75mm or 98mm Motor Mount

[Glasspack]

So I was curious what you think .....

I have a 7.5" diameter Patriot kit with a 75mm motor mount.
The plan has ALWAYS been to use this kit for a LVL 3 Cert when the time is right.

I will tell you right now I am planning to alter this and make it different than a Patriot.............
more like the SAAB Missile. But probably longer ..........




I don't have 75mm or 98mm hardware yet anyway... and really wonder if you think its best to
shitcan the 75mm mount and go with 98mm. I can only think of having more options if I
use a 98mm mount than a 75mm motor mount. But I know its not going to be a frequent flier
and I would think I could still get M/N motors from CTI in 75mm
(All my other hardware 29, 38 & 54 is CTI)



Can you think of any other reasons to not use the 75mm motor mount pieces already in the kit ?
........ and change them out for 98mm. Currently, I think its NOT necessary to change it.....

The kit was a Christmas gift from my two boys a couple years ago............


Thank You

 

[FredA]

Watch your stability with 98mm motors.
Run some sims and figure out if you can stand the extra mass in the aft.

 

[JoePfeiffer]

This strikes me as a question with a straightforward answer. It's pretty easy to find an adapter to put a smaller motor in a larger mount, but I've never seen one that works the other way 'round
If you even remotely entertain the possibility you might conceivably put a 98 in it, use that mount.

 

[David Schwantz]

Myself, I would use the 75. I have 98's and I do use the adapter, but if I were to build them again I would go 75. They will fly many more times on a 75, cost wise.

 

[OverTheTop]

If you don't have any other 98mm rockets I would go 98mm. You can always adapt back down, and you will probably be kicking yourself that you didn't go up to the 98mm. That is what I would do. YMMV.

 

[Neutronium95]

You can always adapt down, you can never adapt up.

That said, 98mm motors are very expensive. Like ridiculously expensive. For a basic 75 4 grain reload like the M1297, MSRP is $375. A 98mm full M in the M1939 is $860, and a N will set you back about $1200. That's not getting into the hardware costs. For my L3 cert, I went with a rocket that had a 75mm motor mount that would also be comfortable flying on most L2 motors, and I don't regret it at all.

I'd suggest deciding whether or not the added mass in reinforcement for a big 98mm motor is worth it if it ends up increasing the minimum motor size, and whether or not you'd ever actually decide to shell out the cash for a 98mm motor. For me 98mm motors are out of my price range unless I'm splitting the costs with someone, so I don't build rockets to fly them.


Edit: There are no commercially available 75mm N motors. There is a pretty good selection of M motors across the 4-6XL casing sizes. The CTI M2245 is the highest impulse 75mm motor on the market today, and it's almost a N, but not quite.

 

[mtnmanak]

Another option since you are using PML tubes - design it to use LOC's MMAS:

https://locprecision.com/products/modular-motor-adaptor-for-7-5-kits
A couple years ago, I bought a Bruiser EXP just to play around with the way they make the MMAS. Now I CNC my own MMAS designs into all my LOC rockets (and make the adapters accordingly).

The design is simple and easy to replicate. Or, you can always buy the modular pieces from LOC when you want to make a new adapter.

This way you don't limit yourself to 75mm or 98mm - you can play with clusters as well.

Here is a description of when I was building my system out using LOC parts:

https://www.mountainmanrockets.com/index.php/tools_tech/loc-mmas/
Also, just want to point out that LOC's 75mm tubes are almost EXACTLY 75mm. My experience is that the Loki 76mm cases will not fit in their 75mm motor tubes, so be aware if you ever thought about buying Loki's 76mm cases. Aerotech and CTI 75mm cases fit in the LOC tubes, no problem.

 

[Glasspack]

mtnmanak, Cool I love that I have always loved the thought of a black powder 4 motor cluster on the older Estes patriot model
I will look more at this

 

[Kallahan11]

M1939

That's not really a fair comparison, the M1297 has 2,722 g of propellant and produces 5,416.6 Ns of impulse while the M1939 has 5,719 g of propellant and produces 10,481.5 Ns. The M1939 is more than twice as much motor than the M1297. The M1297 is still cheaper on a per gram or per Ns basis but its much closer than you implied. A closer comparison would be the M1419, which is the smallest M White Lightning in 98mm, or the L925W which matches well with propellant mass (the M1297w is 2722g, the L925w is 2750g)

You do get more bang for your buck at 75mm, but with a 98 you can always adapt down. Unless you are trying to maximize rocket performance I would not worry about having to add nosecone weight to adjust stability margins for the heavier 98mm components.

	case	Cost	$/Ns	$/g
M1939W	RMS-98/10240	860	12.19	6.65
M1297W	RMS-75/5120	377	14.37	7.22
M1419W	RMS-98/7680	642	12.08	6.35
L925W	RMS-98/5120	388	12	7.09

 

[Glasspack]

Thank you All for the inputs … I am leaning towards the 98mm.

I also am entertaining a modification of my kit to build a different model.
I like the Patriot Missile, don't get me wrong; ….but there are several other
missiles that look way cooler and would make a nice looking flier !