Author Topic: SpaceX Raptor engine - General Thread 4  (Read 1202400 times)

Offline Semmel

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Re: SpaceX Raptor engine - General Thread 4
« Reply #40 on: 04/16/2021 07:40 am »
Raptor SN56 was loaded into the Raptor van and taken away.

The top image from Mary shows one of the raptors and I wonder about the welds in the top right part, the big methane inlet pipe. These welds look very low fidelity, like someone filled the gap between two pipes with metal paste and melted it. I guess thats normal since they are the professionals and I merely want to understand how things work. Would these welds be cleaned on the inside? Or are there perturbances, small cavities and bulges like on the outside? Im just wondering how stuff like this is done. Also, wondering, would it be better to bend the pipe instead of piecing it together with angled sections?

Offline RobLynn

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Re: SpaceX Raptor engine - General Thread 4
« Reply #41 on: 04/16/2021 10:36 am »
I've been thinking about longevity of a Raptor engine.  I would expect that, with time, they would settle into a maintenance schedule of some sort for an engine (similar to the inspection/maintenance schedule that an airline adheres to for a jet aircraft).

I've sort of been assuming that a start/stop cycle probably places more stress on an engine than a period of steady state thrust (similar to how they weigh takeoff/landings more heavily that flight hours when determining maintenance schedules for a jet).  Can someone with more knowledge than I confirm that is the case (or explain why it isn't)?

Transient issues can include:
-Hydrodynamic bearings that work fine at operational speeds but sometimes suffer from rubbing contact at lower speeds or at resonance frequencies.
-Fluid transients that lead to reactions and hot spots occurring in places that they don't during steady state operation.  Potentially a killer when one of the fluids is hot high pressure oxygen.
-Thermal transients as cold things heat up or cool down through their thickness leading to big temperature gradients that induce large thermal strains.  These strains can exceed the elastic limits of the materials (particularly at high temperatures where those elastic strain limits reduce) and lead to dimensional changes and low cycle fatigue (an everyday example of this is gradual belling out of initially flat fry-pans and saucepans with repeated thermal cycles ).  In large power stations this problem leads to very slow gentle ramps in operating powers that can take 10's of minutes to preserve life.  In rocket engines such mitigations are not possible and you get spool up from zero to max power in a second or two with construction from horribly low thermal conductivity materials like titanium, inconels and stainless steels that lead to large thermal gradients, exacerbating the issue.  Thick flanges and anywhere else you get big thickness of material are a particular weakness in this regard.
-Combustion chamber and nozzle wall heat fluxes.  Same thermal strain issue - huge heat fluxes of 10's of MW/mē (maybe as high as 160MW/mē in SSME according to linked paper) lead to large temperature gradients through the walls that cause elastic and then plastic strain due to differential expansion of different layers if the heat fluxes are to high and the walls too thick.  Exacerbated by material weakening and reduction in elastic limits at higher operating temperatures.  This issue - the heat fluxes encountered are linked strongly to chamber pressure.  https://www.sciencedirect.com/science/article/pii/S1000936117301024

It is not clear to me that these thermal fatigue problems have any viable solutions - engines may be inherently limited to quite low numbers of thermal cycles, and it could be that the only path to low cost is making them in large numbers to make them cheap to regularly replace.
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Offline Skyway

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Re: SpaceX Raptor engine - General Thread 4
« Reply #42 on: 04/16/2021 10:48 am »
Looking at the photos of the 3 engines for the SN15, we can see the great advance already achieved. Began to look more like a product, and less like a prototype. And that's not an easy thing.
Everything is fail-proof until it fails.

Offline andygood

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Re: SpaceX Raptor engine - General Thread 4
« Reply #43 on: 04/16/2021 12:00 pm »
Looking at the photos of the 3 engines for the SN15, we can see the great advance already achieved. Began to look more like a product, and less like a prototype. And that's not an easy thing.

Yeah, the 'plumbing' looks much tighter. Everything seems to be within the footprint of the nozzle exit.

