Below is a summary of the conclusions I've reached so far from my testing here in Texas. My main goal now is see what is necessary to achieve consistanly straight vertical flights in any wind condition. Lately, the winds have been very light, but the rockets still want to come out of the tube at an angle. I'm finding it is critical that the rocket have no angular velocity as it exits the tube. So the launch support wires need to fit the launch tube perfectly, with very little, or no clearance. This means that the launch tubes will ultimately need to be made of a material that stays perfectly round... not the thin walled PVC we got in Vancouver. I am using some thick walled PVC here that is made of PVC foam, sandwiched between very thin solid PVC. It is just as stiff as the solid, thick walled PVC, but much lighter. Got it at Low's.
I plan to do more of my testing with Estes D engines, and fewer CTIs now. The CTI engines have so much power that I lose too many rockets. Every G engined rocket I lose is a fairly big expense. With the D engines, it's 1/5 as much. I am hoping to meet one of the ultralight pilots who flys around here, and see if he will do an air search. I finished making T10C, a single D engine rocket for testing consistant straight vertical launches.
SUMMARY OF CONCLUSIONS
1. It is not helpful to make circular fins more than half as tall as their diameter. Doing so just adds weight and drag.
2. It is not helpful to move the circular fin down below the bottom of the rocket.
3. The longer the launch tube, the higher the winds you can launch in, and still get a near vertical flight. 15 meters per second may be enough speed for the fins to become effective, but it may take significantly more to prevent weathercocking.
4. JB Weld is not weakened by the hot engine casing, even if it is glued directly to the engine casing.
5. Tandem engines work. Using tandem engines should make it possible to make the rockets just 38mm, saving a lot of weight on the buoys. Mike can supply engines with the staging mechanism installed, so it's not something we have to figure out.
6. Allowing the rockets to lean in the launch tubes, often gives the rockets angular velocity that makes the rockets go off course.
7. Small launch support fins move the CP up significantly. These fins also make misallignment of any fins, cause the rocket corkscrew.
8. Here is the formula for estimating Center of Pressure of a 38mm minimum diameter rocket with a circular fin that fits a 3" launch tube: To find the CP's distance from the bottom of the rocket, use the OD in mm of the upper half of the rocket as the percentage of the rocket length. The diameter of the bottom half of the rocket doesn't affect CP. So for your 35mm OD upper half rockets, the CP is 35% of the rocket length, measured from the bottom of the rocket. Thus, if your rocket's CG is above 35%, the rockets will fit in a 3" launch tube.
9. Launch support wires need to fit the inside of the launch tube very close. Extra clearance allows the rocket to bounce back and forth as it goes up the tube, giving the rocket angular velocity. The circular fin needs to fit the launch tube closely too.
(The formula in #8 was derived from actual testing, not from any simulation program. It applys only to rockets configured like the ones we are testing.)
I plan to do more of my testing with Estes D engines, and fewer CTIs now. The CTI engines have so much power that I lose too many rockets. Every G engined rocket I lose is a fairly big expense. With the D engines, it's 1/5 as much. I am hoping to meet one of the ultralight pilots who flys around here, and see if he will do an air search. I finished making T10C, a single D engine rocket for testing consistant straight vertical launches.
SUMMARY OF CONCLUSIONS
1. It is not helpful to make circular fins more than half as tall as their diameter. Doing so just adds weight and drag.
2. It is not helpful to move the circular fin down below the bottom of the rocket.
3. The longer the launch tube, the higher the winds you can launch in, and still get a near vertical flight. 15 meters per second may be enough speed for the fins to become effective, but it may take significantly more to prevent weathercocking.
4. JB Weld is not weakened by the hot engine casing, even if it is glued directly to the engine casing.
5. Tandem engines work. Using tandem engines should make it possible to make the rockets just 38mm, saving a lot of weight on the buoys. Mike can supply engines with the staging mechanism installed, so it's not something we have to figure out.
6. Allowing the rockets to lean in the launch tubes, often gives the rockets angular velocity that makes the rockets go off course.
7. Small launch support fins move the CP up significantly. These fins also make misallignment of any fins, cause the rocket corkscrew.
8. Here is the formula for estimating Center of Pressure of a 38mm minimum diameter rocket with a circular fin that fits a 3" launch tube: To find the CP's distance from the bottom of the rocket, use the OD in mm of the upper half of the rocket as the percentage of the rocket length. The diameter of the bottom half of the rocket doesn't affect CP. So for your 35mm OD upper half rockets, the CP is 35% of the rocket length, measured from the bottom of the rocket. Thus, if your rocket's CG is above 35%, the rockets will fit in a 3" launch tube.
9. Launch support wires need to fit the inside of the launch tube very close. Extra clearance allows the rocket to bounce back and forth as it goes up the tube, giving the rocket angular velocity. The circular fin needs to fit the launch tube closely too.
(The formula in #8 was derived from actual testing, not from any simulation program. It applys only to rockets configured like the ones we are testing.)
