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  #1  
Old 12-20-2013, 10:43 PM
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astronot astronot is offline
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Default Question about delay times for RMS reloads

I'm seeking a little bit of clarity on RMS reloads if I may impose on you fine folks for a minute.

This how I understand things and please correct me if I'm wrong.

Let me post the specs of a motor if I may and take it from there.

Specifications:
Reload Designation: F12-5J
Hardware Designation: RMS-24/40
Outer Diameter: .938" (24mm)
Length: 2.75" (70mm)
Loaded Motor Weight: 67 g
Propellant Weight: 30.0 g
Propellant Type: Black Jackô (J)
Total Impulse: 43 N-sec (max)
Burn Time: 3.6 seconds
Delay Time: 5 seconds
Maximum Recommended Liftoff Weight: 15 oz/425 g

Fits: Rockets designed for black powder `D` motors

For it to be a 24mm reload this motor is pretty powerful. 43Ns not too shabby.
Now the way I understand it is, the delay and the propellant are ignited at moment of ignition and liftoff. Correct?
If that is so, with a 3.6 sec. burn time and a 5 sec. delay, then recovery deployment should happen approx 1.4 sec. after motor burn out. Is that the correct way to look at this?

If it is, then why do some websites, like Hobbylinc don't list burn time for their reloads? How am I suppose to choose the correct delay? Say a I want a 3 sec coast after burnout. How would I choose the correct motor for the desired effect?

I'm just asking. Obviously some websites, hobbylinc is not the only one, expects me to do my homework before I go shopping. Fair enough.

Where might I go to discover, uncover, educate myself on these new reloads for the smaller motors. I would love to know what I'm looking for before I shop, and at least learn while I shop. All the data is important but really what is most important to me when shopping is
Reload designation, Max lift off weight, propellant weight, total impulse, delay time, and burn time.

Some of the specs on the motors don't really jive with what I would expect on sites like hobbylinc for instance.

These specs are for an E28-7T

The average thrust of the motor is 6.3 pounds (28 Newtons).
Time delay of 7 seconds between propellant burnout and the firing of the ejection charge.
Uses the exclusive Copperhead single lead igniter.
Motor Diameter: 0.94" (24mm)
Assembled Weight: 1.9oz (54g)
Maximum Liftoff Weight: 10.6oz (301g)
Total Impulse: 40 Newtons

Aerotech Product Number: 52807

And then these specs are listed for an F24-4W

Impulse energy- F-class
Average thrust in Newtons- 24N (5.4 lbs)
Delay in seconds between propellant burn-out and activiation of the ejection charge- 4 seconds
Propellant type- White Lightning
For use in- RMS 24 Motors
Primary use- Single Stage
Motor diameter- .94" 24 mm
Assembled weight- 2.2 oz. 62 g
Maximium liftoff weight- 9.5 oz. 269 g
Aerotech Product Number: 62404

That is very confusing to me. An F class motor has less thrust than an E class motor?

Am I looking at this totally wrong or something?
Thanks in advance for the help.

David
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  #2  
Old 12-21-2013, 02:20 AM
Rex R Rex R is offline
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the stated delay times are measured from motor (propellant) burn out, just like the Estes motors you're used to. yes the delay starts burning at motor ignition But, that has been allowed for by the motor maker, all you have to worry about is the coast time.
yes the F24 has a lower thrust but it has a longer burn (as denoted by the difference in total Newtons 50 vs 40) the Estes E9 has a lower thrust than the D12. running flight simulations in 'Open Rocket' helps a lot in finding the 'right' motor. The hand book of model rocketry has a nice chapter on rocket motors and what all those numbers mean.
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Old 12-21-2013, 02:29 AM
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astronot astronot is offline
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Quote:
Originally Posted by Rex R
the stated delay times are measured from motor (propellant) burn out, just like the Estes motors you're used to. yes the delay starts burning at motor ignition But, that has been allowed for by the motor maker, all you have to worry about is the coast time.
yes the F24 has a lower thrust but it has a longer burn (as denoted by the difference in total Newtons 50 vs 40) the Estes E9 has a lower thrust than the D12. running flight simulations in 'Open Rocket' helps a lot in finding the 'right' motor. The hand book of model rocketry has a nice chapter on rocket motors and what all those numbers mean.
Rex


OK. That clears it up some. They are treating these 24mm motors similar to BP motors of the same size, that why the burn times are not listed. I wish they were though. Is they some place I can find out this info before I buy? Sometimes I want a nice slow burn and then sometimes I don't.

But am I correct in assuming that this is not the case for stated delay times for motors of 29mm and higher. Should I take into account the total burn time of the motor and subtract that from the delay time to find out what my actual coast time will be?

Thanks for addressing this for me.

David
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  #4  
Old 12-21-2013, 08:26 AM
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billspad billspad is offline
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Take a look at this and this. A thrust time curve gives you more information than just numbers.


An E can have more peak thrust than an F but that isn't the whole story.

Delays labeled on motors are always measured from burnout no matter what size the motor.
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Old 12-22-2013, 09:56 AM
mikemech mikemech is offline
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