Introduction
The Saturn V rocket stands alone in the annals of human exploration. It was, and remains, the most powerful launch vehicle ever brought to operational status, a 363-foot colossus of engineering purpose-built for a singular, audacious goal: to land human beings on the Moon. In this, it succeeded perfectly. Every one of the thirteen Saturn V rockets that left Earth did so without a catastrophic failure, a record of reliability unmatched by any new heavy-lift vehicle. For a generation, its thunderous ascent from the Florida coast was the definitive symbol of humanity reaching beyond its terrestrial cradle. It was the machine that turned President John F. Kennedy’s political challenge into a physical reality, the apex predator of the Space Race.
Yet, the story of the Saturn V as merely the chariot of the Apollo program is incomplete. Even as Neil Armstrong took his first steps on the lunar surface, engineers and planners at NASA and its prime contractors – Boeing, North American Aviation, and Douglas – were already looking far beyond the Moon. To them, the Saturn V wasn’t an endpoint; it was the foundation. It was a proven, reliable, and astonishingly capable platform upon which a new, even grander era of space exploration could be built. This conviction spawned a breathtaking array of studies and proposals aimed at evolving the Saturn V into a family of launch vehicles tailored for every conceivable mission, from launching massive space stations into Earth orbit to propelling crewed expeditions to Mars.
These proposed rockets, the studied derivatives of the Saturn V, represent a tangible, engineered vision of an alternate history of space exploration. They were not idle sketches or back-of-the-napkin fantasies. They were the subject of formal, well-funded studies that produced detailed specifications, performance projections, and development plans. This family of unflown giants included concepts that incrementally improved the existing rocket by stretching its tanks and uprating its engines; variants that strapped on powerful solid or liquid-fueled boosters to achieve staggering lift capabilities; and streamlined two-stage versions designed to fill the gap between the Saturn V and its smaller cousins. The most ambitious proposals involved topping the rocket with exotic new upper stages, powered by nuclear reactors or high-energy chemical propellants, to open up the entire solar system.
This article explores that lost future. It digs into the designs, the missions, and the ambitions behind the rockets that could have followed the Apollo program. It answers the central question: What could have been? These studies offer a fascinating glimpse into a future where the momentum of Apollo was never lost, where the industrial and technical might that sent humans to the Moon was leveraged to build an even more robust and far-reaching presence in space. The story of why these giants never flew is not one of engineering failure, but of a significant shift in national priorities that grounded America’s deep-space ambitions for half a century.
The Foundation: Anatomy of a Moon Rocket
To understand the logic behind the proposed evolutions of the Saturn V, one must first appreciate the baseline vehicle itself. The Saturn V was not a single entity but a symphony of three distinct, highly specialized stages, each with its own role, its own technology, and its own industrial architect. Stacked together and topped with the Apollo spacecraft, the vehicle weighed over 6.2 million pounds and stood taller than a 36-story building. Its design was a masterclass in staged combustion, balancing the brute force needed to escape Earth’s gravity with the high efficiency required for an interplanetary journey.
The S-IC First Stage: Earth’s Unrivaled Powerhouse
The journey began with the S-IC, the Saturn V’s first stage. Manufactured by Boeing at the Michoud Assembly Facility in New Orleans, the S-IC was a machine of almost incomprehensible scale and power. Its sole purpose was to perform the most energy-intensive task of any space mission: lifting the fully fueled, 6.2-million-pound rocket off the launch pad and punching it through the thickest, densest part of Earth’s atmosphere. Standing 138 feet tall and 33 feet in diameter – the same diameter as the subsequent second stage – the S-IC was larger than any complete rocket that had come before it.
Its power came from a cluster of five Rocketdyne F-1 engines, which remain the most powerful single-chamber liquid-fueled engines ever successfully flown. Burning a combination of RP-1, a highly refined form of kerosene, and liquid oxygen (LOX), the five engines ignited in a carefully controlled sequence to generate a staggering 7.5 to 7.7 million pounds of thrust at liftoff. The choice of RP-1 was deliberate; while less efficient per pound than liquid hydrogen, its high density meant it could be stored in smaller, lighter tanks, a critical advantage for a first stage where structural weight and aerodynamic profile are paramount. The low-frequency roar from a static test firing of all five F-1 engines at the Stennis Space Center was so powerful it shattered windows in towns miles away.
For approximately two and a half minutes, the S-IC would burn through its more than 4.4 million pounds of propellant, accelerating the massive stack to a speed of over 6,000 miles per hour and an altitude of about 42 miles. Once its fuel was spent, explosive bolts would fire, and a set of eight small solid-propellant retrorockets at the top of the stage would ignite, pushing the S-IC away from the ascending vehicle. Its job done, the colossal stage would tumble back through the atmosphere and crash into the Atlantic Ocean hundreds of miles downrange.
The S-II Second Stage: The Liquid Hydrogen Pioneer
As the S-IC fell away, the second stage, the S-II, would take over. Built by North American Aviation in California, the S-II represented a significant technological leap. It was the largest and most powerful rocket stage ever built to use liquid hydrogen (LH2) as fuel. Liquid hydrogen is the most efficient chemical rocket fuel known, offering far more energy per unit of mass than RP-1. This high specific energy, or efficiency, makes it the ideal choice for upper stages, where every pound of vehicle mass saved translates directly into more payload or higher velocity.
