SpaceX Starship : Texas Prototype(s) Thread 8 : Discussion

[Phantom]

Pardon my ignorance, and I apologize if this has been addressed previously, but what is the point of doing an initial pressure test with gaseous N2 rather than simply using "air"? 

[Slothman]

Pardon my ignorance, and I apologize if this has been addressed previously, but what is the point of doing an initial pressure test with gaseous N2 rather than simply using "air"?

it's available. Air is 80% nitrogen anyways so doing a gaseous nitrogen test isn't much different from just pumping up the tank with air like using a bicycle tire pump. They need the liquid form of nitrogen for cryo tests later anyways so it's on site

[capoman]

About Elon’s tweet about ready in a few weeks, but might be longer for approvals... At the rate they are building Starship, I can see a backlog of ships waiting to launch. In fact, I could see a scenario where they may end up skipping flying a serial or more due to changes and improvements during this period if it ends up being too long getting approvals. It appears the manufacturing is now getting ahead of the bureaucracy needed to fly.

Mind you, some RUD’s can certainly erase any backlog in a hurry...

[enbandi]

Pardon my ignorance, and I apologize if this has been addressed previously, but what is the point of doing an initial pressure test with gaseous N2 rather than simply using "air"?

it's available. Air is 80% nitrogen anyways so doing a gaseous nitrogen test isn't much different from just pumping up the tank with air like using a bicycle tire pump. They need the liquid form of nitrogen for cryo tests later anyways so it's on site

And I think it is purified also. What you call air may contain some water vapour, and also oil sometimes from the compressors. I dont know how clean their tanks should be, but my bet is that they dont want water and oil accumulating at the bottom.

[tommy099431]

https://twitter.com/elonmusk/status/1254432509531652097

[Question: Everything working fine, how long until her first hop?
Answer:] Physically ready in a few weeks. Approvals may take longer.

FAA gonna want a higher Liability Insurance. Anyone know the population within a 20mile radius?

Only one Starhopper permit, that was for last years hop,

Operating Parameters:
(a) SpaceX may operate the Starship Hopper vehicle to an altitude that does not exceed 25 meters AGL, in accordance with its application.
(b) SpaceX may operate the Starship Hopper vehicle for one flight, without further FAA authorization, to a nominal altitude of 150 meters AGL or less, with a maximum propellant load of 30 metric tons at liftoff, in accordance with its application.
Permit Term: The term of the Permit is one year from the original issue date of the Permit.

https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/media/150%20m%20hop%20Permit%20%20Order%20Mod_08_23_2019.pdf

[Karloss12]

Elon, have you used the "planisher" on the SN4 welds yet?

https://twitter.com/elonmusk/status/1254443488898437120

Not yet

twitter.com/flcnhvy/status/1254442280267780096

Do you think some of these issues will be resolved w/ 30X? Or is type of steel not necessarily related to that?

https://twitter.com/elonmusk/status/1254444243315195904

Yes, switching soon to a 300 series alloy with higher ductility at cryogenic temp. 301 is good as sheet, but not as plate. Also, a lot of geometric changes.

[/quote]

I'm picking that it will be plain old dual 304/304L grade, perhaps with some custom tweeks.  It has a lower maximum temperature than grade 301 and about 20% less strength, however superior ductility properties.

The 301 was picked because of its high temperature properties, however it is not a pressure vessel material and was developed for making springs.  Perhaps it is such a tough material to work with that SN4 & 5 won't fly.  Good on SpaceX for trying to make a material fit where industry has designated the material unsuitable (and excluded it from use in Pressure Vessels).  I don't think you can even get 301 in plate greater than 5mm sheet.

The current bottom head has four layers of 301 sheet welded on top of each other around the thrust puck to provide structural reinforcement.  I think this is a bit of a bodge job.  I wouldn't be surprised if SN4 were not able to get off the ground with this weld on top of weld arrangement.

With 304/L stainless, the dome of the bottom head can be formed from 15mm thick plate and then custom machined down to whatever contour is required (removing the need for bodgy doubler pads).  It's more expensive and time consuming than the current arrangement, but not astronomical (and a tiny percentage of additive manufacturing).  It would be a bit like the way Airbus manufactures the top of the A380 wing by machining a huge thick single plate of Aluminium into a single custom contoured panel.

