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s 8 7 1 2 6 5 Figure 4-92: Thruster configuration The minimum force to be produced by each thruster is 2.6 kN. 4.4.5.3.3 Entry modes design 3 4 HMM Assessment Study Report: CDF-20(A) February 2004 page 350 of 422 For the EDL system, the GNC modes are cascaded as in the case of the MSR mission. Manual control is allowed during the final part of the entry, the descent and the landing phases. De-orbit De-orbit De De-orbit orbit mode mode mode Entry Entry Entry mode mode mode Descent Descent Descent mode mode mode Landing Landing Landing mode mode mode Figure 4-93: Entry modes

s 4.4.5.4 Control laws generation HMM Assessment Study Report: CDF-20(A) February 2004 page 351 of 422 For the control laws generation a trade-off has been made between two possible alternatives: Non-Linear Dynamic Inversion and Model-Based Predictive Control. Non-Linear Dynamic Inversion (NLDI) control techniques uses a model of the plant and the system dynamics under control. In case of a nonlinear plant, this technique uses a two-controller level scheme design: a feedback component to linearize the dynamics and a performance enhancement component of the resulting linear system. NLDI control technique computes a model of the dynamics of the vehicle during its flight. Then, it inverts the model to cancel all expected dynamics, and finally it inserts the desired vehicle response to the resulting plant dynamics. Model Based Predictive Control (MBPC) involves four control elements that use a linearized model of the plant under control around a set of well pre-defined trimmed points. The elements are as follows: a process model (a linearized system model obtained experimentally off-line), a predictor equation (a forward algorithm which will run for several steps to predict the behavior of the plant), a known future reference trajectory (previously obtained by other means and off-line), and a cost function (quadratic cost future process output error and controls). For the NLDI solution the controller is able to handle smoothly non-linearities, coupled aerodynamics effects and other uncertainties like Earth atmospheric and gravity disturbances. By having a broad model of the plant, NLDI can cover the full flight envelope, eliminating pointper-point design gain-scheduling. In addition, NLDI can handle a variety of vehicle plants when design evolves or updates. On the other hand, for the MBPC solution the controller is able to minimise the number of constraints when calculating the optimal trajectory and improve the failure forecasting function in the FDIR (fault detection identification and recovery) subsystem. Assuming a linearized model of the plant for a pre-defined interval of the flight, the predictor equation is based on the linearized equations of motion around this steady state flight condition. The NLDI solution requires an acurate model of the non-linear plan (masses, moments of inertia,…), and good aerodynamic data bases for all Mach number ranges (extensive wind tunnel campaigns). The MBPC solution requires a plant linearization on a wide rage of set points along the nominal trajectory, and on-line optimisation problem to be solved on-board inside a dedicated processor. The cost function for the quadratic optimal problem is based on a single criteria (minimum integral of the heat flux). The final selection is done for the Non-Linear Dynamic Inversion (NLDI) shown in Figure 4-94.

s<br />

8<br />

7<br />

1 2<br />

6<br />

5<br />

Figure 4-92: Thruster configuration<br />

The minimum force to be produced by each thruster is 2.6 kN.<br />

4.4.5.3.3 Entry modes design<br />

3<br />

4<br />

HMM<br />

Assessment Study<br />

Report: CDF-20(A)<br />

February 2004<br />

page 350 of 422<br />

For the EDL system, the GNC modes are cascaded as in the case of the MSR mission. Manual<br />

control is allowed during the final part of the entry, the descent and the landing phases.<br />

De-orbit De-orbit De De-orbit orbit<br />

mode<br />

mode<br />

mode<br />

Entry<br />

Entry<br />

Entry<br />

mode<br />

mode<br />

mode<br />

Descent<br />

Descent<br />

Descent<br />

mode<br />

mode<br />

mode<br />

Landing<br />

Landing<br />

Landing<br />

mode<br />

mode<br />

mode<br />

Figure 4-93: Entry modes

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