I’m building Ryan a rocket from scratch. The first step was to decide on an engine since the size and weight of the rocket would need to be built around that.
D12-5, I Choose You!
I chose the Estes D12-5 rocket engines and bought 3 of them from Amazon.
I chose the D engines as a compromise between price and power as well as for convenience. The first letter of the rocket engine name describes how powerful it is. E is more powerful than D which is more powerful than C. C engines cost about $3.00 per engine, D engines are just under $5.00 each and E engines are almost $10 each. $10 per launch seemed too expensive and the fact the D engine supposedly fits nicely inside a piece of PVC pipe sealed the deal.
The next step was to figure out the parameters I had to use to build the rocket. I didn’t want to build a cool rocket and have it only launch 10 feet off the ground.
How High Can I Fly?
I found a fantastic site with all the formulas I needed to calculate how high my rocket would go. My physics skills are rusty, but I can still plug-and-chug formulas like nobody’s business.
I put all the formulas into a spreadsheet which you can download here. You need to enter 6 fields, and it should tell you how high your rocket will go. The fields you’ll need are
- The Mass of Your Rocket in Kg
- The Mass of Your Motor in Kg
- The Mass of the Motor Propellant
- The Diameter of your Rocket in cm
- Your Motor Impulse In Newtons*s^2
- Your Motor Thrust in Newtons
Before you panic since you don’t know those values, they’re pretty easy to look up. A chart on the same site lists the blue italicized values above for Estes motors – other companies provide this information too. Your motor’s thrust (the red bold value) is the first number in its name, eg, the 12 in D12-5.
The two remaining values are the ones I was looking for. Essentially I end up with three unknowns. My rocket’s mass and diameter, and how high it will fly. By playing with the two inputs I can see what the effect will be on the height.
Fun Vs. Safety
I stumbled across the NAR Model Rocket Safety Code while looking up engine info and realized that rule number 1 conflicts with my plan:
Materials. I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.
I want to build a retro rocket similar to the one picture at left. I am considering using thin aluminum sheets and pop-rivets, both of which are metal, of course.
I guess silver spraypaint would get a similar look but I am kind of hoping that I can build a rocket sturdy enough that the kids can play with it as a toy when it’s not being launched. I’m not convinced that I can accomplish that with cardboard and plastic.
Now, off to the hardware store!