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Parabolic flight, minimal forward velocity as input parameters for airflap triggered designs



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header I use airflap triggered designs for a while now. This mechanism triggers for instance the release of a chute or second stage if the forward speed of the rocket during ballistic flight is under the threshold for which it is designed.
The best part of the airflap triggered release is that it is simple, light and rigid.
The drawback is that the forward velocity of the rocket has to decrease below the trigger level of the airflap mechanism. And this does not always happen in practice I found out, the hard-way, with some lawn darts. If your rocket is not launched straight up or the rocket flight is influenced by cross winds.
Then the rocket follows a more parabolic type of flight than straight up and down again.
So i asked myself the question, what is the minimal forward velocity, this is the velocity that works on the air-flap mechanism. Given certain initial launch speeds assumed that the rocket is stable and always in line with its forward speed.
The result is a little surprising to me.


In the graph on the right you see the result from a single flight that is characteristic for a type of rocket I use, the 9L. The rocket mass is 0.8 kg and for this graph it is launched with 80 m/sec under an angle of 75 degrees (this is what i consider a relative worst case launch angle).
It surprised me that the minimal forward velocity was 15.5 m/sec or over 55 km/h!. No wonder that the airflap mechanisme did not operate on some flights.
The airflap I use triggers between 5 and 10 m/sec.

The conclusion of this exercise was that for a reliable trigger with this rocket and initial velocity, the airflap trigger level must be raised to at least 20 m/sec. (72 km/h!). So for a reliable chute-release you may loose up to 20 meter apogee height with an airflap design, because the chute gets out below 20m/sec.
When compared with a timer release mechanism, the timer can do a better job (better is considered as getting the chute out at apogee) if the timer is able to release in a time frame between 4.4 and 6.8 seconds an interval of 2.2 seconds, which is a relative easy job for Tommy-timer designs.
So i decided to change over to a timer release mechanism for launches with initial speeds of 60 m/sec and up.





As reference for other launch angles and velocity's see graph below. The rocket in the model is the same (mass 0.8 kg / CW 0.8) as used in the calculations above.
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From this chart you can read that with an initial velocity of 80 m/sec and a launch angle of 80 degrees the minimal forward velocity is still well above 10 m/sec.
(second curved horizontal markerline that crosses the 80 m/sec vertical markerline in the yellow area).