RMS Delay Minimum Length?

[pick1e]

Removed

 

[CarVac]

I need to drill a delay for a F39 24/40 reload. What I have is F39-9T. I need the delay to be as short as possible as I'll be using it as a booster on my Comanche-3 upscale. The idea is for the motor to eject a chute from the booster after separation, so a short delay is required. My sim shows the booster separating at about 300ft so getting that delay down as far as possible is important.

This chart shows the delays for the F39-9 and F39-6 as .500" and .406" respectively. What I take from this is that if drilling .094" would reduce the delay by 3 seconds, then drilling .188" would reduce it by 6 seconds, giving me a F39-3. That gives me a final thickness of .312". I'd really like to go down to 2 seconds, but that takes my delay thickness down to .281" :eyepop:

That makes me wonder, is there a threshold of minimum thickness of the delay element that should be adhered to so that it can withstand the pressure without blowing through? The drilling instructions I found don't say anything about that.

I wouldn't reduce it below ~4 seconds.

Anyway, the booster should tumble so even if the chute comes out late you should be alright.

 

[troj]

Yes, there is a minimum delay thickness. If AeroTech doesn't have it included in their instructions, I would send them an email and ask.

-Kevin

 

[DizWolf]

From the delay drilling tool press release on https://www.aerotech-rocketry.com/news.aspx?y=2011
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9/21/2011
AeroTech Releases 54mm RMS™ Delay Drilling Tool
AeroTech is releasing a new delay drilling tool for use with its line of 54mm RMS™ reloadable motors.

The 54mm RMS Delay Drilling Tool™ (or RDDT54™, P/N T082510-2) is designed for shortening RMS delay elements to adjust the time delay of a 54mm AeroTech RMS reloadable motor. The RDDT54 consists of a machined drill tool body and a 1/4” stub drill bit with an attached knob. The tool also has a provision for calibration, as the tip of the drill bit must be flush with the end of the tool body for accurate use. Though adjusted initially at the factory, a set screw placed within the knob makes any necessary readjustment easy.

The ends of the tool are counterbored with recesses that are labeled with 4 or 8 second reductions in the delay time. To use, the RMS delay element is placed into the desired end of the tool and held firmly in place, the drill bit is inserted and the knob is rotated clockwise until the bit is fully seated in the delay and no resistance is felt. In addition, a supplied 1/16” thick washer may be placed between the drill tool body and the drill knob to shorten the delay to an intermediate value (i.e., shortening the delay by 2 or 6 seconds).

As is always the case with drilled RMS delays, the delay element should be installed in the motor with the drilled end facing the propellant grains (or away from the ejection charge) for best delay accuracy. Also, no delay should be shortened to less than 4 seconds.

AeroTech is planning to eventually sell most 54mm RMS reloads in only the longest certified delay, so the RMS Delay Drilling Tool will be essential for the future use of the 54mm RMS line of products.

The suggested retail price of the RDDT54 is $14.99. It will also be available in a set, packaged with the previously-released 18-38mm RDDT. Contact your favorite AeroTech dealer to purchase the new 54mm RMS Delay Drilling Tool.
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It's from the 54mm PR, but I'm fairly certain I've seen this warning for all AT loads. (edit: yep, on the 18-38 delay driller PR too: https://www.aerotech-rocketry.com/news.aspx?y=2010)

 

[JohnCoker]

Shouldn't that really be "no shorter than the motor burn time + 4s"? The "4s" statement confuses me because some motors have a longer burn time and you'd end up with a double-ended motor. (Remember that the delay grain will start burning at the same time as the motor grains, just given how these motors work.)

Sorry if that's a stupid question, but it's been a long time since I used motor ejection.

 

[atticus]

Shouldn't that really be "no shorter than the motor burn time + 4s"? The "4s" statement confuses me because some motors have a longer burn time and you'd end up with a double-ended motor. (Remember that the delay grain will start burning at the same time as the motor grains, just given how these motors work.)

Sorry if that's a stupid question, but it's been a long time since I used motor ejection.

But the delay will burn at different rates DURING the motor burn depending on propellant type.
For instance the stock delay length for the E28-7T (Blue Thunder) is 0.438" whereas the E18-7W (White Lightning) is 0.625".
To end up with a 4 second delay you would remove the same amount from each.

 

[JohnCoker]

For instance the stock delay length for the E28-7T (Blue Thunder) is 0.438" whereas the E18-7W (White Lightning) is 0.625".

OK, gotcha; each motor comes with a different delay grain. That makes sense, thanks.

 

[ECayemberg]

...and while the delay grain indeed is ignited simultaneously with the motor, the stated delay (-6, -9, -15, etc) is the number of seconds from motor burnout to ejection charge initiation.

-Eric-

 

[Area66]

The 4 seconds is probably a protection to make sure it will not deploy on full motor trust . I see an AT smoke for 2 or 3 seconds this weekend before fully ignited and launch the rocket, what I suspect is the copperhead fire the delay but not the grains and later the delay fire the grains. Of course the rocket open 2-3 secondes before Apogee.

 

[UhClem]

I need to drill a delay for a F39 24/40 reload. What I have is F39-9T.

You have two problems with what you want to do:

1) NFPA 1125 has specific certification testing requirements for model rocket motors that have user adjustable delays. I can find no evidence that the F39 has passed such testing.

2) NFPA 1122 does not allow for the user to modify the motor. Language was added to NFPA 1127 to allow user adjustable delays in high power rocket motors but NFPA 1122 was not changed.

 

[bill_s]

The F39 was originally available in a -3 delay. It's not made anymore -- don't know if there was a problem, or lack of interest. They were certified for -3, 6 and 9 but the checkboxes on the instructions show 6, 9, 12. It appears they planned to sub the delays post-certification, however no one sells the 12 so perhaps it was never made.