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Discussion in 'Propulsion' started by ATGM, May 16, 2015.
I believe igniter placement in motor affects delay times as well.
Hey, guys, I've got a radical idea!
Why not use electronic deployment and skip all the delay drilling inaccuracies?
Oh, wait. Silly me. There I go making sense, again. I'll stop. Promise.
Seriously, though: If 1/2 second tolerance is required, you're much better off going for the electronic method, rather than the motor ejection. The inherent lack of accuracy in measuring delay adjustments leads to the poor precision you're complaining about. There's more than one way to skin a cat, after all. You can have a precision milling machine setup to make your delay grain adjustments and still not get where you want because of variations in manufacture/motor configuration/igniter installation. So, keep that in mind and use a different method.
The closer the igniter to delay element, the earlier it lights?
^^-- the longer it chuffs, the shorter the delay.
The claim is that the 3 second gaps between drill tool settings doesn't matter because the motor itself is +/- 1.5 second (with shorter delays). 1.5 + 1.5 = 3. You should be able to get it within 2 sec. in reality.
Rockets have to be built to be able to withstand delays + or - a few seconds from optimal delay.
As does the altitude at which the delay grain burns. An AT delay grain at one mile altitude consistently burns 8% slower than one burned at sea level.
I read that, for model rockets, deployment should be at 50 fps or less. That's within about 1.5 seconds from apogee. There might be an impression that since this is on the Aerotech thread etc. we're all talking heavy duty high power type rockets. I am not. Mostly I'm talking about getting 29 mm G reloads in 7 second delay and taking a couple seconds off.
The improvement I get from using a drill bit wouldn't necessarily be worth $20, but it's worth NOT spending it.
plus or minus 1.5 seconds from apogee is recommended for ALL rockets LPR to HPR, depending on materials is how much damage can occur from being earlier or later whether its the airframes or recovery components.
At least reasons:
1). Extra weight (battery, electronics, etc)
2). Extra complexity
3). Extra cost
1/2 second ejecting timing is simply NOT required.
Let me qualify the above statement - I can't think of any situation, nor look back on any of my personal experiences, where 1/2 second ejection timing precision is necessary. It may be desirable, and it's sure nice to time the ejection to apogee, but it's not necessary.
In a perfect world, we would want to separate the airframe and eject some kind of laundry at apogee.
But there are plenty of ways to work around the lack of ejection precisely at apogee, and still have good time.
I always fly the JL Altimeter 3 - it reports apogee to ejection times. <2 seconds is my target. If it's >2 seconds something went wrong either with my SIM or with drilling the delay. Getting a more accurate delay than that I found isn't feasible due to the variations in building the motor, the burn time of they delay grain and the errors in drilling the delay. (Not to mention variations in the flight time of the rocket, etc.) If I recall, the delays are +/- 20% to start with.
For example, I built two G64 reloads at the same time and drilled the delays with the same drill the same way (taking 2 seconds off of the 10 second delay grain). I flew them both in the same rocket within 30 minutes of each other. The 1st flight "apogee to ejection" was .9 seconds!! Perfect.
The second flight, "apogee to ejection" was 2.9 seconds Way off!! But I built both motors at the same time, drilled the delays the exact same way with the same drill and flew both motors in the same rocket within 30 minutes. What did I do wrong on the 2nd reload? Who knows. I certainly can't figure out anything I did differently.
You probably didn't do anything wrong. Delay elements vary. That's kind of an extreme example. Of course the apogee-to-eject time is also dependent on the flight path. Did the second flight weathercock more than the first or otherwise arc over more?
Igniter placement can do it. If it isn't up close to the delay element it could result in a delay of its ignition. Remember that all of those combustion products are trying to get out the nozzle so they give the delay grain a miss.
Also, surface contamination of the delay, such as getting grease on the surface, can really throw off consistency. One thing I’ve noticed is that the more experienced and careful people become, the more consistent their delays burn. I haven’t had an “Aerotech bonus delay*” now for over a decade.
