Scivillage.com Casual Discussion Science Forum

(Jun 15, 2019 06:01 PM)Yazata Wrote: [ -> ]While they are waiting for the engine, SpaceX has been putting up stylish new signs on their various Boca Chica operations. (Sorry the photos are excessively cropped. But you get the idea.) They certainly give a good idea what SpaceX intends to do there.

Where the Hopper is:



At the new concrete pad next to the Hopper:



Where the second vehicle is under construction:



At their central group of buildings:



At the parabolic dishes:

If the headliner is tardy about showing up, hope the arty decor will distract the crowd for awhile.

(Jun 16, 2019 06:28 AM)C C Wrote: [ -> ]If the headliner is tardy about showing up, hope the arty decor will distract the crowd for awhile.

Not for long. We're pretty demanding.

Talk is going around (whose source may be Elon himself) saying that SN5 is currently being "repaired" and that's why the engine is slow making its appearance. Assuming it's true, it does seem to verify the suspicion that there's some problem with the engine.

https://twitter.com/elonmusk/status/1140414413322670080

Unsure what "repaired" means. Did something break during testing? Or is this a reference to ongoing modifications intended to improve unsatisfactory engine performance?

Changes (again) in the road closures.

Yesterday they seem to have been pushed back to June 24, 25, and 26, plus June 27, 28 and 29

Now today, they add a cryptic one for tomorrow, June 19.  

https://twitter.com/SpacePadreIsle/statu...1896465408

They obviously aren't going to fire up the engine if hasn't even arrived by today. So my guess is that they plan to do something with the methane fuel that might conceivably involve an explosion hazard. Tanking tests with liquid methane instead of liquid nitrogen?

Still no engine. But road is closed, flare stack is lit and vapor is visible around the Starhopper steampunk spaceship.

https://twitter.com/BocaChicaGal/status/...7140490241

https://twitter.com/LabPadre/status/1141657421632462850

Speculation is that it's pressurization tests of the fuel system, involving lots and lots real liquid methane (LNG basically). Hence the explosion danger and hence the exclusion zone.

https://www.youtube.com/watch?v=ojwoS874BgI

https://www.youtube.com/watch?v=fvK2_zcEjxQ

Chris Bergin (he has very good inside sources) is reporting on Nasaspaceflight.com that he hears that SpaceX has decided not to use SN5 on the Hopper. They will be pushing back the hugely anticipated untethered flights to July, hoping to use SN6, assuming that unlike the other engines before it, it passes McGregor static testing.

https://twitter.com/NASASpaceflight/stat...0783713280

They definitely seem to me to be having trouble with the hugely complicated Raptor engine design.

(Jun 21, 2019 10:57 PM)Yazata Wrote: [ -> ]Chris Bergin (he has very good inside sources) is reporting on Nasaspaceflight.com that he hears that SpaceX has decided not to use SN5 on the Hopper. They will be pushing back the hugely anticipated untethered flights to July, hoping to use SN6, assuming that unlike the other engines before it, it passes McGregor static testing.

https://twitter.com/NASASpaceflight/stat...0783713280

They definitely seem to me to be having trouble with the hugely complicated Raptor engine design.

Who knows, maybe duds like SN5 and SN4 could still eventually be useful decades from now as part of a display at a SpaceX history museum. Like the refurbished "Apollo-era Saturn IB rocket" that recently returned to public exhibition at the Rocket Garden (Kennedy Space Center Visitor Complex in Florida).

http://www.collectspace.com/news/news-06...tored.html

Nice photo of the rapidly advancing Cocoa FL test-prototype here, its shiny surface reflecting the Florida sky (and the surrounding support structure):

https://twitter.com/spacecoast_stve/stat...6581243905

Another extraordinary aerial photo showing the Cocoa FL vehicle and its assembly site. That is what a spaceship is supposed to look like!

https://twitter.com/SpaceCoast_Life/stat...6076928000

The Texas crew are busily polishing the Boca Chica version, but it's never gonna look this good.

