Entry test missions

HEARTED is important for several reasons. First, it will demonstrate the feasibility and reliability of a European high-speed re-entry system, which is essential for enabling ambitious exploration missions that involve returning samples or payloads from other planets or moons. Second, it will provide valuable data and experience on the aerothermodynamics of re-entry, which is the study of how the air flows around and interacts with the vehicle during its descent. This is a complex phenomenon that involves real-gas effects, shock waves, turbulence, plasma formation, radiation, and chemical reactions. These effects are difficult to predict and simulate with current models and tools, so they require experimental validation. Third, it will test the performance of new TPS materials and structures that can cope with the severe thermo-mechanical environment of re-entry. These include ceramic matrix composites (CMCs), ablative materials, crushable structures, seals, gaps, steps, and singularities. These elements are critical for ensuring the integrity and safety of the vehicle and its payload. Fourth, it will evaluate the performance of the guidance, navigation, and control system that will steer the capsule during its flight. This system will use a combination of flaps and thrusters to adjust the attitude and trajectory of the capsule according to a pre-defined flight profile. The system will also use an inertial measurement unit (IMU) coupled with a GPS receiver for navigation. 

The first Shefex mission launched in 2005 on a Brazilian VSB-30 rocket, reaching over 200 km altitude before splashing down. It tested an unconventional faceted ceramic thermal protection system for 20 seconds at Mach 6.

Shefex II launched in 2012 on a Brazilian VS-40 rocket, reaching 250 km altitude and Mach 10-11 speeds for 45 seconds. It tested an updated faceted ceramic TPS and manoeuvrable reentry.

Shefex III was aimed to reach Mach 20 for 15 minutes, but this required a larger rocket that is not yet available. Further Shefex development is using other international flight opportunities.

A key goal was verifying faceted fiber ceramic TPS, sharp-cooled and uncooled leading edges to reduce future TPS costs by up to 70%. Contour-stable ablative leading edges, thermally stable fin structures, and lightweight fairings were also tested.

The rocket provided around Mach 5.6 velocity for 30 seconds during re-entry from 90 km to 20 km altitude. Flow effects and thermal loads were studied, with ablative coatings protecting fins and leading edges.

The ReFEX vehicle will be around 450 kg in mass, of which the structure of the vehicle itself takes up more than half. The rest comprises the GNS system for autonomous flight, the fins and the avionics. It has a cylindrical body with a length of 2.7 m. It has two canards and a small delta wing at the back with a wing span of 1.1 m. As the ReFEX will be launched on top of a VSB-30, not inside a fairing, its non-symmetrical shape poses an issue, as it has a tail wing. Therefore,  ReFEX will be partially hidden during the ascent by an external fairing structure. This structure covers the two folded wings and adds a find on only the fairing to make the vehicle symmetrical [142,144,145]. Although initially planned to have a parachute to cover the last part of the flight due to volume constraints, it was chosen to remove it and have the data continuously downloaded from the vehicle [142].

The ESA Atmospheric Reentry Demonstrator or ARD was flown in the interstage of the third Ariane 5 rocket. The objective was to demonstrate the ability of ESA to perform a controlled atmospheric re-entry. The capsule was a scale model of the Apollo-style capsule and flew to an apogee of 830km. During the flight, the heat shield reached a temperature of about 900 deg C. The capsule performed multiple manoeuvres during entry demonstrating the ability of ESA to safely return a capsule through the atmosphere. The capsule was equipped with a parachute system for a safe landing. The drogue parachute had a diameter of 5.8 meters. The drogue was followed by a cluster of three 22.9 meter diameter main parachutes. To ensure the capsule remained afloat two balloons were attached and inflated during landing. 

The Jupiter-C reentry vehicle had a conical shape with a base diameter of 1.65 meters and a length of 2.74 meters. Most of the vehicle was covered in an ablative asbestos-reinforced phenolic resin, while the spherical nose was covered with fibreglass-reinforced phenolic resin. Inside of the vehicle’s aluminium structure, there was a recovery system included to allow scientists and engineers to analyze the performance of the ablative materials after flight. The total weight of the vehicle and payload together amounted to a total of 1187 kg.

Prior to any launches, the materials were tested on small samples inside rocket engine exhaust plumes to simulate the reentry heat fluxes, while various vehicle shapes were analyzed in shock-tube high-speed wind tunnels. Between the 20^th of September 1956 and the 8^th of August 1957, three sub-scale (1/3 scale) Jupiter-C reentry vehicles were flown onboard a modified Redstone rocket. The first two sub-scale flights featured a melanine-fibreglass heat shield because the development of the asbestos-phenolic material wasn’t ready yet. However, the third sub-scale flight featured this new material and was recovered successfully, paving the way towards full-scale flight testing.

