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How had the Saturn V lifting capacity changed throughout the Apollo program? The Apollo 11 mission seems to have been very lightly loaded, enjoying only a short time on the surface, single EVA, and no moon buggy.

Would longer EVAs, a buggy, and more life support features have been technically possible for the Saturn V to have lifted in July 1969, or did future Saturn V vehicles have higher payload capacities than the one used to send Columbia and Eagle on their way?

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Small changes were made to the Saturn V launcher over the course of the program -- for example, redundant ullage rockets were deleted, reducing the mass of the interstage between first and second stages from 12,436 lbs on Apollo 8 to 9975 lbs on Apollo 17.

As Uwe notes, the propellant loading on the second stage is the most obvious difference flight-to-flight; the stage would be loaded with enough fuel to perform the mission, plus a safety margin, but no more.

There was a small revision to the fuel injectors in the first-stage engines that was intended to improve performance, flown on Apollo 15-17, but the difference was a fraction of a percent, in fact less than the variation between individual engines on a single rocket.

Up top, the descent engine nozzle of the lunar module was extended slightly on the later missions, which yielded slightly more thrust, and the fuel tanks were enlarged. On one mission, the longer nozzle was actually damaged when it touched down on uneven ground, but this was inconsequential, as the descent stage was done with its job at that point.

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  • $\begingroup$ Flight Evaluation Reports for Apollo lunar missions and Skylab (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730025115.pdf) show liftoff thrust actually increased to the point where engine averages would sit around 7,000kN. Page 59 of this source shows that propellant flow actually decreased by about 1.5% around the 35-38 second region where NASA obtained thrust estimates and adjusted for sea-level conditions. The drop is more pronounced on later flights, which may give the impression that later improvements did little to increase thrust. But yes, individual engines varied a lot. $\endgroup$ – Alastair Haslam Dec 9 '18 at 9:34
  • $\begingroup$ Do you know what the cause of that flow rate drop is? Intentional down throttle when peak thrust is no longer needed, or pressure drop in the tanks, or...? $\endgroup$ – Russell Borogove Dec 9 '18 at 13:58
  • $\begingroup$ The F-1 did not have active throttling so the drop is passive. By 38 seconds a Saturn V has burned through around 500 tonnes of propellant, freeing up almost a quarter of the first stage tank volume that could result in lower tank pressure. Also, early in the flight the pressures in the S-IC feedlines are higher as a result of the tremendous propellant mass above them, despite low acceleration. Late in the flight, it accelerates exponentially as the vehicle becomes lighter and gains thrust at altitude. $\endgroup$ – Alastair Haslam Dec 9 '18 at 14:54
  • $\begingroup$ At 35-38 seconds, there seems to be a sort of middle ground where the engines have yet to gain significant thrust due to lower ambient pressure at high altitude, while still pushing through the lower atmosphere at lowered tank pressures. On the propellant flow graph I mentioned the lowest point occurs at roughly 50 seconds, and not long after is Mach 1 and Max Q. This could indicate some correlation with air resistance lowering acceleration and, hence, reducing the ability for the decreasing liquid mass to push propellant to the engines. $\endgroup$ – Alastair Haslam Dec 9 '18 at 15:04
  • $\begingroup$ The description in the document reads: "For comparison of F-1 engine flight performance with predicted performance, the flight performance has been analytically reduced to standard conditions and compared to the predicted performance which is based on ground firings and also reduced to standard conditions." Importantly, ground firings are static and so cannot simulate the effect of increased feedline pressure to the engines as the vehicle accelerates. Thus, 'standard conditions' cannot account for higher pump pressure, flow rate or chamber pressure on flight vehicles. $\endgroup$ – Alastair Haslam Dec 9 '18 at 15:37
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The possible payload of Saturn V was carefully planed for the whole mission, but the difference of the payloads of the missions Apollo 11 and 17 was less than 3000 kg.

There is a nice table on this website here https://www.bernd-leitenberger.de/saturn-geschichte.shtml , the page is in german, but the table with the comparison of Apollo 8 to 17 is easy to be found.
In this table betankt means stage mass with fuel, leer means stage mass without fuel and Nutzlast means payload. All mass values are in kg. The thrust of the rocket engines was equal for all missions with the exception of Apollo 8 with slightly smaller thrust.
But the propellants mass of stage S II was raised by more than 34000 kg.

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    $\begingroup$ According to your linked page, the increase in payload capacity is due to three factors: 1) Decrease of dry mass (engineers managed to optimise the rocket structure?). 2) Increase of fuel mass (did the earlier Saturn V have unused capacity or was it due to the optimisations to the rocket). 3) The J-2 engine was improved from 1008kN to 1030kN maximum thrust. $\endgroup$ – DarkDust Feb 23 '17 at 12:42
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    $\begingroup$ Dry mass was decreased by removing unneeded features. The one that comes to mind is reducing the number of ullage and separation rockets between the stages. My go-to reference for mass breakdowns is "Apollo By The Numbers": history.nasa.gov/SP-4029.pdf $\endgroup$ – Russell Borogove Feb 23 '17 at 13:46
  • $\begingroup$ The tanks capacity seems to be unmodified, but they were filled closer to the maximum for the later missions. At the first missions, the amount of unused propellants left in the tanks was larger to prevent bubbles entering the turbopumps and rocket engines. With more experience, the rest volume was decreased. $\endgroup$ – Uwe Feb 24 '17 at 19:10

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