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What's the advantage of wider, more expensive HPR motors?

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billdz

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Recently got my L1 and was looking to buy some I motors. The price for 38mm motors is substantially less than comparable 54mm motors. For example, the 38mm I245 is substantially cheaper (about $10) than the 54mm I229. The simulation shows that my rocket will go 90m higher with the I229 and will have a slower velocity off the pad. Are these the primary reasons why the I229 is worth the extra money?
 

Tinker

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I don't know the answer as to why other than maybe production levels? I might suggest putting in the 54mm mount and trying the 38mm adapter for it. I bought a 38mm kit and have been kicking myself for it.

Tinker

Oh, Congrats on your level 1!
 

DavidMcCann

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Short motors are hard to look at like this. You can look at long cases and it's he opposite. Loki 38/1200 loads are 90-115. 54/1200 are about 80

each motor has its own production costs, and there are significant differences in performance. For some rockets, one may be beneficial over another.
 

Bat-mite

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The idea is to have as much of a variety of loads for whatever hardware you have. If you have a 54mm MMT, you get a choice of I thru K motors. You don't need to buy 38mm HW to fly an I. Or, vice versa, you can get a J in 38mm.

Why the price differential? I don't know. Might have something to do with the liner, the nozzle .... But my guess is that they don't want to have as much of a price spread across a particular diameter of motors.
 

rharshberger

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Sometimes the choice of motor is constrained by the dimensions of the rocket. I have a two stage scale project that will require 1 or 2 grain 54mm in the booster to still be able fit recovery and electronics, and still have enough thrust to boost safely.
 

Worsaer

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This is an example where Thrustcurve can be very educational. Those numbers (I245 and I229) are averages. The two motors have very different characteristics, including Maximum thrust. It's worth checking out.
 

Bat-mite

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All great advice, but he has a point. Even if there were two motors, one 38 and one 54, with the exact same thrust curve, the 54 will cost more, just 'cuz it's a 54.
 

rharshberger

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All great advice, but he has a point. Even if there were two motors, one 38 and one 54, with the exact same thrust curve, the 54 will cost more, just 'cuz it's a 54.
54mm I-motors are less expensive than 38mm I-motors, only by a few dollars. Prices based off Balsa Machining Service website.

Edit: the statement above mostly applies to 38mm vs 54mm and the fact some motors aren't the bigger diameter=more expensive.
 
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Exactimator

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All great advice, but he has a point. Even if there were two motors, one 38 and one 54, with the exact same thrust curve, the 54 will cost more, just 'cuz it's a 54.
For that reason, I won't use a short 54mm motor when I can launch with a long 38mm and an adapter.
 

Bat-mite

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54mm I-motors are less expensive than 38mm I-motors, only by a few dollars. Prices based off Balsa Machining Service website.

Edit: the statement above mostly applies to 38mm vs 54mm and the fact some motors aren't the bigger diameter=more expensive.
Well, there you go. I should have done my research.
 

BDB

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Because HPR motors burn laterally from the core to the liner, as opposed to BP motors that burn vertically from bottom to top, a longer, narrower motor case will almost always have a higher initial kick off the pad and a shorter burn time, while a fatter, shorter motor will have less initial thrust and a longer burn time. The former is usually preferable.

Rich, I'm trying to plan an upscale Estes Magnum (2 stage), and I'm running into the same conundrum. A short 54 mm motor in the booster is not ideal, but it may be necessary if I want to keep the scale correct.
 
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billdz

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@rharshberger - I went to the Balsa Machining Service website and I think you have it backwards, the 38mm is cheaper than the 54mm.
@BDB - "
The former is usually preferable." Why do you think so? A slower liftoff and longer burn can be more visually appealing, and with an I motor a cardboard rocket will prefer the lesser initial thrust. Although the higher initial kick would be preferable for a heavy rocket that otherwise might have trouble getting off the pad.
 

