75mm DMS or RMS?

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Dan Kusmer

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Up until this time I've been using 54mm and smaller Aerotech DMS motors (K class and smaller). I hope to get my L3 next year which requires a M (or larger) motor. The smallest diameter M motor is 75mm. This is my first venture into 75mm motors. I noticed that Aerotech has only one M DMS (M1350W) motor and nine M RMS motors. I'm not familiar with CTI, LOKI, or other motors. Should I switch to RMS 75mm motors and incur the cost of buying the casings and end fittings? What are other L3 rocketeers doing? Thanks in advance for your advice.
 

jd2cylman

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The 75/5120 casing is the Swiss Army knife of reload casings.

So, yes, yes you should. You also should get a 54/1706 case. It's the junior Swiss Army pocket knife of reload casings... ;)
 

mrwalsh85

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Technically, you are incorrect. Loki Research has a 54mm M motor. Possibly too aggressive for what you are trying to achieve.

Regardless, it really depends on what you want to do. If you want to have variety to fly different propellants and different size motors, RMS is the way to go. The Aerotech line has a Reload Adapter System for their 75mm casings, allowing you to go down 2 grain sizes. So if you have a 75/6400, you can fly 75/3840 motors. Having the 75/6400 grants you access to 19 different reloads. Adrian wasn't kidding about being the swiss army knife of reload casings.

I had the 75/6400 before someone lost it, and bought me a replacement case (I paid the difference and upgraded to the 75/7680 case since I already have a 75/3840 case, and with the rockets I fly the 75/3840 in, the extra motor casing would be a waste of space. I love the Aerotech 75mm line. The M1315W is a favorite of mine, though I got my L3 on the M1297. Either way, great motors.

Alternatively, if you just want to get your L3 and dip your toes in the larger diameter and only see yourself flying once or twice, the M1350 might be a better choice. I regularly fly big motors, so the RMS was a good investment for me. The RAS also allows me access to more motors, as I indicated above. With the 75/7680 and the 75/3840, I can fly the entire range of Aerotech's RMS reload line.

Adrian is also right. Get the 54/1706 with the RAS. I was just moving some boxes around and realized I have no less than 4 54/1706 motors and I'm pretty sure I have 2-3 more in storage. I have got to stop finding folks getting out of the hobby on accident (no joke, these folks are attracted to me! Guess it shows how long I've been in the hobby...)
 

TheTank

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How often do you think you are going to fly an M (or even a full L)? Look at the cost of the case setup and figure out how many flights till you hit your break-even. Or, if you are not worried about the cost, the system will greatly open up your motor selection capabilities. For me I wasnt really worried about the cost but wanted to be able to fly a wide variety of motors so I bought a 75/6400 case with the RMS system so I can fly anything in the 75/3840, 75/5120, and 75/6400 line. As I typically wont fly anything that large more than once a day, the one case system works out for me and I can fly the full line of 75mm L motors and all but two of the 75mm M's.
 

Dan Kusmer

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Your responses are really appreciated, they all look like sound advice.

For a break-even analysis I looked at the M1500G RMS (4 grains) which seems to be the closest to the M1350W DMS. The M1500G is about $100 cheaper than the M1350W. The 5120 handles 4 grains and the 75/5120 kit (includes fwd and aft fittings) is around $480. This means 5 launches would be the break-even point. Of course if I would go with the 75/7680 and the 75/3840 and a RAS (a combination I'm gravitating to) it would be a lot more launches to break-even.

While not absolutely sure, I believe I'll be doing several 75mm launches and I would like a lot more variety than just the only two 75mm DMS motors Aerotech makes (M1350M and L875DM).
 

Tech 68

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I think I found this somewhere in Aerotech's "Resource" links.
You can go through it all to see what needs to be added. CTI has a 75mm 4 grain M-1830 and M-1101 and has grain replacement "spacers" so you can fly 2 and 3 grain reloads in your 4 grain case (I'm pretty sure CTI is OK with 2 "spacers", but no more than that.
1631763545548.png

I think I recall that in some cases, the 75mm stainless "seal disc" is recommended using AT reloads in a CTI case... again, check.
 
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Dan Kusmer

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I think I recall that in some cases, the 75mm stainless "seal disc" is recommended using AT reloads in a CTI case...
I did not realize you could use CTI 75mm grains in an AT casing. I found the following post where Anthony J. Cesaroni (President/CEO) said for the 75mm and 98mm (with proper o-rings) you can and it's been certified. He said the opposite is not true because AT 75mm and 98mm grains have not been certified in a CTI casing.


This post was back in 2007. Has anybody heard differently since then?
 

jd2cylman

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I only own AT cases in the 75mm & up. I’ve flown several CTI loads in 75mm cases with no problems. I bought extra
o-rings for the seal discs for just such occasions. Sometimes I’ve had to leave out the grain spacers for the seal disc to seat fully. But they’ve always worked.
The regular seal disc would work, but if you’re buying from zero, get the stainless. More durable and easier to clean. As long as you don’t mind the weight penalty.
 

jimzcatz

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The great thing about the CTI 75 and 98mm reloads is they come with orings for the RMS cases!!
 

jd2cylman

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The great thing about the CTI 75 and 98mm reloads is they come with orings for the RMS cases!!
Yup. Except they don’t have the o-ring for the seal disc. You’ll have to provide your own.
 
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