wiki/ lv2c launchdata-2010-06-27

Launch of LV2c in Brothers on June 27, 2010

This was our second "return to flight" launch for the LV2 series. The airframe and recovery systems remained unchanged from last flight. We added a roll control module to as a runner up to full active guidance. We ran an ARTS-2 and a Telemetrum for the flight computers. The motor was an AeroTech N2000W. This page is a container for all the relevant data from the launch of the vehicle including pictures and video.

  1. Launch of LV2c in Brothers on June 27, 2010
    1. Design
      1. Overview
    2. Simulation
      1. Simulated Launch Data
      2. Notes
    3. Pictures
      1. Launch Prep
      2. Launch
      3. From onboard
    4. Video
    5. Launch Data
      1. Flight at a glance
      2. Raw data
    6. Data Analysis/Conclusions
      1. Fin Flutter
      2. Roll Control
      3. Acceleration Data
      4. Velocity
      5. Drag

Design

Overview

We started using Solid Works on academic licenses to model the entire rocket. This was a big success, tracking all of our parts and giving reasonably accurate CG and mass estimations as well as a bill of materials and assisting in the integration of new parts that fit and worked the first time.

We also created a model in OpenRocket off the Solid Works data which resulted in accurate simulations of the flight.

Simulation

We ran a simulation of the flight using OpenRocket. This gave us lots of data for our expected performance.

Simulated Launch Data

This is the data run in advance, not taking into account the measured mass or cg, instead values taken from the Solid Works model.

MKS Imperial
GLOW 29.5 [kg] 65.0 [lbs]
CP (from tip of nosecone) 2.72 [m] 107.0 [in]
CG (from tip of nosecone) 2.56 [m] 100.7 [in]
Burn Time 7.7 [s] 6.45e-6 [fortnights]
Apogee 4616 [m] 15,146 [ft]
Max Speed 358.9 [m/s] (Mach 1.05) 802.8 [mph]
Max Accel 95.7 [m/s2] (9.75 [g]) 314 [ft/s2]

Notes

Pictures

Launch Prep

Launch

From onboard

Video

Launch Data

Flight at a glance

Actual Recoded data from the flight:

Metric MKS Imperial
GLOW 30.8 [kg] 67.9 [lbs]
CG (from NSR seam) 1.357 [m] 53.44 [in]
Apogee (from TeleMetrum) 4784.94 [m] 15,699 [ft]
Max Speed (from TeleMetrum) 374.83 [m/s] (Mach 1.10 - sea level) 838.5 [mph]
Max Accel (from TeleMetrum) 102.93 [m/s2] (10.5 [g]) 337.7 [ft/s2]

Raw data

All of the raw data is in the raw data folder for this launch.

Also check out https://github.com/psas/flight_data-2010.06.27

TeleMetrum

TeleMetrum flight data:

ARTS2

Raw data from the ARTS2 flight computer

Roll (CAN logger)

Opal

In addition to the rocket flight computers we had a piggyback payload of an experimental motion recording device from the medical world. It contains a 6DOF IMU and 3-axis magnetometer. It was never designed for such a harsh environment, but nevertheless has recorded very interesting data.

Data Analysis/Conclusions

Fin Flutter

Careful viewing of the on board camera footage shows the fins fluttering for about one second around frame 31535. Liftoff was on frame 31423. At 30 frames per second this suggests the flutter happening about 3.7 seconds into the flight. This would correspond to the transonic region of flight right as the rocket enters Mach 1 according to both simulations and the flight computers.

The above image shows two nearby frames from the on board camera with the right fin highlighted. In frame 31535 the fin is painted red and in 31537, blue. On the right the two images have been stacked and the fin highlights compared. The fin appears to undergo a flutter with an amplitude of about 4°s.

Conclusion

In general fin flutter is not a good thing. Oscillations can exceed material shear strength and break off fins resulting in catastrophically unstable flight. We are probably fine, but stiffening the fins won't hurt.

Roll Control

The roll control was only a partial success. The mechanical and software systems appeared to work as designed. However we appeared to experience a "control reversal" where fins placed in the counter clockwise arrangement caused the rocket to rotate in the clockwise direction and visa versa. The cause of this problem is aerodynamic in nature and is under investigation.

Acceleration Data

Here is a graph with a comparison of the raw data from the 3 on board computers that had accelerometers. They were all single axis accelerometers aligned vertically. They only capture vertical acceleration and so are only useful from launch to apogee.

Here is the acceleration just during the motor burn. There seems to be an interesting shoulder around the transonic region.

Taking all this data we created an averaged result by putting all the data into 50ms bins:

Download the binned data: accel binned.csv

Comparison with OpenRocket

This was our first launch using OpenRocket as our primary pre-launch simulation engine. Here we compare our recorded acceleration to the simulation. On the whole it did quite well. We suspect that the thrust curve that it had on file for our engine was not very accurate (or at least our motor deviated from it).

Here is a close up of just during the burn.

Velocity

Taking our binned/averaged data we can integrate to find the velocity at each point:

Drag

After the motor burns out we should be able to find all the forces on the rocket; drag and gravity. Gravity is easy to predict and knowing the mass and acceleration we can solve for drag force:

F_d = ma - mg

Knowing the Drag force, height of the rocket, and velocity of the rocket we can estimate the Cd empirically. We simply solve for Cd.

F_d = \frac{1}{2} \rho v^2 C_d A

C_d = \frac{2 F_d}{\rho v^2 A}

This technique does not appear to work very well, unless OpenRocket is just very wrong about drag. Which is possible. The technique is very sensitive to having the correct velocity.