My abacus is broken. Please help the math challenged.

Hello TRF,

My attempt at cloning an Estes kit in G12 turns out to be a 4.2% upscale due to the available G12 body tube outside diameter. The Estes cardboard body tube is 3" OD and my G12 body tube is 3.125" OD so it is a 4.2% upscale. 3.125x100\3 - 100 = 4.2%

Now the use of G12 allows me to use a 54mm mount instead of Estes cardboard and balsa. So the expected Estes D12 motor can now be a CTI 4 grain K2045.

Does this mean my average impulse upscale percentage compared to the D12 is 2045x100\12-100 = 16,941%, and, by using the CTI K740 with a total installed impulse of 1874Ns would mean the total installed impulse upscale percentage compared to the D12 is 1874x100\18-100=10,311%

To me, 16,941% and 10,311% has "errors in your calculations" written all over it.

Thanks for any clarification you can add.

Threemorewishes

Just like you don't upscale angles, you can't scale the motor that way...

Your impulse is going to depend on the weight of the the upscaled rocket.



EDIT: Wait. I misread your question. You want to compare the motor impulse against each other (not scaled).

So (using average impulse) the K2045 is roughly 171.4 times more power than the D12. Or am I misinterpreting
your question again?

Brian I think you answered his question . No you can't up scale motors like that . Put the infomation you have or get the un scaled file Or or rocksim and scale that way . You get templates . It will have the mass for all parts but weigh everything you may have to over ride a mass . The find motors that will sim right

I think the others were suggesting it, but I don't think it is totally clear, that you should start simulating the rocket during the design process. You'll need to know approximately what engines will work in order to make sure you build in the correct motor mount (and allow enough room for the right motors).

If there's room, I usually try to build the rocket with a motor mount that is, at least, one step larger than I think the rocket needs. For example, if I think the rocket will fly okay on a 29mm motor, I'll use a 38mm motor mount (if it'll fit, of course). It's easier to adapt a 29mm motor to fit in a 38mm motor mount than it is to fit a bigger motor in a smaller motor mount!

-- Roger

For a true upscale at 4.2%, your mass would increase by a factor of 1.13137 (1.042^3). Using different materials is immediately going to throw that out the window. I'd sim it.

The upscale example using the body tube outside diameters was just to show my formula.

The comparison is between the D12 motor and the K2045 for average impulse and the K740 for total impulse.

The 17000% more power is what caught my eye. One times more impulse is 100% more impulse so 170 times more is 170x100 = 17000.

This exercise has given me an idea for a competition. The smallest increase in a kits physical size for the largest total or average upscale in impulse.

Thanks

Threemorewishes

Threemorewishes said:

The 17000% more power is what caught my eye. One times more impulse is 100% more impulse so 170 times more is 170x100 = 17000.

Click to expand...

The percentage increase in impulse surprised you because you didn't account for the motor getting bigger. A D12 is a 24mm diameter motor that is 69.85mm long; a 54mm diameter K2045 VMAX motor is 404mm long. The D12 has a volume of 31,576mm³ while the 4-grain 54mm has a volume of 925,249mm³. Using your formula that's a 2830% increase just in propellant volume*, you then have to factor in the energy efficiency difference between the black powder in the D12 versus the AP in the VMAX etc. etc.

(*for the purposes of this example I've grossly simplified things and just used the exterior case dimensions. The propellant volume will actually be less once you account for nozzle, delay grain, ejection charge, Jimmy Hoffa,...)

PhlAsh said:

For a true upscale at 4.2%, your mass would increase by a factor of 1.13137 (1.042^3). Using different materials is immediately going to throw that out the window. I'd sim it.

Click to expand...

Threemorewishes said:

The upscale example using the body tube outside diameters was just to show my formula.

The comparison is between the D12 motor and the K2045 for average impulse and the K740 for total impulse.

The 17000% more power is what caught my eye. One times more impulse is 100% more impulse so 170 times more is 170x100 = 17000.

This exercise has given me an idea for a competition. The smallest increase in a kits physical size for the largest total or average upscale in impulse.

Thanks

Threemorewishes

Click to expand...

If you upscaled everything by 4.2% you don't even get to a 29 mm motor!

Putting a K motor into an Estes D motor design rocket is a complete redesign, especially when you have increased the external dimensions by a mere 4%. The CG/CP relationship is totally different, so the stability will have to be recalculated. The difference between a D and a K is 2 raised to the 7th power or 128 times more power. The rocket will go substantially higher and faster and the shape and design of the rocket might be incompatible with the enhanced speed.

Bob

bobkrech said:

If you upscaled everything by 4.2% you don't even get to a 29 mm motor!

Click to expand...

Oh, I didn't pay enough attention. My mind converted "4.2%" to "4.2x" which is more inline with what I think of as an upscale.

Putting a K motor into an Estes D motor design rocket is a complete redesign....

Click to expand...

That reminds me of Darrell Mobley and his fleet of Estes Maxi-Brute Honest Johns which he flew on H motors. 'Course, he made a few mods to the kits.

-- Roger