Index Home About Blog
From: Henry Spencer <>
Subject: Re: Shuttle Solid Boosters
Date: Sun, 13 Apr 1997 05:50:48 GMT

In article <1997Apr10.112208@telgar>,
C. James Cook <> wrote:
>>I don't remember a single Saturn blowing up because of F-1 failure.
>|> Well, that's because no Saturn ever blew up (although Apollo 13's came
>|> very close due to problems in the second stage)...
>If you have the time, what were the details of this incident?

Summed up quickly, most any history of the flight will tell you that the
center engine of the second stage shut down early.  What wasn't understood
until later was that this was a very lucky thing, because had it kept
running a few seconds longer, it would have either torn itself off its
mounts or broken the second-stage thrust frame, either of which would
probably have caused the disintegration of the stage.

A certain amount of mild "Pogo" oscillation had been seen in that engine
on earlier flights, and a Pogo suppressor had been designed for it, but
the Apollo 13 Saturn V had already been stacked by the time the suppressor
design was ready, and retrofitting it was awkward, so they decided to fly
one more time without it.  What wasn't suspected was that the earlier Pogo
episodes had been on the ragged edge of turning into a much more violent
form.  During the Apollo 13 ascent, that happened... and within two
seconds, there was a 500psi pressure oscillation going in the chamber of a
780psi engine, and two tons of engine (solidly bolted to a thrust frame)
were bouncing up and down three inches at 16Hz.  What saved the flight
was that average chamber pressure fell off enough to trip a low-pressure
switch, and the computer shut the engine down on general principles.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: smaller Saturn: was LM possible?
Date: Wed, 22 Oct 1997 14:29:26 GMT

In article <>,
Graham Nelson  <> wrote:
>> ...the Apollo 6 Pogo oscillation was much more severe than on Apollo 4
>> (where it was so mild that it went unnoticed until people went looking
>> after 6) mostly because 4's dummy was just a tub of ballast while 6's made
>> an attempt to duplicate the LM's mass distribution etc.
>What causes pogo-ing -- is it some kind of resonance effect?

It's an oscillation in which surges in engine thrust cause surges in
acceleration, which cause surges in fuel pressure in the plumbing, which
cause surges in flow rates, which cause surges in engine thrust.  It comes
in several flavors, with slightly different behaviors, but typically
occurs only when the natural frequency of such a cycle matches a resonant
frequency of the vehicle.  Unfortunately, the resonant frequencies of the
vehicle sweep over wide ranges as the tanks empty.

>Was it ever actually understood, or simply fixed in an ad-hoc
>way until all the Saturn rockets had been safely launched?

A little bit of both.

The way to cure Pogo is to add some damping to the cycle, typically by
adding a surge absorber to the plumbing -- just a chamber with liquid in
the bottom and gas in the top, connected to the plumbing, so surges in
pressure push liquid in and out against the gas pressure rather than being
communicated to the engine.  Modern rockets typically have such Pogo
suppressors built in from the start; for example, the SSME has a Pogo
suppressor in its LOX feed (LH2 is too light and too compressible to be
much of a problem -- it supplies its own damping), and has never had a
Pogo problem.

However, all this wasn't terribly well understood in the early 1960s.
Titan had quite serious Pogo, which didn't bother warheads much but had to
be fixed for manned flights, and some early work on Pogo suppression was
done for Gemini.  This came a bit late to influence the Saturn V design,

Pogo damping was added to the Saturn V first stage's outboard engines
after Apollo 6; it turned out that suitable chambers already existed in
the valves, so merely filling them with helium was all that was necessary.
The second-stage center engine was shut down a bit early, starting on
Apollo 10, to avoid a Pogo problem late in the burn that was first noticed
on Apollo 8.  (The first-stage center engine was always shut down early
for acceleration limiting, but that wasn't necessary on the second stage.)
And when a different Pogo problem was seen there on Apollos 11 and 12, and
became so violent on 13 that it nearly tore the engine off its mounts and
(luckily) did cause a premature shutdown, a Pogo suppressor was added to
that engine.  But there was never an attempt to thoroughly de-Pogo the
Saturn V, and in fact it continued to have minor Pogo at times.  Had there
ever been a second production run, changes might have been made then.
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

From: Henry Spencer <>
Subject: Re: Saturn 5 engines
Date: Sat, 14 Mar 1998 05:10:56 GMT

In article <6e177h$dvu$>, pat <> wrote:
>>The first few flights did not cut the second-stage center engine early.
>>On Apollo 8, a Pogo oscillation affecting that engine (and only that
>>engine) showed up very late in the second-stage burn...
>as i recall, that engine shut down essentially accidentally,
>or was the one on A-13?

