Delay question

[Starfire73]

Ok, I’ve been doing this for decades and you would think I should know this, but I’ve come across conflicting information lately and couldn’t find a specific thread with these questions addressed.

1. Does the clock for the time delay in seconds (for the ejection charge) start at motor ignition, or at the start of the coast phase?
2. Is it different for Aerotech vs Estes motors?

Thanks!

 

[Antares JS]

Ok, I’ve been doing this for decades and you would think I should know this, but I’ve come across conflicting information lately and couldn’t find a specific thread with these questions addressed.

1. Does the clock for the time delay in seconds (for the ejection charge) start at motor ignition, or at the start of the coast phase?
2. Is it different for Aerotech vs Estes motors?

Thanks!

1. Start of the coast phase.
2. No.

 

[prfesser]

Ok, I’ve been doing this for decades and you would think I should know this, but I’ve come across conflicting information lately and couldn’t find a specific thread with these questions addressed.

1. Does the clock for the time delay in seconds (for the ejection charge) start at motor ignition, or at the start of the coast phase?
2. Is it different for Aerotech vs Estes motors?

Thanks!

As Antares JS posted, for all rocket motors with a timed delay element, the time begins at the start of the coast phase. BUT...

In an Estes motor the delay does not ignite until virtually all the propellant has burnt. In a coreburning-type motor like most composite motors, the delay is ignited at the same time as the propellant. Which can cause some confusing results.

For example, if a motor chuffs, then ignites, one might see a shorter delay than usual. That's because the propellant didn't reach high enough pressure to sustain ignition, but the delay, ignited at the same time, may continue to burn even at the lower pressure. When the motor finally does reach pressure high enough for sustained combustion, the delay is somewhat shorter than when it started burning. Result: short delay.

Also: in a composite motor under pressure the delay burns faster than it does in the coast phase. A 1/2" long delay in a Blue Thunder motor might provide a 10-second delay, while the same 1/2" long element in a Black Jack motor might give only 8 seconds of delay. That's because the longer burn time of the BJack motor used up more of the delay element at higher pressure. It's nothing to worry about; the manufacturer includes the proper length of delay element for that particular motor.

But as long as everything works right, the delay time is from start of coast to ejection.

Best,
Terry

 

[Starfire73]

Well, I level 1 certified on an Aerotech H135W DMS. I drilled out 6 of the 14 seconds of delay, for an 8 second delay. However, ejection occurred at 5 seconds after the start of the coast phase and so I did some doubling checking. That was an important 3 seconds as I had a JLCR set for 400’, BUT the chute opened at ejection. I think the rocket was going too fast at ejection and it pulled the chute out of the grip of the JLCR.

That was my first time drilling out a delay with the UDDT. Must not be real accurate - and yes i did check calibration before using it.

 

[sghioto]

However, ejection occurred at 5 seconds after the start of the coast phase

The delay element timing should start at motor ignition as both the propellant and delay grain are ignited at the same time usually.
The H135W thrust curve shows a 2.1 second burn duration. So technically the ejection charge should have fired at 5.9 seconds after motor burnout.
Since that's less then 1 second I would consider that within a reasonable tolerance.

 

[Steve Shannon]

The delay element timing should start at motor ignition as both the propellant and delay grain are ignited at the same time usually.

The H135W thrust curve shows a 2.1 second burn duration. So technically the ejection charge should have fired at 5.9 seconds after motor burnout.
Since that's less then 1 second I would consider that within a reasonable tolerance.

No, the portion of the delay grain that burns during the motor burn time is taken into account by the motor manufacturer but is not taken out of the delay time. Assuming that a delay is drilled correctly, drilling a delay to be an 8 second delay means that there should be 8 seconds of delay left after the motor has finished burning, not that the eight seconds of delay time begins when the motor ignites.

The delay is built long enough to burn during the motor's high pressure phase and afterwards. During the high pressure phase, the delay burns at a relatively high rate, such as five times the rate after burnout.

To put that into numbers, a fourteen second delay in a motor that burns for two seconds, would burn 10/32" while the motor burns (5/32 inch per second times two seconds) and 14/32" after burnout (1/32 inch per second for 14 seconds), for a total of 24/32" or 3/4".

Of course the rate while the motor is burning is dependent on the pressure within the motor and very sensitive to igniter placement, chuffing (like Terry explained above), contamination, delay grain seal, etc. We've all seen delays that were too quick and we've all seen delays that take too long. There used to be a term "aerotech bonus delay". I discovered that as I became more experienced I had fewer of those.

 

[Starfire73]

From Estes website FAQ page.

“The final number in the code gives you the timed delay in seconds between the end of the thrust phase and the ignition of the ejection charge.”

 

[Starfire73]

The delay element timing should start at motor ignition as both the propellant and delay grain are ignited at the same time usually.

No, the portion of the delay grain that burns during the motor burn time is taken into account by the motor manufacturer but is not taken out of the delay time. Assuming that a delay is drilled correctly, drilling a delay to be an 8 second delay means that there should be 8 seconds of delay left after the motor has finished burning, not that the eight seconds of delay time begins when the motor ignites.

The delay is built long enough to burn during the motor's high pressure phase and afterwards. During the high pressure phase, the delay burns at a relatively high rate, such as five times the rate after burnout.

To put that into numbers, a fourteen second delay in a motor that burns for two seconds, would burn 10/32" while the motor burns (5/32 inch per second times two seconds) and 14/32" after burnout (1/32 inch per second for 14 seconds), for a total of 24/32" or 3/4".

Of course the rate while the motor is burning is dependent on the pressure within the motor and very sensitive to igniter placement, chuffing (like Terry explained above), contamination, delay grain seal, etc. We've all seen delays that were too quick and we've all seen delays that take too long. There used to be a term "aerotech bonus delay". I discovered that as I became more experienced I had fewer of those.

Yup, that’s what I was thinking… the clock starts after the motor has finished burning.

So, for me I got 3 seconds less than I intended. According to Rocksim - at a 5 second delay (instead of 8) the rocket was still moving at 105’ per second. Enough force to maybe pull the chute out of the JLCR at ejection?

 

[sghioto]

I stand corrected as one who has never used commercial motor ejection.

 

[Steve Shannon]

I stand corrected as one who has never used commercial motor ejection.

It’s completely understandable. Having to design a delay that includes enough length for the motor burn and the delay time post burnout makes it much more difficult to guarantee the delay time. End burning motors are much easier to design a delay grain for.