Ejection delay options & equipment

[stix]

. . . I've had a rocket fitted with a drag switch end up with a parabolic trajectory so it never slowed down enough for the switch to close.

 

Well that's the point right there Neighbor - In the past I've had a few fly like that. I tend to calculate for the "best case scenario" (ie. almost straight up and down) whereas we should consider the worst. It's also very important to have 'known' quantities.

 

That is:

• The motor you're using has 'proven' measured and conisistant performance
• The rocket design has been measured for stability & weight (the free software OpenRocket is good for this)
• Drag coefficient (Cd) should be measured (this is the hardest one)
• Don't launch on a windy day (otherwise the rocket will 'weathercock')

A drag switch won't work effectively (if at all) if the rocket is curving through the air, still at speed.

 

Anyway, the last actual rocket (small 20mm OD) that I launched screwed up somewhere. After doing the calculations, I decided to use a timer to to spark the ejection charge at 1-2secs after calculated apogee. The timer was initiated at launch using a reed switch (magnetic) and a magnet inserted into one of the launch rails - a very clever solution indeed - or so I thought!.

 

After ignition, the rocket took off at speed ('F' class) pretty much straight up but at a slight angle, and out of sight in no time. I've no idea if the ejection charge actually worked because the rocket was never found.

 

In my mind, regardless of success or failure, it's very important to retrieve the rocket and either marvel in your success, or learn something from the failure. After more than an hour of two people searching, It could not be found. Sad.

 

Perhaps the rocket had less drag than I "guessed" and was moving at too much speed and simply ripped the parachute to pieces. The drag on the rocket is probably more important than I realised. This is one thing that's very difficult to measure without building a wind tunnel.

 

Well, afterall this IS rocket science. Estes seem to be able to get it close enough with a simple delay section of grain

 

I do like the idea of a meachanical contraption.

Edited November 24, 2016 by stix

[Redrocketman]

I have now mastered the delay timing and charge for motors up to H maybe I, using epoxy delay grain. Pass fire hole drilled to give certain web thickness for required time. Eg 2/4" delay grain web thickness: 9 sec - to the sec!!!

Far out, again!! 1/4" : 9sec!!