevant to the landing, i.e. can be compensated for in the last few seconds of descent whatever may be going on?

No, because now you have to add an additional receiver on the vehicle and it would be like integrating another sensor.  There are issues with validating the information sent and received.   GPS and landing radars can have redundancy and vote out bad information and don't require additional outside information.  Also, current winds conditions are only at one level, it doesn't the whole story of the air column on the way down. 

Aircraft don't incorporate actual wind conditions during the landing.   The pilot/autopilot reacts to the movement of the aircraft away from the desire path and provided corrections.

[JamesH]

Would doing that make it easier to land accurately, or as others have said, are local wind conditions fairly irrelevant to the landing, i.e. can be compensated for in the last few seconds of descent whatever may be going on?

No, because now you have to add an additional receiver on the vehicle and it would be like integrating another sensor.  There are issues with validating the information sent and received.   GPS and landing radars can have redundancy and vote out bad information and don't require additional outside information.  Also, current winds conditions are only at one level, it doesn't the whole story of the air column on the way down. 

Aircraft don't incorporate actual wind conditions during the landing.   The pilot/autopilot reacts to the movement of the aircraft away from the desire path and provided corrections.

Thanks.

[llanitedave]

The problem is not as difficult as it sounds.

I think it's a fascinating reflection on the progress of technological advancement that we can even make that statement today.

[Hotblack Desiato]

is wind really such a problem?

if it is necessary, the barge could be equipped with wind measuring systems and send the data up to the landing stage.

if there is are more accurate data from within the flight path required, maybe quadcopters could even help (not sure, if the idea is good, or if it just sounds good to me, as it is 3:30 am here, and I'm quite tired): send some quadcopters out, at 50, 100, 200 etc. m height, and a bit offset. the quadcopters have gps, and have the command to stay in position until they get called back to the barge. as they have to steer in order to counter the effects of wind, they can calculate the windspeed, and send it to the barge, which relays it to the stage. if it happens that one of those quadcopters is right in the middle of the trajectory of the stage, the exhaust-stream of the rocket would burn it away. and losing a quadcopter is a minor issue.

(and having the idea posted turns it into prior art  )

[mme]

is wind really such a problem?
...

No, it's not.  This entire thread is an attempt to solve a problem that doesn't exist.  The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates.  It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)

For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target.  The next attempt will hit the barge more directly.  Maybe even "land."  Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.

[JamesH]

is wind really such a problem?
...

No, it's not.  This entire thread is an attempt to solve a problem that doesn't exist.  The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates.  It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)

For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target.  The next attempt will hit the barge more directly.  Maybe even "land."  Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.

Odd how you can say its a problem that doesn't exist, when no-one has EVER landed and recovered a first stage successfully. Close, but no cigar, to quote a phrase.Once a stage has been landed multiple times successfully, then you say its a solved problem, but not yet.

Hoverslam != explode on impact.

[Jim]

if it is necessary, the barge could be equipped with wind measuring systems and send the data up to the landing stage.

No, read the earlier posts.  That is unnecessary and not really viable.

[mme]

is wind really such a problem?
...

No, it's not.  This entire thread is an attempt to solve a problem that doesn't exist.  The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates.  It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)

For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target.  The next attempt will hit the barge more directly.  Maybe even "land."  Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.

Odd how you can say its a problem that doesn't exist, when no-one has EVER landed and recovered a first stage successfully. Close, but no cigar, to quote a phrase.Once a stage has been landed multiple times successfully, then you say its a solved problem, but not yet.

Hoverslam != explode on impact.

The stage exploded because it hit the side of the barge while doing a hard divert.  It was doing a hard divert because it could not maintain an on target trajectory.  It could not maintain an on target trajectory because it did not control over it's primary aerodynamic control surfaces for the last minute of "flight."  It didn't have control of the grid fins because it ran out of hydraulic fluid.

Wind was not the problem.  Complex systems to anticipate the wind are not the solution.  The solution is to have control authority for the entire landing process.

The hoverslam itself is not some intractable problem and the term hoverslam is overly dramatic compared to the reality.  All the stage needs is good information regarding the distance to the ground and it's current deceleration.

I do not mean to imply that the targeting and landing control systems are trivial, just that with enough simulation, testing and experimentation I am confident they can be done and that they can be done reliably.

I feel like I've repeated myself too much so I'll do my best to refrain unless I have something new to add the the conversation.

[MarsInMyLifetime]

My view on the wind concern: Once RTLS becomes routine, first stages will be returning through the same air column they transited just 8 minutes earlier. Whenever a remote barge landing is required, we can be certain they'll be aware of those remote weather constraints as well, and launch only when conditions are within rules at both locations. For a rapidly falling stage, wind effects will be relatively minor and somewhat random anyway (evidenced by the twisting ascent trails we see, where the net drift is essentially null even though there may be shear or corkscrew motions in the air column). Skydivers and returning capsules are good analogs of how minor an issue wind will be for a descent that is basically a freefall most of the way.


And I think more than one smart engineer at Spacex has been working on dispersion mitigation.

[LouScheffer]

For a rapidly falling stage, wind effects will be relatively minor and somewhat random anyway (evidenced by the twisting ascent trails we see, where the net drift is essentially null even though there may be shear or corkscrew motions in the air column). Skydivers and returning capsules are good analogs of how minor an issue wind will be for a descent that is basically a freefall most of the way.

And I think more than one smart engineer at Spacex has been working on dispersion mitigation.

The military makes a kit called JDAM that attaches GPS controlled fins to an otherwise conventional bomb.   It's explicitly designed to work in adverse weather, with no input other than GPS (no lidar, fleets of quadcopters, pre-loaded wind tables, etc.)  It's spec'ed at 13 meters accuracy.

Using this 15-20 year old technology, an F9 with working grid fins (but nothing else) could be expected to hit the ship in almost any wind conditions, with no input other than GPS.  Fancy wind compensation schemes are not needed.

http://web.archive.org/web/20030624083412/http://www.af.mil/factsheets/factsheet.asp?fsID=108