SpaceX has bounced from low temp Carbon fibre to high temp 301.  I think 304/L provides an optimum medium.

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

[TheGull]

I expect the GN2 was fairly cold when they pumped it in.  I saw the top and bottom frost over fairly quickly and all over (not bottom up).  Then the frosting starting going away and the vending shortly after (if memory serves correctly).

Would it be correct to guess that raising the cold GN2 to ambient would induce a nice even pressure gradient suitable for finding small leaks?

What do they use for leak detection?  I note the spraying but they moved so fast and at large quantity that I'm thinking that was really a hydrophobic coating more than a soap bubble test. Yes?  No?

I would assume the GN2 is sourced from the big LN2 tank on the farm, either they pull from the headspace of the tank or they run the LN2 through an expansion. In either case, the GN2 will probably go through a heat exchanger to bring the GN2 up to "ambient" temperature. I do not remember SN4 tanks frosting over until after venting. The only sign of the GN2 test was smoothing of the dents, as pointed out by someone on the LabPadre stream.

Most likely they were spraying soapy water rather than some sort of coating or antifreeze.

Emphasis mine.  When you want large volumes of gas (and you're not making it yourself onsite with, e.g., an air fractionation plant), you have two options. 1) Get large volumes of compressed gas delivered (tube trailers), or 2) get liquid delivered and run it through a gassifier (generally just a big air-warmed heat exchanger).  There's several gassifiers around the build site attached to the larger liquid argon tanks (see BocaChicaGal pic below).  They must go through boatloads of argon gas TIG welding.  There's also a big one at the launch site.  Its tucked in the gap between two of the three vertical black cryotanks at the LOX (? - what I'm the LOX farm is the  tank cluster closer to the road) farm.  See drone pic posted recently by Elon Musk.

The smaller "lab sized" LAr dewars you see all over the site also have gassifiers, they're just not visible externally.  They're typically in the form of tubing soldered or brazed to the inner wall of the outer vacuum jacket of the dewar.

Edit: In case it wasn't obvious enough from the text above ... Image credit: (1) BocaChicaGal; (2) Elon Musk & Co.

On the option 2, just insert the LN2 enougth to boil till the desired pressure wouldn't work? My guess is: The ambient temperature and the huge area of the SS tanks would feed heat enought to boil a little of LN2 increasing the pressure as consequence.  Maybe the process with gassifier is faster. Just a guess.

[philw1776]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

[Karloss12]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

[capoman]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

I don’t think that’s quite what happened. He was expecting it to fly until his team said that it wasn’t fit to fly. Unless there is a reason for SN4 to not fly, it will likely do a hopper-like flight. But if approvals take too long, it’s certainly possible that SN5 could be skipped if SN6 is too far along. Question is, is SN6 waiting for any verification of any technology such as flight surfaces or new thrust area design before it gets past a certain point in manufacturing?

[Vettedrmr]

Pardon my ignorance, and I apologize if this has been addressed previously, but what is the point of doing an initial pressure test with gaseous N2 rather than simply using "air"?

It's also absolutely dry.  Don't think you want any kind of chance of corrosion in a LOX tank.

[tommy099431]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

Not sure, if permits take a couple of weeks id say SN4 static, SN5 flys.
If permits takes a week or two id say SN4 flys & SN5 flys.
I believe that no matter what SN5 flys, Musk already said it will be used to test new aero surfaces & controls, I don't think they scrap a whole ship that they didn't use at all, they will at the very least blow it up/cyro test them or static fire to test thrust structure, if not they'll make it fly.

[Karloss12]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

I don’t think that’s quite what happened. He was expecting it to fly until his team said that it wasn’t fit to fly. Unless there is a reason for SN4 to not fly, it will likely do a hopper-like flight. But if approvals take too long, it’s certainly possible that SN5 could be skipped if SN6 is too far along. Question is, is SN6 waiting for any verification of any technology such as flight surfaces or new thrust area design before it gets past a certain point in manufacturing?

