After several years of theoretical work on manned space-transportation systems, it has been decided to consolidate Europe's technological knowledge by conducting a flight-demonstration project within the Agency s Manned Space Transportation Programme (MSTP). The second qualification flight of Ariane-5 (V502), planned for April 1996, will carry an Atmospheric Reentry Demonstrator (ARD), for reentry technology experimentation. Industrial responsibility for the ARD project rests with Aérospatiale of France, as prime contractor
Although not strictly a prototype of the future European Crew Transport Vehicle (CTV), the ARD project will constitute a major step towards providing greater confidence in Europe s development capabilities. It is a simple unmanned capsule-type vehicle designed to improve our knowledge of certain critical reentry technologies.
To save time by avoiding the need for a long aerothermodynamic shape selection process, an existing shape has been selected. Dimensions and masses have been derived based on Ariane-5 performance capabilities and on ballistic reentry parameters representative of the CTV.
The technology-demonstration objectives are as follows:
Some other challenging issues stem from these primary objectives, such as:
Table 1. Main mission drivers
The ARD vehicle has an external diameter of 2.8 m (Fig. 1) and a maximum mass of 2.8 t. After its release from the launcher, it will perform a sub-orbital ballistic flight, followed by a guided lifting reentry, ending with a final deceleration phase under parachutes and a splash-down in the Pacific Ocean (Fig. 2).
The main driving parameters of these different phases are shown in Table 1.
Figure 1. The ARD vehicle
Figure 2. The overall ARD mission
The pressurised, air- and water-tight structure is composed of four main elements (Fig. 3): the bulkhead structure including radial stiffeners and supporting the heat shield; the conical part carrying the Reaction Control System (RCS), including access doors to equipment items and the Ariane-5 mechanical interface frame attached via a bolted flange; the internal hexagonal secondary structure supporting all electrical equipment; and the back cover ensuring protection of descent and recovery systems during the flight.
All structural elements are made of mechanically fastened aluminum-alloy parts.
Figure 3. The ARD structural design
The heat shield is composed of 93 tiles made from 'Aleastrasil , a compound containing randomly oriented silica fibres impregnated with phenolic resin. These tiles are arranged with one central tile and six circumferential rows (Fig. 1). The conical part and back cover are covered with 'Norcoat-Liège tiles.
Reaction Control System The Reaction Control System (RCS) will provide attitude control during the ARD s ballistic and guided reentry phases. It is derived from the Ariane-5 attitude-control system (SCA: Systè de Contrôle d'Attitude), based upon a hydrazine (N 2 H 4 ) blow-down concept, and is composed of:
The electronics system is also based on the reuse of existing Ariane-5 equipment, i.e.:
The communications and location system, designed to process, store and transmit data to the ground segment during the ARD s flight, is composed of:
The descent and recovery system is designed to decelerate the vehicle (Fig. 4) before splash-down in order to limit the impact loads and to ensure flotation for up to 36 h.
Its elements include:
Figure 4. The parachute deployment sequence
To cover other technology objectives, the ARD vehicle will be equipped with a significant number of measuring devices. Depending on the nature of the measurement, the data will be: transmitted by telemetry antennas (actual emission rate around 250 kbit/s) during periods of visibility, i.e. mainly just before black-out and less than 45 km after black-out, otherwise stored by UCTM during black-out; or stored (all measurements during the entire mission) by the on-board recorders.
The following measurements will be implemented on the ARD (Table 2). In addition, a few functional measurements (equipment temperatures, mission sequences, etc.) have to be taken into account, leading to a total of around 200 measurement channels. The positions of the surface measurements on the vehicle itself have been optimised to meet the technology goals, as discussed below.
The flight conditions will be derived from redundant information sources, including:
Aerodynamic coefficients Information from the SRI and a dedicated tri-axial accelerometer for high altitudes will be used to identify the aerodynamic coefficients. Typical accuracies for the force coefficients will range from 10% at 70 km to 5% at 50 km.
A specific RCS activation plan will be devised to tentatively identify RCS efficiencies as well as derivatives of the aerodynamic coefficient and dynamic stability parameter.
Aerothermodynamics Pressure and temperature measurements (Fig. 5) will be used to qualify the pre-flight predictions of heat flux and pressure distributions. The exact locations of these measurements will be finalised based on the actual TPS topology. These temperature-measurement locations will be used firstly for qualification of the thermal-protection material s behaviour, but also to evaluate aerothermodynamic heat fluxes. Optionally, thermo-indicators could also be used on the cone (temperatures lower than 1500 K) to provide a maximum of temperature mapping and a general overview of the flow-field topology (re-attachment lines, heat flux on protrusions, etc.).
Finally, measurements made on material samples located on the leeward part of the heat shield will be analysed jointly by TPS and gas-surface interaction experts.
The temperature history within the basic TPS materials ('Aleastrasil and 'Norcoat-Liège ) will be recorded through four elements instrumented with five thermocouples and ten with two thermocouples (Fig. 5 and Table 2). The six material samples instrumented with four thermocouples will each comprise four Ceramic Matrix Composite (CMC) samples on the heat shield and two Flexible External Insulation (FEI) samples on the cone. The CMC samples will experience heat fluxes as high as 800 kW/m 2. The heat fluxes on the FEI samples will range between 45 and 70 kW/m 2.
Figure 5. Measurement-sensor locations on the ARD vehicle
Table 2. Measurement plan
At all TPS-related measurement locations, thermocouples will be attached to the underlying structure to check the TPS material s efficiency. Analysis of the heat fluxes encountered during reentry and the temperature histories will assist in the post-flight analysis of TPS materials.
An overall assessment of the robustness of the guidance and control algorithms will be made during the flight. The guidance algorithm chosen is an implicit Apollo-Orbiter algorithm based on a reference deceleration profile. This kind of algorithm allows a good final guidance accuracy with limited complexity and storage requirements. However, in order to distinguish dispersions due to the basic navigation system (based on the SRI) from those generated by the guidance function itself, GPS information will be used.
Some technology measurements will be made on the parachute system (see Table 2) including the following of all deployment phases with a video camera, a specific tri-axial accelerometer (10 g axial, 2g transverse), as well as a few strain gauges on main chute lines.
The ARD should provide a fruitful set of flight data with which to qualify European design, prediction and development tools for reentry scenarios. It represents a major step towards the demonstration of European capabilities to develop, operate and recover such a vehicle.
The ARD Programme has been made possible by a number of contributory factors: