Following in the footsteps of Saturn, SLS prepares for test flight

by William Graham

As NASA’s Space Launch System (SLS) rocket stands ready for its first test flight next week, the Artemis program prepares to follow in the footsteps of Apollo, which first carried humans to the Moon in the 1960s. SLS will fly uncrewed just once before carrying astronauts around the Moon on Artemis II, while the Orion spacecraft will be making only its third trip into space on that mission. By contrast, Apollo saw a series of test flights over the course of a decade, leading up to Neil Armstrong’s “small step” in July 1969.

With NASA confident that Artemis I will be enough to certify the SLS for crewed flights, its single test flight stands in stark contrast to the long series of tests that preceded the Apollo program. There are many factors that have contributed to this decision, including lessons learned from Apollo, NASA’s previous experience with much of the hardware flying on SLS, and advances in technology that have come since the early days of the Apollo program.

While the Saturn family of rockets has become almost synonymous with NASA’s Apollo program, the early development of what would become the Saturn I was carried out by the Army Ballistic Missile Agency (ABMA) years before President Kennedy committed the United States to landing a man on the Moon. The ABMA had already developed the Redstone and Jupiter missiles, and in 1958 they would launch the first US satellite – Explorer I – using the Juno I rocket, a modified version of the Redstone-derived Jupiter-C research rocket.

ABMA’s Juno I and Juno II rockets – the latter based on the larger Jupiter missile – provided access to space for some of the United States’ early space missions, but plans for larger rockets were already being made before Explorer I had even reached orbit. The Juno III and IV, based on the Juno II, were ultimately abandoned, however, the team – led by German rocket scientist Wernher von Braun – was given the go-ahead to proceed with their heavy-lift “Super Jupiter” or Juno V design.

This decision stemmed from a 1957 study of future launch requirements including the proposed X-20 Dyna-Soar spaceplane, heavy military satellites that would need to reach low Earth orbit (LEO), and sending larger and more complex probes beyond Earth orbit. It was to use a cluster of tanks built with tooling from the Redstone and Jupiter programs and would be powered by four Rocketdyne E-1 engines, which were then under development as a backup option for the Titan I missile.

Saturn I and other early rockets at NASA’s Marshall Space Flight Center. (Credit: NASA/MSFC/Fred Deaton)

The name Saturn was adopted in February 1959 at von Braun’s initiative, distinguishing the new rocket from its predecessors in the Juno and Jupiter series, with the name chosen simply because Saturn was the next planet after Jupiter. After the Department of Defense opted not to continue with Saturn, the ABMA was absorbed by NASA, becoming the Marshall Space Flight Center (MSFC) in 1960.

While preparations for the program’s transfer to NASA were underway, the Silverstein Committee was convened to examine the new rocket and make recommendations for its design. By this point, one major change had already been adopted: eight H-1 engines had replaced the four E-1s that had been planned to power the first stage. The H-1, an uprated and simplified derivative of the S-3D that had powered the Jupiter and Thor missiles, was chosen because it could be ready sooner than the E-1. This stage, named S-I, would be at the heart of most of the design options that the committee considered.

The rocket that emerged from the committee was the Saturn C-1. This would pair the S-I booster with a newly-designed S-IV second stage powered by six RL10 engines. Centaur, designated S-V and with two additional RL10s, would serve as the third stage. A larger design, the C-3, was also proposed for later missions to the Moon using the Earth-orbit rendezvous approach. This would evolve into the still-larger C-4 and C-5 designs, with the latter ultimately becoming the Saturn V.

At this time, it was still not certain that Saturn would be the rocket to carry astronauts to the moon, with NASA’s competing Nova rocket designs considered for direct-ascent missions that would allow the crew to launch, land on the Moon, and return to Earth aboard a single spacecraft. NASA opted for lunar orbit rendezvous in 1962, selecting the Saturn V and also authorizing the development of an “Uprated Saturn I” rocket with an enhanced S-IB first stage and the more powerful S-IVB upper stage that was being developed for the Saturn V. This design would ultimately become the Saturn IB.

Graphic from 1962 comparing the Saturn C-1, Saturn C-5, and Nova rocket designs. (Credit: NASA/MSFC)

Von Braun’s team initially took an incremental approach to testing, typical for many missile and rocket development projects in the United States at the time. For the first few flights, only the rocket’s first stage, the S-I, would be tested – with the rocket sporting inert second and third stages. Once engineers were happy with the performance of the first stage on these suborbital tests, a live second stage would be added, and Saturn would shoot for orbit. Then, a series of mockups – or boilerplate – Apollo spacecraft would be added before a planned transition to a live Apollo spacecraft and, finally, crewed test flights to low Earth orbit.