When asked just about the pressure tests (let alone static firing or launching), Elon has to Tweet below.

https://twitter.com/elonmusk/status/1254441948187852800

It’s a little dicey tbh. Thrust dome is being redesigned. Current one has four separate layers of steel in some places!

Fabricating stressed structures from quadruple layers of steel is not good Engineering Practice and almost impossible to stress analyse, even with FEM.  It is a bit of a trial a error guess as to whether this bodgy thrust structure will work.  However, they have hydraulically pushed on the thrust puck structure with success, so that's something.

I sense that the Engineers are slowly and diplomatically maneuvering/advising Elon away from this questionable quadruple layer design and unsuitable 301 material towards something workable.

As Elon is neither a Mechanical Engineer nor Metallurgist and is more of a Steve Jobs type commercial decision maker, it is all his Engineer's fault. 

[ericgu]

Do the tiles actually need to be aligned that carefully? On a traditional capsule or the shuttle, the tiles make up pretty much all of the heatshield and the underlying body is entirely seperate. So the shield is responsible for forming the shock wave, soaking up the heat and shielding the strucure. In some ways, they don't care much what is put behind them.

With the hot steel structure, what specific purpose does the heat shield serve? Keeping the shock wave further from the body and splitting any contact into much smaller and more complex shapes would go a long way in smoothing out heat spikes. This might well be possible with gaps beween tiles, even slighty irregular ones.

Atlantis survived reentry on STS-27 with damage to 700 tiles, including one tile that was fully missing. They got very lucky that the missing tile happened to be over a steel plate as it would have melted through if there was aluminum structure at that point.

https://forum.nasaspaceflight.com/index.php?topic=29360.220

[awests]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk  ✔  @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

I don’t think that’s quite what happened. He was expecting it to fly until his team said that it wasn’t fit to fly. Unless there is a reason for SN4 to not fly, it will likely do a hopper-like flight. But if approvals take too long, it’s certainly possible that SN5 could be skipped if SN6 is too far along. Question is, is SN6 waiting for any verification of any technology such as flight surfaces or new thrust area design before it gets past a certain point in manufacturing?

When asked just about the pressure tests (let alone static firing or launching), Elon has to Tweet below.

https://twitter.com/elonmusk/status/1254441948187852800

It’s a little dicey tbh. Thrust dome is being redesigned. Current one has four separate layers of steel in some places!

Fabricating stressed structures from quadruple layers of steel is not good Engineering Practice and almost impossible to stress analyse, even with FEM.  It is a bit of a trial a error guess as to whether this bodgy thrust structure will work.  However, they have hydraulically pushed on the thrust puck structure with success, so that's something.

I sense that the Engineers are slowly and diplomatically maneuvering/advising Elon away from this questionable quadruple layer design and unsuitable 301 material towards something workable.

As Elon is neither a Mechanical Engineer nor Metallurgist and is more of a Steve Jobs type commercial decision maker, it is all his Engineer's fault. 

I get the opposite feeling. Elon and the Engineers have known that the puck design and material selection have been suboptimal for a while, but they are limited in their resources (time, $$, equipment, etc...) to implement an improved design.

The current design is a quad layered 301 bandaid while they refine other manufacturing challenges and procure the resources to make the refined design.

[KneeJerk]

I caught this being lowered into the cylinder.

The edge corrugations on the cone increase the length of the weld bead and compensate for the weaker metal of the weld filler and reheated (annealed) steel plate. 3/4 hardened SS has ~ 3 times the yield strength as annealed plate. Therefore the length of the Sinusoidal weld bead must be 3 times greater than a simple circle.

The non-symmetry of the edge makes sense if you project a weld bead. The bead itself will be sinusoidal.

In hindsight, these "teeth" on the cone edge are far too small! The teeth must be scaled large enough so the plate is not appreciably annealed within the "teeth", but only adjacent to the weld bead.

The the sinusoidal edge on the CH4 header tank "tutu" has an adequately scaled-up sinusoidal edge.

