Author Topic: Limits on F9 reuse and progress towards it  (Read 95647 times)

Offline FutureSpaceTourist

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Re: Limits on F9 reuse and progress towards it
« Reply #220 on: 12/06/2023 11:37 am »
Article about SpaceXís plans for rapidly increasing launch cadence at Vandenberg:

https://www.noozhawk.com/spacex-launch-rate-at-vandenberg-sfb-could-soar-to-100-by-2025/

Includes quotes from a public talk by Nate Janzen, (manager of launch pad systems and operations for SpaceX at Vandenberg):

Quote
Next year, SpaceX will re-evaluate and conduct analysis with an eye toward certifying the first-stage boosters for 25 to 30 flights, he said.

Offline LouScheffer

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Re: Limits on F9 reuse and progress towards it
« Reply #221 on: 12/07/2023 12:31 pm »
This article says SpaceX are aiming for 25+ flights per booster.

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Next year, SpaceX will re-evaluate and conduct analysis with an eye toward certifying the first-stage boosters for 25 to 30 flights, he said.
Sure. First they "aimed at" 10, then 15, then 20, and now 25+. This is an oversimplification for the general non-space news media.  The reality is almost certainly that they do an inspection of each booster each time it returns. There is no magic number N for which N is acceptable and N+1 is not. The question here is what they will do if a booster is getting "too old" for customer payloads: expend it, retire it, of just keep flying Starlinks?
There may be a magic number in practice.  Previously SpaceX said one of their metrics was that to qualify for the Nth flight, each (non-replaceable) component must survive 4N cycles without failure.  So if this is their metric (perhaps used to reassure customers), then if they tested each part to 80 cycles, then 20 flights is OK but 21 is not.   Of course they can raise this limit with more testing, which might be what they are doing when the "aim" for more reflights.

There are similar magic numbers, such as safety factors and cycle limits in all sorts of engineering.   You need to retire or replace something after a certain number of cycles or hours, not because it would fail, but because it falls out of certification limits that were previously agreed on.   

Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #222 on: 12/07/2023 02:23 pm »
This article says SpaceX are aiming for 25+ flights per booster.

Quote
Next year, SpaceX will re-evaluate and conduct analysis with an eye toward certifying the first-stage boosters for 25 to 30 flights, he said.
Sure. First they "aimed at" 10, then 15, then 20, and now 25+. This is an oversimplification for the general non-space news media.  The reality is almost certainly that they do an inspection of each booster each time it returns. There is no magic number N for which N is acceptable and N+1 is not. The question here is what they will do if a booster is getting "too old" for customer payloads: expend it, retire it, of just keep flying Starlinks?
There may be a magic number in practice.  Previously SpaceX said one of their metrics was that to qualify for the Nth flight, each (non-replaceable) component must survive 4N cycles without failure.  So if this is their metric (perhaps used to reassure customers), then if they tested each part to 80 cycles, then 20 flights is OK but 21 is not.   Of course they can raise this limit with more testing, which might be what they are doing when the "aim" for more reflights.

There are similar magic numbers, such as safety factors and cycle limits in all sorts of engineering.   You need to retire or replace something after a certain number of cycles or hours, not because it would fail, but because it falls out of certification limits that were previously agreed on.
If they make such a commitment to an external customer, then they should keep it. However, they are their own biggest customer by a large margin. They may choose to Starlinks until the booster fails or until it shows actual signs of wear-out. I just hope that if they do this they make a huge deal of this "deliberate testing to failure" they are doing to demonstrate the reliability of younger boosters. Then, when one inevitably fails they can publicly celebrate the result. Given this "extended testing", some external customers may elect to use older boosters if SpaceX chooses to discount them.

They will still need to keep the production line open for awhile or stockpile some new boosters because of commitments to NASA for CCP, which almost certainly has a low upper bound for reuse. There may be a few other Falcon missions in the long tail after Starship is handling all other missions.

