In the ‘90s and early ‘00s, Japan had several research programs running to develop the necessary technology readiness to launch a reusable, orbit-capable space plane called HOPE. After the success of the OREX capsule re-entry experiment in 1994, the HYFLEX mission flew in 1996. In contrast to the ballistic OREX capsule, HYFLEX used a steerable, lifting body configuration during re-entry. The ALFLEX vehicle was flown in the years after HYFLEX to prove the technological capabilities for autonomous landing, subsonic flight and transonic flight for the HOPE spaceplane (see ALFLEX and HSFD). 

The vehicle was 4.4 meters long, weighed 1054 kg and featured two small, tilted wings near the rear of the vehicle. Attitude control was provided by two aerodynamic flaps and a series of RCS thrusters for control outside of the atmosphere. Three types of thermal protection were included: a carbon-carbon heat shield,  ceramic tiles and a flexible insulator.

In February 1996, HYFLEX was launched aboard the J-I rocket. The flight proceeded nominally and all data was transmitted to the ground station. However, after the splashdown in the ocean, a flotation system failure occurred, causing an unsuccessful recovery and loss of the vehicle. Apart from the failed retrieval, the mission was a success and improved Japan’s technology readiness regarding lifting body re-entry. [64, 65]

The first flight, called RLV-TD HEX-01, was conducted in 2016 and demonstrated the hypersonic flight and re-entry capabilities of the vehicle. It was launched on top of a solid rocket booster and reached an altitude of about 65 km. Then, it separated from the booster and glided back to Earth, performing various manoeuvres to test its aerodynamics and control systems. It splashed down in the Bay of Bengal, about 450 km from the launch site. The flight lasted for about 13 minutes and was deemed a success.

The second flight, called RLV-TD LEX, was conducted in 2023 and demonstrated the autonomous landing capability of the orbiter on a runway. The RLV-TD, was released from a helicopter at an altitude of 4.6 km and landed safely on a runway using its own guidance and control systems. 

The third flight, planned for 2024, will demonstrate the powered cruise flight capability of the orbiter using an air-breathing scramjet engine.

When the X-20 Dyna-soar was cancelled without flying the hardware, a new program was created to achieve the goals. This project was the Aerothermodynamic elastic Structural Systems Environmental Tests, or simply named ASSET. Besides testing the heatshield created for the Dyna-soar, the ASSET program also aimed to test flutter behaviour in hypersonic speeds, control during hypersonic speeds using a trailing edge flap and study materials and structures in a re-entry environment [48,49].

Two versions were created, the aerothermodynamics structural vehicle (ASV) and the aerothermodynamic elastic vehicle (AEV). The two versions split the goals with the AEV studying the flutter and control and ASV studying the materials and structures in re-entry environments. Both ASSET vehicles had a height of 1.79 m and a wingspan of 1.53 m [5.]. It had an L/D between 1.2 and 1.4 controlled by a weight that is moved internally [48]. The ASV had a mass of 512 kg and the AEV 555 kg. Another difference between the two vehicles is for the ASV had a parachute recovery system, but the details are unknown, the AEV because of the extra support equipment had no recovery system on board [50].

All ASSET vehicles were launched with variants of the Thor launch vehicle. The AEV was launched on the standard Thor launcher. The ASVs were launched on the Thor-delta. This difference was to increase the re-entry velocity of the ASVs (5-6 km/s) to the AEVs (4km/s) [52]. This ensured the ASVs had a temperature in excess of 2200 C on the nose cap [49,51].

Of the six flights flown two were with an AEV vehicle and four with an ASV vehicle. As previously mentioned the AEV did not have a recovery system and were designed to be lost after the flight. Of the four ASV flights, only one vehicle (ASV-3) was successfully recovered [49]. Of the three vehicles that were lost, one was due to the second stage of the launcher that failed to ignite (ASV-2), another sunk after the flotation system failed (ASV-1) and one was lost due to a parachute failure (ASV-4) [49]. However, except for ASV-3 good telemetry data was received and these flights were judged as a success. The data gathered during the ASSET program eventually would be used to design the space shuttle.

During the mid-1960s the US-Air Force started the PRIME program. This program had the goal to test various elements during the hyper- and supersonic flight regimes. The goals were to test the structure and the heat shield. But the main goal of the, at the time a classified goal, was to test the manoeuvrability during the re-entry [45]. The goal was to be able to have a cross-range manoeuvrability up to 1100km from the target, which was for the retrieval of data from space.

The vehicle itself was a 403 kg lifting body re-entry vehicle made mostly out of aluminium [47]. It was about 2 m long and 0.8 m wide and had a lightweight phenolic ablative heatshield between 20-70mm thick [45]. Once the vehicle was back in the atmosphere it deployed a 1.46 m ballute drogue parachute using a parachute mortar [47]. Afterwards is deployed the 14.3 m main parachute, which was an extended skirt type with an air pick-up cone [47]. The vehicle was recovered in mid-air using a specially equipped JC-130B Hercules [44]. If for some reason the vehicle was not picked up by the recovery aeroplane it also had the capability to land in the ocean. For this floatation bags were installed [47].

In total 4 flights were planned at the start of the program with only 3 being completed [46]. The fourth flight was a backup flight for when one of the first three flights were unsuccessful. This was not the case. However, of the three flights that were flown two of the three vehicles were lost in the ocean. All data was sent down, therefore these flights were still qualified as a partial success. As all goals were achieved.

After this, the investigation of space planes continued with the BOR-2 and BOR-3 vehicles. With the BOR-2 vehicle having improvements on its thermal protection system and the BOR-3 being scaled up in size, being a 1:2 scaled version of the final Spiral project vehicle. The BOR-2 flew four times, of which all were successful. BOR-3 flew twice. Both times an issue occurred. The nose fairing was destroyed during the first flight when flying at about 5km with Mach 0.94. The flight program was fully implemented on the second flight, but due to a parachute failure, the vehicle crashed.

The X-24A was 7.3m long and 4.2m wide. It had an empty mass of 2495kg and an equal amount of propellant onboard. The X-24A was powered by an XLR-11 rocket engine delivering 38kN of thrust in vacuum conditions for 140s [217,219]. 

The X-24A flew a total of 30 flights. Initially, the X-24A flew 12 subsonic unpowered gliding. After this, 28 powered flights were performed. These powered flights reached a maximum altitude of 21.3km and a maximum speed of Mach 1.6. These flights aided in gathering the required information needed for the approach and landing of the Space Shuttle [217,218]. After these 30 flights, the X-24A was rebuilt to allow for hypersonic flights. As a consequence of this rebuild, the vehicle was renamed the X-24B.