Japan's First Sounding Rocket

In the mid-1950s, Japan's first space research group led by Prof. Hideo Itokawa at the University of Tokyo embarked on developing indigenous sounding rockets. Earlier experiments with the tiny Pencil rockets (only 23 cm long, 1.8 cm in diameter) and slightly larger Baby rockets laid the groundwork for a larger vehicle . The impetus came from the upcoming International Geophysical Year (IGY, 1957–58), which required sounding rockets to reach at least 60–100 km altitude for meaningful upper-atmosphere research . Originally, the team had planned incremental steps through rockets code-named Alpha and Beta before tackling a bigger vehicle, but IGY's urgency forced them to skip directly to the “Kappa” rocket series, bypassing those intermediate stages . The result was the Kappa 1 – Japan's first large post-war sounding rocket family, developed at the Institute of Industrial Science (IIS) of the University of Tokyo, and tested at a new beach launch site in Michikawa, Akita Prefecture .

Development of Kappa 1 proceeded rapidly in 1956 under constrained resources. Prof. Itokawa's team chose a solid-fuel design (in contrast to the liquid-fuel sounding rockets like America's early Aerobee and France's Véronique) and leveraged double-base propellants (nitroglycerin/nitrocellulose based) that had been proven in the Baby rockets. By September 1956, the first Kappa 1 vehicles were ready for launch. Three Kappa rockets were fired in late September 1956, all single-stage rockets about 2.7 m long and only 13 cm in diameter . These early flights demonstrated basic functionality but only reached about 5 km altitude each (far short of the 40 km design goal), due in part to short burn times (~3 s) and structural issues like fin flutter . Nevertheless, the successful firings on September 24, 28, and 29, 1956 marked Japan's first launches of the Kappa series and proved the solid-fuel concept viable. Seven Kappa-1 single-stage rockets were launched by the end of 1956 , gradually resolving issues such as motor case overheating (mitigated by an ablative glass fiber/chromium oxide lining inside the steel motor casing) and fin vibrations .

Engineers carrying a Kappa-1 sounding rocket at the Michikawa Beach range (1956). The Kappa's compact size is evident – only ~2.7 m long and 0.13 m in diameter (about 5 inches) . Despite its small stature, this solid-fueled rocket was Japan's first step toward reaching the upper atmosphere.

By early 1957, the Kappa team made iterative improvements. To achieve higher altitude, they experimented with staged configurations. The earliest variant was essentially a Kappa-1 with an added booster stage: in April 1957, a two-stage vehicle called Kappa 2 was flown, consisting of a new 22 cm diameter booster and a dummy upper stage (a passive mass simulator in place of the 13 cm Kappa motor) . This configuration, though technically a two-stage stack, behaved like a single-stage rocket since the second stage was inert. The Kappa-2 booster test reached about 9 km altitude , validating the booster-s performance. Soon after, the dummy second stage was replaced with a live Kappa motor, yielding the first fully functional two-stage Kappa rocket.

Kappa 1 Variants and Technical Specifications

The Kappa 1 series encompassed a progression of models (often denoted by development codes K-1, K-2, K-3, K-4, K-5, K-6 in Japanese sources) which we will designate here as Kappa 1 and its variants 1A, 1B, 1C, etc., for clarity. Each variant introduced new features to incrementally improve altitude performance. All Kappa 1 variants were solid-fueled, pressure-stabilized rockets and retained the same fundamental architecture: a fin-stabilized unguided vehicle launched from an inclined rail launcher. Table 1 below summarizes the key parameters of the main Kappa 1 variants through 1958:

Table 1 – Kappa 1 Rocket Variants (1956–1958): Key dimensions, performance, and propellants. (Note: “Double-base” is a nitrocellulose/nitroglycerine propellant; “Composite” is a modern solid propellant with ammonium perchlorate oxidizer and polymer binder.)

