In the first article, we talked about the "mitosis" of a build, how one idea splits into a complex project. Today, we move from the "Why" to the "How," starting exactly where every good design begins: OpenRocket.
The Philosophy of Intent
Before you click a single button, you have to talk to your "inner chaos monkey" and define your destination.
Philosophy of Intent: In general, this is the practice of defining the "soul" of the project before you ever touch a tool. It is the transition from a daydream to a set of rigid, non-negotiable constraints. In engineering, this is often called a "Concept of Operations" (ConOps), but for a custom project like an upscale rocket, it’s more personal. Of course, you can change your mind, but you need to have a good handle on all the vectors because a small change can have big effects.
Aim Small, Miss Small
For the "Ex-Wife" upscale, my intent is a bit of a paradox:
The Weight Target: Under 1500g for Class 1 compliance so I can fly it at my local small fields.
The Strength Requirement: Robust enough to handle a J450 at Mach 0.75 and survive the "rocky and unforgiving" terrain of Hartsel, Colorado.
The Scale: A 744% upscale of the classic Estes Mosquito.
Step 1: Establishing the Anchor
For most of my designs, I'm not reinventing the wheel. Rather, I am building on the shoulders of giants by using a base simulation of the original Mosquito from K’Tesh on The Rocketry Forum. You can usually find sims that are exactly or near to your target design in the list of links in the reference section at the end of this post. For scaling this model, my anchor will be the body tube because nearly every part is directly affected by the tube dimensions.
Pro-Tip: Immediately "Save As" a new file. Don't be the person who overwrites their reference file and has to go hunting through forum threads again.
Step 2: Design Simplification and Material Selection
Before scaling the sim, I strip the model down to the most basic of parts because I'm not trying to reproduce the original, but rather enhance it. For this model, I reduced the parts down to the basics: Nosecone, Body Tube, and Fins.
For the fins, I found the sim used three separate fin definitions. I deleted two of the three fin entries and set the remaining single fin to a count of 3. This makes future adjustments to the fin configuration straightforward.
You can modify your materials now or later. I bounce around between before and after scaling, but you need to remember to do it. Double-check each one and be detail-oriented. Don't worry about mass values now. We will address those later.
I’ve chosen ASA for my parts short of the body tube. This makes creating certain parts easy relative to other techniques that are difficult to create or require specialized tools. The best part of scratch building a rocket, is that YOU get to decide what is best. Others will question your choices, but you are the designer. Design and build your rocket your way. This will no doubt generate a lot of controversy. Each material has pros and cons, and you must do your research, evaluate each based on your intentions, and make decisions based on your own evidence. I have also elected to use BlueTube for my 4-inch body tube.
Why BlueTube: Always Ready Rocketry (ARR) Blue Tube serves as the "Goldilocks" airframe because it strikes a surgical balance between accessibility and high-power capability. While standard cardboard tubes are generally too fragile for serious impulse, and fiberglass-laminated cardboard is labor-intensive and needs to be near perfect to become a superior choice, Blue Tube offers a monolithic, vulcanized strength that handles Level 2 flight stresses right off the shelf. It avoids the massive "price tax" and health hazards (itchy dust and toxic resins) associated with full Fiberglass or Carbon Fiber, yet remains significantly lighter and more impact-resistant. By providing a durable, RF-transparent, and cost-effective middle ground, Blue Tube allows you to maximize altitude and airframe longevity without the complexity or expense of advanced composites.
I am trying to walk a fine line to maintain versatility. For me, BlueTube is the best balance of weight, strength, and cost for this project (fight me in the comments).
Step 3: Mathing
Next, the math for a 744.53% upscale is straightforward high school algebra:
Scaling Equation:
Target OD ⁄ Original OD × 100 = 744.53%(102 / 13.7) × 100 = 744.525%
Step 4: Refining the Sim Design
Once scaled, I updated the materials to reflect the actual build:
Body Tube: We upscaled the tube and chose BlueTube, but we still need to tell OpenRocket we are using this material. I noted the scaled length, then replaced the placeholder with ARR BlueTube at that exact length.
Nosecone: Switched to 5mm thick ABS (my proxy for the ASA I’ll actually 3D print).
Fins: Set to 6.35mm ABS with 6mm System3 Epoxy fillets. Note: I set the width to the nearest metric value to 1/4-in. This is irrelevant if the fins are printed, but if you wanted to use plywood or composites, it will come into play later.
Motor Tube et al: Added a 54mm tube to accommodate the J450, along with centering rings and fin tabs.
The Current Verdict Right now, the dry weight is ~1000g. With an H135, we’re looking at ~1212g. That gives us about 280g of "wiggle room" for recovery gear, electronics, and the inevitable "oops" weight before we lose Class 1 compliance.
It’s a tightrope walk, but there's a secret advantage: OpenRocket assumes parts are solid (100% infill). By specifying 5 or 6 walls with 25–35% cubic infill in the actual print, we’ll likely see a significant mass reduction compared to the simulation's "worst-case scenario."
Next Steps
In the next installment, we leave the simulation behind and move into Fusion 360. I’ll show you my workflow for turning these "rough draft" dimensions into a flight-ready CAD model that’s actually manufacturable.
I truly hope that this series is helpful to you. I learn as much as I share, so if you have questions, comments, links you want to share, or just want to fight me about material choice, please leave a comment.
Check out more unmedicated streams of consciousness at Frogglicker's Space
Reference:
Links:
OpenRocket Website - The official site for getting OpenRocket. They have tutorials on the site, including how to export parts to a file that is compatible with many CAD applications
RocketReviews.com - Several sims can be found here
JimZ Rocket Plans - Lots of classic rocket plans and kits
Several rocket manufacturers publish their own sims you can use. Please give them credit and consider purchasing from them if you use their sims.
OpenRocket YouTube Tutorials - This is a dynamic search of OpenRocket Tutorials that were published within the last year of when the link is clicked. The results may change, so check it periodically. I cannot endorse or recommend any of these since this list will be constantly changing, but it will get you in the right place.
The Rocketry Forum - Easily hundreds of years' worth of combined knowledge is contained here. If you are just getting started in rocketry or not yet a member, I strongly recommend you create an account, jump in, and ask questions.
(This article uses elements that were generated by AI)