I came across the following section of the Saturn V Flight Manual (pdf):

Section about holddown arms on page 8-8 of the Saturn V Flight Manual

The intriguing part is that they used 12 pins to actually bolt down the Saturn V to the launch pad and the combined engine thrust would pull the pins through a die, gradually releasing itself from the pad, thus preventing a jolt when the holddown arms released.

I cannot find anything about a back-up for this (e.g. severing the pins with explosives), so it seems the principle was well understood. I'd image they'd want to test it anyway, but I fail to see how this setup can be tested without having an actual Saturn V, or at least the lower stage, providing the thrust. I can't imagine what that test rig would look like (except awesome), without being extremely dangerous.

On the other hand, not testing it seems also a big risk.

Question: how was this pin-through-die-based hold-down mechanism of Saturn V tested?

As I'm not a mechanical or rocket engineer, a possible answer can be along the lines of "it's a well known principle, applied many times, never fails".

  • 8
    $\begingroup$ Those historic Apollo program manuals are a serious source of distraction... $\endgroup$
    – Ludo
    Jul 29 '19 at 19:52
  • 1
    $\begingroup$ If you want huge forces, you use hydraulics. Those are well-understood, very controllable and enable a compact test rig. $\endgroup$
    – Hobbes
    Jul 30 '19 at 7:16
  • 1
    $\begingroup$ @Hobbes: I'm underestimating the power of hydraulics then? Or overestimating the power of the 5x F-1? 5x 6.77 MN thrust sounds like an awful lot to me, but I admit I don't have a good reference frame for those numbers. $\endgroup$
    – Ludo
    Jul 30 '19 at 11:15
  • 3
    $\begingroup$ Remember that TWR of a saturn V was only 1.15 - so the bolts would only need to hold down about 450 tons. $\endgroup$
    – Polygnome
    Jul 30 '19 at 22:44
  • 4
    $\begingroup$ Building on what @Polygnome wrote, you could test just one pin. Now you're down to < 40 tons. $\endgroup$ Jul 31 '19 at 2:02

This page describes in some detail the design of the holddown mechanism:

It doesn't say how they were tested but it says they were built by Space Corp. and where tested in Huntsville, with the final test being on May 25, 1965 and tested to make sure it could hold the thrust of 725,747 kilograms.

They didn't need to actually hold against the entire force of the engines because the weight of the fuel and rocket helped as well.

The first hold-down arm arrived at Huntsville on 31 October 1964, and testing began on 20 November. Due to a strike at a subcontractor's plant, the second arm, scheduled for delivery on 19 November, came on the 28th. On 17 May 1965, engineers tested the ability of the first hold-down arm to sustain a vertical thrust of 725,747 kilograms. After the successful completion of all other tests on this arm on 25 May, workers installed and aligned an operational set of hold-down arms on launcher 3 at KSC. The other hold-down arms were ready by the end of the year 38.

38Technical Progress Report Third Quarter CY 1965 (TR-250), 30 Sept. 1965, pp. 3-18.

It is possible that reference #38 will have additional information, but this document is difficult to find online.

On PDF page 10 of a 92 page packet of notes dated March 1, 1965 with handwritten pencil annotations included some notes on test results, although no specifics on how the test was conducted:


4 SATURN V HOLDDOWN ARM FAILURE: At approximately 12 noon on 2/27/65, a Saturn V holddown arm failed while being tested for stress and deflection under an upload condition. The failure of the upper link occurred when the arm had been uploaded to approximately 1,400,000 pounds. The test criteria calls for the. arm to be uploaded to a maximum of 1,600,000 pounds.

The casting material of the upper link of the arm has an ultimate strength of 178,000 p.s. i. Strain gauges on the link in the vicinity of the failure indicated stresses well below this value just prior to the time of failure.

A photograph is attached showing where the failure occurred. (ATTACHED to DR. VON BRAUN’S AND MR. WEIDNER’S COPIES ONLY.)

We are presently working with KSC and Materials Lab of P&VE to determine reason for failure.

enter image description here

This conference paper (full proceedings can be found here) describes the holddown design in a bit more detail, including a detailed drawing of the controlled release mechanism (i.e. the pins in question) and the remark that:

As the pin is drawn through the die at launch, the force decreases from 3.3 X 10^5 newtons (75,000 pounds) to zero after 15.2 centimeters (six inches) of travel.

enter image description here

The picture shows the tapered shape of the pin and this paper suggest that already in the 1920s they had developed fairly accurate models for computing the force required for wire drawing given material properties and the diameter of the wire at both ends of the die. Hence, it is likely that they optimised the shape for a linear force relation with a sufficiently large margin on the required force to avoid the rocket to be "stuck" on the pad. A simple test with hydraulics or other means would quickly verify if the shape was correct.

  • 3
    $\begingroup$ @Sean sometimes an answer can develop over time, let's see how this evolves. $\endgroup$
    – uhoh
    Jul 31 '19 at 2:12
  • 1
    $\begingroup$ @ BigMoneySeth I found some more information after reading your linked source. Since it seems to dovetail with what you have written so far I've appended it here. Feel free to edit further or roll back. $\endgroup$
    – uhoh
    Jul 31 '19 at 2:34
  • 1
    $\begingroup$ I stumbled upon the NAA Technical Reports Server... which is going to kill my productivity. A quick search only came up with this paper: ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760012107.pdf Which has only one very short paragraph on testing. @uhoh: as long-time member I ask you what is the acceptable norm with respect to editing (developing) answers - can I add stuff or better not? $\endgroup$
    – Ludo
    Aug 1 '19 at 7:45
  • 1
    $\begingroup$ @uhoh thank your the edit. And yes that happens all the time over in Stack Overflow. I just wanted to get something started I knew it was gonna be in there somewhere. $\endgroup$ Aug 1 '19 at 13:56
  • 2
    $\begingroup$ Great teamwork everyone. Thanks! $\endgroup$ Aug 5 '19 at 21:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.