Kappa 1 (Initial Model)

The original Kappa-1 was a diminutive single-stage rocket: 2.7 m in length, 0.128 m in diameter, producing about 10 kN of thrust (~1 ton-force) for ~3 seconds . With a launch mass on the order of only a few hundred kilograms, it was at the time one of the smallest sounding rockets ever built. The diameter of ~13 cm (5 inches) is well below the 7-inch figure mentioned – indeed confirming the rocket's remarkably narrow body . The propellant was a double-base solid grain (similar to artillery propellant) cast as a single cylindrical grain. A major challenge was preventing the steel motor casing from overheating and melting, since double-base propellant burns at high temperature over the full internal surface. The team solved this by applying an ablative liner (layers of fiberglass cloth, chromium oxide, and water glass) inside the motor, which protected and cooled the case as the propellant burned . The Kappa-1 had four clipped-delta fins at its base for stability and no active guidance. Its theoretical ceiling was around 40 km altitude, but as noted, early flights in 1956 were limited to ~5 km due to premature burn-out and other technical glitches . These initial flights carried minimal instrumentation (mostly telemetry transmitters) and were primarily to test rocket dynamics and achieve stable flight. By the seventh launch in December 1956, the Kappa 1 was flying nominally (still only ~5 km altitude) and ready for enhancements .

Kappa 1A / Kappa-2 (Booster Testbed)

To push beyond the performance limits of the single-stage Kappa-1, the team added a larger booster stage. Kappa-2, first flown in April 1957, paired a new K-220 booster (220 mm diameter) below a Kappa-1 as the second stage . In this initial configuration, the upper stage was inert (often referred to as K-128JD dummy, packed with wood to simulate the mass of propellant) . Effectively, the booster did all the work and the dummy stage provided realistic aerodynamic and mass properties of a two-stage vehicle. The Kappa-2 vehicle stood ~4.9 m tall and had a liftoff mass of 166.7 kg . In its lone test flight on 24 April 1957, it reached about 9 km altitude , more than a 70% improvement over the single-stage Kappa-1. This test demonstrated the booster's integrity and paved the way for an operational two-stage design. The booster (K-220B) still used double-base propellant, meaning it was essentially a scaled-up version of the Kappa motor cluster (in fact, multiple smaller double-base grains had to be bundled inside the booster because double-base could not be cast in large single pieces ). The success of Kappa-2 confirmed the staging approach and led directly into the first true two-stage flights.

Kappa 1B / Kappa-3 (First Two-Stage Rocket)

The Kappa-3 (which we'll call Kappa 1B) was Japan's first fully functional two-stage sounding rocket. It used the same K-220B booster but now with a live second stage (the standard 128 mm Kappa motor, called K-128J) on top . Kappa-3 rockets were launched three times in mid-1957. Immediately, they encountered the new challenges of staging: on the first launch (May 2, 1957), the second stage ignited but burned abnormally, resulting in loss of telemetry . The subsequent flights in June 1957 achieved staging successfully – reaching 21–22 km altitude – but revealed issues like aerodynamic heating melting the booster's fin leading edges . The vehicle was 4.9 m tall, 22 cm in diameter, and about 170 kg in mass . Its design apogee was ~25 km , though in practice ~22 km was achieved. Despite not yet hitting the 40+ km target, the Kappa-3 proved two-stage solid rockets could be done with the team's resources. All stages of K-1, K-2, K-3 used the old double-base propellant, which was reaching its limits – the largest K-220 booster was heavy (thick steel case for strength) and still limited in total impulse. It became clear that to break the altitude ceiling, a new propellant was needed.

Kappa 1C / Kappa-4 (Transition to Composite Propellant)

The Kappa-4 rocket marks a transitional variant – the first to incorporate a composite solid propellant (a more modern formulation with an oxidizer and rubbery binder) in at least one stage. By late 1957, Prof. Itokawa's group had begun experimenting with polyester-based composite propellants, which promised higher performance and the ability to cast large grains in tailored shapes . The K-4 used a new 330 mm diameter booster (K-330B) loaded with composite propellant for greater thrust, while retaining the 128 mm double-base upper stage from earlier designs . On paper, Kappa-4's specs were a leap: 5.9 m length, 33 cm diameter, and a powerful booster thrust of ~78–105 kN (about 8–10.5 tons) – an order of magnitude greater thrust than Kappa-3's booster. The theoretical apogee was ~80 km if both stages performed optimally . However, the reality in 1957 was more sobering. The composite propellant was still in a trial phase and prone to combustion instability and explosions during development . Moreover, the K-330 booster ended up overweight by ~30 kg due to using a heavy 4130 steel chamber (it was the last motor to use such a heavy steel casing) . This pushed the center of mass rearward and made the rocket aerodynamically less stable . Two Kappa-4 flights in September 1957 were attempted, but neither reached the intended altitude – in fact they likely fell well short (under half the 50 km goal, so on the order of 20–25 km) . Although K-4 did not immediately yield a big altitude gain, it was a critical learning step. It proved the composite propellant could work in flight and highlighted the need for lighter structures. Engineers inferred that using light alloy casings and fully switching both stages to composite fuel would unlock far better performance .