However, LH2 presents immense engineering challenges. It must be kept at a cryogenic temperature of -423 degrees Fahrenheit, and its extremely low density means it requires vast, exceptionally well-insulated tanks. The S-II stage was essentially a gigantic, flying thermos bottle, with a common bulkhead separating its massive LH2 tank from the smaller LOX tank below it to save weight.
Propulsion for the S-II came from a cluster of five Rocketdyne J-2 engines, which, like the stage itself, were pioneers in the use of liquid hydrogen. In the near-vacuum of the upper atmosphere, the five J-2s produced a combined thrust of over 1 million pounds. After igniting moments after the S-IC separated, the S-II would burn for about six minutes. This long, sustained push would carry the remaining vehicle to an altitude of over 115 miles and accelerate it to a velocity of more than 15,000 miles per hour, just shy of what was needed to achieve Earth orbit. Like the first stage, once its propellants were exhausted, the S-II would separate and fall back to Earth, its fiery journey ending in the ocean.
The S-IVB Third Stage: The Versatile Orbiter and Trans-Lunar Ferry
The final propulsive step of the Saturn V was the S-IVB, a smaller but more versatile stage built by the Douglas Aircraft Company. The S-IVB had a unique and critical dual role in the Apollo mission profile. After the S-II stage separated, the S-IVB’s single J-2 engine would ignite for about two and a half minutes. This first burn provided the final push needed to place itself and the attached Apollo spacecraft into a temporary “parking orbit” around the Earth at a speed of about 17,500 miles per hour.
The vehicle would then coast for one or two orbits while astronauts and mission controllers performed a final checkout of all spacecraft systems. Then came the S-IVB’s most important task. At the precise moment, over the Pacific Ocean, Mission Control would give the command to restart the J-2 engine. This second burn, lasting over five minutes, was the Translunar Injection (TLI). It increased the spacecraft’s velocity to more than 24,500 miles per hour, breaking the bonds of Earth’s gravity and sending the Apollo crew on a three-day trajectory to the Moon.
The S-IVB’s design leveraged existing technology to save time and money. It was an evolution of the S-IV stage used on the earlier Saturn I rocket, and its use of the same J-2 engine as the S-II stage simplified logistics and development. After the TLI burn, the Apollo spacecraft would separate from the spent S-IVB. On later Apollo missions, the stage was often sent on a deliberate collision course with the Moon, its impact providing a seismic signal for instruments left on the surface by previous crews.
The F-1 and J-2 Engines: The Power Behind the Glory
The success of the Saturn V rested on the performance of its two revolutionary engine types, both developed by Rocketdyne. The F-1 was a monster of raw power. Standing 19 feet tall and with a nozzle diameter of over 12 feet, each engine generated 1.5 million pounds of thrust and had a turbopump that delivered propellant at a rate that could empty a swimming pool in under a minute. It was a masterpiece of brute-force engineering, designed to overcome the immense inertia and atmospheric drag of the first phase of flight.
The J-2, in contrast, was an engine of efficiency and finesse. As the largest hydrogen-fueled engine of its era, it was the key that unlocked the performance needed for high-energy upper stages. Its most critical feature was its ability to be shut down and restarted in the vacuum of space, a capability that was absolutely essential for the lunar orbit rendezvous mission profile chosen for Apollo. Without the J-2’s restart capability, the Translunar Injection burn would not have been possible with a single launch vehicle of the Saturn V’s size. In a remarkable epilogue to their service, the S-IVB stages from Apollo 13 through 17, each with its single J-2 engine, were intentionally impacted on the lunar surface, creating artificial moonquakes for seismic experiments.
This three-stage architecture, powered by these two engine types, formed the baseline vehicle that successfully carried out the Apollo and Skylab missions. It was this proven hardware that engineers saw as the starting point for an entire family of even more capable launch vehicles.
The triumph of Apollo 11 in July 1969 was a moment of unparalleled national pride and technological achievement. Yet, behind the scenes at NASA, the future was far from certain. The very success of the lunar landing had removed the primary driver of the agency’s massive budget and political support. The “race” had been won. The question that consumed NASA’s leadership and its army of engineers was: what comes next? The answer, they hoped, would be a seamless transition from lunar exploration to the establishment of a permanent human presence in space.
The vision for this post-Apollo era was breathtaking in its ambition. Planners envisioned a logical, step-by-step expansion outward from Earth. The first step would be the construction of a large, permanent space station in low Earth orbit, capable of housing dozens of scientists and astronauts. This would be followed by the establishment of a permanent base on the Moon, a scientific outpost and a proving ground for long-duration habitation on another world. The ultimate horizon goal, the grand objective that would shape technology development for decades, was a crewed expedition to Mars.
In early 1969, President Nixon convened a Space Task Group, chaired by Vice President Spiro Agnew, to chart this future course. The group’s report, delivered in September 1969, laid out a bold, integrated plan. It recommended the development of a reusable Space Transportation System (the Space Shuttle) for economical access to Earth orbit, the construction of a modular space station, and the formal adoption of a crewed Mars landing as a long-range national goal, to be achieved before the end of the 20th century. The Saturn V and its derivatives were seen as the heavy-lift workhorses that would launch the massive components for the space stations, lunar bases, and Mars-bound spacecraft.
