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According to this PDF Cassini-HuygensPDF, we would need to launch 70 tons if we wanted to do a Saturn transfer orbit from a low earth orbit. It says that there are no vehicles in NASA’s stable even remotely close to lifting 70 tons to orbit. Yeah, right! The Saturn V could lift 140 tons! Anyways, why are large rockets being rejected in favour of convoluted gravity assist trajectories (and it might not be cheaper because you have to keep the craft operating longer without science return) and are the cost savings worth other disadvantages to the gravity assist approach?

Update: I’ll make a new question because I wasn’t clear enough about it when I was asking before but I’ll keep it on for the same reason the AI doesn’t let you delete answered questions

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    $\begingroup$ The fact that the Saturn series hasn't been produced since the early 1970s is a factor you may want to consider. $\endgroup$
    – GdD
    Oct 17, 2022 at 12:27
  • $\begingroup$ From a cynical perspective, the last thing congresscritters wanted in the 1970s was a NASA that those congresscritters could not control and were forced to fund. The last Apollo missions were canceled in part because NASA was becoming too popular. Proposed launch vehicles even more powerful than the Saturn V were not funded, human missions to Mars were not funded, and production of additional Saturn V rockets itself was canceled. $\endgroup$ Oct 20, 2022 at 13:06
  • $\begingroup$ Extremely annoying 😠 $\endgroup$
    – A. N Asker
    Oct 22, 2022 at 23:05
  • $\begingroup$ @A.NAsker what they meant by that is no rocket in service right now can lift 70 tons. $\endgroup$ Oct 24, 2022 at 16:37
  • $\begingroup$ That had better change $\endgroup$
    – A. N Asker
    Oct 25, 2022 at 5:00

3 Answers 3

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There's a section with a lot of information about the end of Saturn in The Space Shuttle Decision, in the chapter "NASA's Uncertain Future," subheading "NASA and the Post-Apollo Future," starting page 94. As with many other "why didn't NASA [do what I want]" questions, the answer shakes out to the political environment of the time.

Initial Apollo Applications proposals did promote the continued use of Saturn vehicles:

during 1965 and 1966, the beginnings of a post-Apollo future began to take shape. Not surprisingly, its major features were in line with the initiatives that Webb had suggested in his report to Johnson. Apollo Applications emerged, strongly backed by [head of Office of Manned Spaceflight George] Mueller. For Mars, attention focused on an ambitious automated mission called Voyager that would orbit that planet and then send craft to land on its surface, looking with instruments for signs of life. Plans for Voyager flourished for a time. While initial designs called for use of the Saturn I-B, in October 1965 its officials decided instead to try for the much larger Saturn V.

The President's Space Advisory Committee (PSAC) made a report in reply to that and other proposals that NASA Administrator James Webb had floated in reply to a "what's next" inquiry from President Johnson. Various space centers and their administrators were pulling in different ways: von Braun and Gilruth wanted a space station, not a Mars mission. PSAC recommended continuing to build Saturn Vs at a rate of 4 per year post-Apollo, but also looked askance at the Saturn I:

The payload capabilities of the [Saturn I-B] are not significantly superior to those of the Titan III-M, while the launch costs of the [Saturn I-B] are about double those of the Titan III-M.... For the longer range, studies should be made of more economical ferrying systems, presumably involving partial or total recovery and reuse.

The music changed in 1967. Along with the Apollo I pad fire, the Vietnam War was proving extremely costly. NASA's budget was slashed and efforts were focused on finishing Apollo rather than large-scale future projects. No more super-heavy lift vehicles.

A whole additional answer could probably be put together from the fact that Mariner was launching on lighter rockets while the Saturn manifest was being used by Apollo, so a lineage of interplanetary probes got started on lighter rockets.

Another whole answer could probably be written about the difficulty of trying to directly compare ongoing program costs versus the cost of purchasing a Titan or Atlas when budgets are year-to-year; spending money over time at lower rates can be much easier to justify than the same total cost spent across fewer fiscal years.

