posted 03-05-2012 05:25 PM            

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.

posted 03-05-2012 05:42 PM               

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.

posted 03-05-2012 06:04 PM            

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?

posted 03-05-2012 07:03 PM               

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.

posted 03-07-2012 05:04 AM               

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.