Offline Zed_Noir

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Re: Limits on F9 reuse and progress towards it
« Reply #223 on: 12/09/2023 02:54 pm »
<snip>
They will still need to keep the production line open for awhile or stockpile some new boosters because of commitments to NASA for CCP, which almost certainly has a low upper bound for reuse. There may be a few other Falcon missions in the long tail after Starship is handling all other missions.
Will point out that the production lines for Falcon 9 booster and upper stage is one line with the same tooling use for both the booster and the upper stage. As long as SpaceX is flying the Falcon 9, the production line is available to produce new boosters.

Offline dondar

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Re: Limits on F9 reuse and progress towards it
« Reply #224 on: 12/17/2023 11:33 am »

If they decide to use a hard stop (e.g., 25 flights) instead of a case-by-case analysis, they might choose to expend. Have we ever seen an expended Starlink launch? How many extra satellites do they get? I assume they may be willing to risk Starlinks but not customer payloads.

This article says SpaceX are aiming for 25+ flights per booster.

Quote
Next year, SpaceX will re-evaluate and conduct analysis with an eye toward certifying the first-stage boosters for 25 to 30 flights, he said.
Sure. First they "aimed at" 10, then 15, then 20, and now 25+. This is an oversimplification for the general non-space news media.  The reality is almost certainly that they do an inspection of each booster each time it returns. There is no magic number N for which N is acceptable and N+1 is not. The question here is what they will do if a booster is getting "too old" for customer payloads: expend it, retire it, of just keep flying Starlinks?
I can not imagine any possible reason for enabling launch with a rocket not passing reliability control. They have obligations to FAA, they have obligation to Air Force (launch pad safety), they have simple bona fide financial concerns. Failure costs will  be 10x of any possible gain from  anything reused extra time. They have enough stands to test reliability in controllable manner.

Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #225 on: 12/17/2023 02:03 pm »

If they decide to use a hard stop (e.g., 25 flights) instead of a case-by-case analysis, they might choose to expend. Have we ever seen an expended Starlink launch? How many extra satellites do they get? I assume they may be willing to risk Starlinks but not customer payloads.

This article says SpaceX are aiming for 25+ flights per booster.

Quote
Next year, SpaceX will re-evaluate and conduct analysis with an eye toward certifying the first-stage boosters for 25 to 30 flights, he said.
Sure. First they "aimed at" 10, then 15, then 20, and now 25+. This is an oversimplification for the general non-space news media.  The reality is almost certainly that they do an inspection of each booster each time it returns. There is no magic number N for which N is acceptable and N+1 is not. The question here is what they will do if a booster is getting "too old" for customer payloads: expend it, retire it, of just keep flying Starlinks?
I can not imagine any possible reason for enabling launch with a rocket not passing reliability control. They have obligations to FAA, they have obligation to Air Force (launch pad safety), they have simple bona fide financial concerns. Failure costs will  be 10x of any possible gain from  anything reused extra time. They have enough stands to test reliability in controllable manner.
Agreed. They will inspect every booster, every time. However, ticking off a magic number on a piece of paper is not an inspection, and it does not contribute to reliability. Continuing to use old boosters until one of them finally does fail actually does contribute to reliability of the fleet as a whole, because it provides data and insight into the failure modes of old boosters. Doing this using only their own payloads internalizes the insurance risks.

Offline Robotbeat

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Re: Limits on F9 reuse and progress towards it
« Reply #226 on: 12/17/2023 02:51 pm »
Dan, I think this isnít really true in practice.

They can find a lot of this information with post-flight inspections, looking for near-misses without getting to the point of an actual launch failure.

And if you decide to intentionally push to launch failure with an actual payload (even if itís yours), your ability to argue for high reliability will be limited by the fact that you DID, in fact, have a launch failure.

Nah, you kind of have to do the hard push for searching for the failure envelope early in the flight history because by now, folks expect Falcon 9 to work every time.

Get the failures out of the way early because you wonít be given slack later on.
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Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #227 on: 12/17/2023 03:31 pm »
Dan, I think this isnít really true in practice.

They can find a lot of this information with post-flight inspections, looking for near-misses without getting to the point of an actual launch failure.

And if you decide to intentionally push to launch failure with an actual payload (even if itís yours), your ability to argue for high reliability will be limited by the fact that you DID, in fact, have a launch failure.

