KSP got it RIGHT.... amazingly so...

[kiwi1960]

One thing I hate in KSP is the concertina effect on my rockets...

NASA calls this the POGO EFFECT and apparently, it was a real problem that they had to sort out. It affected mainly the Apollo rockets.

Now I'm wondering if this was added to the game to add realism, or was it an accident... if the latter, then the science must be spot on if its creating in game real would problems...

Either way, even though I hate it, and I'm sure everyone else does too, a thumbs up to the designers for getting it right...

[K^2]

Completely different effects that happen to have similar appearance and potential outcomes purely by coincidence.

POGO effect is due to variations in pressure of fuel and combustion chamber. Since KSP does not model either of these, it cannot replicate POGO effect.

On the other hand, instead of a semi-rigid rocket body, you have perfectly rigid sections connected by spring-like constraints. The constraints can oscillate a little. If some driving force hits a resonance of one of these, your entire ship starts shaking itself apart, which is visually very similar to what happens due to POGO.

So yeah, not on purpose, but somewhat realistic nonetheless.

[Motokid600]

Did NASA ever find a solution to the pogo effect? Or was the solution to just shutdown engines. It's the pogo effect that broke Apollo 13, no?

[Albert VDS]

Apollo 13 happened because they reused Apollo 1 hardware.

[Nuke]

no, apollo 13 had a faulty wiring problem, a manufacturing defect.

[Motokid600]

Ah, Ok. But they did have to shutdown the center stage 2 engine because of pogo I believe.. couldn't have helped lol. Or was it two engines? Either way I can't get over how badass of a rocket the S5 was. Thing could make it to orbit without all cylinders firing. Simply awesome.

[Nibb31]

Did NASA ever find a solution to the pogo effect? Or was the solution to just shutdown engines. It's the pogo effect that broke Apollo 13, no?

POGO effect was mitigated by clever use of materials damping materials and acoustic effects. It had nothing to do with Apollo 13.

[Scotius]

I think they've just built safety valves into fuel pipes, so pressure changes wouldn't affect the combustion process in the engines.

P.S. Ninja'ed

[goldenpeach]

The center engine on Apollo 13 automatically shut-downed because of low pressure in the thrust chamber(according to the flight autopilot).

It it hadn't stopped, the rocket would have been destroyed.

If you want more info(because I don't say a lot of info in this post), I suggest you to follow this link:http://www.universetoday.com/62672/13-things-that-saved-apollo-13-part-5-unexplained-shutdown-of-the-saturn-v-center-engine/

[kiwi1960]

The POGO effect was cured BEFORE Apollo launched. They tested EVERYTHING before it was actually used.

They used a combination of science, physics and intelligence to sort it out. No Apollo mission was EVER affected by POGO...

To demonstrate, a short history lesson.

Apollos 1 -10 never actually landed on the moon, they were, initially, launched into earth Orbit, unmanned, for testing. Then they were manned, still in Earth Orbit, and then the last two (I think) were full dress rehearsals... they were manned, went around the moon but never landed...

Hence, Apollo 11 was the first Apollo craft to land on the moon.... in 1969 ... months before JFK's historic deadline ran out, to get to the Moon before the decade was out, he made that speech in 1961, so, between 1961 and 1969, they tested EVERYTHING....

sadly, in one of the tests, they had a fire and three brave souls died. So they redesigned the capsule....

So by the time they went to the Moon.... manned, they knew EXACTLY what would happen... anything that did happen.... like Apollo 13, was one of those things that couldn't have been foreseen BUT they had plans in place to deal with it, if not, the experience to nut it out fast....

POGO was one of those things that was dealt with to ensure the men went safe, and came back safe.

[Kryten]

The POGO effect was cured BEFORE Apollo launched. They tested EVERYTHING before it was actually used.

They used a combination of science, physics and intelligence to sort it out. No Apollo mission was EVER affected by POGO...

Sure.

Apollo 13 launched on April 11, 1970. During the second stage burn, two episodes of pogo occurred on the center J-2 engine as expected from previous missions, but the third occurrence diverged severely and acceleration at the engine attachment reached an estimated 34 g’s (the accelerometer went out of range) before the engine’s combustion chamber low-level pressure sensor commanded a shut down.

