Starship is Go...

kbd512 · 2020-04-05 02:11:57

Mark,

Well, for now I guess I'll have to take your word for it. I want to see a pressure vessel that holds pressure without exploding or imploding. I would worry more about designing a pressure vessel that holds pressure first and then worry about mass producing it, but that's just me. That has been done successfully by other aerospace companies for decades longer than I've been alive. All of them use specialized welding jigs. I noticed that SpaceX only has the beginnings of such tooling. Maybe they're just experimenting, but time will tell.

elderflower · 2020-04-05 05:28:43

I have the impression that Spacex isn't really into learning from the experience of others. Design of pressure and vacuum vessels, including cryogenic equipment, is not a new subject. Making the same mistakes that others have made before you isn't the best way to progress.

tahanson43206 · 2020-04-05 06:41:05

For elderflower re #602

This post is offered, not in disagreement with you, but instead, to ask for an evaluation of the trades ...

Most certainly it would seem most wise to find out what was done before, and to integrate that into current planning and then practice.

That would take time.

But Elon has said (recently and perhaps previously as well) that he doesn't feel he has the luxury of time.

If I recall the report correctly, I ** think ** he said he is now 40 (or so), and that he wants the project to succeed while he is still alive.

If you were designing the work force for the Starship project, i'm assuming you'd put out a call for experienced people to help you.

But ** then ** there is the small problem of vetting the candidates.

And ** then ** there is the small problem of integrating your chosen candidates into a work force.

My guess (and that's all it is) is that Elon has a team of people he trusts, and he's giving them the opportunity to make mistakes with the implicit understanding they are to ** learn ** from those mistakes.

(th)

SpaceNut · 2020-04-05 09:15:56

elderflower wrote:

I have the impression that Spacex isn't really into learning from the experience of others. Design of pressure and vacuum vessels, including cryogenic equipment, is not a new subject. Making the same mistakes that others have made before you isn't the best way to progress.

tahanson43206
GW and others have said the same over and over again.

The dome is not made from sheet stock welded together as normally a dome is pressed and shaped from a much large piece or its printed and additive means are used on that shape to finish it off.

Cyrogenic standard tanks

OIP.3lcY3P7_ly64SLYWiw00kwHaFj?w=233&h=173&c=7&o=5&pid=1.7]

when they use flat the slices are very small to make them line up better.
OIP.O1OUnCzml4-zOR2pUhizHgAAAA?w=172&h=173&c=7&o=5&pid=1.7

FLAT BOTTOM OR VACUUM INSULATED TANKS?

Bulk storage tanks

GW Johnson · 2020-04-05 11:07:24

If it wasn't 9 meters diameter, you could shear-spin spherical-segment domes from flat sheet or plate the way tank car domes are made. Nobody out there has metal stock or manufacturing machinery big enough to do that at 9 m finished diameter. So welded gores are the only option.

But if the tank end is not spherical, you induce lots of bending stress, even without any point loads supporting thrust structures. Those just make it worse. The combined stresses can get very large very quickly. And bear in mind that elongation capability is reduced at cryogenic temperatures, even with 300-series stainless. Plus, you still get some significant bending at the dome-cylinder joint, because of an inherent geometry-induced strain mismatch, even when you otherwise "do it right".

What I see in the industry cryo tank designs is graduate school-level competency in stress-strain design, which is far beyond what a sophomore-level beginner in structures would know. I do not yet see that level of competency in what Spacex has been designing. Although they have vastly improved over the sophomoric crap they tested in SN-1 and -2.

GW

Last edited by GW Johnson (2020-04-05 11:09:33)

louis · 2020-04-05 16:13:21

I find it odd that Space X haven't learned more from existing pressure tank designers. If Space X's concern is a loss of payload, I would say make the sacrifice. A payload of 85 tons to Mars rather than 100 tons is no big deal. You can add one more Starship to Mission One to make up for the deficiency.

Makes you wonder who is actually designing the tanks...

If it's a fundamental design flaw, then they may have to go back to the drawing board.

louis · 2020-04-05 16:16:26

...Having said that...Musk is claiming on Twitter it was a test configuration error - a full tank sitting on an empty tank, causing it to crush...hmm...well, if so still a pretty basic error for rocketeers!

elderflower · 2020-04-06 09:45:57

The dome end to cylinder joint stress concentration is minimised, I believe, by using a 2;1 ellipsoid rather than a 1/2 sphere. This makes the gores a bit more complex. I doubt that either would be the right pattern, however, when you need to transmit the enormous thrust from the engines and I would look at a conical bottom, with stiffeners to prevent local buckling, and an additional skirt, for this application. For the upper tanks the thrust will already have been transferred to the wall and the tank pressure will support the ends of the tanks, so here the 2:1 ellipsoid is favourite.