The Saturn test program would take place at Cape Canaveral. Launch Complex 34 (LC-34) was constructed between 1959 and 1960. A second launch complex, LC-37, was completed in 1963 with two additional launch pads to provide a backup in the event of an explosion at LC-34. Additionally, this would allow for higher flight rates to be achieved and provide expansion capability for larger Saturn rockets in the future.

The first Saturn I rocket, the SA-1, was delivered to Cape Canaveral in August 1961 and assembled at LC-34. Consisting of a live first stage topped by inert second and third stages, its goal was to prove that the Saturn rocket was flightworthy, assessing first stage performance and the overall integrity of the vehicle. SA-1 lifted off at 11:06 a.m. EDT (15:06 UTC) on 27 October 1961, beginning the first flight test of the Apollo program.

The SA-1 mission lasted eight minutes and four seconds, reaching an altitude of 136 kilometers and splashing down 346 kilometers downrange; it was deemed a great success. The next three launches would use the same configuration, flying similar mission profiles to put the S-I stage and the general configuration of the rocket through a series of tests before NASA would add a live second stage into the mix.

SA-2 lifted off from LC-34 on 29 April 1962 on a repeat of SA-1’s mission. After a successful ascent and reaching an altitude of 105 kilometers, a destruct command was sent to the rocket at two minutes, 43 seconds into the flight. This was not the result of a failure, but instead part of a scientific experiment named Project High Water. The booster’s explosion released a large quantity of water, present as ballast in the vehicle’s upper stages, allowing scientists to study its effects on the ionosphere.

The launch of the SA-1 mission from LC-34. (Credit: NASA/JSC)

Seven months later, SA-3 became the first Saturn I to fly with a fully fueled first stage. SA-3 reached an altitude of 167 kilometers before being destroyed in a repeat of the High Water test from SA-2. The SA-3 mission also tested retro-rockets that would be used during stage separation on later flights.

The final suborbital test was flown by SA-4 in March 1963. This would demonstrate Saturn’s engine-out capability: the ability for it to continue its mission in the event of an engine failure during ascent. The number five engine, one of the inboard H-1s, was programmed to shut down 100 seconds into the flight, with the remaining engines expected to burn for a longer duration to compensate. This was completed successfully, and SA-4 reached an altitude of 130 kilometers.

MSFC’s approach to testing would change after George Mueller joined NASA in 1963, as Mueller carried out a review and restructuring of the agency’s centers and projects. Mueller advocated all-up testing, meaning that the whole rocket would be tested from the beginning, rather than perfecting one stage before moving on to the next. Von Braun was initially skeptical of this approach, considering it reckless, although he was won around by Mueller’s arguments. Writing retrospectively after the end of the Apollo program, von Braun acknowledged that it would not have been possible for Apollo to achieve its objective of placing a man on the moon by the end of the 1960s had all-up testing not been adopted.

This all-up approach to testing has continued into the modern day, with Artemis I serving as an all-up test for the Space Launch System and Orion. While the SLS vehicle that will fly on Artemis I is not the ultimate final version of the rocket, it is still a fully functional launch system in the same configuration that will be used for the first crewed launches, before upgrades are introduced as the program continues to produce the full SLS Block II vehicle.

NASA’s crewed program that preceded SLS, the Space Shuttle Program, used an even more extreme form of all-up testing. For much of the Shuttle program, the orbiters had no way to land themselves, so it was not possible to conduct any uncrewed tests before astronauts John Young and Robert Crippen boarded Columbia for the program’s maiden mission in April 1981, STS-1. To this day, STS-1 remains the only time that a brand new spacecraft system has carried a crew into orbit on its maiden flight.

By contrast, Project Constellation, the planned successor to the Space Shuttle and canceled predecessor to Artemis, did include one partial test flight for the Ares I rocket. The Ares I-X mission – which used a Space Shuttle solid rocket booster, mockup second stage, and Orion spacecraft – served to prove that the design could be controlled during the early stages of flight. Ares I-X launched in October 2009, but in February of the following year, President Obama announced plans to end the Constellation program.

During the Apollo program, the adoption of all-up testing did not immediately affect Saturn I, since it was already proceeding through its test program, but it did allow future missions to be rationalized. As of 1963, up to six uncrewed Saturn V launches were planned before astronauts would be able to fly aboard the rocket, but with all-up testing, this was reduced to just two.

Saturn I would reach orbit with the SA-5 mission on 29 January 1964. This was the first flight of the Block II Saturn I and the first with a live S-IV, as well as the first launch from LC-37. Gone was the inert S-V (Centaur) third stage: Saturn I had been reduced to a two-stage design in 1961, but the suborbital test flights still flew with this present. Instead, the rocket was topped with the nose cone of a Jupiter missile and ballasted with sand.

The launch of the SA-5 mission, the first orbital mission of the Saturn rocket family — via L2 Historical.