[meekGee]

Four weeks seems like a long time to wait for launch approval, when a complete SN4 is sitting on the test stand.  I'm picking that SN4 will be used for static firing only, SN5 will be scrapped and SN6 made from 304/L will fly.

All speculation.

Elon just said that SN4 will hop 150m


Pranay Pathole
@PPathole

Replying to @elonmusk @Cardoso
Will SN4 be doing a 150m hop or 20km?

Elon Musk    @elonmusk
150m

11:36 AM - Apr 26, 2020

Yes I know, that is what he says.  MK1 was scheduled to launch when he knew it couldn't.

Not sure, if permits take a couple of weeks id say SN4 static, SN5 flys.
If permits takes a week or two id say SN4 flys & SN5 flys.
I believe that no matter what SN5 flys, Musk already said it will be used to test new aero surfaces & controls, I don't think they scrap a whole ship that they didn't use at all, they will at the very least blow it up/cyro test them or static fire to test thrust structure, if not they'll make it fly.

Well to fly SN5 they have to take SN4 off the pad, right?

So unless SN4 is unfit to fly, might as well do it like Goddard intended.

[Karloss12]

When asked just about the pressure tests (let alone static firing or launching), Elon has to Tweet below.

https://twitter.com/elonmusk/status/1254441948187852800

It’s a little dicey tbh. Thrust dome is being redesigned. Current one has four separate layers of steel in some places!

Fabricating stressed structures from quadruple layers of steel is not good Engineering Practice and almost impossible to stress analyse, even with FEM.  It is a bit of a trial a error guess as to whether this bodgy thrust structure will work.  However, they have hydraulically pushed on the thrust puck structure with success, so that's something.

I sense that the Engineers are slowly and diplomatically maneuvering/advising Elon away from this questionable quadruple layer design and unsuitable 301 material towards something workable.

As Elon is neither a Mechanical Engineer nor Metallurgist and is more of a Steve Jobs type commercial decision maker, it is all his Engineer's fault. 

I get the opposite feeling. Elon and the Engineers have known that the puck design and material selection have been suboptimal for a while, but they are limited in their resources (time, $$, equipment, etc...) to implement an improved design.

The current design is a quad layered 301 bandaid while they refine other manufacturing challenges and procure the resources to make the refined design.

You may be right.
Elon has to create time and $$ with his mojo to get this done.

A step change in design is required.  A switch of material will make manufacturing a lot easier.  Although the shell will go from 4mm to 5mm thick for 304/L.

The quad thickness monstrosity was created because of the thickness limit of 301.  With a change in material, the design options are much greater.  I dream that there is already a 15mm thick 304/L thrust dome being CNC machined down to the same contours of the quad thickness geometry by a sub-supplier somewhere and it will imminently arrive on the back of a truck.
It's not a component that needs to be fabricated in-house.

[meekGee]

I am not sure if y'all are interpreting "4 layers" as intended.  A simple multi-layer dome seems a bit counterintuitive as a starting point. I don't think there's a material thickness level either.

I think Musk was commenting that in places, he can count 4 layers of structure, and that's needlessly complex, hence a re-design.

I also really call into question the condescending "the Engineers are slowly and diplomatically maneuvering/advising Elon away from this questionable quadruple layer design".  This is not at all the impression I got.  They would not have gotten this far this fast if that were the case.  It is normal to explore a number of directions at the beginning of a project...  And Musk is not acting as "manager" but as "leader".  I wasn't spooked by the transition from Carbon Fiber to Stainless Steel, I am certainly not spooked by the number of iterations on the ship.

In fact it's refreshing.  Other projects spend 10 years on a project "reference design" and don't iterate even once...  And when they do, they are local and incremental changes, because nobody dares disturb the great ghoul that is the system architecture.

[fael097]

I am not sure if y'all are interpreting "4 layers" as intended.  A simple multi-layer dome seems a bit counterintuitive as a starting point. I don't think there's a material thickness level either.

probably this is what he's talking about, you can see 3 layers right there, could be 4 considering internal reinforcement. that never looked optimal anyway. doesn't mean it won't work, sn2 passed the test with this design, but they surely can optimize / reduce weight