GW Johnson · 2020-04-06 11:19:53

Elderflower:

The 2:1 ellipsoid shape is not for stress control, it is for packaging efficiency. It is the "best" tradeoff between shortening how far the dome extends beyond the cylinder end, and incurring bending stresses throughout its membrane for not being spherical. It still has the strain mismatch between dome and cylinder at the joint, just different numbers for 2:1 vs true spherical.

For pressure vessels, the best approach for highest strength/weight is a spherical segment of thin membrane, ending in a circumferential ring that is thick, which welds to the cylinder end. You have to analyze the ring with 2-D axisymmetric or 3-D finite element models to trim it to its min weight. By spherical segment, I mean a shape not tangential to the cylinder at its joint there.

The spherical membrane can use the pi ID = stress x thickness equation to size its thickness. The cylinder wall can be sized for thickness with pi ID = 2 stress thickness, assuming weld strength is the same as parent metal strength. Both the case and the membrane need to smoothly double in thickness over about 6-7 wall thicknesses as they approach the ring. That thickening can be minimized with finite-element analysis, too. If the weld is weaker, you knock down the allowable stress by that factor, in addition to your other safety factors.

In the solid motor business, we NEVER EVER used a membrane dome to absorb a thrust force. We always took thrust forces on the cylinder skirt. You are correct to surmise a conical shape to be best, but it CANNOT be the pressure vessel dome. The highest strength/weight conical thrust "skirt" is really a conical-envelope truss terminating upon the same ring that joins the spherical segment dome to the cylinder. There is a very good reason behind the engine mounting trusses used on the Saturns. And many other rockets.

GW

Last edited by GW Johnson (2020-04-06 11:22:18)

SpaceNut · 2020-04-06 12:04:21

https://www.spacex.com/sites/spacex/fil … ide_v1.pdf

louis · 2020-04-06 17:03:44

Felix's explanation of the SN3 failure:

https://www.youtube.com/watch?v=6TiPETqEdDY

As always very clear.

Also a report on progress with SN4.

elderflower · 2020-04-07 03:06:56

Test protocol problem then. The lower tank was maybe evacuated with no vacuum relief and valves closed. This would happen by condensing air on the interfacial bulkhead when the upper tank was charged. Note that the boiling point of liquid Nitrogen is a bit lower than that of Oxygen, and what condenses at low pressure is about 70% Oxygen, 30% Nitrogen. So pressure in the tank, if it started at atmospheric could end up about 1/2 atmos by this mechanism alone. Also reducing pressure in the tank is simple contraction of the air by chilling. Pressure is proportional to absolute temperature and as the lower tank cools its pressure falls even without condensation.
For GW
Given the number of engines in the bottom of the starship, I don't see how Musk can avoid using a thrust pad on the lower tank end, so this should be conical.

louis · 2020-04-11 14:27:23

I love these videos. The NASA Spaceflight channel has some of the best views....

https://www.youtube.com/watch?v=8Kpek0dASmY

SN4 making good progress.

I am loving the "post it notes" on the dome bulkhead.

louis · 2020-04-14 20:13:42

The pace is not slackening!

https://www.youtube.com/watch?v=DBFJsS-8_3o

louis · 2020-04-15 11:52:44

SN5 production coiming along nicely...

https://www.youtube.com/watch?v=cLiN_bCUy8c

To the tune of Rawhide:

"Keep rollin', rollin', rollin'
Though Covid's come a-strollin'
Keep them rockets rollin', Space X!
Through rain and wind and weather
Hell bent for leather
Knowing your goal is Mars and nothing else
You know we should be terra-forming
Engaged in planet-warming
So tell the Martians we're on our way...
Move 'em on, header tanks
Header tanks, move 'em on
Move 'em on, header tanks, Space X!
Cut 'em out, weld 'em in
Ride 'em in, cut 'em out
Cut 'em out, weld 'em in, Space X!!!"

louis · 2020-04-16 18:50:08

Another good video from Felix. SN4 and SN5 production well advanced and Space X is putting in place what needs to be done to develop mass production in line with Space X's ultimate goals...

https://www.youtube.com/watch?v=t1hnGGaa_mo

louis · 2020-04-23 17:07:06

Roll out of the SN4 - in the middle of a pandemic crisis...gotta hand it to Musk and the Space X crew - they just keep going after the prize.

https://www.youtube.com/watch?v=ZZxrC633mTU

May 4 be the lucky number...

kbd512 · 2020-04-23 20:54:05

Louis,

Let's hope. I'd like to see one of these things actually fly somewhere.

louis · 2020-04-24 03:52:28

Yes, seems a bit pointless just exploding them on the ground - spectacular though that is!

kbd512 wrote:

Louis,
Let's hope. I'd like to see one of these things actually fly somewhere.