Nah, you kind of have to do the hard push for searching for the failure envelope early in the flight history because by now, folks expect Falcon 9 to work every time.

Get the failures out of the way early because you wonít be given slack later on.
Your reasoning assumes that failure rate goes up with age and some particular numerical cutoff will mitigate this. We don't know this. Is a carefully-inspected 5-flight booster more reliable than a carefully-inspected 50-flight booster? I don't know and neither does anyone else. In most types of system a first-flight booster would be less reliable than a pre-flown booster. If this is true for F9, then removing an old booster will increase the risk, because it must be replaced by a new booster.

Offline steveleach

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Re: Limits on F9 reuse and progress towards it
« Reply #228 on: 12/17/2023 03:46 pm »
Dan, I think this isnít really true in practice.

They can find a lot of this information with post-flight inspections, looking for near-misses without getting to the point of an actual launch failure.

And if you decide to intentionally push to launch failure with an actual payload (even if itís yours), your ability to argue for high reliability will be limited by the fact that you DID, in fact, have a launch failure.

Nah, you kind of have to do the hard push for searching for the failure envelope early in the flight history because by now, folks expect Falcon 9 to work every time.

Get the failures out of the way early because you wonít be given slack later on.
Your reasoning assumes that failure rate goes up with age and some particular numerical cutoff will mitigate this. We don't know this. Is a carefully-inspected 5-flight booster more reliable than a carefully-inspected 50-flight booster? I don't know and neither does anyone else. In most types of system a first-flight booster would be less reliable than a pre-flown booster. If this is true for F9, then removing an old booster will increase the risk, because it must be replaced by a new booster.
Presumably there will be a rough limit on the maximum number of flights for some of the core structural components that can't really be replaced or repaired while calling it the same booster. They will likely have done a lot of work around block 5 to make that number as high as possible, which for those simple structural elements could be dozens or hundreds.

Then on top of that you've got the lifetime of all the components that can be replaced while calling it the same booster. Those lifetimes can be much shorter, even down to a single flight.

As for post-flight inspections, you use those when it is more cost effective than using instrumentation, but SpaceX will be pushing the state of the art on that instrumentation as well.

Offline LouScheffer

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Re: Limits on F9 reuse and progress towards it
« Reply #229 on: 12/19/2023 12:39 pm »
Continuing to use old boosters until one of them finally does fail actually does contribute to reliability of the fleet as a whole, because it provides data and insight into the failure modes of old boosters.
This is not clear to me at all.  SpaceX likely already knows of the limiting factor - for almost all pressurized aerospace structures, it's the number of pressurization cycles.  For the SLS tank, it's 22 cycles, for a plane it's about 50,000 cycles, and so on.   At this point, due to metal fatigue, some structure that is uneconomical to replace (the SLS tank, the fuselage of a plane, presumably the F9 tank) will fail if you keep cycling, so this sets the number of allowable flights.

But this can be tested on the ground, without the consequence of failed flights.  This is exactly what plane manufacturers do to set the ultimate life of a plane.  To my knowledge, they never fly a plane until it fails (even a drone model which would not risk the pilots).   That's because they already know the life-limiting failure mode.  And they know they are not being wildly conservative, both from ground tests and because some high-cycle airframes (like the Hawaii example) have exploded when an inspection missed a developing crack.  So they would learn very little from an in-service test to destruction.

In theory, by flying to failure you could find some new failure mode that is unexpected.  But, as most likely will happen, it fails in the usual way (metal fatigue) you've learned nothing in a very expensive way (you lose a second stage, your reputation, and payloads).  It's an engineering judgment as to the odds of an unexpected failure mode, but it looks like SpaceX (and most aerospace in general) has judged that flying to destruction is not worth the risk.

