Choosing the Right Delay

Does choosing the right delay time matter?  You bet.  A delay that is too early or too late means your rocket is flying really fast when the ejection charge goes off and the parachute is deployed.  This puts substantial stress on your parachute, shock cord and attachment and failure can ruin both your day and your rocket. 

Ideally, the delay element burns so that the ejection charge fires at apogee, when the rocket has slowed to a stop.  Unfortunately, in the real world you will often have to choose between a delay that is a shorter or longer than desired. 

Based on rocket simulations, each 2.0 seconds after apogee translates into about an additional 45 mph of rocket speed at ejection; 1.5 seconds adds about 30 mph.  And remember that delay performance can vary from labeled by 1.5 seconds or more, so a delay that should be 1.5 seconds after apogee could actually be 3.0 seconds after apogee!

So, when confronted with choosing a delay that is 1.5 seconds before apogee, or one 1.5 seconds after apogee, how do you choose?  Public Missiles publishes a great chart of altitudes for its kits using various motors.  They recommend choosing the shorter delay for a number of reasons, which includes their experience that simulations overestimate real world delays.  Apogee, in one of their excellent technical newsletters recommends the shorter delay as well.  However, if your flight isn’t perfectly vertical, the flight arc to apogee is longer and thus a longer delay is needed when compared to a vertical flight. 

Now I have observed that coasting rockets slow down when going up, and accelerate when going down.  So in the example above, I would rather eject before apogee, because the rocket is slowing while the parachute is being deployed. 

Parachute style is important too.  Some deploy and open quickly, others take longer (streamers, x-form parachutes).  The stresses they impose on your rocket will change whether the ejection is during ascent or descent, and must be taken into consideration. 

The last word on this goes to Ellis Mountain Rocket Works who said to build your rockets to withstand high ejection speeds.  That is good advice for good flying.