Starship is Go...

RobertDyck · 2025-03-21 13:09:11

The F1 engine of the first stage of Saturn V had a POGO problem. That was the largest rocket engine ever built. The top of the combustion chamber was injector holes alternating fuel/oxidizer. Combustion would oscillate. Just as an explosion be used to put out an oil fire, the explosion pushes the combustion away from the fuel source long enough to extinguish the fire. The same happened inside the engine. The F1 engine had a "burst" or "explosion" in one area of the combustion chamber, which put out the fire/combustion. But the engine was large enough that it didn't extinguish the entire engine. Compression caused a "burst" in another part of the combustion chamber. This caused cyclical bursts moving around the combustion chamber. Amplitude of the bursts increased with each one, until force ruptured the combustion chamber. This caused the engine to literally explode. I believe that's how POGO works. Engineers at Rocketdyne solved that by installing baffles in the combustion chamber. This contained the burst, and the engine basically acted as if it were several separate smaller rocket engines. The baffles didn't have to extend far past the injector plate; just ensure combustion expanded toward the throat, not sideways.

A view up the F1 engine:

GW Johnson · 2025-03-21 13:22:33

Rob:

The process you describe in the F1 is what causes serious thrust oscillations, which can build up and destroy the engine, or might not (just run very rough).

When that thrust oscillation interacts resonantly with either (or both) vehicle structural oscillation modes or organ-pipe fluid oscillation modes in tanks or plumbing, the entire vehicle is at risk of breaking up. The feedback is feed pressure changes feeding back into the rough-burning thrust oscillations. The term "POGO" encompasses all of those.

The Apollo-6 problem had thrust oscillations in the first stage kerosene-LOX F1's that shook the vehicle so violently as to damage the LH2-LOX J-2's in both the second and third stages. The Apollo-13 problem was J-2 engines shaking so violently in the second stage as to cause 1 engine to shut down lest it explode. Different, and yet many of the same things.

The Titan-II man-rating for Gemini was delayed by thrust oscillations that broke up test vehicles in flight. Those were NTO hydrazine storables. I'm not sure, but I think Atlas might have suffered from some of this, too.

GW

RGClark · 2025-03-21 17:58:20

Another expert engineer criticizes the multi-refueling approach SpaceX is taking to get to the Moon and Mars as a bad approach, former high ranking NASA official Daniel Dumbacher:

SpaceX Needs A New Mini-Starship To Land Humans On The Moon And Mars.
By Kevin Holden Platt, Contributor. Kevin Holden Platt writes on space defense…
Mar 17, 2025 at 11:33pm EDT
…
“Our approach today has a very low probability to match the ‘before 2030’ milestone for landing humans on the Moon,” Daniel Dumbacher, who formerly served as Deputy Associate Administrator of NASA’s Human Exploration and Operations Mission Directorate, in charge of the Artemis lunar landings, testified at the hearing.

While he didn’t mention the fiery breakup of SpaceX’s Starship during its January flight demo, Dumbacher, now a professor in aeronautical engineering at Purdue University, said that the ship’s need to be refueled with super-cooled liquid oxygen and methane in low Earth orbit via multiple dockings with still-to-be-developed tankers - a complicated operation that has never been tested - before each flight to the Moon involves an assemblage of complex technologies that might not be perfected within the next five years.

“We might have to build a lander - we might have to scale down the current lander,” Dumbacher told the House, “so that we get to that 2030 landing.”

To avert potentially spiraling problems with testing the colossal Starships during the countdown to this new Moon quest, he said, “I’d get myself a simplified lander - so that I can get to the Moon - that does not require multiple launches.”
…
https://www.forbes.com/sites/kevinholde … -and-mars/

Bob Clark

RobertDyck · 2025-03-21 18:22:33

Gary, that's scary because resonant frequency of the propellant tanks will change continuously has they are drained. Think slide whistle. Or music on water glasses.

GW Johnson · 2025-03-23 09:58:15

Rob:

That's exactly right: the susceptibilities shift dramatically as you burn off propellant. We saw the same things in solids: as the propellant burned back, the internal cavity drastically changed is size and shape, bringing different resonant frequencies into play. You should find it scary. It really is!

With the solids, we had to analyze the shit out of it, with our best shot at instability computer models (which were imperfect at best), and then test the shit out of it, with the right analog instrumentation, all done by people already experienced using that analog instrumentation for instability detection, before we ever let a solid design out the door! Nonmetallized reduced/min smoke required much greater care and attention in this area, because there were no metal oxide particles or droplets to provide any damping action, just the soot. Absolutely NONE of that was a single simple calculation. Test data ALWAYS trumped any analyses, too!

