One of the key systems of the Stratos II+ rocket is the Flight Termination System (FTS). The FTS ensures a safe launch ensuring that the rocket does not fly anywhere where it is not supposed to fly. In this blog post we’ll discuss the details of the FTS.
Safety ensured
To ensure safety during the flight, Stratos II+ is tracked from the ground. In case something goes wrong or the rocket is calculated to touch down outside the safe zone, the rocket flight must be terminated. To be able to do this reliably it is important to have good antennas for best reception of the flight termination system. Even with bad antennas safety is still ensured, because the rocket will terminate it’s flight if it does not receive a proper signal from the flight termination ground station.
Testing the antenna frequency
The length of an antenna determines the frequency it works best at. To get the best reception, a frequency analysis was done to see if the length of the antenna was indeed best to receive with the frequency of the transmitter.
Simulations
As we learned during the launch attempt of Stratos II last year, the system reliability needed to be improved. To do this properly, at first the former antennas were simulated in the same software to be able to compare the results.
During a large part of the flight, the transmitter that sends the signal to the rocket will be pointed at the bottom (engine-side) of the rocket as the rocket ascends. This poses multiple difficulties. First of all, the rocket plume will attenuate (worsen) the signal. Furthermore an antenna like the one that was used last year, has a moderate reception for signals coming from the back of the rocket. Because we don’t want the rocket to terminate flight when it loses signal without a real problem being there, reception needs to guaranteed in all cases.
The new design, called a split-V-dipole, has improved reception at the back of the rocket. Antenna’s on the side of the rocket never give a high signal reception at the back, because the aluminium is blocking the signal. However, the reception that we get with the new design would still be good, even at 200 km from the transmitter. For the 10 to 20 km the motor burns, this should be plenty of signal strength.
FTS antenna mounted on the inside of the motor bay
Validation
To validate the simulation results with the final antenna system, a pattern test was performed in the RF test room at EWI. The measurements done in the test room confirm that the antennas are working as designed. During the last week we also drove the FTS to Portugal 66 km from the launch site. Reception was still fine at that distance, further confirming the proper operation of the FTS.
Installing the FTS antenna assembly in the measurement room
FTS Ground Station
To further improver the system, an interface is made to easily operate the FTS transmitter. The device shown in the picture generates the tones. During flight the rocket is “Armed” and in case an anomaly happens, the rocket can either be terminated by pushing the “Terminate” button or if it looses the arm signal, it will terminate automatically by turning off the engine.
FTS gound station interface