This ambitious roadmap collided with a harsh new political and economic reality. The late 1960s and early 1970s were a time of social upheaval, and the immense cost of the Vietnam War was placing enormous strain on the federal budget. The political unity and sense of urgent competition that had fueled the Apollo program had evaporated. Lawmakers and the public began to question the value of spending billions on space exploration when there were pressing problems on Earth. NASA’s budget, which had peaked in the mid-1960s at over 4% of the federal budget, was slashed year after year. By the early 1970s, it had fallen to less than 1%.
The consequences of these budget cuts were swift and brutal. The first casualty was the Apollo program itself. Three planned lunar landing missions – Apollo 18, 19, and 20 – were cancelled to free up funds for the development of the Space Shuttle. This decision also effectively sealed the fate of the Saturn V. With no more lunar missions on the manifest beyond Apollo 17, and with the Shuttle being promoted as the future of space transportation, there was no longer a justification to keep the massive and expensive Saturn V production line running.
After the launch of the Skylab space station in 1973 on the 13th Saturn V, production was officially terminated. The initial order had been for 15 flight-capable vehicles. The two remaining complete rockets, SA-514 and SA-515, were never flown. Instead of carrying astronauts to the Moon or lifting components for a nascent Mars vehicle, they were turned into museum exhibits, becoming monumental relics of a future that was politically cancelled before it could begin. The shutdown of the assembly line was a definitive turning point. It wasn’t just the end of a rocket program; it was the abandonment of the technological path that the Saturn V derivatives were meant to follow. The engineering studies continued for a time, but without a production line to build the core vehicles, they became purely academic exercises. The disconnect between what was technologically possible and what was politically tenable had become a chasm. The ambition of the engineers had far outpaced the commitment of the nation’s leaders and the public, and the grandest machines ever conceived were left on the drawing board.
More of a Good Thing: Uprating the Core Vehicle
The most straightforward and logical path to increasing the Saturn V’s power was to simply make its existing components bigger and better. This evolutionary approach formed the basis of numerous studies conducted in the mid-1960s. The philosophy was to leverage the existing manufacturing base, launch infrastructure, and proven hardware to achieve significant performance gains with minimal cost and development risk. This family of concepts, often referred to as Modified Launch Vehicles (MLVs), focused on two primary areas: increasing the amount of propellant each stage could carry and improving the performance of the engines that burned it.
Stretching the Tanks: Increasing Propellant Capacity
A fundamental principle of rocketry is that a longer engine burn time results in a higher final velocity. The simplest way to achieve a longer burn is to carry more propellant. For a cylindrical rocket stage, this means making the propellant tanks longer, a process known as “stretching.” Engineers at Boeing, North American, and Douglas conducted detailed analyses on how to lengthen the tanks of all three Saturn V stages.
The S-IC first stage, with its massive 33-foot diameter, was a prime candidate. Studies examined stretching its RP-1 and LOX tanks by as much as 336 inches, or 28 feet. This modification would have increased its total propellant load from about 4.8 million pounds to a full 6 million pounds, significantly extending its burn time and providing a more powerful initial boost. Similar stretches were proposed for the S-II second stage, increasing its capacity for super-light liquid hydrogen and liquid oxygen.
This approach was not without its limits. The most significant constraint was the existing ground infrastructure, particularly the Vehicle Assembly Building (VAB) at the Kennedy Space Center. The VAB’s high bay doors and the maximum hook height of its massive overhead cranes imposed a practical limit of about 410 feet on the total height of a vehicle that could be assembled indoors. While some truly gigantic concepts proposed assembling the final payload and upper stages outside the VAB, this would have been a complex and weather-dependent operation. For most practical purposes, the VAB’s ceiling set a soft cap on how much the core Saturn V stages could be stretched.
The F-1A and J-2S: A New Generation of Saturn Engines
Concurrent with the studies on stretching the tanks, Rocketdyne was working on uprated versions of the F-1 and J-2 engines. These weren’t entirely new designs but rather improvements that incorporated lessons learned from the original production runs, utilized more advanced materials, and simplified manufacturing processes.
The F-1A was the proposed successor to the mighty F-1. By redesigning the engine’s turbopump and injector, engineers could increase its thrust from 1.5 million pounds to 1.8 million pounds. Just as importantly, the F-1A was designed to be simpler and less expensive to build than its predecessor, a key consideration for a sustained, high-flight-rate program. A Saturn V first stage equipped with five F-1A engines would have produced a staggering 9 million pounds of thrust at liftoff. This engine was a cornerstone of many of the most powerful derivative concepts.
For the upper stages, Rocketdyne developed and tested the J-2S. This was a simplified, non-gimbaling version of the J-2 that eliminated a number of components, making it more reliable and less costly. It also offered a performance boost, with thrust increased from 232,000 pounds to around 250,000 pounds. While early improvement studies had dismissed a modest J-2 upgrade as providing only a small payload gain, the more substantial performance of the J-2S made it a very attractive option for uprated S-II and S-IVB stages. The J-2S program reached an advanced stage of development, with several engines being built and successfully test-fired for thousands of seconds. Decades later, the technology and engineering data from the J-2S program would be resurrected to serve as the basis for the linear aerospike engine tested on the X-33 technology demonstrator vehicle in the 1990s.