Offline spacenut

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Re: Limits on F9 reuse and progress towards it
« Reply #230 on: 12/19/2023 01:18 pm »
I think SpaceX will shoot for 25 launches, then thoroughly inspect the rocket, even using x-rays to find micro cracks.  Engines can be inspected and either parts replaced or complete engine replacement.  The tankage will eventually reach a point where fractures cannot be repaired and this the rocket retired, or expended on a final launch. 
« Last Edit: 12/19/2023 01:18 pm by spacenut »

Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #231 on: 12/19/2023 01:48 pm »
Continuing to use old boosters until one of them finally does fail actually does contribute to reliability of the fleet as a whole, because it provides data and insight into the failure modes of old boosters.
This is not clear to me at all.  SpaceX likely already knows of the limiting factor - for almost all pressurized aerospace structures, it's the number of pressurization cycles.  For the SLS tank, it's 22 cycles, for a plane it's about 50,000 cycles, and so on.   At this point, due to metal fatigue, some structure that is uneconomical to replace (the SLS tank, the fuselage of a plane, presumably the F9 tank) will fail if you keep cycling, so this sets the number of allowable flights.

But this can be tested on the ground, without the consequence of failed flights.  This is exactly what plane manufacturers do to set the ultimate life of a plane.  To my knowledge, they never fly a plane until it fails (even a drone model which would not risk the pilots).   That's because they already know the life-limiting failure mode.  And they know they are not being wildly conservative, both from ground tests and because some high-cycle airframes (like the Hawaii example) have exploded when an inspection missed a developing crack.  So they would learn very little from an in-service test to destruction.

In theory, by flying to failure you could find some new failure mode that is unexpected.  But, as most likely will happen, it fails in the usual way (metal fatigue) you've learned nothing in a very expensive way (you lose a second stage, your reputation, and payloads).  It's an engineering judgment as to the odds of an unexpected failure mode, but it looks like SpaceX (and most aerospace in general) has judged that flying to destruction is not worth the risk.
Thanks, this makes sense. When there is a real, well-characterized failure mode with a well-known actual relationship to a usage parameter, then of course they will stop using the booster before the number is reached. flight testing to destruction is not useful unless the actual life-limiting failure mode is unknown. For F9 in particular, do we have reason to think their current limit is the number of pressure cycles?

Offline whitelancer64

Re: Limits on F9 reuse and progress towards it
« Reply #232 on: 12/19/2023 02:32 pm »
For F9 in particular, do we have reason to think their current limit is the number of pressure cycles?

No, the current number is just the current target for flight certification. The limit for metal fatigue is likely higher.
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Offline LouScheffer

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Re: Limits on F9 reuse and progress towards it
« Reply #233 on: 12/19/2023 03:52 pm »
For F9 in particular, do we have reason to think their current limit is the number of pressure cycles?
No, the current number is just the current target for flight certification. The limit for metal fatigue is likely higher.
Way back in 2009, SpaceX stated the tank had been subjected to 150 pressurization cycles.

However it's not clear these were cryo cycles (cool down and then let warm up), or just pressure cycles.  It presumably did not include re-entry stresses, which were unknown at that time.  Also presumably changing the engine layout from square to circular changed the stresses.  So it's unclear how that old analysis might apply to current tanks.

A "cycle" might now include a static fire, then launch, then boostback, entry, and landing burns, though not all of these are required on every launch.    I would imagine (but do not know) that SpaceX is running hardware through these cycles to set maximum flight numbers.  Because they keep tweaking the number of flights up, I now agree they may not have reached the fatigue limit yet (contrary to my suspicion above).  Conversely, maybe they did reach the limit on early boosters, have made changes in the high fatigue areas, and are now searching for the limits of the latest boosters.

Offline R.Simko

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Re: Limits on F9 reuse and progress towards it
« Reply #234 on: 12/19/2023 05:28 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.

Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #235 on: 12/19/2023 05:40 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.
Sure, but this assumes that the number of pressure cycles is the actual reason that an old booster will fail. This may very well be the case and real rocket engineers may be correct to assume it.

If Starship becomes operations soon enough, F9 booster reuse becomes almost moot.

Offline R.Simko

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Re: Limits on F9 reuse and progress towards it
« Reply #236 on: 12/19/2023 05:55 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.
Sure, but this assumes that the number of pressure cycles is the actual reason that an old booster will fail. This may very well be the case and real rocket engineers may be correct to assume it.

