Impulse Economics, some data

Hello,

I like finding ways to use data to help support my decision making. When it comes to picking a motor, there are a myriad of options and many different types of criteria that may be used to select one. Putting aside propellant types, thrust curves, and other considerations, I was curious about how the different motor systems compared in simple terms of $$ per Ns. So I collected some prices from Sirius Rocketry and Wildman, picked a range of motors that I was interested in (mostly LPR to MPR), calculated the $$ per total impulse, and plotted that metric vs total impulse.

No surprise that the cost per total impulse goes down as motor sizes go up, but there are a few things I found interesting in here.

A couple of bullet points:

• In size 38mm, Cessaroni and Aerotech reloads cost about the same. (ignoring the 3 Aerotech 38mm motors that ship HAZMAT free).
  
• The Aerotech 24/60 reloads are more expensive per flight and per impulse than the 24/40 reloads. I don't see any good reason to use the 24/60 case unless you just really need a few more Ns in a 24mm case.
  
• The Economax motors are a better deal than the 29/40-120 reloads. The F67 ships HAZMAT free, but the larger G67 is HAZMAT, so the main advantage of the 29/40-120 reloads is HAZMAT free (and variety).
• In the smaller range of the 29/40-120, you're better off with 24mm single use motors (E15, E20, E30).
  
• The Aerotech 24/40 reloads are a good bargain.
• If you're looking for "D" size reloads, it is more economical to adapt down to the 18/20 case, than use the 24/40 reloads.

Based solely on $$ per impulse, the winners are:
C & D range: Estes black powder
E range: Aerotech 24/40 (also gets you baby Fs)
F range: Aerotech Economax
G range: Aerotech 29/40-120
H range: Aerotech 29/240

For H, I, and beyond, I'd lean towards Cessaroni because the motor loading and cleaning is easier, the cases are cheaper, and the cost is about the same.

Obviously there are many more criteria that go into choosing a motor, but this is one that can be plotted for direct comparison.

Link to original in case it gets downsized

This is why the Loki hazfree motors are such a great deal.

I track my motor use and inventory with a spreadsheet. I have a column for pennies per newton second.

dhbarr said:

This is why the Loki hazfree motors are such a great deal.

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True! And performancehobbies.com has the complete 38/480 motor for $90, which is a fair bit less than the Aerotech motor. They have good prices on their reloads, I think that Loki would win the $$/Ns even without the HAZMAT advantage!

Nice analysis!

I did the same but factored in the casing cost. I have cost columns factoring in the hardware for 1, 3, 5, 10, and 20 flights. Makes a big difference for some of the larger casings that won't get flown as much. If you carried your analysis out to larger motors - especially to 75mm, the price differences between the different vendors is much more exaggerated.

I buy my motors from a local vendor or take delivery at a launch so I don't have to pay shipping or hazmat, but do pay pretty much list price.


Tony

manixFan said:

I did the same but factored in the casing cost. I have cost columns factoring in the hardware for 1, 3, 5, 10, and 20 flights. Makes a big difference for some of the larger casings that won't get flown as much. If you carried your analysis out to larger motors - especially to 75mm, the price differences between the different vendors is much more exaggerated.

I buy my motors from a local vendor or take delivery at a launch so I don't have to pay shipping or hazmat, but do pay pretty much list price.


Tony

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Any chance of sharing? Shot a PM just in case.

manixFan said:

I did the same but factored in the casing cost. I have cost columns factoring in the hardware for 1, 3, 5, 10, and 20 flights. Makes a big difference for some of the larger casings that won't get flown as much. If you carried your analysis out to larger motors - especially to 75mm, the price differences between the different vendors is much more exaggerated.

I buy my motors from a local vendor or take delivery at a launch so I don't have to pay shipping or hazmat, but do pay pretty much list price.


Tony

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Yes, it'd be interesting to see your numbers for HPR.

Unfortunately I have company this weekend and won't have time to dig out my spreadsheet and make it presentable for a few days. My analysis is also very specific to the types of motors I fly so it's not very complete. I make many of my own motors so my analysis is mostly for motors that I don't feel comfortable making, like CTI 6XL motor cases. But once I have a chance to look it over I'll send a copy to interested parties. However it may be a good starting point for anyone if you don't already have something on hand.


Tony

dhbarr said:

This is why the Loki hazfree motors are such a great deal.

mbeels said:

I think that Loki would win the $$/Ns even without the HAZMAT advantage!

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It will not.

Every time I gear up for Xmas sales, I adjust my XLS with the latest iteration of $$/Ns from AT, CTI, and Loki.
Year after year, it comes out the same: AT < CTI < Loki.
With WM sales, it's: AT << CTI < Loki.

YMMV,
a

P.S.: PM me if you want a copy of my XLS.

afadeev said:

Every time I gear up for Xmas sales, I adjust my XLS with the latest iteration of $$/Ns from AT, CTI, and Loki.
Year after year, it comes out the same: AT < CTI < Loki.
With WM sales, it's: AT << CTI < Loki.

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Interesting, I hadn't considered sales prices. I'll be curious to see what is offered from WM this year.

I updated my spreadsheet using current prices from Chris' rocket supplies. His site was the easiest to get all the data from. My local vendor still has the old prices listed which actually skewed the results even more towards CTI.

My results include the cost of the case. I only compare CTI and AT since for most of us that's all we can buy locally and trying to add in shipping just makes things crazy. I only used 2 sizes - 4 grain and 6XL 54mm. Why? Because those are the only motors I routinely buy, I make everything larger and I already have a lifetime supply of 38mm motors.