The HG-3: A Glimpse at a High-Pressure Future
Beyond simply improving the existing engines, NASA also funded research into a true next-generation upper stage engine: the HG-3. Developed by Pratt & Whitney, the HG-3 was a conceptual leap forward. It was designed as a high-pressure, staged-combustion engine. This advanced engine cycle, where some of the propellant is burned in a pre-burner to drive the turbopumps at much higher pressures before being injected into the main combustion chamber, results in significantly higher efficiency and thrust.
The HG-3 was envisioned as a potential replacement for the J-2 engines on the most advanced Saturn V derivatives. A cluster of HG-3s on a second stage would have provided a dramatic increase in performance, enabling the rocket to lift far heavier payloads into orbit or send them on more energetic trajectories into deep space.
Although the HG-3 itself was never built, the research and development into its high-pressure staged-combustion cycle was not wasted. It laid the direct technological groundwork for the Space Shuttle Main Engine (SSME), later designated the RS-25. The RS-25, one of the most efficient and complex rocket engines ever built, powered the Space Shuttle for three decades and now serves as the core-stage engine for NASA’s new heavy-lift rocket, the Space Launch System (SLS). This creates a clear technological lineage, stretching from the ambitious Saturn V improvement studies of the 1960s all the way to the Artemis program’s return to the Moon in the 21st century.
These uprating studies collectively demonstrated a clear and practical path for the Saturn V’s growth. The philosophy was one of evolutionary development, a model where a proven system is incrementally improved to enhance its capabilities. This method of creating a “family” of vehicles from a common core – using a combination of stretched tanks, uprated engines, and strap-on boosters – is more economical and carries less risk than designing entirely new vehicles for each mission class. This same philosophy would later be successfully employed by some of the world’s most reliable and versatile launch vehicle families, including the Delta, Atlas, and Falcon rockets. The Saturn improvement studies were an early, large-scale application of this enduring and effective engineering strategy.
Raw Power: The Solid Rocket Booster Concepts
While stretching tanks and uprating engines offered significant performance gains, the most dramatic and powerful Saturn V derivatives were those that augmented the core vehicle with massive strap-on boosters. The concept is straightforward: for the first one to two minutes of flight, when the vehicle is heaviest and atmospheric drag is highest, large solid rocket boosters (SRBs) ignite alongside the main liquid engines, providing a colossal surge of additional thrust. Once they burn out, they are jettisoned, leaving the lighter core vehicle to continue its ascent. This approach was explored in a series of studies that produced some of the most powerful launch vehicles ever conceived.
The Titan Connection: The MLV-Saturn V-4(S)
One of the earliest and most extensively studied booster concepts was part of the “Modified Launch Vehicle” (MLV) program. Known as the MLV-Saturn V-4(S), this design proposed attaching four large solid rocket motors to the S-IC first stage. To minimize development costs and time, the studies focused on using existing hardware. The boosters selected were the UA1205 motors, 120-inch diameter, five-segment SRBs that were already in use on the U.S. Air Force’s Titan IIIC launch vehicle.
The integration was not a simple matter of just strapping them on. The immense thrust from the four SRBs, combined with the S-IC’s five F-1 engines, would place enormous structural loads on the core vehicle. The S-IC stage would have needed significant strengthening to withstand these forces. The resulting vehicle would have been able to lift heavier payloads, particularly for high-energy missions like a direct flight to the Moon or sending large probes into the solar system.
Engineers also studied more advanced versions of this concept. The MLV-V-4(S)A variant, for example, combined the four Titan SRBs with a stretched S-IC first stage and an uprated S-II second stage. This combination would have pushed the rocket’s payload capacity to low Earth orbit (LEO) to over 160 metric tons, a substantial increase over the baseline Saturn V’s 140-ton capability.
Scaling Up: The Saturn V-25(S)U
As NASA’s plans for post-Apollo missions grew more ambitious, so too did the designs for the launch vehicles needed to support them. A 1968 study by Boeing outlined a vehicle specifically designed to be the workhorse for a crewed Mars expedition: the Saturn V-25(S)U. This rocket was a significant step up in scale and power from the MLV concepts.
The core of the V-25(S)U was a heavily modified Saturn V. The S-IC first stage was to be stretched by an incredible 41.5 feet and powered by five of the more powerful F-1A engines. The S-II second stage would be strengthened to handle the increased loads from the massive payload destined for orbit.
The most prominent feature was the addition of four enormous 156-inch diameter solid rocket boosters. These were not off-the-shelf motors like the Titan boosters; they were part of a dedicated large solid motor development program and had already been successfully test-fired, proving their viability. The combined thrust of this behemoth at liftoff would have been over 16 million pounds. This immense power was necessary for its intended mission: launching the individual components of a nuclear-powered interplanetary spacecraft into Earth orbit for assembly. The V-25(S)U was designed to place a payload of nearly 250 metric tons into LEO, a capability required to lift the massive, hydrogen-filled tanks and nuclear reactor of the NERVA upper stage that would propel astronauts to Mars.
The Behemoths: The Saturn V/4-260 and the ELV
At the absolute peak of theoretical launch vehicle design were concepts that pushed the boundaries of what was considered possible with 1960s technology. The Saturn V/4-260 was perhaps the most powerful of these. It was designed around four colossal 260-inch diameter solid rocket motors – the largest solid motors ever successfully built and test-fired. These monsters, each over 21 feet in diameter, would be strapped to an improved Saturn V core.
