127 wiersze
3.8 KiB
C
127 wiersze
3.8 KiB
C
// --------------------------------------------------------------
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// CU Spaceflight Landing Prediction
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// Copyright (c) CU Spaceflight 2009, All Right Reserved
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//
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// Written by Rob Anderson
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// Modified by Fergus Noble
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//
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// THIS CODE AND INFORMATION ARE PROVIDED "AS IS" WITHOUT WARRANTY OF ANY
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// KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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// IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
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// PARTICULAR PURPOSE.
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// --------------------------------------------------------------
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#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include "pred.h"
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#include "altitude.h"
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#include "run_model.h"
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// get density of atmosphere at a given altitude
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// uses NASA model from http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html
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// units of degrees celcius, metres, KPa and Kg/metre cubed
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static float get_density(float altitude);
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#define G 9.8
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struct altitude_model_s
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{
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float burst_altitude;
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float ascent_rate;
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float drag_coeff;
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int descent_mode;
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float initial_alt;
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int burst_time;
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};
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altitude_model_t*
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altitude_model_new(int dec_mode, float burst_alt, float asc_rate, float drag_co)
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{
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altitude_model_t* self = (altitude_model_t*)malloc(sizeof(altitude_model_t));
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// this function doesn't do anything much at the moment
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// but will prove useful in the future
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self->burst_altitude = burst_alt;
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self->ascent_rate = asc_rate;
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self->drag_coeff = drag_co;
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self->descent_mode = dec_mode;
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return self;
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}
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void
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altitude_model_free(altitude_model_t* self)
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{
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if(!self)
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return;
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free(self);
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}
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int
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altitude_model_get_altitude(altitude_model_t* self, int time_into_flight, float* alt) {
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// TODO: this section needs some work to make it more flexible
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// time == 0 so setup initial altitude stuff
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if (time_into_flight == 0) {
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self->initial_alt = *alt;
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self->burst_time = (self->burst_altitude - self->initial_alt) / self->ascent_rate;
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}
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// If we are not doing a descending mode sim then start going up
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// at out ascent rate. The ascent rate is constant to a good approximation.
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if (self->descent_mode == DESCENT_MODE_NORMAL)
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if (time_into_flight <= self->burst_time) {
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*alt = self->initial_alt + time_into_flight*self->ascent_rate;
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return 1;
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}
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// Descent - just assume its at terminal velocity (which varies with altitude)
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// this is a pretty darn good approximation for high-ish drag e.g. under parachute
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// still converges to T.V. quickly (i.e. less than a minute) for low drag.
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// terminal velocity = -drag_coeff/sqrt(get_density(*alt));
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*alt += TIMESTEP * -self->drag_coeff/sqrt(get_density(*alt));
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/*
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// Rob's method - is this just freefall until we reach terminal velocity?
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vertical_speed = -drag_coeff/sqrt(get_density(*alt));
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// TODO: inaccurate if still accelerating
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if (vertical_speed < previous_vertical_speed - G*TIMESTEP) // still accelerating
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vertical_speed = previous_vertical_speed - G*TIMESTEP;
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if(vertical_speed > previous_vertical_speed + G*TIMESTEP) // still accelerating
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vertical_speed = previous_vertical_speed + G*TIMESTEP;
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previous_vertical_speed = vertical_speed;
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*alt += TIMESTEP * vertical_speed;
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*/
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if (*alt <= 0)
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return 0;
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return 1;
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}
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float get_density(float altitude) {
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float temp = 0.f, pressure = 0.f;
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if (altitude > 25000) {
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temp = -131.21 + 0.00299 * altitude;
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pressure = 2.488*pow((temp+273.1)/216.6,-11.388);
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}
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if (altitude <=25000 && altitude > 11000) {
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temp = -56.46;
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pressure = 22.65 * exp(1.73-0.000157*altitude);
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}
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if (altitude <=11000) {
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temp = 15.04 - 0.00649 * altitude;
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pressure = 101.29 * pow((temp + 273.1)/288.08,5.256);
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}
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return pressure/(0.2869*(temp+273.1));
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}
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