AeroTech Delay element changes

[soopirV]

I will probably lose all credibility by posting this, but... :rolleyes:

First, I know there's a delay drilling tool available now, although none of the max-delay reloads seem to have hit store shelves. Second, I know that it's not a good idea to modify the reload in any way. However, I have determined both with simulation and flights, that one of my birds would fly perfectly on an F39-7T. Problem is, the F39 is not sold with a 7...only a 6 or an 8. Sure, I could drill the 8 to a 7, but I have another bird that would fly great on F24-6W, which is only available as a 7.

Would it be okay/safe/appropriate/legal to swap the delay elements? Buy the F39-6T and F24-7W and create my desired F39-7T and F24-6W?

I would think, "No Prob!", but these are two different propellents, with obvious differences in burn characteristics...so perhaps the delay grains have different burn characteristics too? :o

Thanks for not chasing me out of here with pitchforks and torches (yet!)!! :D

[SCE to AUX]

As far as I know, there are 2 different delay compositions used by AT. "Standard" for most of them, and a "Slow" formulation for a few of them.

The trick is that in a composite motor, the delay grain starts burning from the moment of motor ignition, so a portion of it burns at high ambient pressure (while the main propellant grains are burning), and the remaining portion is consumed at low pressure. The burn rate of the delay material is dependent on the ambient pressure (the higher the pressure, the faster it burns).

Different propellant types burn at different levels of chamber pressure. So the same length of delay grain will give 2 different delay times when used with 2 different types of propellant.

AT sells interchangeable delay kits to get whatever delay time you want. If you have a caliper or micrometer, you can measure the length of the delay grains you have, and figure out which one you need to substitute to get the time value you want. Swapping delay grains and/or drilling them to adjust the delay time is an approved procedure, and shouldn't cause any issues with "modifications" to the motor under the safety codes.

Delay information available here:

http://www.rocketryplanet.com/forums/showthread.php?t=6290

[soopirV]

Thanks Bob, I'm awaiting the confirmation message from Rocketry Planet as we speak (have to be a member to DL that chart). Very exicted to see it, and hope that it prevents me from asking my next stupid question. Before I do, my ego demands I say that I've flown MPR for a few years, but was always happy by choosing recommended motors.

Reading AT's website, there are S,M,L delays related to the "high power & high power style" motors...I have the generic 29/40-120 casing, and am hoping that the 29mm line of motors are compatible with that casing? Also, from what I've gleaned from AT's website is that these particular motors do not require L1 certification...am I reading the data correctly? For example, the G79W shows hardware is 29/120, not 29/40-120 as the Hobby Lines do. Lastly, in the "High Power" section of their PDF catalog, there's one motor, the G104T, that shows it needs 29/100...does that mean what I think it does? Is that a L1 motor, and will it work with my standard casing?

I'm digging because I just finished the AT G-Force, which has only 3 or 4 suggested motors, and I'd like to expand on that within the limits of the law.

Cheers!

Dave

[soopirV]

D'OH! I got my membership approved just after submitting my reply, and I realize the error in my last question- the delays have different ODs.

Is there a similar resource to that delay chart you sent that gives us noobs some idea of what hardware is truly required for the menagerie of reloads that AT offers?

How would one know that they need a 0.610" aft closure, for example, if not for that chart you sent?

I HAVE been researching this as I'm toying with the idea of getting my L1, but really enjoy getting the most bang for my buck, and don't want to shell out for excess hardware that is only good for one particular reload (or just a few).

Thanks again!! :o

[SCE to AUX]

The 29/40-120 is called a "hobbyline" case, and is not compatible with the 29mm "high power style" cases. Totally different threads and forward closure design.

The 29/100 and 29/120 are 2 of the "high power style" cases. If you want to fly the G79W, you will need to buy a 29/100 case, and a set of forward and aft closures.

All the 29mm high power cases use the same forward and aft closures, so you only need one set, and only need to purchase the different length cases to fit different reloads. Most vendors offer these cases as "case only" or "complete motor" (case with closures).

