Understanding effect of burn time on Aerotech motor delays?

broofa

Member
Joined
May 3, 2015
Messages
5
Reaction score
0
Question: If delay times are given as "time from motor burnout", but delay grains start burning at ignition for core-burning motors like Aerotech's RMS line of motors, how is it possible for Aerotech to have standard size delay grains that work across motors with a wide range of burn times?

For example, let's say the standard "L" delay grain from Aerotech has an *actual* burn time of 17 seconds (from ignition). Put that in a J520W motor (1.3 second burn time) and you have a delay time of 15.7 seconds. But put that in a J90W motor (7 second burn time) and you have a delay of 10 seconds.

In theory, that's a pretty considerable variation in delay times... certainly enough that you'd want to account for it when setting your motor delay.

I know this is complicated by other factors, too. For example, I saw a mention that delay grains burn considerably faster when a motor is at pressure than after motor burnout. But it's unclear whether that would temper or exacerbate this issue. E.g. Would a long-burning motor burn the delay grain faster for a longer period of time, leading to an even shorter delay time after burnout?

Anyhow, 'would love to get some insight into how to properly account for this seeming incongruity.
 

dvdsnyd

Well-Known Member
Joined
May 9, 2012
Messages
376
Reaction score
170
Location
MN
The delay grain ignites the same time the motor ignites, so it burns the entire duration of the the motor thrust.
As for how is it possible that they have standard size delay grains for different length motor burns. I don't know for sure, but my hypothesis would be that they alter the formula of the delay grain so that it either burns slower or faster over the same standard length to accommodate different burn times.
One thing to keep in mind - delay grain times can vary as much as 20% from the mean, with the ultimate max being a no more than 3 second deviation from the mean.
 

heada

Well-Known Member
Joined
Jan 18, 2009
Messages
5,513
Reaction score
2,911
Location
Indianapolis, Indiana
NFPA allows +/- 2 seconds or 20% on delays (from memory) Also I think the delay burn rate isn't as pressure dependent as propellant. Lastly they could have different standard delay lengths or delay formulas for long burn motors like the J90 vs standard burn motors, I haven't looked.
 

broofa

Member
Joined
May 3, 2015
Messages
5
Reaction score
0
my hypothesis would be that they alter the formula of the delay grain

Lastly they could have different standard delay lengths or delay formulas for long burn motors like the J90 vs standard burn motors, I haven't looked

These both suggest that the "standard" delay grains aren't actually standard but, rather, are motor-specific, which is not what my personal experience would suggest. From what I've seen fliers and vendors seem pretty comfortable swapping delay grains between different flavors of motor. Thus my working hypothesis is that Aerotech grains of a given designation ("L", "M", etc.) are the same across all motors. Obviously if that's not the case that would explain things.

NFPA allows +/- 2 seconds or 20% on delays (from memory)

Variations due to the unpredictability of manufacturing processes or environmental factors are a different issue. That's understandable. What I don't understand is how the basic math of [delay grain burn time] - [motor burn time] can produce the same result for different motor burn times (assume delay grain burn time is more or less constant). It seems unlikely this is the sort of thing the NFPA had in mind when they made that 20% (or whatever it is) allowance in delay variation.
 

broofa

Member
Joined
May 3, 2015
Messages
5
Reaction score
0
Just look at Aerotech's own documentation:

The delays vary by formula and length. No mystery here.

Oh, okay, so they actually do tune the delay grains size and composition for the different motors then! That's really good to know. As I said in my previous comment (awaiting moderation) I was under the impression delay grains of the same type were interchangeable across motors. Hence my confusion.

And I'm not sure why I hadn't figured this out before. I've actually seen that data sheet several times but for some reason the "length" and "Delay Mat'l" columns didn't register. :p Thanks for clearing that up!
 

heada

Well-Known Member
Joined
Jan 18, 2009
Messages
5,513
Reaction score
2,911
Location
Indianapolis, Indiana
Back before everything turned into adjustable delays, you had to buy your motor with the delay you wanted. If that wasn't available at your vendor of choice, you could buy a RDK for the motor. The RDK was propellant specific. If you had white lightning or black jack, it was 3 options. If you had redline, it was a different 3 options, etc. The options were listed on the back if the card on the reload kit. I think you can still buy RDKs for RMS 29mm and 38mm reloads.
 

tfish

Well-Known Member
Joined
Dec 7, 2009
Messages
3,119
Reaction score
1,583
Multiple delay formulas and lots and lots of different delay lengths..makes it possible. Each propellant formula and motor pressure..burn delays at different rates...during the motors actual burn. Then the delays will burn..by themselves at roughly 1/32 inch per second.

