The test flight on a G74-6 went pretty well last weekend, with some caveats. I wanted a high thrust, short burn motor for two reasons, 1) to be sure it would boost straight before attempting any air starting. I didn't want asymmetric drag to steer the rocket at all. There are a lot of draggy bits on it. And 2) to try and get some data to better estimate the Cd for simulation purposes.
The motor eject at apogee was just about right on, perhaps a touch early, but not too bad. The parachute, however, remained somewhat inverted and tangled. It didn't fully deploy. At 400' I had the electronics set to deploy two additional parachutes, one out of each side pod. One came out and deployed cleanly, but there was no sign of the other. After the rocket touched down, I was staring at it from a distance and noticed a puff of smoke and something looked like it flipped up into the air. Curiously, when I got to the rocket, I saw that the third parachute had deployed, but while it was sitting on the ground. I have no idea how or why the electronics could have triggered that after touching down. The two ematches for the side pods were wired in parallel, perhaps that was a mistake. I will test again with the ematches in series.
One fin was broken from the fast descent, but that will be easily repaired.
I attempted to use the barometric altitude data to estimate the Cd after motor burnout, but it looks like I really need an accelerometer to get a better Cd estimate.
If I do the calculation of Cd = (slope) * 2m / (rho * A) I get Cd = 0.685 which I think is far too low. In Rocksim, if I estimate the Cd by iteratively adjusting the Cd, I get a good altitude agreement using a Cd of 1.15. It may be that I'm not using the same area, I calculated the cross section area manually. If I use only the area of the main body (just the BT-80) tube, I get a number very close to 1.15. I would like to know what cross sectional area Rocksim is using. The boost looked quite straight, but probably not perfectly vertical. But really, I need better data. Accelerometer time....
Perhaps I'll try air starting on the next flight.
The motor eject at apogee was just about right on, perhaps a touch early, but not too bad. The parachute, however, remained somewhat inverted and tangled. It didn't fully deploy. At 400' I had the electronics set to deploy two additional parachutes, one out of each side pod. One came out and deployed cleanly, but there was no sign of the other. After the rocket touched down, I was staring at it from a distance and noticed a puff of smoke and something looked like it flipped up into the air. Curiously, when I got to the rocket, I saw that the third parachute had deployed, but while it was sitting on the ground. I have no idea how or why the electronics could have triggered that after touching down. The two ematches for the side pods were wired in parallel, perhaps that was a mistake. I will test again with the ematches in series.
One fin was broken from the fast descent, but that will be easily repaired.
I attempted to use the barometric altitude data to estimate the Cd after motor burnout, but it looks like I really need an accelerometer to get a better Cd estimate.
If I do the calculation of Cd = (slope) * 2m / (rho * A) I get Cd = 0.685 which I think is far too low. In Rocksim, if I estimate the Cd by iteratively adjusting the Cd, I get a good altitude agreement using a Cd of 1.15. It may be that I'm not using the same area, I calculated the cross section area manually. If I use only the area of the main body (just the BT-80) tube, I get a number very close to 1.15. I would like to know what cross sectional area Rocksim is using. The boost looked quite straight, but probably not perfectly vertical. But really, I need better data. Accelerometer time....
Perhaps I'll try air starting on the next flight.
