Modifications to commercial motor cases--lwhat say the NAR RSO's?

I'm pondering the idea of building a HIGHLY instrumented rocket (Strain gaged fins, pitot tubes, pressure ports, air temp. nose temp, etc) . something in the L2 power range. Among the measurements I would like to record are engine chamber pressure and engine case temperature. Would like to drill/tap a port into a plugged forward closure (AT type 38 or 54 mm) to get a pressure transducer port (maybe 1/16" dia into the chamber) , and a blind drilled hole somewhere to take a thermocouple or RTD probe. I am using electronic ejection, and don't need an ejection well. To be flown on commercial AT loads, not EX.

Would an RSO at a NAR sanctioned launch allow such a setup to fly? What about an entirely user-fabricated (turned from Al barstock on a lathe by a competent machinist) forward closure?

If I flew such a thing, would NAR insurance cover the flight?

Nope, you'll have to go Tripoli for a project like that. From the NAR website:
"Motors. I will use only certified, commercially made rocket motors, and *will not tamper with these* motors or use them for any purposes except those recommended by the manufacturer. I will not allow smoking, open flames, nor heat sources within 25 feet of these motors."

It's not that far of a ride down from Jersey, come fly it with MDRA and then you have no worries.

CoyoteNumber2 said:

Just fly it. Unless you have a big decal on the side of your rocket that says "I modified my casing," no one is going to notice or care. (Unless your RSO read this thread already, then you're out of luck.)

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As a NAR RSO, if I know there was an unauthorized modified casing in the rocket I would never let it fly on my range. It wouldn't be the first time I've seen modified hardware at a launch, and it wouldn't be the first time I disallowed a rocket from flying because of it.

There is so much wrong with a "just fly it" attitude, but it all comes down to your ethics. If your ethics tell you it's okay to try to slip one by at a NAR sanctioned launch just remember that a single action like this could jeopardize the entire club or even the NAR in the event something did go wrong. It would be highly unlikely that the NAR insurance would cover any incident involving knowing used modified hardware since we explicitly do not allow it. If you want to try something like this please take it to a Tripoli EX launch, where this type of thing is more common.

Don't get me wrong, I think what you are attempting to do is interesting. There is the right time and place to run these experiments, but a NAR sanctioned launch isn't it.

CoyoteNumber2 said:

Just fly it. Unless you have a big decal on the side of your rocket that says "I modified my casing," no one is going to notice or care. (Unless your RSO read this thread already, then you're out of luck.)

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My sentiments exactly!!!

NAR - No way, break one of their rules, you may as well sign up that you don't care for any of the others.

Tripoli - Why not? I may be wrong, but I believe that it would fall under experimental, so, yes on experimental days.

Geez. As a NAR RSO I'd have to say "no", and regret it like mad. I'd love to see that project and the results.
You're going to post photos/data once you do this, right?

Would an RSO at a NAR sanctioned launch allow such a setup to fly?

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Key phrase is "NAR sanctioned launch". An RSO should not knowingly allow this to launch.

What about an entirely user-fabricated (turned from Al barstock on a lathe by a competent machinist) forward closure?

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Again, no. I believe the "certified, commercially made.." statement applies to the motor hardware as well.

If I flew such a thing, would NAR insurance cover the flight?

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No. See insurance FAQ's or Q and A's

SCE to AUX said:

I'm pondering the idea of building a HIGHLY instrumented rocket (Strain gaged fins, pitot tubes, pressure ports, air temp. nose temp, etc) . something in the L2 power range. Among the measurements I would like to record are engine chamber pressure and engine case temperature. Would like to drill/tap a port into a plugged forward closure (AT type 38 or 54 mm) to get a pressure transducer port (maybe 1/16" dia into the chamber) , and a blind drilled hole somewhere to take a thermocouple or RTD probe. I am using electronic ejection, and don't need an ejection well. To be flown on commercial AT loads, not EX.

Would an RSO at a NAR sanctioned launch allow such a setup to fly? What about an entirely user-fabricated (turned from Al barstock on a lathe by a competent machinist) forward closure?