The sheer size of these boosters created a unique and formidable engineering challenge. They were so tall that their attachment points would be halfway up the S-IC’s delicate liquid oxygen tank, creating structural loads that were nearly impossible to manage. Boeing’s engineers devised an ingenious, if complex, solution. Instead of stretching the S-IC stage, they would place additional propellant tanks for the S-IC’s F-1 engines on top of the solid rocket boosters themselves. In this “propellant cross-feed” system, the F-1 engines would first draw fuel and oxidizer from the tanks on the SRBs. Once those tanks were empty, they would be jettisoned along with the spent solid motors, and the F-1s would switch to drawing propellant from the S-IC’s internal tanks. This arrangement solved the structural load problem and dramatically increased the first stage’s burn time. The resulting vehicle would have had a liftoff thrust of over 36 million pounds and could have placed an astounding 362 metric tons into low Earth orbit – more than twice the payload of the original Saturn V.
A related, slightly less extreme concept was the Saturn V-ELV, or “Earth Launch Vehicle.” Also intended as a launcher for a Mars mission, it would have used a stretched Saturn V core augmented by four 120-inch UA1207 solid boosters, a type derived from the later Titan IV program. The ELV was designed to lift 200 metric tons to LEO, enough to support a robust campaign of interplanetary exploration.
The consideration of these massive, solid-augmented launch vehicles in the 1960s had a significant and lasting impact. The extensive engineering analysis of the aerodynamics, structural loads, acoustics, and ground support requirements for a vehicle combining a liquid-fueled core with large solid strap-on boosters provided a direct and invaluable foundation for the design of the Space Shuttle a few years later. The Shuttle’s architecture – a liquid-fueled core stage (the External Tank) augmented by two large SRBs – was not a novel concept developed in the 1970s. It was a direct technological descendant of the Saturn V improvement studies.
Furthermore, these studies revealed the hidden complexities of scaling up a launch vehicle. The sheer mass of a rocket like the Saturn V/4-260 would have required a completely new, more powerful crawler-transporter to move it to the pad. The acoustic energy from its launch would have been so immense that existing launch pads would have been destroyed; planners even considered building a new launch complex in the middle of a man-made lagoon to use the water to absorb the shockwave. These challenges demonstrated that at a certain scale, simply adding more power creates a cascade of secondary logistical and infrastructure problems that can be even more difficult and expensive to solve than the rocket itself.
Not all of the proposed Saturn V derivatives were designed to be bigger and more powerful. A significant branch of the family tree explored the opposite approach: creating smaller, more economical vehicles by removing one of the Saturn V’s core stages. These “Intermediate” Saturns were intended to fill the vast payload gap that existed between the Saturn IB, which could lift about 21 tons to LEO, and the full Saturn V, which could lift about 140 tons. This gap was seen as a major inefficiency; using a massive Moon rocket for a medium-heavy Earth orbit mission was like using a sledgehammer to crack a nut. By reconfiguring the existing Saturn V stages, NASA could create a modular family of launchers to suit a wider range of missions.
The Stubby Saturn: The INT-20
The Saturn INT-20 was the most prominent of these intermediate concepts. The design was simple in principle: remove the huge S-II second stage from the stack and place the S-IVB third stage directly atop the S-IC first stage. A new, conical interstage adapter would be needed to connect the 33-foot-diameter S-IC to the 21.7-foot-diameter S-IVB. The result would have been a shorter, “stubby” two-stage rocket with a powerful heavy-lift capability for Earth orbit missions.
The rationale for the INT-20 was primarily economic. It would provide a launch vehicle capable of placing between 60 and 70 metric tons into LEO, a payload class ideal for launching large space station modules, heavy scientific observatories, or logistics flights to an orbiting outpost. This capability would come at a fraction of the cost of a full Saturn V, since it eliminated the expensive S-II stage entirely.
A key engineering consideration for the INT-20 was that the S-IC first stage was now massively overpowered for the much lighter vehicle it was lifting. The rapid acceleration would have created excessive aerodynamic stress on the vehicle as it flew through the lower atmosphere. To solve this, engineers studied versions of the INT-20 that used fewer than the full five F-1 engines on the first stage. A three-engine variant was considered, as was a five-engine version that would shut down two or three of its engines partway through the ascent. The most practical and widely studied configuration was a version with four F-1 engines, which provided a good balance of liftoff thrust and manageable acceleration.
The Skylab Precedent: The INT-21
A second intermediate concept, the Saturn INT-21, took a different approach. This two-stage vehicle would consist of the S-IC first stage and the S-II second stage, but would omit the S-IVB third stage. This configuration was designed for missions that required lifting the absolute heaviest possible payloads to low Earth orbit, but did not require the S-IVB’s ability to perform an in-space burn for injection to a higher orbit or an interplanetary trajectory. The INT-21 could deliver a payload of around 116 metric tons to LEO, nearly the same as the three-stage Saturn V, making it an ideal launcher for a single, massive space station.
Unlike all the other proposed derivatives, a vehicle almost identical to the INT-21 was actually flown. On May 14, 1973, the Saturn V designated SA-513 lifted off from the Kennedy Space Center carrying its final payload: the Skylab space station. Skylab itself was constructed from a modified S-IVB stage, with its hydrogen tank converted into living quarters and laboratory space for the astronauts. Because the payload was a non-propulsive version of the third stage, the launch vehicle that carried it was, in effect, a two-stage rocket consisting of an S-IC and an S-II.