Tenchars

[Moach]

it wasn't EVERYTHING they tested, you know...

one particular bit of very critical hardware would self-destruct upon a single use, so the one carried aboard had really never been tested until it was actually used

that was the LEM's ascent engine, btw

they did test a bunch of exactly-alike models... but I don't think anyone ever felt much at ease with that whole concept.

well, we got *pointerToAVeryLongListOfKerbals stranded on the Mun to clearly depict just what is the worse that could happen (not counting the ones who are dead)

and Kerbal-Pogo might not be the same, but it sure is fun... well, fun for me - I don't think those k'nauts are screaming of excitement up there

Edited December 2, 2013 by Moach

[kiwi1960]

Sure.

Tenchars

POGO was cured, I never said eliminated. More like, it was controlled. The worst of it was cured. Nothing, as Apollo 13 proved, was perfect.

[Kryten]

There's 'not perfect', and then there's vibration severe enough to destroy vital parts of the vehicle. If that's how you define cured, your doctor must be making an absolute killing off of you.

[magnemoe]

What is the KSP effect we talk about, how stages kind of compresses during launch or re-ignite?

Or telescoping where the bottom stage pass trough upper stage blowing both part up.

[Albert VDS]

no, apollo 13 had a faulty wiring problem, a manufacturing defect.

Did I say Apollo 1... I meant Apollo 10. They used Oxygen Shelf from the A10 CM.

[Nuke]

in apollo 1 the wiring problem was in the crew module, in a pure oxygen environment. spark+velcro+pure oxygen environment at 1 atmosphere means boom. this happened during a ground test (i dont think the rocket was even fueled at that point).

apollo 13 had a faulty wiring problem in the o2 tanks, they blew up when they were stirred.

13 also had an engine shutdown, but that system worked the way it was supposed to. they just had to burn the other 4 engines a little longer to compensate.

i think everyones getting their faults mixed up. idk what any of this has to do with pogo.

[Kryten]

The engine shutdown on Apollo 13 was caused by pogo. The oscillations destroyed the engine.

[kiwi1960]

Why argue, are you wanting to be right all the time?

Cured, as in safe to use, the best they could do, having made it workable...

But nothing is space flight is ever 100% ... nothing is ever perfect....

I quote from "Apollo, expeditions to the Moon" (Dover Edition)

(Page 55)

The Perils Of Pogo

An important Marshall facility was the Dynamic Test Tower, the only place outside the Cape where an entire Saturn V vehicle could be assembled. Electrically powered shakers induced various vibrational modes in the vehicle, so that its elastic deformations and structural damping characteristics could be determined. The Dynamic Test Tower played a vital role in the speedy remedy of a problem that unexpectedly struck in the second flight of a Saturn V. Telemetry indicated the during the powered phases of all three stages a longitudinal vibration occurred, under which the rocket alternately contracted and expanded like a concertina. This "pogo" oscillation (the name derived form a child's toy) would be felt particularly strongly in the command module.

....

ONCE THE PROBLEM WAS UNDERSTOOD, A FIX WAS QUICKLY FOUND.

---

AFTER A FEW WEEKS OF HECTIC ACTIVITY, A POGO FREE SATURN FLIGHT NUMBER 3 SUCCESSFULLY BOOSTED THE APOLLO 8 CREW TO THEIR CHRISTMAS FLIGHT INTO LUNAR ORBIT

End quote.

I'd rather take the word of the Head of NASA that it was cured than you, any day of the week. Have a nice day.

[Kryten]

If something turns up again, it's not cured. Apollo 13 had pogo that was just as severe as that of Apollo 6. Seriously, what would you rather trust; a technical report commissioned by NASA itself on this exact problem, or a small part of book on the entire program written for the general public? It's not even got anything to do with the 'head of NASA', so I don't know what you're rambling on about there.

[goldenpeach]

The engine shutdown on Apollo 13 was caused by pogo. The oscillations destroyed the engine.

Not exactly.

They had POGO effect but it didn't destroyed the engine because it was shutted-down early, although it was a very close call.

[Kerbart]

Why argue, are you wanting to be right all the time?

Cured, as in safe to use, the best they could do, having made it workable...

But nothing is space flight is ever 100% ... nothing is ever perfect....

Probably because somebody insists on using the word "cured" when it obviously wasn't?

Mitigated? Maybe. But if your car starts shaking uncontrollable over 40mph and $2000 the garage calls you to tell you that they "fixed" it and it turns out that the "fix" is post-it note that says "do not exceed 40 mph" I doubt you're happily going to pay that bill and tell them "thanks for fixing it!"

Shutting down the engine before it rips apart the entire rocket N1 style is mitigation. It's not curing. Curing is preventing the problem from happening again. And obviously, it did happen again.