Kappa 1D / Kappa-5 (Hybrid Propellant Configuration)

By early 1958, efforts were made to fix K-4's shortcomings. The next variant, Kappa-5, was something of a hybrid: it used a composite-propellant upper stage and a double-base booster. Essentially, it combined a new lightweight second stage motor (called K-150 for its 150 mm diameter) atop the proven K-330 booster from K-4 . The K-150 upper stage was made with an aluminum alloy case and filled with polyester composite propellant – this motor was developed through static tests (denoted K-150-T) to ensure reliability . In March–May 1958, at least two Kappa-5 launches were conducted as intermediate tests . Kappa-5 managed to reach roughly 30 km altitude – still shy of IGY's 60 km requirement, but it validated the superior performance of the composite fuel, as engineers noted. The success of the K-150 motor in flight gave the team confidence to also replace the booster's propellant with composite in the next iteration . In terms of configuration, Kappa-5 was similar in size to K-4 (~5.9 m long, 33 cm first stage) and had an estimated mass around 300–350 kg. The booster's performance was slightly under K-4's (being still double-base), but the efficient second stage made the overall flight more effective, nearly doubling K-3's altitude. These K-5 tests were essentially the final stepping stone before the milestone achievement to come.

Kappa 1E / Kappa-6 (First High-Altitude Success)

The culmination of the Kappa-1 series was the Kappa-6 – the version that at last propelled Japan into the IGY scientific program. Debuting in June 1958, Kappa-6 was a two-stage rocket in which both stages used composite propellant, a first for Japan . The first stage (booster) was 25 cm in diameter (the K-250 motor, essentially a refined composite booster) and the second stage was about 15–16 cm in diameter (the K-150 motor from K-5, now standard) . The overall length was about 5.4–5.6 m and launch mass ~255–270 kg . Thanks to the high-energy composite fuel and lighter structures, Kappa-6 could loft a scientific payload of ~20 kg to the targeted altitude. In its first successful flight on June 16, 1958, a Kappa-6 reached an apogee of around 60 km – finally exceeding the minimum altitude for IGY observations. Over the next year or so, the team launched a series of Kappa-6 rockets (over a dozen by 1960) , achieving apogees in the 60–70 km range. These rockets carried instruments to measure upper-atmospheric winds, temperatures, and cosmic rays at altitudes tens of kilometers above the ground . With Kappa-6, Japan officially joined the IGY efforts; the data collected on high-altitude winds and cosmic ray intensity was Japan's contribution to the global science campaign . The composite propellant (now using ammonium perchlorate oxidizer and synthetic polymer binder) proved to be the “game-changer,” described as “the greatest weapon” in achieving the jump in altitude . It is worth noting that even with the new propellant, development was not without setbacks – many early composite motor tests had ended in explosions and fragmented hardware before the formula was perfected . The Kappa-6's success was the result of a year-long intensive effort to tame this new propellant .