Offline Izan Ramos

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Re: SpaceX Raptor engine - General Thread 4
« Reply #44 on: 04/16/2021 01:26 pm »
I made a thread where I show changes I've seen on the new raptor variants hopefully this has some interesting info
https://twitter.com/IzanRamos2002/status/1383045888986189826
« Last Edit: 04/16/2021 04:32 pm by Izan Ramos »

Online edzieba

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Re: SpaceX Raptor engine - General Thread 4
« Reply #45 on: 04/16/2021 02:27 pm »
As well as the small changes, the newest variant has all new Methane turbopump and preburner castings.

Offline Prae_

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Re: SpaceX Raptor engine - General Thread 4
« Reply #46 on: 04/16/2021 05:04 pm »
I've been thinking about longevity of a Raptor engine.  I would expect that, with time, they would settle into a maintenance schedule of some sort for an engine (similar to the inspection/maintenance schedule that an airline adheres to for a jet aircraft).

I've sort of been assuming that a start/stop cycle probably places more stress on an engine than a period of steady state thrust (similar to how they weigh takeoff/landings more heavily that flight hours when determining maintenance schedules for a jet).  Can someone with more knowledge than I confirm that is the case (or explain why it isn't)?

Transient issues can include:
-Hydrodynamic bearings that work fine at operational speeds but sometimes suffer from rubbing contact at lower speeds or at resonance frequencies.
-Fluid transients that lead to reactions and hot spots occurring in places that they don't during steady state operation.  Potentially a killer when one of the fluids is hot high pressure oxygen.
-Thermal transients as cold things heat up or cool down through their thickness leading to big temperature gradients that induce large thermal strains.  These strains can exceed the elastic limits of the materials (particularly at high temperatures where those elastic strain limits reduce) and lead to dimensional changes and low cycle fatigue (an everyday example of this is gradual belling out of initially flat fry-pans and saucepans with repeated thermal cycles ).  In large power stations this problem leads to very slow gentle ramps in operating powers that can take 10's of minutes to preserve life.  In rocket engines such mitigations are not possible and you get spool up from zero to max power in a second or two with construction from horribly low thermal conductivity materials like titanium, inconels and stainless steels that lead to large thermal gradients, exacerbating the issue.  Thick flanges and anywhere else you get big thickness of material are a particular weakness in this regard.
-Combustion chamber and nozzle wall heat fluxes.  Same thermal strain issue - huge heat fluxes of 10's of MW/mē (maybe as high as 160MW/mē in SSME according to linked paper) lead to large temperature gradients through the walls that cause elastic and then plastic strain due to differential expansion of different layers if the heat fluxes are to high and the walls too thick.  Exacerbated by material weakening and reduction in elastic limits at higher operating temperatures.  This issue - the heat fluxes encountered are linked strongly to chamber pressure.  https://www.sciencedirect.com/science/article/pii/S1000936117301024

It is not clear to me that these thermal fatigue problems have any viable solutions - engines may be inherently limited to quite low numbers of thermal cycles, and it could be that the only path to low cost is making them in large numbers to make them cheap to regularly replace.

Thermal Flux is obviously most extreme in the combustion chamber itself, and solutions to reducing this have been studied. Given Raptor is using coaxial injectors, SpaceX can utilize a larger coaxial injector with a "Thicker" post of hot GOX to reduce the overall heat flux experience inside the combustion chamber, this does however come at a cost  of efficiency that has to be made up somewhere else.

"The results of the simulation are consistent with existing experimental data in the chamber pressure, combustion stability, as well as wall heat flux. This paper's results have shown that the larger liquid oxygen post thickness and swirl coaxial injector can significantly reduce the heat flux while influence of the injector number is relatively small. Furthermore, the decrement of the liquid oxygen post thickness can improve the combustion efficiency."

Effect of coaxial injector parameters on LOX/methane engines: A numerical analysis
Chinese Journal of Aeronautics
Volume 31, Issue 8, August 2018, Pages 1667-1678

Main point being, there are optimizations to be made within Raptor to reduce these extreme effects, many being direct advantages of FFSC, but they can be quite exotic and counter intuitive. That being said, do I believe a raptor will ever fly "1000 times", no. I do think they can make them fly a few dozen times.
« Last Edit: 04/16/2021 05:07 pm by Prae_ »

Online Alberto-Girardi

Re: SpaceX Raptor engine - General Thread 4
« Reply #47 on: 04/16/2021 05:24 pm »
Yeah, I think there is still a lot of flux in the design. They won't ramp up production until they get a mature design. Higher production means automation, and there no sense putting all that automation effort and cost in until you get to a reasonably fixed design. It's possible that they are doing batches of iterations right now, testing the entire batch to get good handle on data and performance, then tweaking and making another batch. I'm sure there are certain components that are reasonably settled on, but Raptor output really won't ramp up until all or most components are mature.