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  • $\begingroup$ Very large rockets are coming back into vogue, though SLS isn't slated to fly any interplanetary probes as far as I know. Hard to say what Starship will do beyond Starlink and HLS. New Glenn/New Armstrong are even more of a wildcard. Saturn's definitely gone though. $\endgroup$
    – Erin Anne
    Oct 18, 2022 at 5:29
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    $\begingroup$ @A.NAsker Gravity assists aren't really that convoluted. Yes, there's a fair amount of math involved and you need to simulate a lot of different scenarios, but that's just table stakes to a space program. Getting a bigger rocket might let you cut corners on optimizing your course (a little), but it won't make it easier to analyze your stack for dangerous pogo oscillations or calculate whether your payload can survive max-Q. (Quite the opposite, in fact.) $\endgroup$
    – Cadence
    Oct 18, 2022 at 11:58
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    $\begingroup$ Basically, it boils down to a simple premise: math is cheap, rockets are expensive. If you can save some rocket by doing some math, it would be wasteful and irresponsible not to. $\endgroup$
    – Cadence
    Oct 18, 2022 at 12:03
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    $\begingroup$ @Cadence well put. hope to see more of you around Space SE $\endgroup$
    – Erin Anne
    Oct 19, 2022 at 22:25
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    $\begingroup$ Yet another whole answer could be written about Congress not wanting yet another J. Edgar Hoover over whom Congress had no control. (The control was the other way around; Hoover had photos and other evidence of congresscritters fooling around, taking bribes, et cetera.) Wernher von Braun was starting to look like another J. Edgar Hoover in the sense that von Braun's public appeal could force Congress to act in ways they didn't want to act. $\endgroup$ Oct 20, 2022 at 12:53
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Well, arguably it was used for the deepest space manned missions, Apollo. But that was its purpose, there weren’t just extras lying around for whatever nasa wanted and it didn’t have a payload faring designed for anything else.

Rockets like the falcon 9 are made as one size fits all, the Saturn V was a custom tailored design.

Thus, the preferred method is a one size fits all rocket, simply put it’s cheaper, and the gravity assists can allow it to gather much more data. If the voyager spacecraft had only gone to one planet they would have had little use to scientists.

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    $\begingroup$ I'm not sure if I agree with that last statement, if the Voyagers had been able to visit Jupiter and Saturn only it still would have been worthwhile, as it was the grand tour was a scientific bonanza. $\endgroup$
    – GdD
    Oct 17, 2022 at 12:42
  • $\begingroup$ Downvoted because I was asking about the advantages and disadvantages of the use of gravity assists rather than large rockets and you did not really answer that $\endgroup$
    – A. N Asker
    Oct 17, 2022 at 13:08
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    $\begingroup$ @A.NAsker - I don't see your question asking about the advantages and disadvantages of gravity assists. It appears to ask why super heavy lift vehicles weren't used, which is what Topcode has answered. $\endgroup$
    – Rory Alsop
    Oct 18, 2022 at 12:14
  • $\begingroup$ It asks about both $\endgroup$
    – A. N Asker
    Oct 22, 2022 at 11:11
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Can be answered reading NASA Planetary Decadal Survey (warning, 26 MBytes):

https://science.nasa.gov/files/science-red/s3fs-public/atoms/files/Decadal-Strategy-Planetary-Science-and-Astrobiology-2023%25E2%2580%25932032.pdf

See Chapter 22, the paragraph "Program balance considerations"

To summarize - there are many science groups advocating to launch probes to different places in Solar system. So usually the better balance is to lauch two or more missions to diverse destinations than to launch one expensive mission (in our case - expensive because of the rocket cost).

Up to the current time superheavy rockets costs were huge. Launching a probe on superheavy means you are spending money that could be used to build and launch a second probe to an anoter destination. Actually Europa Clipper mission was initially mandated by the Senate to launch by SLS, and science community wasn't happy with this, because it meant excessive spending. Now the plan was changed and Europa Clipper will fly on Falcon Heavy instead. It means longer flight to Jupiter, 7 years instead of 3, but significantly cheaper.

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    $\begingroup$ At over a billion dollars per launch (inflation adjusted), the Saturn V was uneconomical for anything but sending astronauts to the Moon. At over two billion dollars per launch (some say it's over four billion), the SLS is uneconomical even for that purpose. $\endgroup$ Oct 18, 2022 at 8:26
  • $\begingroup$ @DavidHammen the incremental per-launch cost for a SLS + the Orion needed to carry astronauts is \$4.1B. SLS alone would be \$2.2B, if there were a spare SLS core, which there isn't...they've all been allocated to SLS/Orion Artemis flights. oig.nasa.gov/docs/IG-22-003.pdf $\endgroup$ Oct 18, 2022 at 11:27
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    $\begingroup$ Note that on top of being uneconomical, SLS's large solid boosters caused excessive vibrations that made it unsuitable for Europa Clipper. So, it was too expensive on its own (by multiple entire interplanetary probe budgets), would have required expensive redesign work to reinforce Europa Clipper, and they would have spent so long waiting for a SLS core that Falcon Heavy would actually get it to Jupiter sooner. $\endgroup$ Oct 18, 2022 at 12:00

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