A few simple questions for conspiracy theorists

[ka9q]

I think of pogo as an (undesired) mechanical oscillator.

Thrust increases a little, which also increases acceleration.
The increased acceleration increases propellant pressure at the turbopump inlets.
The increased pressure increases the propellant flow rate.
The increased flow rate increases thrust.

This vicious circle repeats until a limit is reached, e.g, on flow rate through the pumps. Then it continues:

Thrust decreases a little, decreasing acceleration.
Propellant flow rate decreases.
Thrust decreases.

And this repeats until a lower limit is reached. Thrust thus oscillates, possibly very rapidly and in an extreme way that could destroy an engine or at least tear it out of its thrust mounting.

A static test of a rocket engine can't simulate the changing longitudinal accelerations and the resulting fuel pressure fluctuations of pogo, so the problem has to be solved mainly through analysis and modeling. The only other way is an expensive flight test.

[ka9q]

Thinking more about it, I suppose it might be possible to design a static test stand that could simulate pogo. You'd measure engine thrust and vary the propellant inlet pressures to simulate their responses to the increased thrust.

You'd have to do this very quickly. You might control the pump inlet pressures by changing the pressurization in the (fixed) propellant tanks, but I can't imagine how you'd do that at a few dozen hertz. And this would not simulate any change in momentum of the propellants already flowing through the lines, or the dynamic response of the vehicle structure that can oscillate like an accordion.

I guess computer modeling has gotten good enough to resolve pogo in the design phase so there's no real need for ground testing. If not, it'll usually come out in the early flight tests.

[JayUtah]

Analysis or flight test or both.  This was the Aries-1 dilemma.  Analysis, chiefly via FEA, showed longitudinal oscillations (in this case purely structural, since it was a solid-fueled vehicle).  But the lack of FEA convergence couldn't be assertively attributed to actual mechanical properties or to the natural instability of FEA models.  So the only option was a flight test, which was nominally successful.

As for problems cropping up in development, there's a maxim that says if you don't run into problems in development, you're not engineering hard enough.  Which is a pithy way of saying that in order to make something of sufficient value to be notable or profitable, you have to bite off enough of a problem to make it unsure whether you're going to get there without difficulty.  That was the spirit of Apollo.  They were biting off huge problems, and the notion that they would get there without incident or serious difficulty is a gross misunderstanding of what engineering development is.

[ka9q]

Exactly. One of my company's founders put the same idea in a slightly different way:

"If you haven't tested it, it doesn't work!"

[JayUtah]

You'd have to do this very quickly. You might control the pump inlet pressures by changing the pressurization in the (fixed) propellant tanks, but I can't imagine how you'd do that at a few dozen hertz.

You put an accumulator on the propellant line and vary the pressure of the gas bubble using a cylinder or other variable-volume apparatus connected to the top of the accumulator.  You can use a roller cam, or a set of them, to quickly actuate the piston or bellows or whatever.  I've even seen rotating eccentrics used for this in lower-pressure applications.

[Luke Pemberton]

That was the spirit of Apollo.  They were biting off huge problems, and the notion that they would get there without incident or serious difficulty is a gross misunderstanding of what engineering development is.

I'm not an engineer, but the argument that the problems they encountered is evidence that the success of Apollo was prohibitive can be easily countered:

'They were building something that was going to the Moon, it's not like dusting crops.'

[ka9q]

You put an accumulator on the propellant line and vary the pressure of the gas bubble using a cylinder or other variable-volume apparatus connected to the top of the accumulator.

Why didn't I think of this? I know that a helium gas accumulator was used to damp pogo in the S-IC, so it makes sense that one could also be modulated to deliberately create it in a test.

I suppose you could also use the same thrust mounting as on the actual vehicle to let the engine move as it would in flight. This would reproduce the dynamic part of the acceleration on the engine itself, though the static acceleration would still be 1g. To change that, you'd need a big-ass centrifuge...