I computed the cost for 1, 3, 5, 10 and 20 flights. For the 4 grain motors, CTI has a per flight cost advantage out to 5 flights, by 10 flights the advantage has shifted to AT by a small margin. At 10 flights, the cost difference is less than $3 a flight. For the 6XL motors, the cost advantage is still in CTI's favor, but only out to about 3-4 flights. But it's really almost apples to oranges since the CTI motors are so much more powerful.

However, if you look at cost per 100 Newtons, the CTI motors are a better bargain in almost every case, no matter how many flights. If your goal is performance, CTI cases have reloads that offer higher performance than the AT cases. And I did not include the cost of things like extended forward closures, which may be required for some AT motors.

In the end I buy the motors that do what I want them to do, not so much based on price. Of course the results may be very different for different motor sizes, especially based on hardware costs. I was more than happy to buy a Loki 38mm case for use with just a single reload - the K627, regardless of how economical it will be on a per flight basis.

I'm not comfortable posting the results, but if you want a copy, let me know and I'll send it to you.


Tony

mbeels said:

Interesting, I hadn't considered sales prices. I'll be curious to see what is offered from WM this year.

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That is a very important variable in cross shopping: compare the actual street prices, not MSRPs!

AT motors are regularly sold at significant discounts from MSRP. BMS has them at 25% off day in and day out. WM holiday sales kick that upto 40% off for some.
Discounted CTI motors are becoming an engendered spices.
Loki's are never discounted, except for one time I found them at 5% off at one place, which is now back to MSRP.

AT & Loki have non-Hazmat packaged HP motors, though that barely matters when you buy in bulk.

manixFan said:

I computed the cost for 1, 3, 5, 10 and 20 flights. For the 4 grain motors, CTI has a per flight cost advantage out to 5 flights, by 10 flights the advantage has shifted to AT by a small margin. At 10 flights, the cost difference is less than $3 a flight. For the 6XL motors, the cost advantage is still in CTI's favor, but only out to about 3-4 flights.

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So what you are observing is that AT motors are cheaper per reload, but hardware is more expensive upfront.
Agree completely.

As of the last holiday season, I have 54mm 4-grain CTI's K-motors @~1700 Ns coming in at 0.067 $/N, while similar impulse AT motor is 0.045 $/Ns.
My break-even between the two is at 2.1 flights.

manixFan said:

However, if you look at cost per 100 Newtons, the CTI motors are a better bargain in almost every case, no matter how many flights. If your goal is performance, CTI cases have reloads that offer higher performance than the AT cases.

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I would disagree with both of the above assertions, but frankly, it doesn't matter.

Fly whatever makes you happy!

a

afadeev said:

<snipped>...I would disagree with both of the above assertions, but frankly, it doesn't matter.

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They are not assertions, they are observations. I won't argue pricing as I did not take sale prices into consideration but frankly don't think that works for the vast majority of flyers - most pay regular price.

However, the performance issue is clear cut, at least in the two case sizes I compared. For each case CTI has the better performing motor, and for the 6XL case, it's not even close. For the 4 grain case, each company has 10 reloads. Picking a midpoint of 1520 newtons, AT has 6 motors under and 4 over, while CTI has 4 under and 6 over. And while the AT reloads my be less expensive per reload, when compared on a per Newton basis, most are more expensive than a similar CTI load when you factor in case prices. The big standout for AT is the K1103X, for CTI it is the K740. AT also has the disadvantage of sometimes needing different forward closures, which adds to the overall cost of the motors.

I'll be happy to send you my spreadsheet so you can review my numbers.


Tony

manixFan said:

They are not assertions, they are observations. I won't argue pricing as I did not take sale prices into consideration but frankly don't think that works for the vast majority of flyers - most pay regular price.

However, the performance issue is clear cut, at least in the two case sizes I compared. For each case CTI has the better performing motor, and for the 6XL case, it's not even close. For the 4 grain case, each company has 10 reloads. Picking a midpoint of 1520 newtons, AT has 6 motors under and 4 over, while CTI has 4 under and 6 over. And while the AT reloads my be less expensive per reload, when compared on a per Newton basis, most are more expensive than a similar CTI load when you factor in case prices. The big standout for AT is the K1103X, for CTI it is the K740. AT also has the disadvantage of sometimes needing different forward closures, which adds to the overall cost of the motors.

I'll be happy to send you my spreadsheet so you can review my numbers.


Tony

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Tony,
You keep talking about the cost per Newton (which is a unit of force, not impulse), but I suspect you really mean Newton-second, which is impulse and proportional to the amount of propellant. Can you confirm that?

Steve Shannon said:

Tony,
You keep talking about the cost per Newton (which is a unit of force, not impulse), but I suspect you really mean Newton-second, which is impulse and proportional to the amount of propellant. Can you confirm that?

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Dang, yes, Newton-seconds. (I actually measure per 100NS to get more meaningful numbers). I just use N in my spreadsheet which is clearly not technically correct. When I started getting interested in going for altitude, I looked at which motors had the most impulse and that’s when I discovered that on average, the CTI motors were better for my purposes than most AT motors, especially in the 6XL case. The L935 is really a killer motor in that respect. Most folks I’m sure don’t care and fly the motors they like. I fly plenty of AT motors in the 4 grain 54mm case because I like them, regardless of cost per foot of altitude.


Tony