Error (?) in motor adapter instructions prompts some thoughts.

[WizardOfBoz]

So I purchased a Madcow DX3, with a 38mm motor mount and a PML motor retainer. I don't have level 1 cert, so I got a 38-29mm motor adapter so I could actually test launch before putting an H into the thing. In dry fitting the adapter together, I realized that if I put it together as the instructions suggested, I would need a very complex retainer to hold the 29mm motor in place. I made a drawing to help me think, and to communicate the thoughts. If the discussion below is useful, great. If not, ignore. If I'm wrong, I'd love to hear why (a better or more proven idea would be super).

So in diagram a), I show a standard 29mm motor, with a shoulder on the aft end to serve as a thrust ring. RM indicates the registration line of the shoulder. This is where the aft of the motor tube meets the motor and RM sets the position of the motor and motor nozzle.

As I'm fairly new to high powered (or medium powered here) rockets, I'd be a bit reticent to create an adapter that moves the motor forward or aft. I figure that the rocket designer knew where they wanted the motor, and put it there. So in diagram b), I've put the adapter registration line (RA) is aligned exactly with RM. This keeps the motor in the exact same place as a larger motor would be. It does require a spacer because the shoulder ring on the adapter (green) is wider than the motor shoulder (roughly 1/2" and 1/4"). A washer along would retain the motor, but the spacer keeps the it from shooting aft when the ejection charge goes off.

To avoid the need for a spacer, one could move the inner motor tube back 1/4" or so. This is diagram c). The back of the motor shoulder then corresponds to the back of the out motor adapter tube and shoulder. A flat washer (41mm by 25mm or so) can then keep the motor secure for ejection. The shift is small - about 6mm for my adapter kit. Apogee sells an adapter kit and Tim van Milligan put a little scalloped wooden washer in the kit to allow retention, apparently using drawing c arrangment.

Diagram d) shows how my instructions suggested making it. If one does this the inner motor tube sticks out so far that the 38mm motor retainer can't work without a very complex "top hat" retainer. Or duct tape. And the motor is moved aft about 5/8", changing both the nozzle position and the CG a little bit. If the instructions would have worked I never would have dove into this!

Also, my adapter kit doesn't have a washer. I'd probably buy the Apogee product based upon completeness.

For now, I'm going with the arrangement in diagram b. The best idea for a spacer I've found is a copper tubing union for a 1" tube, sawn to be about 7mm long. This with a 1-5/8 fender washer, drilled out to 1 inch ID. One might try CPVC for the spacer but motors are supposed to be allowed to get to 200°C, and CPVC has a service temperature limit of 200°C.

Comment or ignore as the Spirit moves you.

 

[Titan II]

Comment or ignore as the Spirit moves you.

Long time multiple DX3 owner here. Don't waste your time with the 29. Stick a small H in it and fly. That bird is so well proven there is nothing to go wrong if you do not do something stupid. That is the Spirit talking. Have fun.

Addendum....just realized I gave you the same advice in an earlier thread.

 

[WizardOfBoz]

Long time multiple DX3 owner here. Don't waste your time with the 29. Stick a small H in it and fly. That bird is so well proven there is nothing to go wrong if you do not do something stupid.

Thanks Titan II. Encouraging to note that you're a repeat customer on the DX3. It seems really nice. The 29mm motor option is just so that I can fly the thing now, as I'm not certified. I will try to get to the range to certify in a month or two to get that taken care of.

Do you fly yours with an ebay and double deploy?

By the way, you may be underestimating my ability to do something stupid. I'm a bit concerned that launching a nearly 1500g rocket with a G80 is "something stupid". OpenRocket suggests an altitude of 485 feet. This would require pretty exact ejection timing and even then, is there enough height to ensure parachute deploy? Or would I be safe in getting the chute out in time with a G80 (albeit unspectacular).

Thanks again.

 

[Titan II]

Do you fly yours with an ebay and double deploy?

Yes

By the way, you may be underestimating my ability to do something stupid. I'm a bit concerned that launching a nearly 1500g rocket with a G80 is "something stupid". OpenRocket suggests an altitude of 300 feet.

I will be gentle....that would be stu...! Ahem...I recommend you do not do it.

 

[Art Upton]

So is the altitude simed to 485 feet or 300 feet, different quotes here; just asking.

 

[Art Upton]

Does your motor vendor have any 38mm G motors? Single Grain generally.

Even if they don't try a Sim on some.

 

[WizardOfBoz]

So is the altitude simed to 485 feet or 300 feet, different quotes here; just asking.

Sorry - my memory was faulty. I was recalling a G57 I think. The G80 prediction was 485ft (145m). You caught me in between writing the error and correcting it.

 

[WizardOfBoz]

Does your motor vendor have any 38mm G motors? Single Grain generally.

Even if they don't try a Sim on some.

Yeah, I went through several lists and have a pretty complete accounting of the Pro-38 and RMS-38 motors. Indeed, all the ones I see are single grain.
For my certification flight I'll probably try to find one of the "specials". Surprised that there's not more of these: if I were CTI or Aero I'd want my customers to be buying consumables...