AT also offers a package they call the "29mm special", which consists of a 29/240 case, a standard aft closure, a special 2-piece forward closure, and a set of machined aluminum "grain spacers". This package allows you to fly all of the 29mm high power style reloads for the 29/240, as well as ALL the ones for all the smaller 29mm cases. With this package, you can fly all the MPR reloads, and also choose from 8 different "H" reloads if you want to go for the L1 cert.

http://www.rocketryplanet.com/content/view/3281/30/

The best resource for figuring out what reloads go into what hardware would be Aerotech's own catalog. The 2011-2012 version is supposedly coming soon, but the 2009-2010 version is available on their website:

http://www.aerotech-rocketry.com/products.aspx

All the 29mm high power cases use the same .610" diameter delay grains. AT eventually plans to ship all reloads with the full length delay, and let the user drill the delay grain as needed. If you get into HPR much beyond a simple L1 cert flight, motor delay accuracy becomes a LOT less critical, because deployment is usually controlled by an electronic altimeter which senses apogee. Motor ejection is usually either disabled (omit the powder or use a plugged forward closure), or used as a backup (usually with a long delay). At their best, it is difficult to get better than +/- 20% accuracy using a pyrotechnic delay. Just too many variables to contend with. Electronics is really the way to go if you want accurate deployment every time.

The G104T reload is one of the "special cases", in that it is only a "G" motor, but it is considered a high power motor because it has an average thrust greater than 80N. Dealers *should* only be selling these to people with an L1 cert or higher. Same with the recently released G138T that fits your 29/40-120 hobbyline case.

Hope all this helps!

[ghrocketman]

For every approx 1/32 you drill into the delay, you reduce it by 1 second.
Aerotech does not reccommend reducing the shortest delays they offer.
You can always modify a medium or long high power delay to the time you want.
Go ahead and try it; most of us already do.
You can do the same for the 18mm, 24mm, and 29mm hobby line delays too.
MOST of the time Aerotech delays run long time-wise anyway; so keep that in mind.

[GregGleason]

If I'm not mistaken, the delays for the AeroTech 24/40 RMS F39T reload are 6 and 9.

If it were me, I would go with the 6-second delay, since AT delays tend toward long. The NAR S&T records a delay of 6.27 seconds for the 6-second delay for the F39T.

Greg

[Sandy H.]

For every approx 1/32 you drill into the delay, you reduce it by 1 second. [snip]

I recently flew an H170M motor for the first time and the instructions state .025" for each second of delay removal. In my case, I wanted an 8, so the stock 14-8=6 second reduction or .15". Using the 1/32 rule, I would have drilled .1875", which would have ended up being a 7.5 second reduction. I know that there are tolerances that make all of the above only valid in a perfect world, but I was surprised that it was off that much. I imagine a lot of the new instructions will specify the amount to drill, but don't know for sure.

Just thought I'd mention it, since I just used that load and noticed the different recommendation.

Sandy.

http://www.aerotech-rocketry.com/uploads/a81d7528-7754-486a-b88f-4689f1ecdc39_38_360m_in_20065-2.pdf

[SCE to AUX]

I imagine a lot of the new instructions will specify the amount to drill, but don't know for sure.

I'm sure the official AT instructions will call for the use of the official AT drilling tool, which is calibrated directly in seconds, rather than inches of drill depth.

http://www.apogeerockets.com/Aerotech_Delay_Drilling_Tool.asp

AT will need to provide a MINIMUM allowable delay spec for each reload type. This is important because the delay grain has to withstand the chamber pressure during the burn, so there is some minimum "web thickness" that needs to be present during the propellant burn, in order to prevent the chamber pressure from blowing through the delay column and venting through the forward closure, prematurely firing the ejection, destroying the closure, and probably trashing the rocket as well.

So far, I've only needed to use the drilling tool with the new G138T reload, which is only being sold with a 14 second delay. I fly this load in a modified Estes Big Daddy with a downlink videocamera package in the nosecone. The delays I have been getting seem to correspond pretty well to whatever I set, within the typical tolerances for motor delays.

Haven't needed to use the drill tool on any of the HPR 29/38 mm delays, as I usually only fly those in rockets using electronics, so I either use the longest available delay grain (purchased separately) if I want motor ejection as a backup for drogue deployment, or just install whatever delay the reload comes with, and pack the ejection charge cavity with grease rather than black powder.

[Jerry Irvine]

AT will need to provide a MINIMUM allowable delay spec for each reload type.I have not seen this written down anywhere but it is about 4 seconds to provide sufficient bulkhead for most motors.

They cheat a bit and say never go below our minimum delay, but since that is typically 6 there is some overhead. A well characterized combination can go down to 3 under 29mm.

This thread should be in the FAQ.

Jerry

[gpoehlein]

I would also like to add that, although NAR prohibits making "unauthorized" modifications to motors (removing ejection & delay to turn a regular motor into a booster, cutting an inch off an 18mm motor to make it a shorty, etc.), modifications to motors that are "authorized" by the motor manufacturer (drilling AT delay grains to shorten the delay) is legal by NAR standards. I'd have to double check if such modified motors were contest legal or not, but I know it's OK to fly them at sport launches.

Greg