After the success of SA-5, Saturn I was ready to carry its first Apollo payloads. SA-6 flew the A-101 mission in May 1964 with a boilerplate Apollo Command and Service Module (CSM) – an instrumented structural mockup of the Apollo spacecraft. An engine failure late in first stage flight allowed Saturn I to again demonstrate its engine-out capability, and the mission was flown successfully. Another boilerplate CSM was launched by SA-7 in September as A-102. In both cases, Saturn I placed its payload into a very low orbit from which it quickly decayed.

The final three Saturn I launches carried a trio of Pegasus micrometeoroid detection satellites in addition to Apollo boilerplates. Built into the S-IV stage, each Pegasus deployed a pair of instrumented “wings” which were used to record impact events in orbit. The A-103, 104, and 105 missions were flown by SA-9, SA-8, and SA-10 respectively, launching in February, May, and July of 1965.

By this stage, plans for crewed missions aboard the Saturn I had been abandoned, with the rocket’s payload capacity limiting what could be carried aboard an operational spacecraft – and therefore the usefulness of such a mission. Instead, the focus for these missions would switch to the more powerful Saturn IB rocket, which was ready to fly in 1966, and thus the three Pegasus missions brought Saturn I’s service to a successful conclusion.

The AS-201 mission was an all-up test of the Saturn IB and the first flight of the Apollo spacecraft. It also marked the debut of the S-IVB stage, which replaced the S-IV second stage that had been flown on Saturn I. Powered by a single J-2 engine, this would be the stage that would power Apollo towards the moon when used in conjunction with the Saturn V. The version used on Saturn IB differed slightly from that used on Saturn V – the principal difference being that Saturn V’s S-IVB could be restarted, while Saturn IB’s could only fire once.

Launched on 26 February 1966, AS-201 followed a suborbital trajectory with both stages firing before the Apollo spacecraft separated. After reaching an apogee of about 490 kilometers, the Service Module fired its engine to increase its re-entry speed. The Command Module then separated from the Service Module, deploying its parachutes and splashing down in the Atlantic Ocean after a 37-minute flight.

Recovery divers work to prepare the AS-201 Command Module for retrieval following the AS-201 mission. (Credit: NASA)

AS-203 flew next in July 1966. This was the first Saturn IB mission to reach orbit, but it did not carry a payload. Its primary purpose was to study how the S-IVB would perform in orbit and how its liquid hydrogen propellant would behave. In order to get to the Moon, Saturn V would need to restart its S-IVB stage, and AS-203 was intended to validate that the stage would be in a condition to restart when the time came – even though the Saturn IB version of the stage could not restart itself. Objectives included assessing the performance of baffles and deflectors designed to prevent propellant slosh and ensuring that necessary components could be chilled down in preparation for restart. This was successful and paved the way for S-IVB’s use on the Saturn V.

The final development flight before crew were expected to ride the Saturn IB was AS-202 on 25 August 1966. This was another suborbital test flight with a full Apollo spacecraft, but it followed a different trajectory. After separating from its launch vehicle, the Apollo spacecraft performed its first burn to raise its apogee to over 1,140 kilometers. It then performed a series of burns to increase its re-entry velocity in order to provide a more extreme test for the heat shield. After a 93-minute mission, the Command Module was successfully recovered.

Orion’s heat shield has undergone a similar test that performed on Apollo during AS-201 and AS-202 as part of the Exploration Flight Test 1 (EFT-1) mission that was flown in December 2014. With SLS still under development, Orion launched aboard a United Launch Alliance (ULA) Delta IV Heavy rocket, completing two orbits of the Earth and reaching an apogee of 5,800 kilometers before being recovered in the Pacific Ocean. EFT-1 tested the Orion capsule but flew without a functioning service module with Delta IV’s upper stage providing propulsion during the mission instead.

The first crewed flight of the Apollo program was to have been flown in early 1967 by Virgil “Gus” Grissom, Ed White, and Roger Chaffee. Officially designated AS-204 at the time, this mission is known to history as Apollo 1. The mission was to have been an orbital test flight of the Apollo spacecraft, launched atop a Saturn IB from LC-34B. On 27 January 1967, with liftoff still several weeks away, the crew boarded their capsule for a “plugs out” test. During the test, a fire broke out aboard the spacecraft, tragically claiming the lives of all three astronauts.

Four days after the Apollo 1 fire, the interstage for Saturn V SA-501 arrived at the Kennedy Space Center. This was the last piece of flight hardware to arrive at the Cape for the rocket’s first test flight, although the investigation that followed Apollo 1 and the subsequent discovery of manufacturing defects in other Apollo spacecraft resulted in Apollo 4 being delayed. Once the rocket and spacecraft were ready, they were stacked inside the Vehicle Assembly Building (VAB) and rolled to Launch Complex 39A on 26 August 1967 to begin tests at the pad.

