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  Saturn V hold down posts: dies and pins

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Author Topic:   Saturn V hold down posts: dies and pins
RAARO
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Posts: 3
From: New York, USA
Registered: Mar 2012

posted 03-05-2012 05:25 PM     Click Here to See the Profile for RAARO     Edit/Delete Message   Reply w/Quote
I have been reading a lot about the Saturn V vehicle launch sequence. Some documents talk about some kind of metal pins attached to the launch pad that are pulled through dies that are fixed to the bottom of the first stage. Apparently, this was to limit the acceleration for the first 150 mm just after hold down release.

I have been trying to find some more info on these pins and dies, but have found nothing more than that the amount of pins varied between flights.

Has anyone come across any info or picture on these devices? Maybe a technical document exist?

Tykeanaut
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From: Worcestershire, England, UK.
Registered: Apr 2008

posted 03-05-2012 05:42 PM     Click Here to See the Profile for Tykeanaut   Click Here to Email Tykeanaut     Edit/Delete Message   Reply w/Quote
When thrust had been confirmed by the onboard computers, the rocket was "soft-released" in two stages: first, the hold-down arms released the rocket, and second, as the rocket began to accelerate upwards, it was slowed by tapered metal pins pulled through dies for half a second. Once the rocket had lifted off, it could not safely settle back down onto the pad if the engines failed.
Source: Wikipedia. Any use?

RAARO
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From: New York, USA
Registered: Mar 2012

posted 03-05-2012 06:04 PM     Click Here to See the Profile for RAARO     Edit/Delete Message   Reply w/Quote
I was going to quote that in my first post, that's about as much info about it as I can find, there's a similar thing in the Apollo 15 flight summary.

I'm trying to find some documentation about why it was used, I mean why did the stage need to be soft released? Could it have something to do with the sudden acceleration at release being to violent and cause pogo oscillating or excessive sloshing of fuel?

That's about the only thing I can imagine it being used for, but it would be nice to know what the real story was, and maybe some before and after pictures of the pins...

mikej
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Posts: 374
From: Germantown, WI USA
Registered: Jan 2004

posted 03-05-2012 07:03 PM     Click Here to See the Profile for mikej   Click Here to Email mikej     Edit/Delete Message   Reply w/Quote
I did some research on this topic some time ago, planning on writing a page for my website (but hasn't quite progressed that far yet).

I've found that there's no real consistency with respect to what they're called: I've seen them called (slow, soft, or controlled) release (mechanisms, pins, or rods). This certainly complicates keyword searches!

Whatever they were called, they were used to "reduce transient stresses resulting from abrupt disengagement of a vehicle from its launch stand." This dynamic transient could be enough to damage the vehicle.

To prevent such damage, they put a number of dies on the S-IC's thrust structure which engaged pins mounted on either side of the holddown arms. After the holddown arms released the Saturn, the rods were stretched through the dies, slowing the initial acceleration over the first 6 to 7 inches off the pad.

As you mentioned, the number of slow release pins varied over time. SA-501 (Apollo 4) had 16 pins; it took 1.17 seconds to free the vehicle from the pins and limited the transient to +/- 0.2g.

The amount of force necessary to clear the pins was larger than anticipated. For SA-502, they reduced the number of rods from 16 to 12 and started lubricating the rods. It then took 0.54 seconds to break free, resulting in +/- 0.4g. SA-503 also used 12 rods.

Documentation after SA-503 is more limited, but I seem to remember an indirect reference on one vehicle (I think it was 506) indicating that the later vehicles used only 8.

Information on the liftoff transient loads are available in each missions' Flight Evaluation Report (available through the NASA Technical Report Server).

I never found any photos of the full and complete mechanisms. Photos of the portion of the die attached to the S-IC in the VAB are fairly common (once you know what to look for); a very good one is about 2/3 down the page on this blog posting by Jonathan Ward.

Diagrams of the mechanism are much easier to come by. Some of the better ones can be found at page 161 (page in PDF) of Mission Operation Report: Apollo 8 (AS-503) Mission, page 366 in the PDF of the 9th Aerospace Mechanisms Symposium, and page 70 in the PDF of Symposium on Transient Loads and Response of Space Vehicles.

Once you know what to look for, you'll also notice the dies on the S-IC's thrust structure in launch footage. If you happen to have the Mighty Saturns: Saturn V DVD set, there are plenty of clips which show the dies on the S-IC.

I have the original Spacecraft Films (i.e., before the Fox distribution deal) Mighty Saturns. Among the Apollo 8 footage on Disc 3 is a film clip which shows a pin being extruded through its die. In the frame, the pin is located right below the tail service mast. You can watch the pin get longer and longer, until it is lost as the dislodged frost obscures.

If you've read this far and are still interested in more, drop me an email and I'll send you a copy of my notes and an (even more) exhaustive list of diagrams.

RAARO
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From: New York, USA
Registered: Mar 2012

posted 03-05-2012 08:01 PM     Click Here to See the Profile for RAARO     Edit/Delete Message   Reply w/Quote
Exactly what I was looking for! Thanks!

One of those pins would be a really cool thing to have, the last thing that keept the mighty Saturn V on the ground! Thanks again!

moorouge
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Posts: 1490
From: U.K.
Registered: Jul 2009

posted 03-07-2012 05:04 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
This is not nearly as detailed as previous posts and comes from a reply by Doug Van Dorn on UnmannedSpaceflight.com:
The most fascinating pad-release mechanism I ever heard of was for the Saturn V. After the explosive bolts were blown and after the hold-down arms released the vehicle, the Saturn V still had one more process to go through. Huge soft-iron rivet-head bolts were attached to the rocket and through slots in the launcher which were smaller than the bolt heads. As the vehicle lifted off, these bolts were pulled through the slots and the soft iron was literally pulled through the slots, deforming like thick taffy as they pulled through. This attenuated vibrational shocks and especially any tendency for the rocket to lift off in very short, rhythmic "spurts" which could shake the whole stack dangerously. So, the final contact between a Saturn V and the ground was four huge soft-iron bolts that were forced to extrude through slots in the launcher.

moonguyron
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From: salado, tx, usa
Registered: Jan 2011

posted 03-09-2012 08:13 PM     Click Here to See the Profile for moonguyron   Click Here to Email moonguyron     Edit/Delete Message   Reply w/Quote
You can see a picture and read a description of the release mechanisms in the book Saturn V Flight Manual by Periscope Film. This is a copy of NASA's flight manual for vehicle 506. See page 8-8 and fig. 8-7.

mikej
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Posts: 374
From: Germantown, WI USA
Registered: Jan 2004

posted 03-10-2012 02:50 PM     Click Here to See the Profile for mikej   Click Here to Email mikej     Edit/Delete Message   Reply w/Quote
The Flight Manuals for SA-503 and SA-507 are available online through the NASA Technical Reports Server.

The diagrams are on page 190 of the SA-503 PDF and page 188 of 507's. (The SA-503 Flight Manual is a higher-quality scan.)

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