Aeroespacial

SENER was the main contractor for the IXV’s GNC system: the mind that controls the mission’s trajectory

11/02/2015 (Spain)
SENER was the main contractor for the IXV’s GNC system:  the mind that controls the mission’s trajectory

The European Space Agency’s IXV (Intermediate eXperimental Vehicle), an atmospheric re-entry demonstrator, successfully took off on board the European launcher, VEGA, from the Kourou Space Center in French Guiana the afternoon of February 11th. As planned, it reached an altitude of 413 km, leaving the Earth’s atmosphere. A few minutes later, the autopilot guided the vehicle back towards the Earth through a safe re-entry trajectory up to 26 km above the surface. At this altitude, the process of releasing a sequence of three parachutes began to slow down its descent for a safe splashdown in the Pacific Ocean, approximately one hour and forty-two minutes later. A system of flotation devices enabled a full recovery of the vehicle, which was performed by the Aries ship.

This was a very significant milestone for the European Space Agency (ESA), as the mission had the objective of demonstrating re-entry technology – of vital importance on manned and unmanned missions that require a safe return to Earth – for a hypersonic vehicle with a pure lifting body. Specifically, the IXV mission followed a trajectory that was fully representative of any return mission from a low Earth orbit, for example a return from the International Space Station (ISS), involving speeds of over 26,700 km/h. Modern European technologies for hypersonic and supersonic flight were tested in real conditions during the voyage, such as technology for aerothermodynamics, thermal protection systems or guidance, navigation and control (GNC) systems.

Among these technologies, the GNC system was of particular significance, being the most critical application on the mission. It includes autonomous flight control that is based on the aerodynamic actuators (flaps) and is assisted by thrusters for attitude control. In this respect, SENER and Elecnor DEIMOS were responsible for what can be considered the intelligence of the spacecraft, which pilots the vehicle from the moment of its separation from the VEGA launcher until the opening of the parachutes. SENER led the consortium, also integrated by GMV (which was responsible for the navigation function), that delivered the complete process of detailed design, development, integration and verification of the GNC. This was under coordination from Thales Alenia Space Italia (TAS-I) as the prime contractor for the mission. The GNC of IXV is the result of a framework agreement between SENER and Elecnor DEIMOS that takes advantage of the technological capabilities of both companies.

The GNC in operation

The GNC for the IXV took into account some highly demanding technical requirements that were specific to this re-entry mission: the IXV vehicle is a pure lifting body, weighs 1.957 kg and has a lift to drag ratio of 0.7. During its voyage, it followed a suborbital equatorial trajectory with a 5º inclination and reached an altitude of 412 km in its apogee. The re-entry phase lasted approximately 21 minutes and was controlled through a combined use of thrusters and aerodynamic actuators, which reduced the speed down to 1.6 times the speed of sound (Mach 1.6). At this point, successive openings of parachutes were initiated, helping the vehicle to eventually come down to its expected location in the Pacific Ocean.

In order to pilot such a mission, the vehicle boarded a set-up of sensors and actuators that comprised: four thrusters giving 400 Newton each; two aerodynamic flaps, located at the base of the vehicle; an Inertial Measurement Unit (IMU) with gyroscopes and accelerometers; a GPS receiver; and a Descent and Recovery System (DRS) made up by a set of parachutes, floatation and location devices.

The Flight Management system was responsible for scheduling the internal GNC functions operation in accordance with the pre-programmed flight phases and the calculations of various parameters provided by the Navigation function (IMU acceleration and attitude measurements, GPS position and speed, and an algorithm that improves the altitude estimate during the GPS blackout phase, which is based on Drag Derived Altitude pseudo-measurements).

The Guidance function calculated the reference attitude for the vehicle’s control for it to follow the desired trajectory up to the point where the parachutes were to be released. As regards the Flight Control system, it used thrusters to correct the vehicle’s attitude around three axes during the orbital phase and the yaw during atmospheric re-entry. The aerodynamic actuators (flaps) were used to trim and control the vehicle on the longitudinal and lateral axes during re-entry.

Without doubt, the GNC was one of the vehicle’s most complex components, and its correct function was one of the reasons behind the mission’s success. SENER was entrusted with this responsibility because of its experience in missions such as Minisat and Planck, the company having progressed from being a subcontractor for attitude control system on Planck (AOCS), to the main contractor on the GNC for IXV, and also the main contractor for the complete Proba-3 mission.

GNC/AOCS systems is one of three areas in which SENER specializes as a supplier of production and engineering services for the space industry, the other two being precision mechanisms and optical systems. With nearly 50 years of experience in Space, SENER has delivered over 260 devices and systems that have been successfully launched in satellites and space vehicles for agencies from the USA (NASA), Europe (ESA), Japan (JAXA) and Russia (Roscosmos).

The significance of the IXV mission

Despite being a low-cost, experimental vehicle, IXV is a pioneering pure Lifting Body vehicle, in that it was the first to achieve a controlled and autonomous lifting re-entry, in a context of other re-entry vehicles of a ballistic nature with less lifting capacity (capsules), or the manned winged vehicles that are far greater in their cost and complexity (such as the Space Shuttle) and have been used to date by other space agencies.

This ESA project puts Europe at the forefront of re-entry technologies, which are needed to continue moving forward in the exploration of our solar system, enabling missions whose scope is to return to Earth with samples taken from distant environments such as Mars, asteroids, comets or other planets.

Thanks to the competitiveness of its industrial network, Spain has played a particularly important role in the IXV program. It has in fact been the second most active country in the mission, with both the technology it has provided and its contribution of around 18%. Currently, Spain is the fifth largest European space power and has a high degree of specialization as a supplier of equipment, instruments and systems for satellites.

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