Counter-UAV
Kratos
Kratos started from the high-speed drone and grew into something bigger and more powerful: 12S, 12-inch props, and enough lift and thrust to carry a 1 kg payload. Its mission is counter-UAV. Development is paused for now, but we have a prototype and a lot of data from bench testing.
How I sized it
I scaled up the HSFD shell and sized everything around the payload: props, motors, battery, ESCs and wings. For that I used prop data, the propeller equation and prop-induced drag, with the body drag coming from CFD.
The tricky part is that everything depends on everything else. To find the top speed you need the angle of attack, and to find the angle of attack you need the speed. The thrust works the same way. It depends on the RPM, but a prop at a given RPM makes less thrust as the air coming into it speeds up, so the speed you’re solving for changes the thrust you have to work with.
So it takes several iterations to get an accurate model. I interpolated drag and lift from the CFD runs and thrust from the prop data, and kept going around the loop until the lift held up the weight, the thrust matched the drag, and the speed stopped changing.
At first I added a wing up front, but the CFD showed the body was already making enough lift, so I took it off.
Rev A to Rev B
For Rev B I moved the wing down and added fins to the bottom of the motor cones, which are also the landing legs. The fins are there for stability at high speed. I took the forces and pitching moments from CFD to find the center of pressure at each angle of attack, and the static margin went from slightly negative on Rev A to positive on Rev B. It ended up with less drag and more lift too.
- Drag
- −7%
- Lift
- +4%
- Lift to drag
- 4.4 → 4.9
- Static margin
- −1.8% → +3.1%
In the surface pressure plots, the blue on top of the wing is the low pressure that makes lift. In the drag plots, the red shows where the flow is losing energy, and Rev B has a lot less of it.
Film
The strike commercial
We shot this with the high-speed drone (7-inch props) standing in for Kratos (12-inch props) while Kratos was still being designed.