Also, Alan Whitmore and I did some testing. The actual delay grain material from Aerotech burns at a very consistent rate as long as nothing external changes, but very slight changes in motor performance cause huge changes in the delay times. Delay grains burn loosely five times faster while the motor is pressurized than when only the delay is burning in ambient pressure. A motor that burns one fifth of a second longer removes a whole second more from the delay. So, I believe that failure to consistently and immediately ignite the motor is probably the greatest detriment to consistent delays. Igniter placement can affect that greatly.
*I used to blame these on Aerotech. I’ve had to eat those feelings.
If I recall, the G64 has a "C-Slot" grain. (I'm out of G64's at the moment so I can't check) If those slots aren't well aligned there is a chance the igniter could hit the bottom of the top grain and stop there. That might explain the two different behaviors from the two motors.
I've had a "healthy" paranoia of getting contamination on the delay grain. I may take it just a bit far by using nytril gloves when building the motor so I can keep the grease on the gloves not my skin & remove the gloves when handling the delay grain.
I believe that something outside of the motor build process was causing the different results. I'm reasonably convinced that it's not the delay grain burning significantly longer but instead something in the environment or in my process that is causing the difference. How I insert the starter could explain it, especially with the C-Slot grains.
I'm going to try marking the igniter where it is verified at the top of the grains, then double checking when I install the starter on the pad. That way I can either verify or eliminate that as a cause. It shouldn't be too hard to mark the point with a sharpie and then check on the pad.
The G64s that I remember were one monolithic grain with a C-slot. There would be no misalignment in assembly.
A 20% difference can be a huge amount in a longer motor delay... nearly three seconds on a 14 second AT DMS motor. For any delay over 7 seconds, you need to be thinking about using electronic deployment.
They are made in 2 grains now to avoid hazmat shipping. You have to tape them together to align the slot.
Let's all be the same and do things the way John does.
Context, Jarrett, context. The discussion was about 1/2 second delay adjustments. I was pointing out that if you need that level of precision, you're better off using electronics. Others have rebutted my point quite effectively. Day late and a dollar short.
I'm always late and short on plenty of things, nothing new there.
For fun, who the hell would need 1/2 second precision anyway?
No Earthly idea, myself! I was just trying to bring a bit of levity to the discussion. As you can tell, I don't always succeed in my goals.
I know Aerotech is running behind in production again and was curious if Aerotech has considered pre-orders? I do not know if this is cost prohibitive or if there needs to be a certain number of orders before production can begin? Does anyone know if clubs do club orders from Aerotech; (i.e. One club member becomes licensed and places an order once a certain dollar amount is reached)?
We got a guy like that. He does pre orders whenever AT gets ready to make a price hike.
Not sure where you are ordering from but we currently have a 2-4 week lead time.
Charlie, where are we at on 38mm end burn closures? Waiting on that for a warranty replacement from you guys as well ascthe rest of the hardware.
A couple days ago, I posed the question regarding the status of Explosives Regulatory Division of Natural Resources Canada authorization for the Quest/Aerotech Q-Jets for purchase and use in Canada. It has been three years that our American compatriots have had access to these model motors. About a year ago, Gary did comment to me on Facebook that some work was being done, and upon consulting the current ERD Authorized list, which regrettably is almost a year out of date, there is no mention of any authorized Quest motors. It would be interesting to know which if any of the newer Aerotech/Quest products have been submitted for testing. Such as the F67 Enerjet (Econojet) and any or all available Q-Jets. There are many of us chomping at the bit to get access to these products.
Thanks to Bernard Cawley for encouraging me to attempt to contact Charlie directly, regarding news of any further activity post Gary's post to Facebook.
NAR 26894 L2
CAR S04 L3
Garth, thanks for the comments. Charlie is no longer working for RCS/AeroTech. We will make contact with one of our Canadian representatives who helps with the authorizations and see where we stand with the newer motors.
I will ask Karl where we stand on this and report back. Thanks for your patience.
Well that's some news, at least to me. Thanks for starting this thread Charlie and good luck with whatever's next.
What is Charlie doing now?
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