Later Evolution and Legacy

Although the scope of this report is the Kappa 1 series, it is important to recognize that this series laid the foundation for more advanced Japanese sounding rockets that followed. The very next iteration, the Kappa-7 (1959), was a one-stage, 42 cm diameter vehicle mainly used to carry heavy instrumentation to moderate altitudes (~50 km) . By 1960, the Japanese team introduced the Kappa-8, a much larger two-stage rocket (42 cm booster, 25 cm upper stage) that opened the door to near-space altitudes. In September 1960, a Kappa-8 flew to around 150–200 km , penetrating the ionosphere and carrying cosmic-ray detectors – an achievement observed by NASA scientists visiting Japan . The Kappa-8 and its later derivatives (Kappa-9 and Kappa-10 series) demonstrated improved solid motors, staging techniques, and even three-stage configurations. By the mid-1960s, the final Kappa rockets (such as Kappa-10 and its high-altitude variant Kappa-10S) were reaching apogees of 350 km and beyond – with one record flight hitting 742 km in 1965 . Table 2 below shows the growth from Kappa-1 to the later models for context:

Table 2 – Evolution of Kappa Sounding Rockets: From the modest Kappa-1 to high-altitude multi-stage Kappas in the 1960s. (Kappa-9L: “Lower-performance” 3-stage, Kappa-9M: improved 2-stage; Kappa-10S: special 3-stage high-apogee flight.)

The legacy of Kappa-1 is significant. It established Japan's competency in solid-propellant rocketry and set the stage for the Lambda rocket series, which would attempt orbital launches by the late 1960s. Notably, technologies proven in Kappa 1's variants – such as lightweight composite propellants, multi-stage ignition, spin stabilization (introduced in Kappa-9), and payload recovery techniques – fed directly into the later Lambda 4S vehicle that launched Japan's first satellite in 1970. The Kappa rockets were also Japan's first practical scientific tools in space research, carrying instruments for atmospheric physics and astronomy experiments throughout the 1960s.

In summary, the Kappa 1 sounding rocket and its variants (Kappa-1 through Kappa-6) transformed Japan from a newcomer in rocketry (with only a 5 km reach in 1956) to a contributor to space science (60 km+ by 1958). With a core diameter of only 0.13 m (~5 inches), the original Kappa-1 was a remarkably small vehicle , yet it spawned a whole family of larger successors. Through methodical iteration – adding stages, switching propellants, and refining designs – the Kappa series achieved success. By 1958, Kappa-6 rockets were routinely flying to the edge of space, firmly establishing the foundation of Japan's rocket development program . The story of Kappa-1 is thus one of rapid progress in the face of technical hurdles, and it represents Japan's first “giant leap” (albeit by a very small rocket) toward space exploration.

References

1. JAXA/ISAS, “Under the Limelight – The Kappa Era”, History of Japanese Space Research. – Discusses the accelerated development of the Kappa rocket for IGY and technical evolution from K-1 through K-8 .
2. JAXA/ISAS, “Kappa Rocket Start (History Chapter1)” (in Japanese). – Detailed narrative of early Kappa launches at Akita, including technical hurdles like motor case ablation and fin flutter .
3. Encyclopedia Astronautica – Kappa (archived). – Comprehensive technical summary of Kappa rocket variants, launch counts, and performance figures .
4. Japanese Wikipedia, “カッパロケット” (Kappa Rocket). – Provides variant-by-variant data (in Japanese) on dimensions, masses, and flight outcomes for K-1 through K-6 .
5. Galactic Journey blog, “The Kappa Sounding Rocket” (Feb 2025). – Historical overview of Japan's early space efforts, describing Itokawa's role and the Kappa series' significance in IGY .
6. Kotobank Encyclopedia, “カッパ・ロケット”. – Highlights major milestones: Kappa-6 reaching 60 km in 1958, Kappa-8 reaching 200 km in 1960, etc., underscoring the rapid progress of the Kappa family .
7. JAXA/ISAS, “Completion of Kappa-6” (History Chapter1-4-4, in Japanese). – Describes the K-5 and K-6 development, including the first 60 km success in June 1958 and the specs of K-6 (25 cm and 16 cm motors, 5.4 m length, 255 kg) .
8. Asahi Shimbun, “Rocket Development: Fall Down Seven Times, Get Up Eight (Part 2)” (2019, in Japanese). – Notes the naming of Kappa (from the Greek “K”) and the first successful launch in September 1956, followed by K-6 reaching 60 km in 1958, allowing Japan to join the IGY project .
9. R. Akiba, JAXA, “A Passion for Rocketry” (2005). – Recollections of early Japanese rocketry; mentions H. Itokawa with a Kappa-8L rocket in 1962, reflecting on how far the program advanced from the tiny Kappa-1 .