BDB

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@rharshberger - I went to the Balsa Machining Service website and I think you have it backwards, the 38mm is cheaper than the 54mm.
@BDB - "
The former is usually preferable." Why do you think so? A slower liftoff and longer burn can be more visually appealing, and with an I motor a cardboard rocket will prefer the lesser initial thrust. Although the higher initial kick would be preferable for a heavy rocket that otherwise might have trouble getting off the pad.
You're right. I was just thinking of maximizing speed off the rod to combat instability. It's really just a matter of preference and flying conditions.
 

markkoelsch

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This is an example where Thrustcurve can be very educational. Those numbers (I245 and I229) are averages. The two motors have very different characteristics, including Maximum thrust. It's worth checking out.
This is also why the way CTI lists their motors is best - total impulse motor designations
 

RKeller

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Some cost more, some don't. The "wider" motor will have more options, and you can always adapt down. That being said, 38mm covers a huge range of fun. I just fly whatever does what I want for the least $$$
 

rharshberger

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@rharshberger - I went to the Balsa Machining Service website and I think you have it backwards, the 38mm is cheaper than the 54mm.
@BDB - "
The former is usually preferable." Why do you think so? A slower liftoff and longer burn can be more visually appealing, and with an I motor a cardboard rocket will prefer the lesser initial thrust. Although the higher initial kick would be preferable for a heavy rocket that otherwise might have trouble getting off the pad.
You are correct, sort of, the smaller total Ns motors in 38mm are cheaper than the the 54mm I motors in the 54/452 case. But if you compare apples to apples (54/452 to 38/480 case I motors) or as close as they can be the two sizes of motors are the same price, Warp9 propellants in both sizes are the same as well. It kind of makes sense since the propellant masses are nearly the same. My earlier statement was wrong, thank you for catching it.
Example:
AT 38/480 I300T with 222g propellant and a total of 426.5 Ns for $44.99 ship now price
AT 54/452 I229T with 196g propellant and a total of 407.0 Ns for the same price
 

Coop

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Keep in mind when you're buying a reload you're buying the single-use parts that go along with it to make it work. Machining a 54mm nozzle is going to cost more than a 38mm nozzle just because it comes from a larger stock of material.

The advantage of a larger diameter is motor choice, and various ways to use the propellant. Longer burn times can be not only a lot of fun, but be the preferred choice in certain projects. There's some propellants that don't work in narrow cases, too: C*, for example, is available in 1-grain 54mm as the I-165. If you want to fly this propellant on your L1, you're going to need to get the 54mm.

The adage: "You can adapt down, but not up," has a lot of merit. If I could do my L1 again, I'd throw a 54mm in my L1 bird to let me fly the complete range.


Later!

--Coop
 

rharshberger

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Why is that? Just curious..
Probably something to do with erosive burning, one of the EX crowd like G_T, Mike Fisher, Fred Azinger or several others could probably explain why certain propellants are not suited to higher L/D cases (length/diameter).
 

jadebox

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Recently got my L1 and was looking to buy some I motors. The price for 38mm motors is substantially less than comparable 54mm motors. For example, the 38mm I245 is substantially cheaper (about $10) than the 54mm I229. The simulation shows that my rocket will go 90m higher with the I229 and will have a slower velocity off the pad. Are these the primary reasons why the I229 is worth the extra money?
Here's a comparison of the two motors:

https://www.rocketreviews.com/compare-motors-139095.html

As the graph shows, the time thrust curves are very different.

And the chart shows that the 54mm motor has more propellant (206g versus 181g) and provides more Newton-Seconds of thrust (412NS versus 351NS) than the 38mm motor. The 54mm motor provides about 15% more total impulse and costs about 20% more than the 38mm motor. From that metric, it seems that the 38mm is a better bargain (but not by much).

The average thrust of the two motors is actually closer than the designations suggest - 238N to 240N.

-- Roger
 
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blackjack2564

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By the way.....AT changed from 54mm nozzles to 38mm nozzles in the 1 grain [soda can] motors several years ago , so they could lower the price on 54mm 1G reloads!
If I remember correctly, at the time a Medusa 54mm nozzle was 18.00 and a 38mm nozzle was 1.75 from RCS.

Hence the special small hole rear closure [54mm]needed to utilize the 38 nozzle.

I fly the Blue Thunder 1 grain a lot. Love the almost neutral burn thrust curve, use it on fields with lower waiver or windy days. No need for adaptors in my 54 mm birds.
 
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