If I flew such a thing, would NAR insurance cover the flight?

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actually, if you use 38mm aerotech, they use 14/4-20 threads i think, and this is a common transducer thread... you may be okay to just bore from the inside of the closure to the threaded portion. tape in your transducer and grease up the port, and go fly. then you dont have to worry about tolerances on your closure...

albeit at a research launch

Salvage-1 said:

NAR - No way, break one of their rules, you may as well sign up that you don't care for any of the others.

Tripoli - Why not? I may be wrong, but I believe that it would fall under experimental, so, yes on experimental days.

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If you modify a commercial motor, as by the example in this thread, I do not think that qualifies it for EX.

EX is meant for totally homemade stuff, not slightly modified commercial.

Perhaps if EX meant "anything goes - we don't care what you do for a motor", then this could count as EX. But that's not what EX is, as I understand it.

- George Gassaway

Isnt a CATO the worst that can happen?

It isnt like people are going to be right next to it when you launch, and CATOs happen once in a while anyways.

Considering that he'd be modifying the motor to collect additional data, I'd say that's exactly a Research motor

A research motor, per the Safety Code, is "any non-certifiable motor made for personal use that may or may not contain commercially available components"

Ergo, a modified Commercial Motor is permissible

If the case is modified, then it is no longer in a certified configuration, and therefore must be treated as a Research motor. So, no, it cannot and should not be flown at a NAR launch.

The whole "Just fly it" attitude is garbage. The rules are not difficult to follow. Fly it at a TRA Research launch.

Then why does the NAR even both with R&D reports, if they prevent people from doing just that?

It's a long shot but there is a way you could do it at a NAR launch. You would have to get the motor manufacturer to do or sanction the modification, get the certifying organization to sign off on it, and get the NAR Board to go along with it. If you manage to pull that off you have a great future in sales.

By the way, doesn't a plugged forward closure still take the delay grain?

Thanks for all the input. I figured that "no way" would be the answer, but wanted opinions from real people, rather than just a straight regurgitation of the NAR HPR safety code language.

Looks like I'll have to spring for TRA membership (with TRA insurance) before instrumenting a motor case. I fly with METRA and occasionally MDRA, so access to a research friendly launch isn't a problem.

This whole project would be more of a technology demo to show exactly what kinds of measurements and transducers are available, and how to do signal conditioning and interfacing in a relatively small package. I do that kind of stuff for a living (university instrumentation shop), and thought that a fully instrumented HPR rocket would be a cool showpiece for students who drop by for help with their projects (maybe stir up enough interest to start a USLI team?). Have access to all kinds of surplus sensors, strain gages, transducers, etc. Thinking of using a Raspberry Pi and a USB DAQ board as a datalogger, and adding measurement channels a few at a time. Just put off the "NAR verboten" ones until later in the project, I guess.

billspad said:

By the way, doesn't a plugged forward closure still take the delay grain?

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That depends on the motor. For most of them, yes. Some of the Warp9 motors use a special plugged forward closure that does not have a delay well. There is no delay grain for those. But for a normal plugged closure, the answer is yes.

No need for TRA membership if you are going to fly with MDRA as long as you are certified to the appropriate level by one of the two. MDRA is non-affiliated and carries its own insurance that is independant of NAR or TRA for this exact reason.

Disaster_Guy said:

No need for TRA membership if you are going to fly with MDRA as long as you are certified to the appropriate level by one of the two. MDRA is non-affiliated and carries its own insurance that is independant of NAR or TRA for this exact reason.

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I'm always wary about the "indie" club insurance policies actually covering individual flyers, rather than the club itself. Especially given the tendency of insurance companies to find every loophole to avoid paying a claim. The NAR policy specifically covers the individual member, as does TRA insurance, AFAIK.

SCE to AUX said:

I'm always wary about the "indie" club insurance policies actually covering individual flyers, rather than the club itself. Especially given the tendency of insurance companies to find every loophole to avoid paying a claim. The NAR policy specifically covers the individual member, as does TRA insurance, AFAIK.

