Author Topic: SpaceX's Falcon 9 Landing Failure  (Read 941 times)

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Offline Homer J Simpson

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SpaceX's Falcon 9 Landing Failure
« on: December 06, 2018, 02:03:01 am »





 
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Offline raptor1956

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #1 on: December 07, 2018, 12:58:52 am »
They've had a pretty good run of recovery but nothing's perfect.  They lost the center core of the Falcon Heavy at the beginning of the year and this core, though by some measures not fully lost, is not likely to ever fly again given the salt water treatment the engines will have received and the likelihood that the tube structure received some damage after it tipped over and slapped the water -- the tube is very thin and needs to be stiffened with gas pressure in much the same way as a thin walled can of Coke is stiffened by gas pressure. 

I still haven't heard what actually went wrong and it may be a while before we do though it's certain they will try to figure that out.

Although I think it unlikely they will ever reuse any of the engines I hope they at least test a few to see how they fare. 


Brian
 

Offline ajb

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #2 on: December 07, 2018, 01:04:33 am »
Via Twitter:
Quote from: Elon Musk
Grid fin hydraulic pump stalled, so Falcon landed just out to sea. Appears to be undamaged & is transmitting data. Recovery ship dispatched.
https://twitter.com/elonmusk/status/1070386062164283392

Quote from: Elon Musk
Pump is single string. Some landing systems are not redundant, as landing is considered ground safety critical, but not mission critical. Given this event, we will likely add a backup pump & lines.
https://twitter.com/elonmusk/status/1070388894875545600

Given how far out of control the stage seemed to get initially, it's fairly impressive how well it recovered once the landing burn started and the engine gimballing became the dominant control authority.  It would be interesting to know what sort of contingencies the guidance system is prepared for, in terms of adjusting the transition schedules between the cold gas thrusters, grid fins, and gimballing.
« Last Edit: December 07, 2018, 01:15:59 am by ajb »
 

Offline CatalinaWOW

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #3 on: December 07, 2018, 01:20:24 am »
The biggest impact of this is likely to be the response of those at or near landing sites that are forced to realize that large metal pieces might land on their head.
 

Offline EvilGeniusSkis

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #4 on: December 07, 2018, 06:22:27 pm »
This event combined with the SSO-A launch where booster 1046 was launched on its third flight with no problems seem to indicate that re-used boosters are more reliable than brand new boosters. What i find more interesting is the use of liquid helium as hydraulic fluid. Not only does LHe not act as a lubricant, it also seems like it would be hard to stop the creation of gas bubbles in the lines due to the LHe boiling. My previous assumption was that RP-1 was used as a hydraulic fluid, because it is the most oil-like substance on F9.
 

Online dmills

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #5 on: December 07, 2018, 06:46:07 pm »
If there is really no oil system on board, what lubricates the turbopump bearings? I would have expected there to be some sort of oil carried?

Helium looks unlikely as a working fluid, but I could maybe see it as a pressurant for a total loss system.

Regards, Dan.
 

Offline SeanB

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #6 on: December 07, 2018, 07:03:19 pm »
You can use boundary layer lubrication, just need a high enough pressure fluid ( anything including a gas under high pressure) that will form a film between metal to metal contact, and then have a backup system, either a solid lubricant like PTFE or molybdenum sulphide to handle the start up and shut down conditions of low pressure, and of course you can also use bearings of some sort with the same lubrication regime. At the speeds the pumps operate at this would be enough just having a small scroll on the shafts both sides of the seals and putting in the liquid to make them both self priming and self maintaining. That it is at 3K is not really much difference than if it was long chain carbon molecules at room temperature, your choices of materials just get a little more expensive and exotic.
 

Offline Neilm

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #7 on: December 07, 2018, 08:28:30 pm »
The biggest impact of this is likely to be the response of those at or near landing sites that are forced to realize that large metal pieces might land on their head.

There aren't any people near landing sites.

The flight profile of the Falcon is to come down ballistically somewhere safe (over the water in this case) then translate accross to the landing zone. This translation requires both the landing engines and the grid fins. Most of the translation as I understand is due to the lift from the rocket body as it is controlled by the grid fins. If either system fails to work, the falcon lands in the water.
Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe. - Albert Einstein
 

Offline CatalinaWOW

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #8 on: December 07, 2018, 09:43:32 pm »
The biggest impact of this is likely to be the response of those at or near landing sites that are forced to realize that large metal pieces might land on their head.

There aren't any people near landing sites.

The flight profile of the Falcon is to come down ballistically somewhere safe (over the water in this case) then translate accross to the landing zone. This translation requires both the landing engines and the grid fins. Most of the translation as I understand is due to the lift from the rocket body as it is controlled by the grid fins. If either system fails to work, the falcon lands in the water.

I guess I'll buy it. But you are saying that there are no possible errors/failure modes which will result in the ballistic trajectory being off by a few miles (which in view of the distances traveled appears to be a few percent).   Near is in the eye of the beholder.  When I was trying to get safety certification for a test on a far smaller and less energetic piece of hardware 10 miles was considered near.  It all depends on who owns the damage/injury liability and how much risk they are willing to accept.
 

