Apollo 15 Saturn V mods to increase payload capacity & the GREAT potential hazard one of them caused

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Winston

Lorenzo von Matterhorn
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About Apollo 15 Saturn V modifications to increase payload capacity, the probable rupturing of the 1st stage's forward LOX tank dome due to one of them, and a possible 1st and 2nd stage collision if just one of the four (reduced from eight) 1st stage retrorockets had failed to ignite.

Apollo 15 Press Kit (lots of interesting content):

https://history.nasa.gov/alsj/a15/A15_PressKit.pdf

Part of that content:

Significant Vehicle (Saturn V) Changes

Saturn V vehicle SA-510 will be able to deliver a payload that is more than 4,000 pounds heavier than the Apollo 14 payload. The increase provides for the first Lunar Roving Vehicle and for an exploration time on the lunar surface almost twice that of any other Apollo mission. The payload increases were achieved by revising some operational aspects of the Saturn V and through minor changes to vehicle hardware.

The major operational changes are an Earth parking orbit altitude of 90 nautical miles (rather than l00), and a launch azimuth range of 80 to 100 degrees (rather than 72 to 96). Other operational changes include slightly reduced propellant reserves and increased propellant loading for the first opportunity translunar injection (TLI). A significant portion of the payload increase is due to more favorable temperature and wind effects for a July launch versus one in January.

Most of the hardware changes have been made to the first (S-IC) stage. They include reducing the number of retro-rocket motors [from eight to four), re-orificing the F-1 engines, burning the outboard engines nearer to LOX depletion, and burning the center engine longer than before. Another change has been made in the propellant pressurization system of the second (S-II) stage.

Three other changes to the launch vehicle were first made the Apollo 14 vehicle: a helium gas accumulator is installed the S-II's center engine liquid oxygen (LOX) line, a backup cutoff device is in the same engine, and a simplified propellant utilization valve is installed on all J-2 engines.

These changes prevent high oscillations (the "pogo" effect) in the S-II stage and provide more efficient J-2 engine performance. For Apollo 15 a defective cutoff device can be remotely deactivated on the pad or in flight to prevent an erroneous "vote" for cutoff.
[but in the post-flight crew debrief, they mentioned a 10-15 hertz minor vibration during the S-II and S-IVB stages which hadn't been experienced before; my guess is that the added payload had caused a different vehicle resonant frequency and that vibration might have been worse if not for the anti-pogo mods to the S-II which may have been a reason for the anti-pogo mods as the vehicle dynamics simulations may have predicted the problem. - W]

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Retro-rocket motors on the first stage were changed back to eight because the second stage wasn't far enough from the first at ignition and (apparently from tracking film) ruptured the LOX tank dome on the first stage. I'd love to see the onboard film of that, but can't find it online. From an airborne tracking camera:



Here's a fantastic, detailed article revealing once again the beauty of engineering and failure analysis:

APOLLO 15 - STAGING ANOMALY

https://gwsbooks.blogspot.com/2015/05/apollo15-staging-anomaly.html

Quoting from his book series which I'm definitely going to check out, "Growing up with Spaceflight- Apollo Part Two":

“Boosting Apollo 15 was a Saturn V quite a bit different from the vehicle that had boosted Apollo 14. Nearly all of the changes in the launch vehicle monikered as AS-510, had been made to save weight and add payload capability. Most apparent from the external perspective was the absence of the four ullage motors that had previously been located at 90 degree intervals around the skirt that attached the S-IC first stage to the S-II second stage.

“Those four solid propellant rocket motors were designed to fire .5 seconds after shutdown of the first stage as triggered by the Saturn V’s Instrument Unit (IU). Then .2 seconds later the IU commanded the firing of the shaped charges that would separate the first and second stages. The Ullage motors would continue to burn for a total of about 3.87 seconds producing about 22,900 pounds of thrust each. The objective of the motors was to apply a continued acceleration to the S-II stage and thus seat the fuel and oxidizer in the bottoms of their tanks. Data from past flights indicated that those four Ullage motors were not needed, so they were deleted from all future Saturn V launch vehicles beginning with AS-510. Internally, the S-II stage had another ullage system change when fixed orifices replaced its ullage pressure regulators.

“Meanwhile, down on the huge S-IC first stage, four of the eight solid-fuel retro rockets were deleted. The retro rockets aided the separation of the stages by countering any residual impulse from the spooling down of the F-1 engines and helped to counter the stage’s inertia. Data indicated that four retro rockets could do the job as easily as eight. Additionally the F-1 engines were “reorificed” beginning with AS-510 bringing the Saturn V’s total thrust up from 7.5 million pounds to 7.68 million pounds. All of these changes, although sounding somewhat large, only increased AS-510’s payload capability by 1,410 pounds, but 400 of those extra pounds allowed the Lunar Rover to ride to the Moon.

[big snip]

“In fact when the plume of the S-II engines contacted the S-IC it was so powerful that it knocked out the telemetry being transmitted from the departing stage. The Radio Frequency (RF) signal continued, but the antenna gain was muted. Such RF signals without gain would indicate that the cable bundles had been blown away, or that the RF canister itself had been damaged or blown away. This loss had no impact on the mission since the S-IC had already been jettisoned.

“The real alarm was sounded after the mission at MSFC when it was calculated that if one of the four S-IC retro rockets had failed to ignite the two stages could have re-contacted during the staging sequence. That would have resulted in a catastrophic failure of the S-II and a mission abort using the escape tower. It was immediately decided that the four retro rockets that had been deleted from the S-IC would be reinstated for the next mission; Apollo 16.”

Now - here is the part that I could not put into the “Growing up with Spaceflight- Apollo Part Two” book, because there is no NASA documentation to prove it
[this is where the onboard film would be revealing - W], there is only my own comparison of TV video.

So violent was the close proximity ignition of the five J-2 engines of the S-II stage that not only did it wipe out the telemetry RF units but it also ruptured the LOX tank dome on the S-IC stage. This is clearly seen in the staging footage as a secondary plume erupts from the S-IC that is not seen on any other Saturn V staging. In order to prove this I watched the staging videos from all of the Saturn V stagings where the event can be seen (Apollos 9, 12 and Skylab 1 were obscured by weather and Apollo 17 was a night launch so no plume was visible) and none have the brief secondary plume.

At staging there was 31,135 pounds of residual LOX in the tank, under pressure. At that point the S-IC was ~72Km or 44 miles in altitude and well outside of most of the earth’s atmosphere. The release of 31,135 pounds of LOX could easily account for the secondary plume seen in the video. That would mean that the upper LOX tank dome was compromised by the staging event.

Presented here is a side-by-side of the Apollo 14 staging sequence and the Apollo 15 sequence. The images are matched frame-by frame so you can see the difference:


[Go to the linked site to see the multiple graphs, figures and images]

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"Growing up with Spaceflight" book series:

https://www.amazon.com/s?k=Growing+up+with+Spaceflight&ref=nb_sb_noss_2

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Unrelated, but I found this in the process of researching the above. Two cameras were used. This is the real time view, not slo-mo. Apparently not Apollo 15 due to no LOX tank dome rupture or major plume:

"I have been wanting to do this for a long time, ever since I learned that there were 2 films made on the same flight from opposite sides of the rocket. I sped the original film up as near as I could to real time. The original was filmed at 15x normal speed. The lens on the cameras were very wide angle lenses. Took me a while to figure out how to rotate the video get the aspect ratio right, and have the 2 films match up. This was made even more of a pain as the 2 films have a slightly different frame rate and length. plus I think I had to flip one of them. Leaving this silent."

 
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