Both of our launches on the 17th of February went extremely well. We are having trouble publishing our launch video but will post a link to the YouTube video when it is live. Our 2.2” diameter 62” rocket was expected to reach an altitude of 2100 feet on a 234 Ns H motor with a max speed of 250 mph. The J motor had an impulse of 700 Ns, we were expecting it to reach about 5900 feet with a max speed of 700 mph.
Our on board altimeter recorded both flights allowing us to review data from the flights including temperature, pressure, velocity, and altitude. The H motor launched our rocket to 2068 feet with a max speed of 272 mph. Our rocket soared to 6,839 feet on the J motor with a max speed of 760 mph (that’s 7 mph away from going supersonic).
Notice our first flight was only 32 feet below the simulation while the second flight was 939 feet above the simulation. This is due to OpenRocket’s inability to accurately account for drag coefficients as velocity reaches the speed of sound (767 mph).
This past Thursday, we received materials for our main competition rocket and ordered a pixhawk autopilot system for our drone. Construction of this rocket will be essentially the same as our previous rocket on a larger scale with four main differences.
- Our first rocket did not have a drogue parachute, this rocket will. A drogue parachute is a relatively small parachute with a hole in the middle. The drogue chute will allow our rocket to fall fairly straight and slow, allowing us to safely eject the drone and main parachutes at our desired altitude.
- There will be two independent avionics and ejection systems inside this rocket. They will both be set to eject at the same heights. The redundant system allows us to have one system fail while still safely returning our rocket to the ground. Since this rocket is substantially longer, wider, and heavier than our previous rocket, more involved safety measures must be put in place.
- This rocket will be carrying much more weight than our previous rocket, as well as carrying a drone. This will shift around our center of gravity and we must be careful to keep mass as centered as possible. We will put two centering rings on the forward and aft sections of our drone which will keep it from rocking around while inside. The drone itself will be placed behind the nose cone and in front of the main parachute. This will allow the drone to be pushed out by the parachute after the nose cone is separated by the ejection charges.
- Our competition rocket is using a P75-4G (Pro 75 four grain) reloadable motor. While the mechanical construction process is unaffected by this, it will result in a heavier load on our rocket. The majority of commercially available high powered motors (L and above in my experience) do not come as single use motors like our H and J did. Instead, they are single use solid cylinders classified by their number of grains that are then placed inside of a reusable motor tube. All of the anodizing and engineering that goes into a tube able to withstand numerous thousand degree high-pressure burns drives the price of these tubes way up. Our L motor case was $309, about $5 less than the cost of the fiberglass rocket itself. Each reload for this case will cost $280.