[Luke Pemberton]

Is it true that when they fixed the fuel instability problem but the engineers were stilla little baffled by how it was caused in the first place? I understand injector plate designs based on what they believed would work were developed, but the actual cause was unknown (if that makes sense). I picked up this snippet in a BBC docudrama, but I find it hard to believe that the engineers just kept changing designs in the hope that they'd find something that worked.

[ka9q]

I think that's right; they simply didn't have the tools to really understand what was happening inside the combustion chamber.

I do know that combustion instabilities can be very difficult to fix largely because they can be intermittent and/or rare, and thus never occur in ground testing or even in numerous flight tests. The amateur radio satellite group I'm involved with lost a spacecraft in 1980 on the Ariane L-02 flight due to a combustion instability in a first-stage engine. The first test flight was perfect, but it only carried a dummy payload because nobody wanted to risk theirs on an untried rocket.

I think the F-1 engineers devised a method to initiate an instability by firing small explosive charges in the combustion chamber. I can't imagine how they did that.

Here's some footage of an Atlas with serious combustion instability problems. Note the left booster engine in the second camera view:

[Luke Pemberton]

I think the F-1 engineers devised a method to initiate an instability by firing small explosive charges in the combustion chamber. I can't imagine how they did that.

That's right. I understand the idea was that if the F1 carried on working after the explosive charge was set off, then they could be certain that they had solved the instability.

[ka9q]

In electrical engineering terms, they conducted an "impulse response" test. I've done many as an EE, but never have I generated the impulse with an explosive...

[gwiz]

How do these people actually believe 100% that so many people could actually hold such a massive lie forever! Perhaps it's because they can actually tell lies themselves simply and fluidly, as easy as breathing each breath, with no remorse, no guilt, no shame. And perhaps they perceive it to be a normal 'state of affairs' so everyone else, without fail, is a habitual (and very, very, very, accomplished) lair themselves.

I seem to remember a psychological survey that showed that conspiracy theorists were indeed more likely than average to resort to underhand methods.

[onebigmonkey]

I forget which one of them it is, but at least one of these fraudsters routinely describes himself, and consequently gets described, as a 'NASA Consultant'.

The inference from this for people who don't think too hard about it is that he is employed in some capacity by NASA, when in fact the correct interpretation is that he is consulted by people about NASA. I could also describe myself as a NASA consultant, because when people on the internet ask questions about NASA, I answer them.

This does not means I have any connection with NASA, nor does it mean that my answers are necessarily correct.

[Luke Pemberton]

I forget which one of them it is, but at least one of these fraudsters routinely describes himself, and consequently gets described, as a 'NASA Consultant'.

Ralph Rene. You'll find the explanation here, but I've quoted the material below to save you visiting the site.

http://ralphrene.com/biography.html

Over a decade ago, the Rand Corporation contacted him pleading for contributions of free inventions or thoughts relating to space for NASA.  Two years later he received from the superintendent a free, full sized, thick, glossy page, full color NASA propaganda document.  To his complete surprise he found his name printed in the middle of page A-51.  To deserve this questionable honor, at least one of his ideas had to pass three sequential screening committees. By intonation this listing made it seem that he was a booster of a NASA's manned Mars mission.

If I recall, NASA did not contact him personally, it was a general invite for ideas and Ralph replied. Ralph does have two patents so he wasn't quite a useless sack of sorry DNA. I understand that being a patent holder would have led to him being 'invited' to put forward ideas as the RAND communication ramped up mass mailing from data searches of the patent records.

[JayUtah]

Is it true that when they fixed the fuel instability problem but the engineers were stilla little baffled by how it was caused in the first place? I understand injector plate designs based on what they believed would work were developed, but the actual cause was unknown (if that makes sense). I picked up this snippet in a BBC docudrama, but I find it hard to believe that the engineers just kept changing designs in the hope that they'd find something that worked.

There was very little theory in combustion mechanics through the 1950s and 1960s.  Quite a lot of it was trial and error.  There were at least a half-dozen baffle-plate candidates and three different orifice ratios and impingements that made it to the construction stage.  Today we understand a great deal more, but our FEA systems are still not at the point where we can do a full-up, accurate fluid-dynamics simulation of what happens inside a thrust chamber.