OpenRocket - Ejection Delay Start Time ?

[kjhambrick]

All --

Another quick Q.

When does the Ejection Delay Start in OpenRocket ?

It appears to start after motor burn-out which would be correct for end-burners.

OTOH, most AP Motors light the delay when the motor is ignited.

Is there a way to delay the delay start time ?

Thanks.

-- kjh

 

[neil_w]

When does the Ejection Delay Start in OpenRocket ?

It appears to start after motor burn-out which would be correct for end-burners.

OTOH, most AP Motors light the delay when the motor is ignited.

True, but the rated delay is relative to the burnout time of the motor, is it not? An E15-4 should eject 4 seconds after burnout (approximately ), not after ignition, otherwise it'd be useless. So the existing reported timing should be essentially correct as currently implemented.

Unless I'm missing something.

 

[Steve Shannon]

All --

Another quick Q.

When does the Ejection Delay Start in OpenRocket ?

It appears to start after motor burn-out which would be correct for end-burners.

OTOH, most AP Motors light the delay when the motor is ignited.

Is there a way to delay the delay start time ?

Thanks.

-- kjh

The motor ejection delay is measured during certification and reflects the time after the motor’s thrust drops to near zero, even though the delays on core burners are burning while the motor burns.

 

[kjhambrick]

True, but the rated delay is relative to the burnout time of the motor, is it not? An E15-4 should eject 4 seconds after burnout (approximately ), not after ignition, otherwise it'd be useless. So the existing reported timing should be essentially correct as currently implemented.

Unless I'm missing something.

Neil --

First, thanks to you and your team for OpenRocket !

It is an amazing piece of work

As for delays ...

The Estes E16 is a Black Powder End-Burner so yes, the delay is ignited after the propellant burns-through.

It should be right-on.

OTOH, most AP Motors are Core-Burners and the delay is ignited right along with the Propellant.

The Aerotech Delays that I am familiar with come in Standard Lengths and the burn rate is predictable which is why one can adjust the Delay Time by drilling the Delay.

But in practice, it 's complicated because AP burn rates depend on the pressure in the motor casing.

I fly with electronics and without motor ejection and I always fly the longest Delay available for the Motor.

This way I've got a smoke trail for as long as possible.

It matters when I fly small rockets very high -- too high to see at apogee

I also pack as much chalk as my drogue tube will hold when apogee is very high and it is expected much after the end of the delay smoke.

And when apogee is later than the delay, at least I know how long to watch the 'empty sky' for the cloud of chalk.

Or not ... oh no ... incoming !

And that's why I even noticed the Ejection Markers in OR -- it is nice to know when I will lose my smoke trail.

Anyhow, no big deal, I am sure there are other things to work on

Like I mentioned in my 'Time to Open the Chute' Thread, I am still learning OpenRocket and wondered if I missed a setting somewhere.

Thanks again !

You and the OR Dev Team are awesome !

-- kjh

p.s. speaking of time ...

One other thing I would find useful on the Rocket Design View is to show the time to Apogee along with the distance:

Example

Apogee:             14596 ft (at 25.1 sec)       // add time to Apogee here
Max velocity:       1984 ft/sec (mach 1.79)
Max acceleration:   2294 ft/sec^2

 

[kjhambrick]

The motor ejection delay is measured during certification and reflects the time after the motor’s thrust drops to near zero, even though the delays on core burners are burning while the motor burns.

Thanks Steve.

Interesting discussion !

I've been away from HPR for 20+ years.

In the olden days the motor delays for AP core-burning motor were 'approximate burn times of the delay slug'

Since the delay is ignited with the propellant on a Core-Burner, is delay time the time from motor burn-out or from ignition ?

Thanks again.

-- kjh

 

[SolarYellow]

In the olden days the motor delays for AP core-burning motor were 'approximate burn times of the delay slug'

This is still the case. It's just reported based on testing.

Since the delay is ignited with the propellant on a Core-Burner, is delay time the time from motor burn-out or from ignition ?

Thanks again.

-- kjh

Not Steve, but go back and read what he wrote again. The times are reported from motor burnout.

 

[kjhambrick]

This is still the case. It's just reported based on testing.



Not Steve, but go back and read what he wrote again. The times are reported from motor burnout.

Thanks Solar Yellow.

-- kjh

 

[Steve Shannon]

KJH,
As you pointed out the delay grain in most high power motors is burning the entire time the motor propellant is burning. The burn rate is sensitive to internal pressure; it burns faster while the motor is burning and then slows to a rate that’s close to 1/32 inch per second after the motor burns out and no longer pressurizes the case. Using the stock delay lengths and the delay times for Aerotech motors I estimated that the burn rate under pressure is about five times faster than after burnout.
Of course different motors burn for different time periods and produce various pressures while burning.
So, the manufacturers nowadays simply put in a long delay and then allow users to drill them out, 1/32 inch at a time until they are the delay they need. They estimate how long the long delay will burn and the certification process verifies it within a second or two.
Even so the delay time is not terribly consistent. There are many reasons for that, including the altitude at which the delay is burning.
Alan Whitmore (head of TMT) and I tested some pieces of Aerotech delay grain material. We bought full length grains, which are a few inches long, and had Aerotech cut them in two and ship them to us so we were using exactly the same grain. He burned his samples at near sea level and I burned mine at my elevation in Montana which is slightly more than a mile. His half samples burned at a rate that was 8% faster than my half samples.

My conclusion is that anything that you want to be done precisely should use an altimeter.

 

[kjhambrick]

Thanks for that 1/32 inch per second number, Steve.

I had forgotten exactly what the burn rate of the AT Delay Grains was.

But when I read your post, it immediately hit me

Nice experiment between you and Alan.

Knowing the properties of AP Propellant ( or Delay Grains ), I knew the burn rate would depend on the Pressure.

Like you said, anything requiring precise deployment timing requires electronics.

Thanks again !

-- kjh