This Skylab launch vehicle is often referred to as an INT-21, and for all practical purposes, it was. The only technical difference between the rocket that launched Skylab and the formal INT-21 study was the location of the Instrument Unit (IU), the ring-shaped computer that served as the rocket’s brain. In the INT-21 study, the IU would have been relocated to the top of the S-II stage to control the rocket. For the Skylab launch, because the payload was a modified S-IVB, the IU remained in its normal position atop the payload, serving as the control system for both the launch vehicle and the orbiting station.
The INT-20 and INT-21 concepts highlight a strategic vision for a flexible and cost-effective national launch capability. By mixing and matching a common set of flight-proven stages, NASA could have created a family of vehicles capable of launching a wide spectrum of payloads. This modular approach would have maximized the return on the immense investment made in the Saturn V’s tooling, manufacturing facilities, and launch infrastructure. The subsequent decision to retire the Saturn family entirely and focus solely on the Space Shuttle represented a complete departure from this philosophy. Instead of a versatile family of vehicles tailored to different missions, NASA opted for a single, “one-size-fits-all” system. While the Shuttle was a technological marvel, its fixed payload capacity proved to be too small for the truly massive payloads envisioned in the post-Apollo plans and too large and expensive for many smaller satellite launches. The abandonment of the modular Saturn family approach left the United States without a super heavy-lift launch capability for nearly five decades.
Reaching for the Planets: Advanced and Exotic Upper Stages
The grandest ambitions of the post-Apollo era were not focused on Earth orbit, but on the planets. Crewed missions to Mars and ambitious robotic probes to the outer solar system would require sending massive payloads on high-energy trajectories, a task that pushed the limits of even the uprated Saturn V concepts. To meet this challenge, engineers looked beyond the Saturn family’s own components, proposing to top the rocket with entirely new, high-performance upper stages powered by the most advanced propulsion systems of the day. These concepts represent the most forward-looking of all the Saturn V derivatives, vehicles designed not just for the Moon, but for the solar system.
The Nuclear Option: The Saturn V-NERVA and the Mars Mission
The centerpiece of NASA’s plans for a crewed mission to Mars was a revolutionary propulsion technology: the nuclear thermal rocket. The NERVA (Nuclear Engine for Rocket Vehicle Application) program was a joint effort between NASA and the Atomic Energy Commission to develop a flight-ready nuclear engine. The principle was both simple and powerful. Instead of a chemical reaction, a compact nuclear reactor would be used to heat a propellant – in this case, liquid hydrogen – to extremely high temperatures. This superheated hydrogen would then be expelled through a nozzle to generate thrust.
The advantage of a nuclear thermal rocket is its extraordinary efficiency. Because the energy source (the reactor) is separate from the propellant (the hydrogen), the propellant can be chosen for its ideal properties, namely low molecular weight. The result is a specific impulse – a measure of engine efficiency – of around 825 seconds, roughly double that of the best chemical engines like the J-2. This means a nuclear rocket can produce the same change in velocity using far less propellant, or achieve a much greater change in velocity with the same amount of propellant. For long, high-energy interplanetary missions, this advantage is transformative.
Numerous studies focused on replacing the Saturn V’s S-IVB third stage with a NERVA-powered stage. This vehicle, sometimes called the Saturn C-5N, would have dramatically increased the rocket’s ability to send heavy payloads into deep space. The NERVA stage was the linchpin of the entire crewed Mars mission architecture of the late 1960s. The mission plan was complex, requiring multiple launches to assemble the interplanetary vehicle in Earth orbit. Several launches of an uprated, booster-assisted Saturn V, such as the V-25(S)U, would be needed to lift the various components: the crew habitat, the Mars landing vehicle, and several fully-fueled NERVA stages. Once assembled in orbit, these NERVA stages would fire in sequence to propel the expedition on its months-long journey to Mars.
The NERVA program was remarkably successful. Over more than a decade, a series of reactors and complete engine systems were built and ground-tested in the Nevada desert. These tests demonstrated stable operation, high performance, and the ability to restart, a key requirement for a Mars mission. By the end of the program, engineers had tested an engine, the NERVA XE, that was deemed to have met all the requirements for a human mission to Mars. The technology was ready for flight integration. However, like the Saturn V itself, the NERVA program fell victim to the shifting political winds and budget cuts of the early 1970s. The program was cancelled in 1973, just as it was reaching maturity.
The Deep Space Workhorse: The Saturn V-Centaur
While NERVA was the high-end option for crewed interplanetary flight, planners also studied a more conventional, near-term solution for launching heavy robotic probes: the Saturn V-Centaur. The Centaur was America’s first high-energy upper stage, pioneering the same liquid oxygen and liquid hydrogen propellant combination used on the Saturn’s S-II and S-IVB stages. Developed by General Dynamics, the Centaur had a long and storied history as a reliable and powerful upper stage for the Atlas and Titan families of rockets. It was the Centaur that launched many of NASA’s most iconic robotic explorers, including the Surveyor landers to the Moon, the Viking missions to Mars, and the Voyager grand tour of the outer planets.
A 1968 study from the Marshall Space Flight Center proposed placing a Centaur stage inside the payload fairing of a Saturn V, where it would act as a high-energy fourth stage. After the S-IVB placed the Centaur and its attached payload into a parking orbit, the Centaur’s own engine would ignite to provide the final, highly efficient “kick” needed to send a heavy spacecraft on a fast trajectory to Jupiter, Saturn, or beyond.