Between the 18^th of May 1958 and the 28^th of May 1959, a total of three successful full-scale Jupiter-C vehicles were recovered using the asbestos-reinforced phenolic heat shield. The latter two also carried two primates onboard that returned safely back to Earth. This success confirmed the excellent performance of ablative materials during the harsh reentry conditions and led to many more ablative material flight tests and eventually also crewed reentry vehicles with ablative heat shields.

The instrument package included accelerometers, pressure sensors, temperature sensors, radiometers, and mass spectrometers. These instruments were subjected to about 80g of acceleration and measured the pressure, temperature, and density changes with altitude changes, as well as the mean molecular weight of the gas encountered.

The radiometer collected data on the composition of the atmosphere through a quartz window by measuring emissions from the hot shock layer created in front of the probe. The radiation detection method was used to determine the amount of oxygen, nitrogen, and carbon dioxide present. A separate experiment used two of the radiometer's nine channels to determine the amount of water vapour in the Earth's atmosphere by measuring the absorption of infrared radiation reflected from the Earth's surface.

During entry, atmospheric samples were drawn into a specially designed inlet located in the nose of the probe, where a mass spectrometer determined the composition of the gas. The circular centre section of the nose was made of beryllium, while an ablator made of silicon elastomer covered the aluminium structure of the remaining part of the nose. The beryllium section protected the radiometer quartz window and the mass spectrometer inlet from contamination by ablation products. The afterbody structure was made of a fibreglass honeycomb with a silicone elastomer ablator for thermal protection.

To get re-entry data for the HOPE space plane, the Orbital Re-entry Experiment (OREX) was created. The goal was to gather data on various aspects, all having to do with the re-entry phase of the flight. These objectives included gathering data on the aerodynamics and aerodynamic heating as well as communications during the black-out and GPS navigation during re-entry.

The OREX vehicle has a blunt-shaped cone. It has a diameter of 3.4 m and weighed 865 kg at launch and 761 kg at re-entry [38]. On top of the research about the re-entry phase, the OREX vehicle was also intended to test the thermal protection system for the nose of the HOPE spaceplane. It was launched in 1994 into a 450 km circular orbit before it re-entered. After the re-entry, a parachute system was deployed, but it was not known to be type or size [41]. It finally splashed down in the central pacific ocean. [39, 40].

The Demonstrator of Atmospheric Reentry System and Hyperbolicvelocity (DASH) was created to verify the MEUS-C sample return vehicle. This vehicle would test the TPS system as well as the deorbit engines. The plan was to launch the vehicle on an H-IIA launcher as a piggyback payload [62,63].

The vehicle itself consisted out of two parts. The actual re-entry vehicle and the payload bus with the -deorbit engines. The total vehicle had a wet weight of 89 kg. The re-entry vehicle had a diameter of 0.4 m and a height of 0.2 m [63]. It weighed 16.5 kg and only consisted of a thermal protection system, space for a scientific sample and a cross parachute [63]. The TPS material was a carbon phenolic type capable of surviving the expected 10 MW/m^2 re-entry heat [62,63]. Although the size of the cross parachute is not given, it can be calculated to be somewhere between 3 m² and 4.5 m² [63].

The test was to be completed on the 4^th of February in 2002. Unfortunately, the DASH vehicle failed to separate from the HII-A on its way to the target GTO orbit [62,63]. From subsequent investigation gave the possible problem of the mix-up in the pin-assignment in the cable sending the separation signal [62].

SRE 1 was a technology demonstrator mission launched in 2017 by ISRO to test various re-entry and recovery technologies. The 500 kg capsule was launched using a PSLV C7 rocket along with a couple of other satellites. The capsule orbited earth for 12 days before re-entering the atmosphere. The capsule was successfully recovered, and various design aspects of SRE-1 have been adopted in RLV and Gaganyaan.

The capsule was powered with solar panels and had 8 ADCS thrusters. One of the main experiments of the mission was the reusable thermal protection system which endured an entry at 8km/s into the atmosphere. The Capsule used Ablative Phenolic tiles for the nosecone which experiences temperatures up to 2000 C. The sides of the capsule were covered in Silica tiles that relied on radiative cooling and experiences temperatures up to 1400 C. The capsule also used low-density ablative material on the far edges.

The capsule used a 3-stage parachute system comprising of two drogue parachutes and one main parachute, slowing it down from a velocity of 100 m/s to 12 m/s. The capsule was equipped with an onboard floatation system and released fluorescent dye after touchdown for easy aerial detection. There were plans for SRE 2 and 3 but they were cancelled as the focus shifted to RLV and Gaganyaan.