Elon has cast a little light on what's happening with the Raptor engines.

https://twitter.com/elonmusk/status/1143019549492744203

He says, "Raptor liberated its oxygen turbine stator (appears to be mechanical, not metal combustion failure), so we need to update the design & replace some parts. Production is ramping exponentially, though. SN6 almost done. Aiming for an engine every 12 hours by end of year."

My suspicion (I'm not an engineer) is that this problem might go beyond a simple broken part on one particular engine. There might be a more serious problem with the Raptor design if they have been breaking turbopumps prior to SN5 as well. And all the previous engines do seem to have failed for as yet undisclosed reasons.

The information that it's the oxygen turbine is a little worrying since my understanding is that the oxygen-rich preburners run extraordinarily hot. So hot that the United States thought in the 60's-80's period that they were impossible, until they learned in the 1990's that the Russians were already using them. Superheated high-pressure oxygen is very reactive, even with metals. So very high-tech alloys must be used to prevent the oxygen from literally burning the metal away (Elon's "combustion failure").

Apparently, at least in SN5, the heat and stresses caused a stator to break. Stators are rings of fixed turbine blades interspaced with rings of rotating blades ('rotors') in a turbine, intended to convert rotational energy in the gas/fluid (technically gases are fluids) imparted by the rotating blades, into linear energy. (Stators make turbines much more efficient. All jet engines have them.) The Raptor rocket engines essentially have small jet engines feeding their fuel-rich and oxygen rich exhausts into the rocket's main combustion chamber. Elon's text suggests that examining the pieces revealed that the metal hadn't been eaten away in a high-temperature oxygen-combustion process. Meaning that the defect should be easier to remedy (by redesigning the stators, perhaps making them thicker) than if the metal alloy itself had to be reformulated.

If this is what's causing the problem, the good news is that it's solvable. The bad news would be that SpaceX thought that they had already solved it (and perhaps did on sub-scale test engines) but may not have solved it for full-scale production Raptors. I expect that SpaceX engineers are working night and day on it.

But it does suggest that the untethered Hopper flights might be further away than everyone had hoped. Probably months off.

Maye Musk, Elon's mom, was in Boca Chica looking at a grain silo which, if you just squint your eyes enough, looks a little like a spaceship. She looked at a water tower too.

https://twitter.com/mayemusk/status/1147781786270760960

Elon accompanied her. He sounds optimistic about SN6 and suggests that it should be arriving in Boca Chica and untethered flights will start fairly soon. His latest update, presumably referring to the ongoing McGregor test-firings says:

"Exciting progress in Boca! Hopper almost ready to hover. Based on tonight's test, looks like 600 Hz Raptor vibration problem is fixed."

https://twitter.com/elonmusk/status/1147790758449012736

Unclear quite how this 600 Hz vibration problem relates to the broken stator. Conceivably the vibration was coming from the turbopump and the stator was the source of the vibration. So fixing it might have fixed the vibration too.

Now that they know what to look for, the NSF engineering geeks point to the difference between this clear photo from Elon's latest tweet:



And this photo from an earlier tweet (compare the shock diamonds in the exhaust and the exhaust's margins):



In retrospect, the lower photo may or may not show significant vibration in the rocket nozzle. That could lead to mechanical failure of many parts and would certainly reduce the lifetime of the engine. (Which is intended to be reusable like an airliner's jet engine.)

The engineers say that vibration problems like this aren't unexpected in late testing stages of engine design, since there are so many parts and so many
potential vibration modes. Fortunately, most of these problems can be dealt with by relatively simple design changes.

I'd be very interested in whether they can fire SN6 for any extended amount of time. According to Teslerati, 1 through 5 all failed at between 50 and 100 seconds of run time. The acid-tests are whether 6 can be run for extended periods without something coming apart, whether it can be stopped and restarted, and whether it can be throttled up and down (necessary if they want the single engine Hopper hops to ever land propulsively). So while Elon sounds ebullient at good news finally coming from McGregor, they still have a lot more work to do. I personally think that the Hopper hops might still be some time off.