It was on Apollo 13.  A slightly different form of Pogo showed up in that
same engine on Apollo 11, and showed up more strongly on Apollo 12.  A fix
(a Pogo suppressor on the center engine) was in the pipeline, but 13's
booster had already been stacked and retrofitting it was awkward, so they
decided not to bother.  And on 13, they discovered that the Pogo on the
previous flights had been teetering on the edge of a far more violent
form, which is what caused the shutdown.  (And 13 was lucky on that one:
had the engine's average chamber pressure not drifted far enough in a few
seconds to alarm the Saturn V's computer -- which wasn't smart enough to
detect the Pogo -- the second stage would probably have come apart in a
few more seconds.  That Pogo was *really* violent.)

>>...The Saturn V upper stages really needed a full set of
>>Pogo suppressors; they were never completely free of Pogo.
>so what's a Pogo supressor?

Essentially, you put a shock absorber in the propellant lines which are
resonating, to damp the oscillation.  In a LOX/LH2 stage, the problem is
invariably in the LOX system, because the LH2 is too squishy to have much
tendency to oscillate, and too light to cause much trouble if it does. So
you put in a chamber containing a few baffles, connect the bottom to the
LOX plumbing, and arrange for the top to be filled with gas, so any
oscillation makes the LOX slosh up and down through the baffles.  It's
fairly standard for a modern engine (e.g. the SSME) to have a Pogo
suppressor designed in from the start, just to head off any problem.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: Henry Spencer <>
Subject: Re: Saturn 5 engines
Date: Sat, 14 Mar 1998 04:58:25 GMT

In article <>,
Jason C Goodman  <> wrote:
>What's a Pogo oscillation?

Oversimplifying a bit...  Suppose combustion-chamber pressure surges a bit
for some reason.  That increases back pressure against the fuel coming in,
which slows fuel flow.  That reduces chamber pressure, which lets the flow
pick up again, etc.  There are also less direct interactions, e.g. greater
chamber pressure means greater thrust and higher acceleration, which
boosts hydrostatic head of fuel coming down from the tanks and tends to
increase flow.  Lots of feedback loops that might oscillate.

Now, if the natural frequency of one of those potential oscillations
happens to match a resonant frequency of the vehicle or the fuel-feed
system... you get oscillation, potentially violent (several Gs), at
relatively low frequencies, typically a few Hz.  That's Pogo.  (The name
is not an acronym; it comes from the resemblance of the motion to that
imparted by a Pogo stick.  And the name of the stick comes from Walt
Kelly's cartoon character.)

Such a match in oscillation frequency is particularly likely because the
resonant frequencies of the vehicle sweep over a wide range as the tanks
empty.  In fact, it's possible to have an oscillation come and go several
times in a single flight.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: (Henry Spencer)
Subject: Re: Saturn 5 pogo
Date: Fri, 4 Sep 1998 14:30:21 GMT

In article <6sn6s5$m6j$>,
 <> wrote:
>Until the problem was permanently fixed for A14, pogo was a persistent
>problem for the Saturn 5.

No, it was a persistent problem, period.  The Saturn V was never entirely
free of Pogo.  They cured the worst Pogo problems but never mounted a
systematic attack to eliminate it once and for all.

>however, different flights seemed to have suffered
>from pogo to varying degrees. AS501 hardly suffered at all, while AS502
>suffered engine shutdown on the SII and damage to the SLA panels.

Actually, although there were suspicions at the time, the first-stage Pogo
on 502 did not cause either the upper-stage engine trouble or the SLA
panel damage.  Those were unrelated problems.

The first-stage Pogo was fairly thoroughly cured after 502, and did not

>AS503 & 504 were reasonably uneventful...

Both 503 and 504 had Pogo in the second-stage center engine late in the
burn, which motivated the early cutoff of that engine on later flights.
(A less drastic fix was tried on 504 but didn't work.)

>but AS505 experienced vibration all the way
>through to S4B shutdown...

505 had no first-stage or second-stage Pogo, but had the only case of
Saturn V third-stage Pogo.  Since the third-stage Pogo had never been seen
before, they tentatively concluded that it was a fluke, requiring no
corrective action.  Although people continued to be nervous about it
throughout the rest of the program, it never reappeared.

>and the ills of AS506 are well-known!

Actually, 506 (Apollo 11) had a little bit of second-stage center-engine
Pogo, but only the engineers really noticed it.  The third-stage vibration
problems on 506 were not Pogo at all, but chattering in a vent valve,
which was fixed for later flights.

507 had more second-stage center-engine Pogo, and a suppressor was
designed.  But 508's Saturn V was already stacked, and retrofitting the
suppressor would have been awkward, and they decided to fly one more time
without it.

