Delay Calc

[jimdanforth]

So after a 15 year sit-out (I certified 10 days before 9/11) I'm back in. Going to recertify with my old corporal since it flew great back then. Going with an H180 for a nice easy flight. My altimeter has failed all tests, and a replacement will not make it here before Saturday's attempt. Sooo, just going with simple motor ejection. But Rocksim give the following data:
Time data

• Time to burnout: 1.401 Sec.
• Time to apogee: 7.396 Sec.
• Optimal ejection delay: 5.995 Sec.
• Shouldn't I actually need a 7.396 second delay since it burns at ignition? I always just used anecdotal (somone else's flight) to figure it if I wasn't using my altimeter.

 

[Bat-mite]

No, delay times given take motor burn into account. Drill your delay to six seconds.

 

[blackbrandt]

No, delay times given take motor burn into account. Drill your delay to six seconds.

^^^^ Time to apogee is time from ignition to apogee.

 

[NateLowrie]

The delay is from ignition. If it takes 7.3 seconds to apogee, drill to 7 seconds and you will get close. Note that delays can have a 10% differential in burn time as specified by the manufacturing.

 

[rstaff3]

I have always known the delay to be from propellant burnout to ejection. I sanity checked with Estes and AeroTech docs and that's what they say.

 

[markjos]

I have always known the delay to be from propellant burnout to ejection. I sanity checked with Estes and AeroTech docs and that's what they say.

Yes - this. Burnout to ejection. That's one reason it's so important to distinguish between the simulation values for "Time to Apogee" and "Optimal Delay" often seen in OpenRocket and Rocksim. Time to Apogee includes motor burn time, while Optimal Delay does not.

Mark

 

[NateLowrie]

My bad, I am mistaken.

 

[jimdanforth]

As an extension to this discussion:
Using https://www.rockethead.net/black_powder_calculator.htm and
https://www.rocketryforum.com/showthread.php?73431-4f-Black-Powder-Questions
I'm getting around 5.2 Grams of FFFG, or BOUT 5 cc (i HAVE A LEE'S dipper set).
Going to fly my R&D Corporal on H180, and I understand it has about 2 grams of powder for the ejection charge. Do I just manually measur and correct that when building the motor? To go from 2->5 seems like a big jump, but there is a lot of volume to this rocket. I built it in prep for level 2 cert.

 

[AlfaBrewer]

5g is a lot. What size is the airframe and how much space are you trying to pressurize?

 

[jimdanforth]

4" x 50" - parachute that consumes about 8-10" of tube.

 

[cls]

2 grams of powder is quite enough.

 

[Bat-mite]

4" x 50" - parachute that consumes about 8-10" of tube.

Remember that the volume you are pressurizing is from the top CR to the bulkhead. I seriously doubt you have 50" above the top CR, right?

 

[Banzai88]

5.2 grams in a 4 inch tube 50 inches long is going to produce about 16psi and 200 pounds force on the nose cone. How many shear pins of what size are you trying to shear?

 

[jimdanforth]

Wan't planning on sheer pins. Just used friction fit last time. Here's the rocksim file:
https://www.rocketreviews.com/rocksim-rocket-rd-corporal.html
It's really built just like the smaller rockets. The last centering ring is quite low. It does choke down for the coupler, but still reatins about a 2" passage, which opens back to 4". I built it for dual deployment, but as mentioned, the altimeter is toast.

 

[AlfaBrewer]

I'd suggest a temporary stuffer tube through the coupler to direct the ejection more towards the payload section. You may have some issues with the pressure wave going through the coupler passage if you don't.

As is often said, ground test your flight setup.

 

[Banzai88]

I'd tape the nose cone on, install the bulkheads on the coupler, and use the motor charge for single deployment.