Semantics? Maybe. But most engineers are semantic hecklers because it makes the difference between a design that meets requirements and one that does not. (Well it did return the astronauts back to earth, didn't it? "Yes, but we meant ALIVE" Well you didn't say that anywhere!)

[Lohan2008]

no, apollo 13 had a faulty wiring problem, a manufacturing defect.

Ditto this ^

It is disrespectful to say NASA re-used Apollo 1 parts

[sjwt]

A little less over reacting please, and I think its a typo for Apollo 10

see the wiki..

"The oxygen shelf carrying the oxygen tanks was originally installed in the Apollo 10 Service Module, but was removed to fix a potential electromagnetic interference problem. During removal, the shelf was accidentally dropped about 2 inches (5 cm) because a retaining bolt had not been removed"

[rdfox]

OK, quick comment here. First off, POGO vibration (so named because it's similar to what you feel while riding a pogo stick--do they even still make those?) exists in *all* rockets, be they liquid- or solid-fueled. Solid-fuel rockets are less vulnerable to it purely because of their simplicity--they're just a big burning can of boom, so unless the vibration hits their natural resonant frequency and breaks up the casing, they're not going to break. Liquid-fueled rockets are more vulnerable because they have more complex parts, and thus have a lot more resonant frequencies to worry about.

The POGO-induced problems on Apollo 6 and Apollo 13 were *not* the same. On 13, the pogo oscillations in the second stage hit the resonant frequency of the stage's thrust structure, and got to a level that was threatening to damage it, so the automated systems performed an early Center Engine Cut Off to (successfully) try and break up that resonance. (CECO was a normal procedure on the first- and second-stages of the Saturn V, done to limit acceleration to 4.5g during the late stages of the burn without adding the complexity of throttling the engines; on 13, the second-stage CECO was performed early.)

On Apollo 6, however, the POGO vibrations hit the resonant frequency of two things--the Spacecraft/Lunar Module Adapter (the fairing over the Lunar Module that connected the Service Module to the third stage), and the fuel lines for the J-2 engines used in the second and third stages. This caused structural damage to the SLA (though not enough to cause a failure), but caused more serious issues for the engines. See, the engineers knew that the fuel lines would face heavy vibration and oscillation during their burns, and designed for this by putting two sections of rubber bellows in the line, which would allow it to flex and stretch/compress to absorb those distortions.

In ground testing, this worked just fine, and they had no trouble on Apollo 4, either. However, during Apollo 6, because the POGO was strong and at the right frequency, SOMEthing went wrong, and they lost two engines on the second stage due to fuel starvation, and the third stage engine refused to restart for the simulated Trans-Lunar Injection burn. Frantic analysis and ground testing in a vacuum chamber isolated the cause: the liquid hydrogen fuel was freezing the rubber in the bellows and causing them to crack under vibration. This hadn't been detected earlier because all ground tests to this point had been conducted in air, allowing ice to form on the bellows, which then damped out the vibration. In the vacuum chamber, with no water vapor to freeze, no ice formed, and the bellows ruptured, puking LH2 out into the stage itself and starving the engine of fuel.

A quick redesign followed, replacing the bellows with bends in the line that could provide similar flex, retrofitted to all the other Saturn Vs (there was plenty of time to do this before the next Saturn V launch), and no more problems were experienced with this failure mode for the remainder of the Apollo program.

Side note: The dropped oxygen shelf wasn't actually directly related to the actual Apollo 13 accident. The sole cause was that the subcontractor that made the thermostat for the service module oxygen tanks had failed to redesign it to operate on higher voltage after NASA decided--and informed all contractors and subcontractors--to change the spacecraft's ground power from 28VDC to 60VDC. When, following a tanking test, the #2 oxygen tank refused to vent on its own (possibly related to the drop, though it had vented fine in the past and would vent fine again after a later test), NASA used a standard procedure to vent it and safe the spacecraft--turn on the tank heaters to warm it up and boil off the contents. The thermostat, not designed for the higher voltage used on the pad, promptly welded itself shut, and instead of the heaters cycling on and off to warm up the tank, they instead stayed on continuously for EIGHT HOURS. Temperatures inside the tank soared (somewhere between 600 and 1000 degrees F), and the insulation on the wiring inside the tank burned off as a result. Thus, that tank became a bomb due to the now-exposed wiring inside it, and eventually, activating the stirring fan inside the tank caused a spark that ignited either some residual insulation or the tank lining, causing the tank to overpressurize too quickly for the burst disc to handle, resulting in the tank exploding, and the rest is history.