Offline james_s

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #9 on: December 07, 2018, 11:55:46 pm »
If there is really no oil system on board, what lubricates the turbopump bearings? I would have expected there to be some sort of oil carried?

Helium looks unlikely as a working fluid, but I could maybe see it as a pressurant for a total loss system.

Regards, Dan.

Dental drills turbines ride on air bearings, it's a total-loss system using the same air that is used to spin the turbine. Automotive turbochargers use engine oil but the bearings are not bearings in a traditional sense, they are quite loose and rely on the pressurized oil to create a layer for the shaft to float on. I suspect virtually any fluid that could tolerate the temperature without decomposing would work, regardless of whether or not it makes a good lubricant.
 

Offline Neilm

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #10 on: December 08, 2018, 06:40:29 pm »
The biggest impact of this is likely to be the response of those at or near landing sites that are forced to realize that large metal pieces might land on their head.

There aren't any people near landing sites.

The flight profile of the Falcon is to come down ballistically somewhere safe (over the water in this case) then translate accross to the landing zone. This translation requires both the landing engines and the grid fins. Most of the translation as I understand is due to the lift from the rocket body as it is controlled by the grid fins. If either system fails to work, the falcon lands in the water.

I guess I'll buy it. But you are saying that there are no possible errors/failure modes which will result in the ballistic trajectory being off by a few miles (which in view of the distances traveled appears to be a few percent).   Near is in the eye of the beholder.  When I was trying to get safety certification for a test on a far smaller and less energetic piece of hardware 10 miles was considered near.  It all depends on who owns the damage/injury liability and how much risk they are willing to accept.
After the falcon has entered the atmosphere this is true as at this point it is mostly going straight down. While the rocket body has a certain amount of lift, the landing zone is towards the edge of the flight envelope (plus some margin one would have thought).  The rocket is flying back through the area that will already have been cleared due to the rocket launch anyway. If the earlier boostback burn (when it turns around to head for the coast) fails the Automatic Flight Termination System will blow up the rocket. If you listen to the webcast, after the entry burn you hear the call "AFTS has safed". Meaning that they have now disabled it as the booster is only going to land somewhere safe.

If you want to see a visualisation of the flight check out the flight club 3d simulation of this at https://flightclub.io/result/3d?id=596096c1-a6a9-46a9-a127-6fc9a258c895. The red lines are where the engines are firing, the blue bits when the vehicle is coasting.
Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe. - Albert Einstein
 

Offline raptor1956

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #11 on: December 08, 2018, 09:34:05 pm »
The biggest impact of this is likely to be the response of those at or near landing sites that are forced to realize that large metal pieces might land on their head.

There aren't any people near landing sites.

The flight profile of the Falcon is to come down ballistically somewhere safe (over the water in this case) then translate accross to the landing zone. This translation requires both the landing engines and the grid fins. Most of the translation as I understand is due to the lift from the rocket body as it is controlled by the grid fins. If either system fails to work, the falcon lands in the water.

I guess I'll buy it. But you are saying that there are no possible errors/failure modes which will result in the ballistic trajectory being off by a few miles (which in view of the distances traveled appears to be a few percent).   Near is in the eye of the beholder.  When I was trying to get safety certification for a test on a far smaller and less energetic piece of hardware 10 miles was considered near.  It all depends on who owns the damage/injury liability and how much risk they are willing to accept.
After the falcon has entered the atmosphere this is true as at this point it is mostly going straight down. While the rocket body has a certain amount of lift, the landing zone is towards the edge of the flight envelope (plus some margin one would have thought).  The rocket is flying back through the area that will already have been cleared due to the rocket launch anyway. If the earlier boostback burn (when it turns around to head for the coast) fails the Automatic Flight Termination System will blow up the rocket. If you listen to the webcast, after the entry burn you hear the call "AFTS has safed". Meaning that they have now disabled it as the booster is only going to land somewhere safe.

If you want to see a visualisation of the flight check out the flight club 3d simulation of this at https://flightclub.io/result/3d?id=596096c1-a6a9-46a9-a127-6fc9a258c895. The red lines are where the engines are firing, the blue bits when the vehicle is coasting.

The visualization angle makes it look more straight down than it actually is -- the boost-back burn has to zero out the downrange velocity then reverse it to get back to where it started.  If the flight profile on the return had it come straight down they'd need to spend additional fuel to zero out the horizontal velocity a second time and they don't do that.  If you watch the booster as it descends through the atmosphere you can see that the tube is not straight up and down but instead is pointed at an angle.  They want to add only enough horizontal velocity to get back to home with just a bit more for margin.  Everything they do is at the margin as the landing itself is called a hover-slam -- no wasted hovering, they come right down and the landing gear only deploy within a few seconds of touch-down.  They rely on aero drag to reduce energy as much as possible to reduce the amount of fuel needed.


Brian
 

Offline Homer J Simpson

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Re: SpaceX's Falcon 9 Landing Failure
« Reply #12 on: December 09, 2018, 01:42:49 am »
 


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