Actually, the same thing is true of liquids, just the details differ.

I suspect the "new space" outfits will be seeing a lot of POGO-type troubles as time goes by, because none of the youngsters at those outfits know anything substantive about instability (since that remained about 80-90% art, never written down). And none of those youngsters knows anything about the analog instrumentation required to diagnose it, or anything about how to go about doing this kind of work with that analog instrumentation.

Myself, I think SpaceX accidentally dodged the instability bullet with their kerosene-fueled Merlins, because of the damping effect of the soot cloud in the chamber flame. A soot cloud is inherent when burning long-chain carbon things like kerosene. Vaporizing droplets can assist with that damping effect. But with the methane-burning Raptors, there is no soot on the flame, and the droplets evaporate far quicker.

Since SpaceX hires no one past age 40 to 45, how would any of their engineers who design these things know anything at all about such instabilities? Or even recognize them? Much less how to deal with them when they crop up? And they DO crop up!

"Build it, break it, build another" is a strategy that will always fail, if you cannot recognize the problem to be corrected.

GW

Last edited by GW Johnson (2025-03-23 10:04:47)

RobertDyck · 2025-03-23 11:06:37

You could offer your services. Elon would probably consider 40 hours/week "part time". Even 20 hours/week could help them. You just need to identify the problems, let the youngsters learn from your experience, and do the hard work to work out the details.
SpaceX Careers

Last edited by RobertDyck (2025-03-23 17:30:54)

tahanson43206 · 2025-03-23 11:50:20

For RobertDyck re #2031

A casual reader of your post might assume from your wording that GW had NOT offered his services.

This post is intended to correct that misunderstanding.

Please do not continue this in public.

(th)

GW Johnson · 2025-03-23 15:40:39

Rob:

I tried but could not make the link you provided work. Since I come from the slide rule days, that should be entirely understandable, and yet totally outside anything SpaceX is familiar with. I will try some other way.

Tom:

I never applied at SpaceX in any of the last several years, after finding out some years ago that they hire no one over age 40 to 45, because they demand chronic 70-80-hour weeks. Oldsters cannot do that. So they do not want them.

Both:

The problem is now that SpaceX is in deep kimchee with what looks like POGO troubles with Starship. No one there in California or any other site would have the slightest idea how to recognize this as a problem, much less have any idea how to approach solving it. That is because (1) no one has faced this trouble in some decades, and (2) this is 80-90% art never written down decades ago. All the people who knew are long dead or retired, like me. Even even what I know is only tangentially useful, as it deals with solids, not liquids. The real expert in solid instabilities was my friend and mentor W. Ted Brooks, who died many years ago.

The real experts in liquid instabilities would have been those who designed the J-2's and F-1's for Saturn-5. They are all long dead. Period. 10% or at most 20% of what they knew was ever actually written down anywhere at all. It was development stuff, not production stuff, back then.

Production: 50% art, 40% science, 10% blind dumb luck. Only the science was ever written down.

Development: art and luck factors very much higher.

GW

Last edited by GW Johnson (2025-03-23 15:43:31)

GW Johnson · 2025-03-23 16:50:59

Today I finally did make the video work of Flight Test 8 on the SpaceX website. I DO NOT do X or any of that other social media BS. I never will.

I saw no surprises, except that the Starship upper stage loss occurred right before the end of the ascent burn, with little propellant left aboard. I did see first stage engine losses on both ascent and descent, excepting the 3 gimballed engines dead center. That makes me wonder if we didn't see POGO in both stages.

If the control software is formulated correctly, ascent oscillations risking engine damage might cause shutdowns that do not recur during descent. Except that there were 2 engines that failed to restart during the 13-engine initial boost-back burn. 11 were enough to slow it so that the "design 3" could recover it on the tower.

The upper stage Starship did not seem to suffer problems until right before the end of its ascent burn onto the ascent ellipse. Then it lost 1 vac engine, then all 3 SL engines (with the thrust vector gimbal capability), then another vac engine, then it obviously tumbled totally out of control until self-destruct ended this.

Bear in mind that this ascent ellipse has apogee at LEO altitude, but a perigee at, or very near, the surface. THAT is what ensures automatic de-orbit, if you do not do a circularization burn at apogee. The ascent ellipse perigee is well down in the atmosphere.

GW

Last edited by GW Johnson (2025-03-23 16:52:37)

RobertDyck · 2025-03-23 17:45:55

Gary, how does the Starship siphon work? When I see engines not restart, but they do restart later, I suspect a propellant feed problem. Do they have a slosh problem?

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#2026 2025-03-21 13:09:11

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#2033 2025-03-23 15:40:39

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