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.
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?
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.
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.
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.
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.