 

[WizardOfBoz]

Does your motor vendor have any 38mm G motors? Single Grain generally.

Even if they don't try a Sim on some.

My day job is simulation (simulating disease and drug effects in human physiology). Even so, I'm pretty amazed at how much data I can generate with the sim program. I'll check out a single grain reusable. But I have to weigh my rocket in - I suspect that a 3 grain tube with two spacers would knock me over the 1.5 kg limit!.

 

[Art Upton]

My day job is simulation (simulating disease and drug effects in human physiology). Even so, I'm pretty amazed at how much data I can generate with the sim program. I'll check out a single grain reusable. But I have to weigh my rocket in - I suspect that a 3 grain tube with two spacers would knock me over the 1.5 kg limit!.

Why would you not just use a 1 grain tube, or a 2 grain with a single spacer?

 

[WizardOfBoz]

Why would you not just use a 1 grain tube, or a 2 grain with a single spacer?

Art, I think we are agreeing - that's the point I was making. I want to try this out with a single use or single grain reusable motor, so that the motor and propellant weight doesn't put me over 1500g. If I'm going to invest in reusable motors (and I suspect I will) I'd get the starter kits that CTI and Aerotech sell. They have 3 and 6 grain tubes, with spacers so that you can use from 1 to 6 grain reloads. So I was sticking with disposables as
1) I didn't necessarily want to buy a single grain reusable motor when I would end up with the starter kit
2) A single grain reusable probably weighs more than the equivalent disposable (or am I wrong).

If the weight were ok I'd just get the kit. Until I certify though I'll try this out with disposables. Make sense, or no? It's late, and my wife is positive for Covid, and I may have it. Point is I may be confiused.

 

[John Kemker]

Put a Loki G70 in it!

38mm G motor and it has a neat purplish flame.

 

[mikec]

So I purchased a Madcow DX3, with a 38mm motor mount and a PML motor retainer.

You really got a PML retainer? If you had gotten an Aeropack retainer, you could have used an Aeropack adapter without any fuss. And I'm not sure what kind of adapter you actually did get.

As for placement of the motor, put the motor in and measure where the CG actually is, it's likely you'll be fine.

At any rate, I fly my Super DX3 on 29mm motors regularly and it's fine, but it does take some effort to stay under the 1.5 kg limit.

 

[WizardOfBoz]

You really got a PML retainer? If you had gotten an Aeropack retainer, you could have used an Aeropack adapter without any fuss. And I'm not sure what kind of adapter you actually did get.

As for placement of the motor, put the motor in and measure where the CG actually is, it's likely you'll be fine.

At any rate, I fly my Super DX3 on 29mm motors regularly and it's fine, but it does take some effort to stay under the 1.5 kg limit.

This is encouraging. For starters, I'll not be using an ebay, and I don't have a 38mm motor casing so I'm using disposable.

 

[SolarYellow]

Generous bevels on those fins can take some of the fat out of the 1/4 inch plywood.

 

[bjphoenix]

As I'm fairly new to high powered (or medium powered here) rockets, I'd be a bit reticent to create an adapter that moves the motor forward or aft. I figure that the rocket designer knew where they wanted the motor, and put it there. So in diagram b), I've put the adapter registration line (RA) is aligned exactly with RM. This keeps the motor in the exact same place as a larger motor would be.

In general there is usually some latitude in motor position. Motor position of course affects center of gravity location which then affects stability. A little bit of change in motor location won't have that much affect on CG or stability. If you can find a sim file for your rocket you can check this yourself.
Now if you are using a smaller motor than the largest motor the rocket is designed to fly with you have a lot of latitude because your smaller motor and adapter will weigh a lot less than the larger motor and you will have a lot more stability margin. This will give you the ability to let the motor stick out farther.

 

[WizardOfBoz]

bkphoenix, thanks for that. Makes sense.

Great idea about playing with the stability using simulation. Simulation is my day job, so I'm pretty comfortable with it. Again, I'm new, so I don't want to step in the middle of the Hatfields and McCoys, but what are the salient differences between RocSim and OpenRocket? I run Linux, so I'm using OpenRocket right now. But I can duel boot or VM a windows machine if RocSim....rocks.

Even if a few millimeters doesn't make a difference, I plan to have the thrust shoulder registration about as exact as I can. I'm going to purchase some aluminum disks, and try brazing them to a section of 1" nominal copper straight coupling (often called a repair coupling). Because this should give a decently strong, reliable, and attractive retainer, but also because... I want to try brazing.

Right now I'm (overdesigning) a launch pad. That is 96" of 2010 t-slot extrusion on a sawhorse mount.

 

[bjphoenix]

what are the salient differences between RocSim and OpenRocket?

I use OR because I'm frugal, I'm not familiar with benefits of RS. You could ask here under the software subforum or do some internet searching. RockSim is popular so I assume it has features worth the price.