Long Tom - Open Rocket - and engine selection

[David_Stack]

Good Afternoon All;

Working toward completion of my Estes reboot of the Centuri Long Tom. Decided to take this opportunity to also begin playing around with Open Rocket. There is no Open Rocket file for the Long Tom, but there is a .rkt file for it available on Rocket Reviews that Open Rocket will read.

Uploaded the file and began running simulations with various engine combinations. In the course of doing so I noticed data that now has me questioning fundamental laws of physics...

This particular Rocksim file for the Long Tom details a rocket weight (empty) of 139 grams, 175 grams with a B6-0/B6-4 configuration, 180 grams with a C6-0/B6-4, and 185 grams with a C6-0/C6-5.

All of those configurations result in a liftoff weight well in excess of what is recommended per the Estes Engine Chart (~113 - 115 grams). For that matter, even the empty weight of the rocket is in excess of that recommended liftoff weight.

Is the Rocksim file that much in error in its empty airframe weight (Estes specs the Long Tom at 91 grams, but I've heard their numbers are borderline unachievable for mere mortals (or anyone who wants more than a dusting of paint,,,))? Unfortunately I don't have a scale to get an accurate weight of my model (it's on the 'to do' list). Is the data (weight) being 'corrupted' because I am using a .rkt file instead of a .ork one? Or is this 'operator error' and I am guilty of misinterpreting the information being presented to me in Open Rocket and/or the Estes Engine Chart?

Looking forward to feedback...

Thank you,
Dave

 

[neil_w]

Can you post the file?

Oh, and get a scale ASAP. I have this one, dirt cheap and works great.

 

[David_Stack]

Good Evening Neil;

This is the file that I used.

Believe me, as soon as the schedule allows, I'm venturing to Harbor Freight/Office Depot/Staples or similar in search of... I know, there is Amazon, etc., but I'm more of a 'want to see it in person' sort. Thanks for the link, it at least provides another point of reference.

r/
dave

 

[neil_w]

OK, I'll take a look.

Believe me, as soon as the schedule allows, I'm venturing to Harbor Freight/Office Depot/Staples or similar in search of... I know, there is Amazon, etc., but I'm more of a 'want to see it in person' sort. Thanks for the link, it at least provides another point of reference.

For me, a little $12 scale doesn't justify spending time going shopping, but your call. Harbor Freight has a couple of similar items on their website, not sure if they have in-store. Staples also has similar, for twice the price (https://www.staples.com/brecknell-e...lack-silver-1-1-lbs-capacity/product_24400284), but almost certainly not in-store to look at.

 

[neil_w]

Here's an updated file; I corrected some wall thicknesses for body tubes (they were waaaay too thick) and materials (the booster fins were listed as cardboard, also way too heavy.) Also I adjusted the body tubes to proper BT50 and BT55, which I *assume* is what you'd be building with. For some incredibly annoying reason, I am unable to save it as an ORK file, only as a RKT.

Note that the model assumes balsa nose and transition; plastic will increase weight somewhat. I don't know what the Estes Long Tom has.

Anyway, my updated model comes out to a mere 2.4 oz (68 g) empty, 4.3 oz with two C motors. Still a bit high for a C6 booster, just build light and launch on a very calm day. Or if you put a B6 (or even an A8) in the sustainer, you can improve your margins a bit more.

 

[rharshberger]

RocSim and ORK files are notoriously inaccurate for YOUR rocket......unless you input the data yourself...

 

[David_Stack]

Thank you Neil!

Those numbers look MUCH better, and allay some concerns.

Now I just need the weather to improve (remnants of Hurricane Sally coming through) so that I can get outside and spray some paint...

r/
Dave

 

[mooffle]

Sub gram scales are often in the kitchen area of places like target or kohls advertised as spice scales.