# Differences

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parachute_testing [2015/07/22 17:38] ana |
parachute_testing [2016/05/22 14:10] (current) sam |
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- | ====== Parachute Testing ====== | + | this page has moved to [[orb:parachute_testing|Parachute Testing]] |

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- | ====== Oscillation of a Parachute ====== | + | |

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- | {{ :parachute.png |}} | + | |

- | Pvs stands for parachute vortex shedding. The pvs is the cause of resonance of the system. At high altitudes, | + | |

- | the free-stream density is sufficiently low to result in a relatively small ratio of apparent mass to payload mass, | + | |

- | causing the system center of gravity to be located very close to the payload. This result in a long momentum arm | + | |

- | between the canopy and the system center of gravity, such that the pvs can dominate the dynamics of the complete | + | |

- | system. | + | |

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- | $f_{pvs}= \frac{S_t . V_{term}}{d_{ratio} . d_o}$ | + | |

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- | $S_t$ stands for Strouhal number and $d_o$ is the nominal diameter of the parachute. | + | |

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- | ====== Constraints ====== | + | |

- | * In our modelisation we won't consider the PVS oscillation due to the mass of the rover 2Kg and the altitude. | + | |

- | * The parachute that we'll implement will reduce the terminal velocity in 90%. | + | |

- | * For an altitude smaller than 1,6 Km we can apply the general dragging formula $2.W=c.\rho.V_{term}^2.A$ where W stands for the weight, $\rho$ is the air density, c the dragging coefficient, | + | |

- | * The recomending $V_{term}$ or landing velocity is between $3.5{m}{s}$ to $4.5{m}{s}$. | + | |

- | * The dragging coefficient for a half-sphere $c$ is 0.47. | + | |

- | * The density of the air increase with low temperatures; | + | |

- | * The area of the parachute can be obtained: $A=\frac{2.g.M_{rover}}{c.\rho.V_{term}^2}$ | + | |

- | * For a round parachute the diameter is obtained: $d=\sqrt{\frac{4.A}{\pi}}$ | + | |

- | * The spill hole to gain stability should have a diameter of 20% of the total diameter of the canopy. | + | |

- | * If the area and/or the diameter of the canopy are fixed, we can change the diameter of the spill hole in function of the terminal velocity, reducing the total area in the formula above. | + | |

- | * Dynamics of free fall with air resistance, says that a body will arrive to the terminal velocity after 5 seconds, with out depending on the altitude. | + | |

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- | **References: | + | |

- | * F. M. WHITE and D. F. WOLF. "A theory of three-dimensional parachute dynamic stability." | + | |

- | * A Method to Characterize Parachute System Instability Modes by Michael Bernatovich Dr. Ian Clark 5/6/2013 | + |