Technical Blog: Launch Safety (part two)

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A rocket, more so than other vehicles, is subject to extensive safety requirements and procedures. In part two of this technical blogpost we will discuss how safety considerations determine the countdown and flight of Stratos II.

A certain trajectory

Stratos II is an unmanned vehicle, as such it could just deploy its parachute on a mechanical timer, which is standard practice on DARE’s first year rockets but also on some larger rockets such as the Rexus sounding rocket. For this system however a digital system  was designed. With the use of electrical components found in smart phones and some proper system engineering a more advanced system makes parachutes  much more controllable and robust. The rocket knows what it is doing and can deal with rapidly changing situations. This means that an off nominal flight trajectory does not automatically result in a mission failure. Fundamental to this is:

  • The flight termination system, continuously monitoring the connection with the base.
  • The motor control unit, an electronic system evaluating  and controlling the state of the valves in Stratos II and measuring tank and combustion chamber pressure.
  • The main control unit, an electronic system in the top section of Stratos II, measuring atmospheric pressure, translational accelerations and rotational speed.

T-60 seconds

System status is checked for the last time before the ignition sequence is started. A computer connected to Gabriel (DARE’s launch box which is used to control the launch activation) at the launch command post reads out the system status.  Does the flight abort system have a connection to the base?  Are the batteries charged enough? Are all the valves in their correct pre-flight positions? Is the tank pressure within limits for launch?


Flight abort system:As with most professional rockets, Stratos II also has a flight abort system. It is a way of making sure that the rocket is kept within the controlled airspace and safety zone  regardless of any, unforeseen technical malfunctions.  The system works with two signals, the first one checks continuously if  the rocket has a connection with mission control at the launch site. In case of a loss of signal the rocket has a three second window to reacquire its connection. Otherwise it automatically results in mission abort.  The second signal allows the base to manually terminate the flight if it starts to deviate from the allowed trajectory. The trajectory is  continuously monitored by the range safety officer using the optronic and radar systems and compared it to the simulated flight path .

T-10 seconds

Ten seconds before launch, the launch button is pressed at the launch control post. The rocket starts its internal mission clock, but if anything happens that is not part of the procedures the safety officer only  needs to release the button to abort the launch.

T-9 seconds

Nine seconds before launch the rocket switches to internal power.  The electrical umbilical is still attached, which allows the launch crew to continuously monitor the state of the system.

T-4 seconds

The rocket puts itself in the flight state. A loss of signal from the umbilical that would normally abort the launch now requires a specific disarm command from the launch crew. This is to prevent the vibrations of engine start up to create a false abort. The main valve bypass line is opened.

T-3 seconds

The Igniter is fired. It is a small pyrotechnic charge the size of a firecracker that ignites the initial flow of nitrous oxide, allowing the combustion chamber to preheat before the main valve is opened and the chamber is flooded with cold oxidizer.  Preheating the chamber ensures that the inflowing nitrous oxide will burn in a stable manner, lowering the chance of a flameout..  The team is able to asses visually whether there is a misfire.

T + 0 seconds  Launch

The main valve opens and Stratos II takes off. It reaches 100 km/h within 0.7 seconds after launch. It accelerates so fast that the components weigh seven times as much compared to when they were still on the pad. The rocket keeps in contact with the launch site through its flight abort system and sends down the flight data to the ground station.  The flight abort system is active for 35 seconds or ten seconds after burnout. After this time the rocket is on a well determined ballistic flight and through most of its critical moments.

T+ 300 seconds

Stratos II is half way to space.