If Starship becomes operations soon enough, F9 booster reuse becomes almost moot.

Additional pressure testing of an old booster certainly won't guarantee more flights, but more importantly, it may show that Spacex is to close to the edge of reuse.

I can't wait until Starship is flying regularly, but I think F9 is going to be flying for a long time yet. :)

Offline steveleach

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Re: Limits on F9 reuse and progress towards it
« Reply #237 on: 12/19/2023 06:09 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.
Sure, but this assumes that the number of pressure cycles is the actual reason that an old booster will fail. This may very well be the case and real rocket engineers may be correct to assume it.

If Starship becomes operations soon enough, F9 booster reuse becomes almost moot.
I suspect the assumption is based on the fact that most other parts of the booster can be replaced as they start to show signs of wear, but its not really practical to replace the main tanks etc.

Offline SweetWater

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Re: Limits on F9 reuse and progress towards it
« Reply #238 on: 12/19/2023 07:00 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.
Sure, but this assumes that the number of pressure cycles is the actual reason that an old booster will fail. This may very well be the case and real rocket engineers may be correct to assume it.

If Starship becomes operations soon enough, F9 booster reuse becomes almost moot.

I for one will be shocked if Falcon 9 is completely retired before the end of the decade.

I am confident that SpaceX will eventually get Starship operations worked out; however, at the moment, SpaceX is limited to - IIRC - 5 test flights out of Boca Chica a year. While they are building out Starship facilities at the Cape, I can't believe that NASA will permit them to launch, must less land, at the Cape without several (3+) successful landings from the Boca Chica site, and nor should they - the facilities (SpaceX and otherwise) at the Cape are far too important to risk damaging in a mishap with an experimental vehicle.

Even then, and even with a vehicle intended for 'full and rapid reuse' it will take them time to really work out operations. It took SpaceX ~7 years from Falcon 9 first flight to first re-use. It will also be a very long time before NASA puts astronauts on a vehicle without a robust launch escape system.

Offline DanClemmensen

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Re: Limits on F9 reuse and progress towards it
« Reply #239 on: 12/19/2023 07:35 pm »
At some point, Spacex will decide that they are at the limit of the number of times that they can safely fly a booster.  It is my suggestion, they then take that booster that has been flown the most times and then put it through a series of pressure tests until it fails.  They might find, that the booster has many more flights left in it, that it can safely fly.  The additional pressure testing might also reveal that they were actually too close to the edge and that with just a very few additional pressurization's, it will fail.

This additional ground testing to failure of a well used booster, could also point out the weakest point of the booster, without Spacex risking a payload or their reputation.
Sure, but this assumes that the number of pressure cycles is the actual reason that an old booster will fail. This may very well be the case and real rocket engineers may be correct to assume it.

If Starship becomes operations soon enough, F9 booster reuse becomes almost moot.

I for one will be shocked if Falcon 9 is completely retired before the end of the decade.

I am confident that SpaceX will eventually get Starship operations worked out; however, at the moment, SpaceX is limited to - IIRC - 5 test flights out of Boca Chica a year. While they are building out Starship facilities at the Cape, I can't believe that NASA will permit them to launch, must less land, at the Cape without several (3+) successful landings from the Boca Chica site, and nor should they - the facilities (SpaceX and otherwise) at the Cape are far too important to risk damaging in a mishap with an experimental vehicle.

Even then, and even with a vehicle intended for 'full and rapid reuse' it will take them time to really work out operations. It took SpaceX ~7 years from Falcon 9 first flight to first re-use. It will also be a very long time before NASA puts astronauts on a vehicle without a robust launch escape system.
F9 will continue to fly CCP missions until ISS is retired, presumably in 2030.
5/yr is not a limit. It was the nominal value use in the PEA.
SpaceX does not need perfection for Starship to start launching Starlinks. Starlink launches comprise more than 60% of F9 launches.

BUT, yes I will be shocked if the tenth Starship flight occurs before 2027.  So, four years of >100 F9 flights/yr.  Call it 500 more F9 flights, average 25 flights/booster, for a total of 20 new boosters.

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