Rocket Testing in the pandemic

Since my post a week ago the world has turned upside down. COVID-19 pandemic has gripped the world, the leadership of the USA FINALLY started to take it seriously, all business are pretty much shut down – including the bars!, travel is curtailed… etc… So my trip to launch rockets down in New Mexico with family will need to wait.

But a budding rocket program waits for no pandemic! Onward!

Subscale Testing

Right after posting the previous article about this, I was thinking I could shrink the size of the rocket and accommodate a smaller series of motor. The full size was sized to take a 29x114mm rocket motor. The smaller motors used in smaller/lighter rockets are generally 18×70. Now, as we covered in the previous article, I am bad at math, but I have calculators. So with a few attempts I found funny enough, an 18×70 motor is exactly 62% the size of a 29×114 motor. So if I downsize everything on the rocket 62% I can do a subscale test with zero re-design of parts, just smaller parts.

This will let me not only build some smaller rockets (better for smaller kids), but also test pretty rapidly as the small rockets print almost 2x as fast, and use less than 1/2 the material (given how 3D printing scales and works)

To print the rocket smaller, I simply took the same 3D models and in the slicer (the app that takes the 3D models and converts them into Gcode for the 3D printer) to scale the model down 62% in all dimensions. With the scale down there is a slight tightening of tolerances compared to the big rocket, so I had to sand some parts a little, but overall, everything worked on the first print!

Printing the small rocket also gave me some flexibility easily to test some other print settings, and I’ve found a way to take even more wight off of the big rocket.

Static Testing the Rocket

One of the things I have been concerned about after watching some videos of how much fire and force ejection charges can have on rocket motors, is the potential that my “piston” setup could legit explode my rocket. I would rather test this on the small rocket before throwing an expensive and high energy F motor in the big one.

So I did a static test of the rocket zip tied to a saw horse on my rooftop. I used a small B6-4 motor as the ejection charges are roughly the same size between the same diameter of motors. Below is a slow mo video of the test:

You can key through the video using the < and > keys on your keyboard to see the ejection sequence.

Above is the image sequence of the ejection. You can see the piston moving forward a bit before fire enters into the chamber and the piston fully extends. The piston also stops the fire behind it clearly. This entire ejection sequence takes under .1 sec. (The slow mo video above is 240 fps)

The ejection worked perfectly over all. The materials I printed with handled the temperatures and pressures just fine. No actual damage was left on the rocket other than some discoloration inside.

So the test validated the ejection design I did works, the motor mount is stable and solid, and the material can handle the heat and pressures.

Launch accommodations

So one thing I realized when scaling down the rocket, is the launch lugs are an issue. The big rocket was initially designed for a 3/16″ rod to slide on. The bigger rocket pads I need for this all have 1/4″ rods though. So initially I was going to buy a 3/16 rod for a larger pad. Then I realized if I bought a 1/8″ rod, scaling down 1/4″ by 62% made it fit a 1/8″ rod perfectly. So I had to redesign the mid section tube to accommodate ea 1/4″ on the big rocket and reprinted it.

Next up: (within the bounds of the CV-19 quarantine/social distancing rules)

Sub Scale Launch testing – I am going to use this small rocket to determine the center of pressure. Since the rocket is the exact proportions of the big rocket, I can determine on the small one where the center of balance needs to be in order to be ahead of the CoP on this rocket. I have some other B and C class motors to launch this small rocket with.

Stretching the big rocket – Since I created my own mid body for the rocket, I am able to now have some freedom to modify it. Depending on the initial tests with D and E series motors on the big rocket, I may print a stretched mid body to make the rocket even bigger.

Nose cone tweaks – I want to add a camera, and potentially a sensor package (arduino based) to the nose cone to read data on the flight up and down. This requires me to design my own nose cone… So that’s going on.

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