I agree, since they plan the full stack for NET July (if this is true this summer will be simply beautiful) they have enough time to have 28 engines, only making production a bit faster, which IMO is happening now. Maybe with no payload they need less engine on SH, but I don't think a few engine will be the proble, since the production is probably in batches.
I would just like to point out that according to the information Chris received, the target for an orbital flight is by July 1st, not NET July 1st.
Yes, you are right, but knowing spacex they are a little bit too optimistic with their schedule.
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Offline ETurner

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Re: SpaceX Raptor engine - General Thread 4
« Reply #48 on: 04/16/2021 05:40 pm »
[...] In rocket engines such mitigations are not possible and you get spool up from zero to max power in a second or two with construction from horribly low thermal conductivity materials like titanium, inconels and stainless steels that lead to large thermal gradients, exacerbating the issue.  Thick flanges and anywhere else you get big thickness of material are a particular weakness in this regard.
-Combustion chamber and nozzle wall heat fluxes.  Same thermal strain issue - huge heat fluxes of 10's of MW/mē (maybe as high as 160MW/mē in SSME according to linked paper) lead to large temperature gradients through the walls that cause elastic and then plastic strain due to differential expansion of different layers if the heat fluxes are to high and the walls too thick.  Exacerbated by material weakening and reduction in elastic limits at higher operating temperatures.  This issue - the heat fluxes encountered are linked strongly to chamber pressure.  https://www.sciencedirect.com/science/article/pii/S1000936117301024

It is not clear to me that these thermal fatigue problems have any viable solutions - engines may be inherently limited to quite low numbers of thermal cycles, and it could be that the only path to low cost is making them in large numbers to make them cheap to regularly replace.
From what you say (thanks, very informative!) it seems that lifetimes may be shorter for some of the big, simple, solid parts (the combustion chamber, etc.) than for some of the complex subsystems with moving parts (turbopumps, etc.). If so, then what gets replaced and what gets kept during refurbishment could be a bit counterintuitive.

Offline oldAtlas_Eguy

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Re: SpaceX Raptor engine - General Thread 4
« Reply #49 on: 04/16/2021 09:02 pm »
[...] In rocket engines such mitigations are not possible and you get spool up from zero to max power in a second or two with construction from horribly low thermal conductivity materials like titanium, inconels and stainless steels that lead to large thermal gradients, exacerbating the issue.  Thick flanges and anywhere else you get big thickness of material are a particular weakness in this regard.
-Combustion chamber and nozzle wall heat fluxes.  Same thermal strain issue - huge heat fluxes of 10's of MW/mē (maybe as high as 160MW/mē in SSME according to linked paper) lead to large temperature gradients through the walls that cause elastic and then plastic strain due to differential expansion of different layers if the heat fluxes are to high and the walls too thick.  Exacerbated by material weakening and reduction in elastic limits at higher operating temperatures.  This issue - the heat fluxes encountered are linked strongly to chamber pressure.  https://www.sciencedirect.com/science/article/pii/S1000936117301024

It is not clear to me that these thermal fatigue problems have any viable solutions - engines may be inherently limited to quite low numbers of thermal cycles, and it could be that the only path to low cost is making them in large numbers to make them cheap to regularly replace.
From what you say (thanks, very informative!) it seems that lifetimes may be shorter for some of the big, simple, solid parts (the combustion chamber, etc.) than for some of the complex subsystems with moving parts (turbopumps, etc.). If so, then what gets replaced and what gets kept during refurbishment could be a bit counterintuitive.
To give some perspective on some of the outward observable properties an engine has to meet to achieve a nominal 10 flight without refurbishment (the M1DFT in it's latest version). It must be able to handle 75 or more start stops. 15 or more full duration burns. 40 or more short duration of a couple of seconds to less than 15 seconds. With a bunch of start abort stop <than a couple of seconds events making up the remainder between the start/stop events and the short and long burn time events of ~20 such events.

