Because as great as the Saturn V was, it had serious shortcomings stemming from several fundamental facts:
- It was a rush job. We went to the Moon less than eight years after Kennedy announced we would do it. That meant we had to assemble a working system using parts at hand where possible. Fortunately, the F1 engine had already been in development for several years, or we might have failed. The engine used by the service module was twice as powerful and three times as heavy, as ideal for a lunar mission—but it was already almost ready when we decided to use lunar orbit rendezvous. Okay, technically, that’s not part of the Saturn V, but it’s just an example. Similar examples exist all throughout the program.
- We had no prior experience building large boosters.
- We had very little prior experience doing anything in space.
- We had extraordinarily primitive computers, which not only meant heavier, simpler onboard computers, it meant lots of details that went into every aspect of the booster design we based on gut and experiment, without the fuller understanding that would be available today.
- We still used fairly crude, high-power analog systems for collecting and transmitting telemetry, which affected the entire design of the launch vehicle from batteries to wiring, to equipment bays, to radio wattage.
- The Saturn V was built around a fairly nascent high-performance rocket technology, which affected every aspect of its design.
Did you catch how Saturn’s upper stages taper? Those stages are smaller, so of course, they taper, right? Wrong. They taper because the S-IV upper stage was already in development, and it was easier to uprate it for the Saturn V than start over with a new booster. But remember this,
Making a rocket stage long and skinny makes it heavier. All things being equal, a squat, wide stage uses less material for the volume it contains, and the interstage used to connect it to its neighbors will also use less material if it has straight walls instead of a taper.
There is naturally a trade-off between material efficiency and aerodynamic cross-section, and various construction issues. We wouldn’t want to build a booster with spherical tanks, but it generally makes little engineering sense to narrow the stages on a launch vehicle—unless it’s needed to splice together a big launch vehicle from smaller existing parts.
This was common in the early days and has become rarer with time as the space industry has matured because it reduces efficiency and increases production costs. The space shuttle and NASA’s new SLS are both designed to be built using facilities built for the Saturn first stage. That saves money.
In the case of Apollo, production costs weren’t as important as the speed of delivery. The S-IVB upper stage was made by a completely different company than the rest of the booster—which added cost and increased opportunities for integration problems, but eliminated concerns over not being able to reuse jigs, etc.
The optimal design is for the entire booster to have the same diameter and have only a single tapering interstage below the payload—or accommodate any change in diameter in the payload fairing. As far as I know, this principle is used in all modern launch vehicles.
On the issue of rocket engines: Saturn used kerosene for its first stage because
- no one had built sufficiently powerful engines burning anything else, and
- kerosene is easy to store and move and holds a lot of energy per unit volume—so the tanks don’t need to be as big.
Using kerosene reduced the size of the booster, which saved more weight than it cost due to the choice of kerosene instead of hydrogen. The space shuttle switched to hydrogen, which in my inexpert opinion, was a mistake.
Hydrogen, overall, is fairly expensive to produce, store, and handle. It’s also rather dangerous, as it ignites over a wider range of fuel/air ratios than any other fuel I’m aware of. The result was that the shuttle’s performance enhancements from using hydrogen (which has the most energy per unit mass of any fuel) were eaten up by increased tankage mass and increased handling costs.
Space-X, in particular, in addition to using fundamentally more efficient engines, is pioneering what may turn out to be the big solution to this problem. They use methane, which is cheap and is midway between hydrogen and kerosene in terms of safety and energy per unit mass, and then they pre-chill it to pack more energy into the same unit volume. This is brilliant, and if they can do it routinely and safely, it could be a game-changer in the large space launch industry.
And NASA’s SLS? Well, at best, it could become what Saturn would have become had it remained in service. Right now, though, it frankly seems rather uninspired and is heavily handicapped by NASA’s need to spread contracts over as many congressional districts as possible. This limitation may ultimately destroy their ability to compete in economically large space projects.