Tony
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
8,495
Reaction score
6,923
Location
Butte, Montana
As far as I know there are only two different Aerotech delay compositions. Aerotech lists one as being the same as one of their Smokey propellants.
The delay is sensitive to pressure. The head of TMT and I split some full length grains and burned one half of each at his elevation and one half at mine. He’s at about sea level and I’m at just over a mile high. The burn rate at his elevation, which is higher pressure, was consistently 8% faster than at mine.
Also, taking the length of a delay grain, using the commonly accepted roughly 1/32” per second burn rate after internal pressure has dropped, and looking at the burn time of a popular motor I was able to calculate that the delay grain burn rate while the motor is burning is about 5 times that of the delay grain after motor burnout when it is surrounded by ambient air pressure. So, a motor with a ten second delay will have 10/32 inch of delay that burns after the motor burns out. The remainder of the delay burns during the few seconds the motor burns. It’s a greater challenge with long burn motors.
So, a motor that chuffs, or a motor that’s flown to a much higher altitude, or a motor that just doesn’t fully come up to pressure will greatly affect the delay time. It’s simply not possible to guarantee delay times better than the NFPA 1125 requirements and I’m honestly surprised they do as well as they do.
 

Tractionengines

Well-Known Member
TRF Supporter
Joined
Aug 29, 2020
Messages
806
Reaction score
766
Location
Northeast Ohio
Just to make a note. This HAS been said earlier in this thread, but I will say it a little differently. 1) Pressure and delay grain burn rate go hand-in-hand. Higher pressures = faster burn of the delay grain. 2) Also most of the time internal pressure, and average thrust go hand-in-hand. Faster burning fuel = shorter burn time & higher motor pressure.
So...a given delay grain in a high-thrust, short burn motor, with higher pressure; will burn at a high rate, but for a short time and end with a specific thickness left for the coast phase. The same delay grain in a low-thrust, longer burning motor, with lower pressue, will burn slower but for a longer time ending with a similar amount of thickness remaining for the coast phase.
Within the wide tolerance range and relatively short times of our hobby, these "balance out" amazingly well in general.
 

Pete.D

Ex Rocket Scientist
TRF Supporter
Joined
Oct 7, 2020
Messages
152
Reaction score
107
Location
Chittenango, NY
Just to make a note. This HAS been said earlier in this thread, but I will say it a little differently. 1) Pressure and delay grain burn rate go hand-in-hand. Higher pressures = faster burn of the delay grain. 2) Also most of the time internal pressure, and average thrust go hand-in-hand. Faster burning fuel = shorter burn time & higher motor pressure.
So...a given delay grain in a high-thrust, short burn motor, with higher pressure; will burn at a high rate, but for a short time and end with a specific thickness left for the coast phase. The same delay grain in a low-thrust, longer burning motor, with lower pressue, will burn slower but for a longer time ending with a similar amount of thickness remaining for the coast phase.
Within the wide tolerance range and relatively short times of our hobby, these "balance out" amazingly well in general.

This statement assumes that a high-thrust, short duration motor has greater chamber pressure than a low-thrust long duration motor. This is not necessarily the case; chamber pressure is also dependent upon nozzle dimensions. Although I'm not involved with motor design, it seems to me that to maximize efficiency the chamber pressure would be kept as high as possible within the physical limitations of the case and nozzle, and in accordance with the burn properties of the chosen propellant. Therefore a low-thrust long-duration motor of a given total impulse would have a smaller nozzle while maintaining chamber pressure via core and/or propellant selection.

Excluding the super-long burning ("moonburner") motors, burn duration of the certified engines we use don't vary by more than a few seconds, so maybe burn duration doesn't have a substantial impact upon total motor delay burn rate when compared to other things that dominate the delay tolerance, like initial temperature, altitude and general manufacturing tolerances. It's probably all encompassed within the stated 20% delay tolerance.
 
Top