The Merlin's have a soot problem due to RP1. So a Raptor should be able to easily beat this set of capabilities. Meaning the in the early versions of Raptor the start/stop event may be the primary limiter to number of flights prior to refurbishment as being discussed but is likely to start with a good for ~10 flights point and go quickly higher from there.

Offline RobLynn

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Re: SpaceX Raptor engine - General Thread 4
« Reply #50 on: 04/17/2021 12:43 am »
The fundamental limit on engine life is likely to be thermal cycles (low cycle thermal fatigue), so static fires and engine relights for landing will be expensive in reduced life - likely costs on order of $10k+ per ignition.  Not massive in the context of several $millions per flight, but probably worth reducing in every way possible over long term.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline vaporcobra

Re: SpaceX Raptor engine - General Thread 4
« Reply #51 on: 04/17/2021 07:01 am »
Brief history of Starship development from SpaceX's HLS award announcement:

Quote
SpaceX has manufactured and tested more than 60 of Starship’s Raptor engines, accumulating nearly 30,000 seconds of total test time over 567 engine starts, including on multiple Starship static fires and flight tests.

https://www.spacex.com/updates/starship-moon-announcement/index.html

New baseline for cumulative Raptor testing since February 2019! From an official history in SpaceX's SN8 launch announcement, we know that ~16,000 seconds and ~330 engine starts were completed in 2020 alone.

https://twitter.com/13ericralph31/status/1336040590526451713
« Last Edit: 04/17/2021 07:03 am by vaporcobra »

Offline FutureSpaceTourist

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Re: SpaceX Raptor engine - General Thread 4
« Reply #52 on: 04/17/2021 04:04 pm »
https://twitter.com/artzius/status/1383450305560338435

Quote
Raptor Engines 1-50 as seen in Boca Chica, Texas.

As of, April 16, 2021

Edit to add:

https://twitter.com/artzius/status/1383465282711134221

Quote
Raptor Engines 51-100 as seen in Boca Chica, Texas.

As of, April 16, 2021
« Last Edit: 04/17/2021 05:01 pm by FutureSpaceTourist »

Online Alberto-Girardi

Re: SpaceX Raptor engine - General Thread 4
« Reply #53 on: 04/18/2021 07:53 am »
I'm waiting to see an official look up the skirt of Lunar Starship. To date, every depiction I've seen shows 6 engines: 3 SL Raptors and 3 Vac Raptors. Except that Lunar Starship is a one way vehicle. Once it leaves earth, it will not be coming back. And because Lunar Starship will only ever operate in a vacuum - like the 2nd stage of F9, only vacuum engines are needed after being boosted to altitude - any sea level engines, whether Raptors or otherwise, would be out of their element and quite literally useless. So if there's a reason to send engines that can't function properly in a vacuum all the way to the moon, I'd really like to know what it is.

Sea level engines aren't "useless", merely slightly less efficient.  Isp of Vacuum Raptor is 375, Isp of SL raptor in a vacuum is 355, the difference of 20 ISP that Elon was referring to as "a lot of hard work".   Try running this through the rocket equation, you'll see that the 20 Isp delta helps with one less fueling launch under many scenarios, that's really all it saves.

Furthermore, Vacuum raptors don't gimbal, and there probably isn't enough room for their bells at the center.  6 raptors are also needed to boost anything with payload for the first 60 seconds of second stage burn, because thrust/weight ratio is less than one with 6 and less than half with only 3, and thus gravity losses will start to become significant without all 6 Raptors.

I doubt SpaceX is going to redesign something as finicky as the puck for Lunar Starship to fit 6 vacuum raptors.  It'll be bog standard 3+3.

Is possible to make Raptors Vac gimbalable? The problem preventing it is the too big nozzle?
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Offline enbandi

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Re: SpaceX Raptor engine - General Thread 4
« Reply #54 on: 04/18/2021 01:09 pm »
....

Is possible to make Raptors Vac gimbalable? The problem preventing it is the too big nozzle?

