Terminal Entry and Landing GNC Flight Demonstrator for Re-usable Re-entry Vehicles: REVLANGNC

In the context of an increasing worldwide and European effort to improve the affordability of space transportation, DEIMOS Space is leading an activity to further develop and mature GNC technologies for future European reusable space applications.

Building on past experience, in the REVLANGNC project the design of innovative GNC solutions is consolidated for the terminal entry (TAEM) and descent and landing phases (D&L). The recovery strategy is based on parafoil systems that will provide the control authority needed in the last phases of the descent to allow for the return vehicle to achieve a soft and precise landing. The consolidated GNC solutions are tested in application scenarios fully representative of the recovery of critical systems:

• A lifting body vehicle performing a pinpoint landing, where the GNC shall steer the parafoil toward a predefined, fixed, landing site.
• The recovery of a launcher fairing through mid-air retrieval, where the GNC shall steer the parafoil toward a predefined location and maintain a given heading in order to allow a recovery vehicle to approach and perform the retrieval manoeuvre

The GNC technologies under development are based on innovative solutions to be able to cope with the challenges imposed by autonomous systems. End-to-end hybrid Navigation assures the continuous estimation of the vehicle state throughout all the phases of the flight, making use of measurements provided by the available sensors (IMU, GNSS, altimeter, and FAD(S), if available). The Guidance function uses online optimisation techniques to systematically recompute the best path to target depending on the actual flight conditions. Robust control design techniques are adopted to attained the challenging required levels of performance and robustness.

During 2020, the REVLANGNC activity has successfully completed several critical steps toward the GNC verification and in-flight testing.

At first, the ESA GNC design review 1 was successfully passed in May 2020: the proposed Guidance, Navigation, and Control solutions have been extensively tested using an internal DEIMOS functional engineering simulator (FES), in which a detailed parafoil model has been integrated, exploiting the verification experience and qualified models from other programmes. The performance levels obtained for the proposed GNC solution showed the capability to obtain a very precise accuracy at landing, below 200 m, depending on the capability to correctly estimate the wind. The GNC review therefore confirmed the suitability of the proposed GNC solution for the targeted applications.

Then, in September 2020, the ESA GNC design review 2 demonstrated the suitability of the REVLANGNC algorithms to the in-flight testing, by carrying out several analyses of the functioning of the GNC solution in the context of the targeted flight scenarios:

• A parafoil drop test with a scaled down parafoil to test in-flight the capability of the parafoil GNC to perform a pinpoint landing
• High altitude tests with an UAV to test the TAEM GNC in TAEM-lie conditions above 20 km of altitude

Models of the parafoil and UAV test platforms were used to perform Model-In the Loop (MIL) analyses and flight predictions for the flight test scenarios.

After the DR-2, the different GNC functions were coded and the GNC S/W tested in closed loop and nominal and non-nominal conditions. The Test Readiness Review 1 certified the correct behaviour of the GNC in a S/W-In the Loop (SIL) test configurations.

The REVLANGNC activity is now ready to move onto the integration of the GNC S/W on a processor for PIL testing, a necessary step before the start of the fligth test campaign.