Introduction

While it’s true that NASA has collected data on their rockets for years, very little of this data pertains to the modern high power rocketry hobby. The data such as fin stress analysis on computer controlled fins made of high dollar materials and costly processes can be found, but these are not the fins we use in our hobby. Moreover, much of the data is not made public. The most illusive data pertains to Mach transition. This has three major components, each having its own unique inherent characteristics; subsonic, mach and supersonic.

 

With this in mind, I am building a data collection rocket. This rocket will be used to learn as much as possible about what happens to our rockets during flight. I will be able to plot the change in drag resistance through Mach transition, motor thrust changes at high altitudes, fin flutter, and much more. Why? Because you can never have too much data.

 

Equipment on hand:

I have been collecting equipment and sensors for over three years. I have been buying things as I found closeouts or great deals on Ebay. Some things were donated for the project by corporations. Everything I have is brand new and calibrated to NIST.

 

Load Cell - Omega Model LC321-2K

This will be used to measure drag coefficient through Mach transition. This will be measured by collecting motor thrust data and acceleration through mach transition and plotting it against known motor thrust profile. As the rocket passes through Mach transition, the drag coefficient changes as a shock wave briefly envelopes the vehicle. This will be evident as the load on the airframe increases.

Omega Model LC321 Load Cell for Motor Thrust

 

Precision Strain Gauges - Omega Models SG-50/120-LY40, SGD-7/350-DY11, SGD7/350-XY11

These will be used to measure fin flutter and airframe bending. This will be measured in strain with a ½ Wheatstone bridge, having one strain gauge on each side of a fin. (See diagram) This phenomenon is caused by resonance frequency in the fins. This causes lateral movement in the fins whereby the fins begin to flutter. I will have fins made of both carbon fiber and G10, having the same thickness, to see if there is a difference in response to fin flutter. Fin flutter is the leading cause of HPR rocket failures at mach speeds.

Omega Strain Gauges For Fins and Airframe

Wheatstone Bridge

Wheatstone Bridge in Use

 

Micro Thermocouples – Omega Model CHAL-005

These will be used to measure surface and motor case temperatures. This will be measured by thermocouples imbedded in strategic areas on the rocket. Many of our rockets returned with burnt paint due to Mach speeds. This data will be collected to find the temperature seen and help determine the highest heat areas. The other use will be to measure motor case temperature.

Omega Thermocouples for Motor Case and Leading Edges

 

Pressure Transducers – Omega Models PX41TO-2KG5V and PX4202-3KG5V

These will be used to measure motor pressure. As motors gain altitude, their thrust profiles change. This change is due to lower ambient pressure seen at higher altitudes. No nozzle geometry is perfect for all altitudes. I happen to have these two pressure transducers and I haven’t decided which one to use. Why collect this data? Because we can!

Omega Pressure Transducers

 

Fin Strain Test Fixture

It was decided to build a test fixture for the fin strain gauges. This would allow the fin strain gauges, the signal conditioner and the logger to be tested prior to flight. With it, I was able to make sure that the system will work and there are no bugs in the firmware and it will be stable at all predicted frequencies.

The fixture was made from two 1" thick plates of AISI 1046 welded together. A plate made of hardened AISI 4137M (my own secret recipe with lots of molybdenum and a dash of calcium) was mounted to the top using 5/16 socket head cap screws. The fixture is mounted to the test frame with a 1" socket head cap screw. (The 6" thick test frame had the threaded holes in it already.) A general sketch of the fixture can be seen in the picture.

Once the stability of the electronics is verified, a baseline was established. To do this, the actuator and fin is started at zero. Then the actuator was moved to various distances, both up and down. The output voltage from the strain gauge is recorded so that we know that X distance = Y voltage.

Sketch of Fixture (Click for larger, easier to read image)

Test Fixture in Position A

Test Fixture in Position B

 

Fins

The fins are being made by Mick Kelly. This fin design has flown at 2343 fps by Greg Mills. We will be adding a fins page once they are complete.