This Saturn V-Centaur combination would have offered a 30% performance improvement for deep space missions compared to the standard three-stage Saturn V. It could have launched a 39-ton payload on a Mars flyby trajectory or sent probes to the outer planets that were far larger and more capable than what was possible with the Atlas-Centaur or Titan-Centaur rockets. While the Centaur was briefly flown on early Saturn I test flights (where it was designated the S-V stage and carried only water as ballast), the Saturn V-Centaur combination was never built. NASA opted instead to use the less powerful but readily available Titan IIIE-Centaur for its outer planet missions in the 1970s.
These studies to integrate NERVA and Centaur with the Saturn V reveal a pragmatic and mission-focused engineering philosophy. NASA’s planners were not constrained by programmatic boundaries. They looked across the entire American aerospace landscape for the best available tools to accomplish their goals. The Saturn V was the powerful first and second stage; NERVA and Centaur were the specialized, high-performance upper stages for interplanetary travel. The willingness to integrate these disparate systems, developed by different contractors for different initial purposes, demonstrates a flexible and ambitious approach to building the most capable launch systems possible. It was a vision of a truly integrated national space exploration capability, with the Saturn V platform at its core.
A Hybrid Future: The Saturn-Shuttle
As the 1960s drew to a close, NASA’s focus began to shift from the expendable, single-use philosophy of the Apollo program to a new paradigm centered on reusability and routine access to space. The result of this shift was the Space Transportation System, or Space Shuttle. During the early 1970s, as the Shuttle’s design was being finalized, engineers explored a fascinating hybrid concept that attempted to bridge the gap between the old world of Saturn and the new world of the Shuttle. This concept, known as the Saturn-Shuttle, proposed using the massive first stage of the Saturn V as a booster for the Space Shuttle Orbiter.
A New Kind of Booster
The final design of the Space Shuttle consisted of the winged Orbiter, a large, orange External Tank that fed propellant to the Orbiter’s main engines, and two solid rocket boosters (SRBs) strapped to the sides of the tank. The SRBs provided the majority of the thrust at liftoff. However, before this design was finalized, NASA considered an alternative to the SRBs: a modified Saturn V S-IC stage.
In this configuration, a large interstage adapter would be mounted on top of the S-IC. The Shuttle’s External Tank would then be attached to this interstage, with the Orbiter mounted to the side of the tank, much like in the final Shuttle design. At liftoff, the S-IC’s five F-1 engines would ignite, lifting the entire stack. After about two and a half minutes, the S-IC would separate and fall into the ocean, and the Orbiter’s own three main engines would continue to fire, drawing propellant from the External Tank to push the Orbiter into orbit. This would have created a two-stage-to-orbit system, leveraging the proven power of the Saturn V’s first stage.
The Flyback S-IC
The most ambitious and visually striking version of the Saturn-Shuttle concept took the idea of reusability a step further. In this “flyback” configuration, the S-IC stage would be fitted with large, deployable wings, a tail, and its own jet engines for powered flight in the atmosphere. After boosting the Shuttle stack and separating at high altitude, the massive stage would deploy its wings, start its jet engines, and fly back to the Kennedy Space Center. It would then land horizontally on a runway, like an airplane, where it could be inspected, refurbished, and prepared for its next launch.
This was an incredibly complex proposal. A detailed study concluded that the S-IC would need a wing with a surface area of at least 700 square meters to generate enough lift for a controlled landing. The added weight of the wings, landing gear, and jet engines would have reduced the overall payload capacity, but the potential cost savings from reusing the most expensive part of the booster stage were thought to be significant.
Rationale and Cancellation
The driving force behind the Saturn-Shuttle studies was the search for a lower-cost, reusable booster system for the Shuttle program. The Saturn V’s S-IC was a known quantity with a perfect flight record. Using it, even in an expendable form, was seen as a potentially less risky alternative to developing the large, new solid rocket boosters that were ultimately chosen. The flyback S-IC represented the ultimate expression of the reusability goal that was at the heart of the Shuttle program.
Ultimately all versions of the Saturn-Shuttle were cancelled. The engineering complexity and development cost of the flyback S-IC were deemed to be prohibitively high. Even the simpler, expendable version was judged to be more expensive over the life of the program than the solid rocket booster approach.
The Saturn-Shuttle studies represent a fascinating and critical moment of transition in NASA’s history. They were an attempt to merge two fundamentally different philosophies of spaceflight: the Apollo era’s reliance on expendable, super-heavy-lift rockets and the coming era’s focus on a reusable space transportation system. These studies show engineers grappling with this paradigm shift, trying to adapt the powerful and proven hardware they knew so well to the new requirements of the future. The rejection of the Saturn-Shuttle concept in favor of the SRB-based design marked the final, definitive end of the line for the Saturn V’s hardware. It was the moment NASA fully committed to the unique architecture of the Space Shuttle, closing the book on the Saturn family and its potential evolution forever.
The End of the Line: A Legacy of Unflown Giants
The vast and varied family of Saturn V derivatives remains one of the great “what-ifs” of space exploration. These were not mere fantasies; they were the products of rigorous engineering analysis, backed by a national industrial capacity that had just achieved the impossible. The reason these powerful rockets never flew has little to do with their technical feasibility and everything to do with a fundamental shift in national priorities that occurred in the shadow of Apollo’s success.