And so they found out just how bad Pogo could be, when 508's second-stage
center engine went berserk.  Within a few seconds, it came within a
hairsbreadth of tearing the second stage apart.  Fortunately, its overall
performance deteriorated badly enough in the process that the computer
decided something was wrong with it and shut it down.  This episode would
have gotten more attention if it hadn't been overshadowed by later events
on the same flight -- Apollo 13.

On later flights, the suppressor was fitted only to the second-stage
center engine, and the rest of the second-stage engines still showed a
little bit of Pogo at times.  A more thorough fix might have been
attempted if a second production batch of Saturn Vs had been built.

>Why was there this variation between flights?  Was it the payload
>distribution, the vibration characteristics of individual engines?

It's thought likely that the great increase in first-stage Pogo from 501
to 502 was due to payload changes -- 501 just had a tub of ballast where
the LM was supposed to be, while 502 had a more realistic dummy LM that
simulated the real LM's dynamic characteristics tolerably well.

The other variations were probably just a matter of individual engines and
individual stages.  While efforts were made to build them all the same so
that changes in properties would be minimized, Pogo is sensitive to small

>Also the
>Saturn 5 experiencing pogo seems to have come as a surprise; did this not
>show up in static testing?

Nope.  Vibration characteristics in a static test of a firmly-anchored
stage are very different from those of free flight.  There were attempts
to test for Pogo sensitivity, but the problem was not then well understood
and they carefully measured the wrong things.
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

From: (Henry Spencer)
Subject: Re: Saturn V Centre Engine Cutoff
Date: Thu, 18 Jan 2001 22:13:24 GMT

In article <kip96.1750$>,
Christopher M. Jones <> wrote:
>> Modern engines usually have Pogo suppressors designed in from the start,
>> but the problem was not well understood in the 50s and 60s.
>If you don't mind, how exactly do Pogo suppressors work?

There have been many design proposals, but the ones actually used
generally add, essentially, a captive gas bubble open to the liquid
somewhere near the pump inlet.  This basically acts as a spring, absorbing
small pressure surges rather than letting them propagate.

(More formally, it greatly lowers the first resonance frequency of the
propellant line, moving it down below the resonant frequencies of the
vehicle structure, while leaving the line's higher frequencies more or
less alone, so they stay up above the structure frequencies.)

Modern designs generally have a small cavity above the LOX feed line just
where it enters the pump, and tap a bit of warm oxygen gas out of the
engine to keep that cavity full of gas.  LOX is denser and less
compressible than most fuels, *especially* LH2, so it's more likely to be
the problem area, and a suppressor in the fuel line is often unnecessary.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

From: (Henry Spencer)
Subject: Re: Saturn V Centre Engine Cutoff
Date: Fri, 19 Jan 2001 20:26:15 GMT

In article <3a689039$>,
Thomas Hartline <> wrote:
>> None.  Its engines had Pogo suppressors from the start.
>Actually, I got a hold of a Critical Design Review package dated Sept 1976,
>which talks about adding the Pogo Suppresion System to the SSMEs.  The
>effectivity was supposed to be Test engines 0005 and subsequent and all MTPA
>engines.  I have also seen  old cut-away views of the original design
>submital which did not have the Pogo.

Oho, interesting!  That's early enough in the SSME history that the author
of the discussion I saw may not have known about it.

When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

From: (Henry Spencer)
Subject: Re: The Pre-Astronauts by Craig Ryan
Date: Sun, 21 Jan 2001 01:18:18 GMT

In article <94cnp7$5a$>,
Bruce Sterling Woodcock <> wrote:
>> If we did Saturn again now, we'd certainly put Pogo suppressors on all the
>> engines, which would eliminate those problems.  But the Saturns did
>> remarkably well, much better than their immediate predecessors.
>How did Saturn manage to fly with the Pogo Effect?  I was always
>under the impression this was a show-stopper unless you knew how
>to deal with it.

It doesn't always happen, and it's not always strong.  Some rockets have a
lot of it, some have hardly any.  The Saturns were, I would guess, the
last major rocket program which took the "we'll fix it if it happens"
attitude; nowadays, it is normal to install suppressors everywhere, even
though it does add a bit of mass and complexity, to prevent the problem
completely.  The problem with "we'll fix it if it happens" is that Pogo is
terribly sensitive to small hardware changes, so that even a careful test
program doesn't necessarily rule it out.  Of the first eight Saturn Vs,
no two had quite the same Pogo behavior.

The Saturn V ended up with suppressors on all its first-stage engines
(after Apollo 6) and on the center second-stage engine (after Apollo 13),
but not elsewhere.  The second stage continued to have minor Pogo late in
its burn, and people worried that the third-stage Pogo seen on Apollo 10
might come back (it never did).  Had there ever been a Block II run of
Saturn Vs, Pogo suppressors everywhere might well have been one of the
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

Index Home About Blog