Would the Saturn V have been able to send more mass to TLI if it had a lower earth parking orbit?

Question

After reading into the topic a little, I've come across a number of increases to the Saturn V's LEO and TLI performance over the course of the Apollo program. Early moon missions sent between 133t and 137t to 185km parking orbits, and could then throw about 47t to the moon. Later missions (Apollo 15, 16 and 17) orbited 140t to 141t at 167km, and each sent around 48.6t to the moon. These later Saturns were slightly uprated compared to those of earlier flights.

So, had the SA-514 and SA-515 Apollo Saturn Vs been improved a little further for the later planned missions (Which were, of course, cancelled), how much mass would they have been able to send to 167km LEO and to the moon?

Also, could these LEO and TLI masses be increased by lowering the earth parking orbit to 150km? (This would depend on whether S-IVB hydrogen venting could overcome atmospheric drag during the time before the TLI burn...)

Answer 1

Yes, you would get a little more performance from a lower parking orbit. The standard is 100 nmi (185.2 km), but on MER (Mars Exploration Rover) we got the launch vehicle folk to lower the parking orbit to 80 nmi for, as I recall, on the order of a 1% increase in delivered mass to Mars injection. 1% might not sound like a lot, but every kg counts. Later we backed off to 90 nmi when we had better knowledge of the spacecraft mass.

We also got them to reduce the altitude at which the fairing was ejected.

We considered, but rejected, making the parking orbit elliptical.

We also allowed for a slightly lower probability of commanded shutdown, i.e. an increased probability of running out of propellant on the second stage. This could be accommodated probabilistically by spacecraft maneuvers, resulting in a net win.

These are all small changes, but when you are trying to eke out the last few kg as we were at one point on MER, you try stuff like this.

Answer 2

It's not clear what improvements you're thinking about for late Apollo missions. It's unlikely that significant further changes would have been made for two additional missions.

The performance increases across the flown missions came mostly from minor modification of the first-stage engine fuel injectors allowing a higher propellant flow rate and hence greater thrust, combined with somewhat greater fuel loads on all three launcher stages.

NASA was able to increase the weight of the Apollo payload stacks simply because they gained confidence over the course of the program that the launcher was reliably meeting and exceeding its performance specifications.

A number of future evolutionary developments of the Saturn V were considered. The first of these involved using uprated F-1A engines, plus stretched first and second stages, and would have increased payload to LEO by some 7 tons, but this would be an entirely new rocket, not a modification to the already built SA-514 or SA-515 vehicles.

As described in Mark Adler's answer, a lower parking orbit can give about 1% payload mass advantage on a translunar or Mars mission.

Answer 3

could these LEO and TLI masses be increased by lowering the earth parking orbit to 150km?

Yes. According to this paper*, a trade-off was done between mass-to-LEO, orbital lifetime and thermal limits:

The choice of 100 n.mi. orbit for the altitude of the circular orbit was arrived by a trade-off between launch-vehicle-injected payload capability and a combination of orbital lifetime and heating limits. The lower the parking-orbit altitude, the higher the launch-vehicle-injected payload capability. However, as the parking orbit is lowered (for consideration of possible dispersions), the minimum required orbital lifetime and the spacecraft and launch vehicle thermal limits are approached.

(emphasis mine)

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*) The paper is behind a paywall and I have not yet been able to find a copy of it. There may be more in-depth analysis in the remainder of the paper. If someone has access to it...