The primary cause was the collapse of the political consensus that had driven the space race. With the lunar landing achieved, the Cold War imperative that had justified NASA’s massive budget evaporated. The Vietnam War and domestic social programs competed for federal funds, and NASA’s budget was cut dramatically and repeatedly. The ambitious, multi-decade vision of the Space Task Group – with its space stations, lunar bases, and Mars missions – was deemed unaffordable. The political decision was made to focus on a single, more narrowly defined goal for the post-Apollo human spaceflight program: a reusable, LEO-centric vehicle that promised routine, economical access to space. That vehicle was the Space Shuttle.
This strategic shift led directly to the most critical and irreversible decision: the shutdown of the Saturn V production line. A rocket is more than just a set of blueprints; it’s a complex ecosystem of specialized tooling, supplier networks, and a highly skilled workforce with years of hands-on experience. Once the Michoud Assembly Facility was retooled, the massive jigs and welding machines for the Saturn stages were dismantled, and the thousands of engineers and technicians who built the rocket moved on to other projects, that industrial capability was effectively lost. Restarting production years later would have been nearly as expensive and time-consuming as designing a new rocket from scratch. The physical capacity to build the giants was gone.
Yet, the immense engineering effort that went into the Saturn V derivative studies was not wasted. The concepts, analyses, and technological paths explored in those studies created a deep well of knowledge that directly influenced the design of American launch vehicles for the next 50 years. The legacy of these unflown giants is written in the DNA of the rockets that followed.
The extensive studies on augmenting the Saturn V with large solid rocket boosters provided the technical confidence and design heritage for the Space Shuttle’s iconic SRBs. The Shuttle’s entire launch architecture was a direct descendant of the MLV-Saturn concepts. Today, the Space Launch System (SLS), the rocket designed to return Americans to the Moon, carries that legacy forward with its own massive, five-segment SRBs.
The work on advanced, high-pressure liquid-fuel engines like the HG-3, intended for uprated Saturn upper stages, was the direct technological precursor to the Space Shuttle Main Engine (RS-25). The RS-25, one of the most efficient rocket engines ever built, is now the core engine of the SLS. The very philosophy of designing an evolutionary family of vehicles, with different configurations for different missions based on a common set of core components – the central idea behind the Saturn improvement studies – is now a standard principle in launch vehicle design and is the explicit strategy for the evolution of the SLS, with its planned Block 1, Block 1B, and Block 2 configurations.
Ultimately, the story of the Saturn V derivatives is a powerful case study in how long-term technological progress can be shaped, and sometimes derailed, by short-term political and budgetary decisions. In the early 1970s, the United States possessed both an unparalleled super heavy-lift launch vehicle and a clear, technically sound roadmap to evolve it for missions to the farthest reaches of the solar system. The decision to abandon that path in favor of the Space Shuttle was not merely a choice between two different rocket designs; it was a choice that fundamentally altered the scope and ambition of human space exploration for generations. It created a half-century capability gap in super heavy-lift launch, confining American astronauts to low Earth orbit and placing the dreams of lunar bases and Mars expeditions on hold. With the development of the Space Launch System, NASA is, in many ways, finally rebuilding the capability that was discarded when the last Saturn V assembly line went cold. The designs of these new vehicles carry the echoes of the unflown giants of the 1960s, a testament to a future that was imagined long ago.
Summary
The Saturn V rocket, the icon of the Apollo program, was envisioned not as a final achievement but as the starting point for a new era of space exploration. In the mid to late 1960s, NASA and its contractors conducted extensive studies into a wide range of derivative vehicles that would have expanded on the Saturn V’s immense capabilities. These concepts formed a coherent and ambitious vision for the future, a technological roadmap for establishing a permanent human presence in space.
This family of proposed rockets included several distinct categories. Some were straightforward “uprated” versions, featuring stretched propellant tanks and more powerful engines like the F-1A and J-2S to increase the core vehicle’s performance. Others were colossal giants, augmented with powerful solid rocket boosters of varying sizes – from existing Titan motors to the largest solid rockets ever fired – designed to lift the massive components of interplanetary spacecraft for missions to Mars. A third category consisted of “intermediate” two-stage vehicles, like the INT-20 and INT-21, which reconfigured existing Saturn V stages to create a modular family of launchers for a variety of Earth orbit missions. The most forward-looking concepts involved replacing the Saturn’s upper stage with advanced propulsion systems, such as the nuclear-thermal NERVA engine for crewed Mars expeditions or the high-energy Centaur stage for ambitious robotic probes.
None of these powerful rockets ever flew. A shift in national priorities in the post-Apollo era, coupled with severe budget cuts, led NASA to focus on the development of the Space Shuttle and to prematurely terminate the Saturn V production line. This single programmatic decision effectively closed the door on the evolution of expendable heavy-lift vehicles for decades.
The Saturn V derivatives leave behind a powerful legacy. The engineering work invested in them directly influenced the design of the Space Shuttle and its successor, the Space Launch System. They remain some of the most capable and ambitious launch vehicles ever designed, a fleet of giants existing only on paper. They are a tangible reminder of a bold future that was almost within reach, a tantalizing glimpse of a history that might have been.