Mick Kelly's fins during construction

Data Acquisition

 

Datapaq – 9000 Models DP9069 and DP9161A

I have two data recorders from by Datapaq. (see www.datapaq.com) They each record 6 temperature channels at a rate of 1 sample every 0.1 seconds for 12 hours.  One has a range of -238F to 932F and the other has a range of 32F to 2500F.

 

Datapaq Model 9000 temperature controller

 

Strain, Load Cell and Pressure Transducer Data Acquisition

The data acquisition actually consists of two components, a signal conditioner and a data logger. The signal conditioner sends conditioned power to the gauges and collects the return signal. It then takes the return signal and sends it to the data logger.

The signal conditioner was designed and built by David Shultz.

For the data logger, we went with the Logomatic from SparkFun Electronics. With names like that, how can you go wrong! This unit has 10 channels and can record from 150 Hz if all 10 channels are used, and up to 1,500 Hz if only one channel is used. The data is stored on an SD card. This data is then downloaded to a computer for study.

 

SparkFun Electronics Logomatic Data Recorder

 

Motor Mount Design with Load Cell

The motor mount system is unlike any design out there. It is designed so that the thrust from the motor is transferred to a load cell through the motor mount tube. This was achieved by not gluing the motor tube to the centering rings. It is secured to the rocket at the forward end through a load cell. The load cell can accurately measure loads up to 3,000 psi, and tolerate spikes of up to 5,000 lbs.

 

The design and construction of the motor mount bulkhead was critical. Normally, all the thrust generated by the motor is transferred from the motor, through the motor mount tube, into the centering rings, and finally, into the airframe. In this rocket, all of the thrust generated by the motor, is transferred through just one bulkhead. The MMT bulkhead was made of alternating layers of 1/16 birch ply, carbon fiber, and fiberglass.

 

The first thing that came to mind for motor retention was Aero Pack. They make the easiest to use, most reliable, and strongest motor retainers on the market. When the project was first brought to their attention, they quickly offered to help. Aero Pack donated a motor retainer. The motor retainer had to be custom machined so that it worked with the "sliding motor mount" system. This allowed the motor's thrust ring to impart its entire load to the motor tube.

 

Motor Mount Design (Click for larger, easier to read image)

 

Wire

The wire used throughout this project was donated by MTS. The strain gauge signals are particularly sensitive to EMF, so the highest quality double shielded wire was needed. The thermocouple wire obtained for the project is 50' of Type K, 0.010" PVC coated.

 

Equipment Needed

 

Rocket Build Components

I don’t yet have the airframe, nose cone, and all other rocket parts. This is mostly because I need to see how much space I need for all of the electronics. I’ve got well over $10,000 worth of equipment for this project already. Any donations will be rewarded with first hand data, involvement with specifics of data collection, and large “sponsor decals” on the rocket.

 

Want to contribute?

The project is being discussed on the Rocketry Planet discussion forum for Rocket Materials Testing. You can join the discussion there, or you can contact me directly via the contact form.

Project Contributors to Date

Contributor	Website	Contribution Description
Bob Buchalski, Hangar 11	www.hangar11.com	Phenolic tubing
Bob Mosley, AeroPack	www.aeropack.net	Custom 75mm motor retainer
Darrell Mobley, Rocketry Planet	www.rocketryplanet.com	Webmaster and project web hosting
David Schultz	Design Diagram	Design of data acquisition electronics signal conditioning
Jim Urata	None	Feather weight screw switches
Mick Kelly	CompositeRockets	One set of composite fins
Michael DiVinti	None	Centering rings
Mike Lacorte, MTS	www.mts.com	Double shielded multi strand wire
Paul Robinson, AMW Pro-X	www.amwprox.com	Custom forward closure, tapped for pressure transducer
Rob Hornsblow, Datapaq	www.datapaq.com	Two Datapaq 9000 data loggers