Any truth to these two stories during Apollo development?

[Everett]

I've read a pair of stories relating to Von Braun and the Saturn V /Apollo CSM development, and I'm not sure if they're fully true or not. (Note I'm going from memory here, so some inaccuracies may be my several year old memory.)

The first was that, midway through LM/CSM development, the spacecraft were overweight, compared to the design estimate. (Everything grows in weight during the design sequence.) The engineers are fighting over kilograms for various systems, and somebody decides to go and as Von Braun if a little weight could be saved from the booster. His reply was (at least along the lines of) "oh, it might be possible to save a couple of tons..."

The second, and related story, in that the CSM/LM combination was originally designed to mass 35 tons. Von Braun and his team are starting design on the booster, and are looking at the weight estimate and noticing that it's increasing almost daily, and decide that there's no way the combination will come out to 35 tons. So, they design the booster for 45 tons, and (this is where some interpretation comes in) more or less only told/allow those working on the CSM/LM stack to "learn of"/use the additional mass as needed. (So if they ran the mass budget dry, they got another ton or two, instead of simply saying "you have 45 tons to work with" at the start, and then allowing them to design for 45 tons and run the spacecraft mass over that.) This was a good thing, as the CSM/LM combination actually ended up weighing about 45 tons.

Did this actually happen? My guess is that there's a grain of truth that grew into part of space program lore.

[ka9q]

With regard to booster mass savings, the big question is where you'd save that much. It would be a lot easier on the S-IC first stage, but it would also turn into a smaller increase in payload mass. But a kilogram saved in the S-IVB would translate directly to a kilogram more for the payload.

Look at the flight reports for the Saturn Vs. Many S-ICs shut down with quite a few tons of propellant still in the tanks and lines, but real-world engineering demands margins and unavoidable losses. For example, at separation the S-IC on AS-506 (Apollo 11) had 934 kg of LOX in the tank, 14,717 kg of LOX below the tank in lines, and 3,616 kg of gaseous oxygen in the tank ullage. There were 6,759 kg of RP-1 still in the tank, 5,946 kg below the tank, and 250 kg in ullage (which may have been helium, I'd have to check). One reason for the large amount of LOX in the lines was that the LOX tank was above the RP-1 tank, with several large pipes running through it.

This may sound like a lot but it's only a small fraction of the 1,478,371 kg of LOX in the tank at ignition.

[bknight]

Indeed as from https://history.msfc.nasa.gov/saturn_apollo/documents/First_Stage.pdf
page 4, the engines burned more than 2000 gal of LOX each second, so they would need to leave some so any individual engine wouldn't starve and cause an abnormal pitch or yaw.

[QuietElite]

Indeed as from https://history.msfc.nasa.gov/saturn_apollo/documents/First_Stage.pdf
page 4, the engines burned more than 2000 gal of LOX each second, so they would need to leave some so any individual engine wouldn't starve and cause an abnormal pitch or yaw.

Indeed. Leaving some unburned fuel is probably better than a "rapid unplanned disassembly" of your rocket 

[Everett]

I'd guess that if both stories were true, the mass 'saved' was actually payload capacity that the design was capable of in the first place. Perhaps it's not so much a question of "saving" mass as "increasing" mass, with still enough delta-V left over to do the mission. (Lowering the altitude of the parking orbit by a few tens of km, perhaps?)
(If literally true, the he likely used the word "save" because he was asked if he could save any mass, and so that's the word he used in reply when he was specifically referring to increasing payload.)

[ka9q]

(Lowering the altitude of the parking orbit by a few tens of km, perhaps?)

Exactly this was done on some "J" missions (Apollos 15-17) to increase the payload capacity to the moon. Apollo 11 used a 185 km parking orbit. This is extremely low by usual standards, but they were in it only about 1.5 orbits and they also had some small thrust from propellant boiloff venting to help compensate. Apollos 16 and 17 used an even lower parking orbit of 166 km, buying them 317 kg of extra payload capacity (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090016334.pdf).

The fact that they used a parking orbit at all was a concession to practicality; it made the launch windows much wider, and it gave them a "plateau" in the mission to conduct systems checks before committing for the moon. It would have been even more efficient to go directly into a translunar trajectory with a subterranean perigee (which would already be behind them at shutdown).

[Allan F]

https://gwsbooks.blogspot.dk/2015/05/apollo15-staging-anomaly.html?m=1

Stageing incident on Apollo 15 - deleting the ullage motors on the 2. stage and the separation motors on the 1. stage wasn't such a good idea after all.