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MDRA's launches are not run in accordance with the Tripoli Research Safety Code, and thus Tripoli's insurance doesn't cover the launch.

-Kevin

I fail to see what the hubbub is about... the main thing I see in this whole argument is rules for rule's sake...
Looks like if you're at MDRA you're fine...

As someone previously mentioned, what's the worst that could happen?? A cato?? Those happen far too often anyway, either from improperly assembled motors or faulty equipment (worn out closures/threads/casings). I guess there is the remote possibility that you could lawn dart on somebody's new Mercedes or something and be out of pocket, but I take it you're NOT going to be flying "cutting edge" designs with unproven stability or having a high degree of complexity, like airstarts, composite staging, shear pins, or other such things that would make the flight 'complicated' and thus less reliable and more prone to such a malfunction...

Still, stuff happens and I can see where you'd feel better having the insurance behind you...

I find it somewhat amusing that HPR basically started as a "maverick" and technically illegal activity outside the safety code and the rocketry organization back in the 60's, 70's, and 80's, until it was FINALLY codified and "brought in from the cold" and basically "made legal" in the late 80's... how many flights took place prior to that outside the safety code boundaries and technically "illegal"??

Don't get me wrong, I'm all for the safety codes, but I'm for more about MAKING SENSE than rules for rules sake... I find it even more amusing (rather troubling, actually) that folks who'll choke on you modding your motor casing to accept a transducer and would throw a shoe at you for bringing it to the RSO table, will smile and pat some shmuck on the head wanting to fly some off-the-wall unproven stuff with inadequate safety precautions, low-reliability design, excessive complexity, or just plain ill-conceived ideas with little likelihood of success and virtually NO benefit other than the "hey, cool" factor for the risk taken, and send them out to the pad... (say, like, oh, somebody having the hairbrained idea that it's a good idea to use shear pins on a HPR flight on a field surrounded by homes and businesses when the waiver is only to 2500 feet anyway... so shear pins are TOTALLY UNNECESSARY and add unneeded complexity to the flight, lowering reliability substantially, and with insufficient testing... the rocket slid right through the check-in and flew, didn't have enough BP to shear the pins, and streamlined in to someone's backyard offsite, and demolished their back porch awning...

Yep, I feel safer already! Follow the letter of the law, but not its intent... Don't even think about looking at the risk/reward quotient... Geesh...

OH well, we return you to your regularly scheduled program...
That's just my take on it anyway...

Later! OL JR

georgegassaway said:

If you modify a commercial motor, as by the example in this thread, I do not think that qualifies it for EX.

EX is meant for totally homemade stuff, not slightly modified commercial.

Perhaps if EX meant "anything goes - we don't care what you do for a motor", then this could count as EX. But that's not what EX is, as I understand it.

- George Gassaway

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If you modify the components of the motor to include the ability to obtain scientific measurements, is it not a research activity?

When I "kitbash" a commercial reload for delay elements or o-rings or a liner that i had not planned on needing at the last minuit. when i fly that motor with different o-rings or liner, i have to fly it under a research flight card.

https://www.tripoli.org/LinkClick.aspx?fileticket=79KCbI0ADyY=&tabid=86

13. I want to ‘kit-bash’ some commercial motor parts to create a ‘custom’ motor is this allowed at a TRL?
Yes, this is considered a Research motor. However, this is not allowed at non- TRLs since the motor is not certified.

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luke strawwalker said:

Don't get me wrong, I'm all for the safety codes, but I'm for more about MAKING SENSE than rules for rules sake... I find it even more amusing (rather troubling, actually) that folks who'll choke on you modding your motor casing to accept a transducer and would throw a shoe at you for bringing it to the RSO table, will smile and pat some shmuck on the head wanting to fly some off-the-wall unproven stuff with inadequate safety precautions, low-reliability design, excessive complexity, or just plain ill-conceived ideas with little likelihood of success and virtually NO benefit other than the "hey, cool" factor for the risk taken, and send them out to the pad... (say, like, oh, somebody having the hairbrained idea that it's a good idea to use shear pins on a HPR flight on a field surrounded by homes and businesses when the waiver is only to 2500 feet anyway... so shear pins are TOTALLY UNNECESSARY and add unneeded complexity to the flight, lowering reliability substantially, and with insufficient testing... the rocket slid right through the check-in and flew, didn't have enough BP to shear the pins, and streamlined in to someone's backyard offsite, and demolished their back porch awning...