As I remember, the Vac nozzle will be welded to the outer skirt, it was mentioned somwhere (guess that means whatever attachment method not exactly welding). That makes gimballing a bit difficult.

But aside the attachment, there arent enogh space there to gimball outward.

Online Alberto-Girardi

Re: SpaceX Raptor engine - General Thread 4
« Reply #55 on: 04/18/2021 02:45 pm »
....

Is possible to make Raptors Vac gimbalable? The problem preventing it is the too big nozzle?

As I remember, the Vac nozzle will be welded to the outer skirt, it was mentioned somwhere (guess that means whatever attachment method not exactly welding). That makes gimballing a bit difficult.

But aside the attachment, there arent enogh space there to gimball outward.

So gimbaling outward is impossible, but is impossible to gimbal it inwards too? Why they want to weld it to the skirt? What will be the advantage?
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Offline Rekt1971

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Re: SpaceX Raptor engine - General Thread 4
« Reply #56 on: 04/18/2021 02:49 pm »
....

Is possible to make Raptors Vac gimbalable? The problem preventing it is the too big nozzle?

As I remember, the Vac nozzle will be welded to the outer skirt, it was mentioned somwhere (guess that means whatever attachment method not exactly welding). That makes gimballing a bit difficult.

But aside the attachment, there arent enogh space there to gimball outward.

So gimbaling outward is impossible, but is impossible to gimbal it inwards too? Why they want to weld it to the skirt? What will be the advantage?

I don't think that "welding" is the right word, they will be fixed to the skirt. Probably to allow maximum width of the nozzle and reduce vibration (my speculation).
« Last Edit: 04/18/2021 03:08 pm by Rekt1971 »

Offline cscott

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Re: SpaceX Raptor engine - General Thread 4
« Reply #57 on: 04/18/2021 06:44 pm »
Speculation: stronger structure and thinner bell if the outer engines are attached to each other and the skirt. If not attached, vibrations would tend to make adjacent bells contact each other, which could lead to damage as vac bells are typically extremely thin.

Offline rsdavis9

Re: SpaceX Raptor engine - General Thread 4
« Reply #58 on: 04/18/2021 07:57 pm »

So gimbaling outward is impossible, but is impossible to gimbal it inwards too? Why they want to weld it to the skirt? What will be the advantage?


If you watch the SSME(space shuttle) engines when they start up they vibrate in and out in x-y directions. This is because they are over expanded operating at sea level. This is the reason VAC engines that are very over expanded at SL aren't fired at sea level. This is caused by flow separation. This is caused by insufficient exit pressure to move the ambient atmosphere out of the way. So the ambient atmosphere oscillates in and out of the bell.

Speculation:
So the idea is if you attach the bell at one point you can damp out the flexing from this flow separation.
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Offline _MECO

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Re: SpaceX Raptor engine - General Thread 4
« Reply #59 on: 04/18/2021 08:41 pm »

So gimbaling outward is impossible, but is impossible to gimbal it inwards too? Why they want to weld it to the skirt? What will be the advantage?


If you watch the SSME(space shuttle) engines when they start up they vibrate in and out in x-y directions. This is because they are over expanded operating at sea level. This is the reason VAC engines that are very over expanded at SL aren't fired at sea level. This is caused by flow separation. This is caused by insufficient exit pressure to move the ambient atmosphere out of the way. So the ambient atmosphere oscillates in and out of the bell.

Speculation:
So the idea is if you attach the bell at one point you can damp out the flexing from this flow separation.
I think this is the sort of thing where we're all hopelessly out of our league in when it comes to speculation. SpaceX is supposed to be king when it comes to in-house developed simulation and effects modeling software. As in, that's one of the major technical aspects that allowed them to do away with a lot of hardware testing and which gave them such an agile lead over other aerospace companies.

I suspect that they have vastly sophisticated numerical methods for simulating the way a Vacuum Raptor jiggles in operation. I can't speak for any of you guys but I'm going to basically just sit there and stare mouth agape as they continue to execute propulsion magic. If they decide to weld Vac Raptor to the skirt of the vehicle I'm going to excitedly nod my head and agree.

« Last Edit: 04/18/2021 08:42 pm by _MECO »

 

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