[bknight]

https://gwsbooks.blogspot.dk/2015/05/apollo15-staging-anomaly.html?m=1

Stageing incident on Apollo 15 - deleting the ullage motors on the 2. stage and the separation motors on the 1. stage wasn't such a good idea after all.

Spot on to the implication of the design changes.  From the post flight report http://klabs.org/history/history_docs/jsc_t/apollo_15_saturn_v.pdf
page xxii:

Analysis indicates that with an S-IC stage having only four
retro motors, failure of one retro motor to ignite would result in
marginal separation distance and in the 3g case, recontact of the two
stages. Consequently, S-IC-11 and subsequent stages will be equipped
with eight retro motors rather than the planned four.

I hadn't read that before today.

[Allan F]

I wonder what the implications of running the first stage all the way to engine shutdown by fuel starvation would be. Would the engines shut down in unison (which would be good) or one after another with varying thrust (which would be bad).

[ka9q]

I wonder what the implications of running the first stage all the way to engine shutdown by fuel starvation would be. Would the engines shut down in unison (which would be good) or one after another with varying thrust (which would be bad).

I've wondered the same thing. Obviously you're not worried about damaging the engines, but you don't really want to apply any body rates to the stack. The LVDC (IU computer) commands a "tilt arrest" before staging to keep the stack in a constant inertial orientation. I think this starts at center engine cutoff though I'm not sure. It continues through staging until after the interstage adapter and probably the tower are jettisoned. (There was significant concern about the interstage adapter striking the second stage engines as it slides off.)

Now that it's in vacuum, the guidance system finally switches to a closed-loop mode where it computes and commands the attitude needed to get to where it wants to be at shutdown from where it actually is at any given moment. Until then (including all of first stage flight) the IU blindly executes a pre-programmed attitude program necessary to minimize aerodynamic forces, especially through the max-Q region.

I.e., during first stage flight the IU doesn't care where it actually is or what direction it's going. It just gimbals the engines as needed to point in a certain direction at any given time. This starts with the yaw maneuver immediately after liftoff (to improve tower clearance), then the roll maneuver to launch azimuth, and finally the pitch program to fly a gravity turn at the lowest possible angle of attack.

[Jason Thompson]

I wonder what the implications of running the first stage all the way to engine shutdown by fuel starvation would be. Would the engines shut down in unison (which would be good) or one after another with varying thrust (which would be bad).

Funnily enough I've just read the Haynes Manual Saturn V edition (the Haynes Manual books on spacecraft and rockets are really good volumes, generally accessible to all but containing a lot of technical info), and it mentioned exactly this. Essentially studies had indicated that there was no reliable way to ensure that all five engines would run out of fuel or oxidiser at the same time, and that even if they could, a controlled shutdown was preferable to the uncontrolled splutter and choke of an engine running out of propellant.

I guess when you're dealing with millions of pounds of vehicle going at thousands of miles an hour with three people on it, you want to be sure every part of the flight happens in a controlled fashion, even if that has a penalty in terms of unused propellants.

[Jason Thompson]

I've wondered the same thing. Obviously you're not worried about damaging the engines,

I don't know about that. From what I've been able to glean about the design of the F-1 there was a significant risk of catastrophic failure if the turbopumps forcing the LOX and RP-1 into the combustion chamber suddenly found themselves trying to pump GOX or fuel vapour while at full speed for pumping liquid fuel and propellant. The sudden drop in fluid viscosity would likely lead to over-revving and disintegration, with significant risk of damage to adjacent engines or other components.

If I was riding it I'd rather know that the process involved a controlled shut-down than 'yeah, we ride it until the back end falls apart, then we stage...'

[Allan F]

Sensors in the fuel pipes which shuts the valves to the preburners when no more fluid is passing through?

[Jason Thompson]

Potentially, but that would have to account for the fact that the last dregs of propellant are probably not going to present a single simple transition from liquid to vapour, or may have bubbles in. Fluid dynamics are likely to be quite complex at that point, I'd have thought, especially with cryogenic propellants. It would be simpler (and hence safer) to just have a level sensor in the tank or a timer that shuts everything off at a certain point with a margin of unused propellant with all the lines still full.

[bknight]

IIRC it was the level sensor in the LOX and/or the fuel tanks that commanded the shut down.  More than likely the LOX tanks as they were further from the engines.  The appropriate decision to shut them all down at the same time, allowing the gas turbines to naturally slow instead if an abrupt stop, which would likely result in a catastrophe.