The maiden launch of the Saturn V from LC-39A in November 1967. (Credit: NASA)

After completing pad testing, Apollo 4 was cleared to launch. It lifted off on 9 November 1967, marking the maiden flight of the iconic Saturn V rocket. The rocket performed perfectly, with the S-IC first stage and S-II second stage burning as planned before the S-IVB inserted itself and the Apollo CSM into low Earth orbit. After a two-orbit coast phase, the S-IVB demonstrated its restart capability, increasing the apogee of its trajectory to 17,200 kilometers and lowering the perigee to ensure re-entry occurred at the end of the revolution.

The CSM separated and made its own series of burns, increasing its re-entry velocity to simulate a return from a lunar mission. The Command Module splashed down in the Pacific Ocean eight hours and 37 minutes after liftoff.

Despite Apollo 4’s performance, at least one more Saturn V test flight was called for before astronauts would be allowed aboard the rocket. Apollo 6 lifted off from LC-39A on 4 April 1968. This had a planned orbital apogee of 515,000 kilometers – beyond the orbit of the Moon – while the mission plan called for the Apollo spacecraft to demonstrate a direct-return abort after separating from the S-IVB.

Apollo 6 was the closest a Saturn rocket ever came to a launch failure and can arguably be regarded as at least a partial failure. Multiple issues occurred during ascent, beginning with severe pogo oscillations on the first stage. During the second stage burn, an issue developed with the number two engine which ultimately required it to be shut down. Due to a wiring issue, the shutdown command also closed an oxidizer valve feeding the healthy number 3 engine, causing it to shut down as well.

The S-II was able to complete its burn on three engines, and the S-IVB made an extended burn to reach a parking orbit that was somewhat more eccentric than had been planned. After completing two orbits of Earth, the S-IVB was to have restarted to boost the CSM onto its simulated trans-lunar trajectory, however, it failed to ignite. Instead, the Service Module’s propulsion system was used to fly a similar trajectory to Apollo 4, with the Command Module splashing down after a nine-hour, 57-minute flight.

Despite the issues with Apollo 6, NASA was confident that no further test flights would be needed. In the time between Apollo 4 and Apollo 6, the Lunar Module had carried out its own test flight – Apollo 5 – having been boosted by the same Saturn IB that had originally been stacked for Apollo 1. All elements of the Apollo program were now ready to begin crewed missions.

Artemis I will serve a similar purpose to the Apollo 4 and Apollo 6 missions, serving as a check-out of the SLS rocket, the Orion spacecraft, and the European Service Module. A significant difference, however, is that it will visit the Moon and enter a distant retrograde orbit around it. With a planned duration of just over 42 days, Artemis I will be in space longer than the combined flight time of all Apollo capsules at the time of Apollo 11’s launch.

SLS and Orion stand atop LC-39b in August 2022, awaiting the launch of Artemis I — via Thomas Burghardt for NSF

Apollo incorporated four crewed flights building up to the first Lunar landing: Apollo 7 tested the CSM in low Earth orbit (LEO) before Apollo 8 took it into lunar orbit, Apollo 9 performed a crewed test of the Lunar Module in LEO, and Apollo 10 was a full dress rehearsal in lunar orbit for Apollo 11. By contrast, Orion’s first crewed flight, Artemis II, will make a flyby of the Moon, and the first landing is expected on the next flight, Artemis III.

During the era of the Apollo program, often the only way to test something was to fly it. With modern advances in computer technology, it is far easier to make realistic simulations without needing to launch actual hardware. While this can never truly account for every variable during a real mission, it will no doubt give engineers more confidence in the systems they have developed for Orion and Artemis.

Likewise, while Saturn was a completely new rocket – incorporating new engines that had never before flown and cryogenic propellants all on a far larger scale than any rocket that had come before it – SLS has the advantage of drawing from many years of flight heritage. As a Shuttle-derived launch vehicle (SDLV), its first stage is powered by four RS-25D engines taken directly from the Shuttle where they served as Space Shuttle Main Engines (SSMEs). SLS’s twin solid rocket boosters are five-segment versions of the solid rocket boosters that flew on the Shuttle stack, using flight-proven segments left over from the Shuttle program.

The upper stage for the Block I SLS is the Interim Cryogenic Propulsion System (ICPS), which is based on the Delta Cryogenic Second Stage (DCSS) flown aboard United Launch Alliance’s Delta IV rocket. Its RL10B-2 engine is part of the same line as the RL10A-3 that was used on Saturn I’s S-IV stage during Apollo and has also served with distinction on Centaur from the 1960s through to present-day flights on Atlas rockets.

This impressive pedigree puts SLS in good stead for its role in the Artemis program. However, the Artemis I test flight is still an important hurdle that it and Orion must clear before NASA can proceed with its goal of sending the next humans back to the Moon.

(Lead image: The launch of the SA-4 mission on a Saturn I rocket. Credit: NASA)

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