Oldfart1939 · 2020-04-24 08:30:18

The whole thing about the Starship design is utilizing monococque construction, similar to the original Atlas. Internal pressurization is required to hold everything together and there were some spectacular failures of early Atlas tests. Basically like a beer can when full and pressurized, one can stand on a full one with body weight. Empty they are crushed.Same learning curve for SpaceX as for Convair back in the 1960's.

Last edited by Oldfart1939 (2020-04-24 08:31:00)

louis · 2020-04-24 14:49:58

I was interested to hear the SpaceXcentric guy say that the SuperHeavy will be easier to build because it's basically a "Falcon 9 made of steel"...Not sure I was aware of that being the case.

Presumably that's a scaled up Falcon 9.

Good if true, if it means the development time for the SuperHeavy will be relatively short.

GW Johnson · 2020-04-25 12:52:06

The Superheavy will be very much like a scaled-up Falcon-9 core made out of stainless steel. You can't get aluminum of the type they use in stock that large, plus it has a short service life due to fatigue and heating exposures.

The stainless will have a much longer service life, being far more fatigue-resistant, and far, far more heat-resistant. It's just heavy, which you counter by going to a pressurized-balloon structure and thinning-down the thickness.

What Oldfart1939 refers to is exactly correct. That is indeed what Convair had to learn how to do in the 1960's: learn how to handle both the material, and the inflated structures. That led to the Centaur stage still in use today, and to the original Atlas missile itself. Both were pressurized stainless-steel balloons that would crush under their own weight if not inflated.

The Superheavy and the Starship cannot be quite as thin and light as the Centaur and original Atlas, because they have to take higher stress loads in order to fly back at all, much less be reusable dozens to hundreds of times. But with the stainless, that potential is there, and it is not with aluminum-lithium. Nor with composites (cannot take the heating or hold the cryogens). Nor with titanium (cannot be formed, only "carved", and it's not as heat resistant as the stainless. It's no better than mild carbon steel, actually).

Spacex still has a long way to go learning how to use the material (which is new to them), and actual pressure vessel design (also new to them excepting their helium pressurant tanks, which is not the same structural problem at all).

I wish them success. They will need all the luck and qualified advice they can get. And qualified advice they won't get so much from NASA, who paid contractors to supply those smarts and things, never doing it for themselves. Those contractors no longer exist, and the actual people with the know-how and experience are long dead or retired.

The closest to a qualified pool of people are the builders of Centaur today. But consider this: that is a build-to-print stage that no one is modifying (or didn't you notice?), developed long ago by people now dead or retired.

Spacex is having to reinvent the art of stainless steel balloon structures that is actually more extensive than the science of it. This is a clear example of my oft-repeated claim that rocket science ain't science, it is 40% science (stuff written down), 50% art (crucial info never actually written down), and 10% blind dumb luck.

And that's in production. In development, the art and luck fractions are even higher. And it applies to any kind of engineering, not just rocketry.

GW

Last edited by GW Johnson (2020-04-25 12:53:59)

Mark Friedenbach · 2020-04-25 15:40:37

ULA is developing a new Centaur stage.

GW Johnson · 2020-04-25 16:04:38

They might be changing the engines, but I rather doubt they are changing the pressure-balloon tankage design. If they do, they will end up spending a lot more time in development than they think.

GW

kbd512 · 2020-04-25 16:10:53

GW,

As Mark pointed out, ULA developed a new 2-engine Centaur upper stage for Starliner. It wasn't a carbon copy of the 2-engine Centaur from the past since it incorporated changes required for man-rating, even though it was substantially similar in its basic design. The all-new 4-engine upper stage ULA is developing for SLS, even if SLS never flies, should be something NASA retains for use with Starship. ULA and SpaceX really should swap notes and use the parts of the fabrication processes that make the most sense for both companies.

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