Yep, I feel safer already! Follow the letter of the law, but not its intent... Don't even think about looking at the risk/reward quotient... Geesh...

OH well, we return you to your regularly scheduled program...
That's just my take on it anyway...

Later! OL JR

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I've got nothing wrong with Bob running the experiment, quite the opposite in fact. I think it's a really cool idea. However, his question was whether or not it was allowed at a NAR sanctioned launch. The answer is No.

Here's a problem. Pressure transducers and thermocouples need direct contact to the pressurized area/heated area, but the aluminum cases used in commercial motors will melt fairly quickly on direct contact to the flame.

The reason liquids work so well is that the instrumentation is put either on non-inflamed areas, or on areas that are actively cooled.

No it's not allowed at a NAR sanctioned launch, but unless you case the transducer into the top of the delay grain and watch the pressure spike, you're going to be in for a forward closure failure and a lot of ruined equipment anyway.

Was considering the thermal issues, and planned on at least a couple static fire tests followed by careful inspection before actually flying the modified motor.

My initial thought was that a 1/16" dia port wouldn't present all that much surface area to be damaged by the heat during the relatively short burn time, and the fact that there would be no FLOW through it (pressure transducer being essentially a plugged port), the port shouldn't erode much at all. If erosion proved to be a problem, would probably use an inconel or other high temperature alloy sleeve pressfit into the bore on the internal face. Was planning on the transducer being mounted directly onto the outside of the closure.

Not sure if combustion products would plug the port enough to interfere with the pressure readings. That would have to be determined through testing, as well.

The temperature measurement is simply a blind well drilled in from the outside, leaving enough bulkhead thickness to withstand the pressure plus a considerable safety factor. Not looking to measure temperature inside the chamber (not sure if any thermocouple would even survive that environment), just a representative "motor case temperature" measurement.

I haven't done this myself but I recall watching Paul Robinson from AMW do it years ago. He had the pressure transducer at the end of a piece of tubing and filled the tubing with silicone grease to isolate the transducer from the heat.

An open delay well might allow enough convection that you could get some heat damage. If it's a flat surface I think you'll eliminate that problem. Consider the forward bulkhead of a hybrid that sits in the top of the fuel grain. They don't even get damage to the anodizing.

Of course if you're not getting enough heat to damage the bulkhead then your thermocouple isn't going to get an accurate reading. I've got no idea what the temperature range inside the combustion chamber is but they make thermocouples that go up to 4000F.

billspad said:

I haven't done this myself but I recall watching Paul Robinson from AMW do it years ago. He had the pressure transducer at the end of a piece of tubing and filled the tubing with silicone grease to isolate the transducer from the heat.

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The folks I know who run transducers all use the silicone grease to protect the transducer.

The key is that you have to make sure there are zero bubbles in the grease, or it will throw off your readings.

-Kevin

AT uses 5/16" threads not 1/4"-20

JD

ClayD said:

actually, if you use 38mm aerotech, they use 14/4-20 threads i think, and this is a common transducer thread... you may be okay to just bore from the inside of the closure to the threaded portion. tape in your transducer and grease up the port, and go fly. then you dont have to worry about tolerances on your closure...

albeit at a research launch

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A CATO is, by far. NOT the worst that can happen. The worst that can happen... Happened at a boy scout launch last month. Or was that in November?

cherokeej said:

A CATO is, by far. NOT the worst that can happen. The worst that can happen... Happened at a boy scout launch last month. Or was that in November?

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DING DING DING.