Burn time and delay time

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runty

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let's say I'm using a CTI motor with a 13 second adjustable delay. I go for -7 on the delay adjustment tool, yielding 6 seconds.

Does that 6 seconds get added onto the burn time, resulting in closer to 7-8s depending on the rocket? Or is it 6 seconds from the moment of igntion?
 
And keep in mind that most (all?) sim software knows this, so when they recommend a delay time it's time after burn-out, not time from ignition. So be aware that you might be compensating for something the sim software has already taken into account and may end up with a non-optimal delay. It's certainly true for Open Rocket's "Optimum delay" column and ThrustCurve's "Delay" column, those are the only sim programs I've used.
 
And keep in mind that most (all?) sim software knows this, so when they recommend a delay time it's time after burn-out, not time from ignition. So be aware that you might be compensating for something the sim software has already taken into account and may end up with a non-optimal delay. It's certainly true for Open Rocket's "Optimum delay" column and ThrustCurve's "Delay" column, those are the only sim programs I've used.

Do they really though? I'm running sims for my L1, and one specific sim says this: for a motor that burns for 1.2s, it calculates apogee at 9.1 seconds, and recommends a delay of 8.8 seconds. This is openrocket.

A total delay of 8.8 seconds from the moment of ignition would work well, since apogee is just tenths of a second later. But if I drill my delay to around 8.8 (or 9) seconds, my chute won't eject until almost a full second after apogee.
 
okay, so in my example the ejection charge would go off at about 7-8 seconds (depending on motor)? thanks guys!
 
As noted there is some natural variability in delay times. My guess is that your software is making an assumption that a second after apogee is a better risk than a second or two before apogee. There probably isn't a lot of difference but that is the choice that I would make as well.
 
Do they really though? I'm running sims for my L1, and one specific sim says this: for a motor that burns for 1.2s, it calculates apogee at 9.1 seconds, and recommends a delay of 8.8 seconds. This is openrocket.

A total delay of 8.8 seconds from the moment of ignition would work well, since apogee is just tenths of a second later. But if I drill my delay to around 8.8 (or 9) seconds, my chute won't eject until almost a full second after apogee.

The numbers OR and ThrustCurve give are "close enough" usually, on a motor delay + or - less than a second is within the range for motor delays since they are not exact even when drilled. Motor delays have an acceptable error factor (I don't remember what it is ) but a second more or less is within the range.
 
Do they really though? I'm running sims for my L1, and one specific sim says this: for a motor that burns for 1.2s, it calculates apogee at 9.1 seconds, and recommends a delay of 8.8 seconds. This is openrocket.

A total delay of 8.8 seconds from the moment of ignition would work well, since apogee is just tenths of a second later. But if I drill my delay to around 8.8 (or 9) seconds, my chute won't eject until almost a full second after apogee.

Hmmm. Probably worth double-checking what OR thinks the burn-time is, maybe it has it wrong (or just being complete, make sure that you have the right number). For example here's the OR sims for one of my rockets, you can see by comparing them that it's using the motor burn time for the delta (the last two are long-burn J's, so the delta is quite significant).

Screen Shot 2015-10-03 at 9.17.53 AM.png
 
Hmmm. Probably worth double-checking what OR thinks the burn-time is, maybe it has it wrong (or just being complete, make sure that you have the right number). For example here's the OR sims for one of my rockets, you can see by comparing them that it's using the motor burn time for the delta (the last two are long-burn J's, so the delta is quite significant).

View attachment 273510

Also double check the Cd's that OR is using for the rocket and the one ThrustCurve is using, if they aren't the same then the coast times can be longer or shorter and different (this came up in another thread as a reason for the two programs showing different altitudes, but it would also effect the delay times.)
 
let's say I'm using a CTI motor with a 13 second adjustable delay. I go for -7 on the delay adjustment tool, yielding 6 seconds.

Does that 6 seconds get added onto the burn time, resulting in closer to 7-8s depending on the rocket? Or is it 6 seconds from the moment of igntion?

Optimum Delay = Time to Apogee - Motor Burn Time

A sim uses an engine file that includes a thrust curve versus time for you selected motor, and includes the initial motor mass and the burnout motor mass.

Using the information you input for your rocket , the sim uses the equations of Newtonian physics to calculate the ideal trajectory of your rocket flight which includes the apogee altitude and time to apogee. As the thrust curve contains the motor burn time, the time, the difference between the time to apogee and the motor burn time is the optimum delay time.

Actual motor delay times depend on the burn rate of the delay charge which depends on the ambient temperature and pressure. There is an allowable +/- 20% variability in the delay time. The minimum variability is +/- 1.5 seconds for delays less than 7.5 seconds and not more than +/- 3 seconds for motors with more than 15 seconds.

Your recovery system should be designed to survive a deployment that is +/- 5 seconds off apogee. That means you deployment velocity could be as high as 32 x 5 = 160 feet per second, however under most circumstances you should be able to select an ejection delay time within +/- 2 seconds of apogee using the standard delays available for you motor. If the motor you want to use does not have a delay within +/- 2 seconds of your calculated apogee, you should double check to see if the motor is actually appropriate for you rocket.

If the wind is strong, you choose a shorter delay because your rocket will weathercock and not reach the no-wind apogee.

Bob
 
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