The Flight Profile of the REXUS Rocket

We want to take the time and give you a brief summary of the countdown and a short preview of the launch and flight profile of REXUS 9:

The Countdown

The countdown officially starts at T-2 hours, meaning 2 hours before lift-off. In the first minutes of the countdown the launch area is cleared, communication checks are performed and the payload and experiments are checked out. At the same time the recovery helicopter is adviced to fly to Esrange. Approximately one hour into the countdown the roof of the mobile rocket launcher (MRL) is removed and the rocket is brought to an upright position for launch. This is followed by a lot more checking of the rocket and its subsystems. Finally, EXPLORE’s master safety valves are armed at T-20 minutes.

15 minutes prior to launch all personnel not directly involved in the launch activity must leave the safety zone and return to their assigned buildings. On the entire rocket base no one is allowed to stay outside during the launch for safety reasons. 

10 minutes prior to launch the excitement rises: Power on for all experiments. EXPLORE will come to life and start sending status messages. This gives the launch crew of the EXPLORE team the chance to check out critical system parameters and confirm that the experiment is in good shape and ready for launch.

3 minutes prior to launch the EXPLORE cameras are turned on and go into recording mode. At the same time, the experiments are switched to the rocket’s internal power system. This is another important milestone, because a power loss during switching could lead to an unwanted restart of the microcontroller onboard EXPLORE.

At T-25 sec the ESRANGE Operations Officer will give permission for launch. At T-5 sec the vehicle officer down in the „block house“ next to the launcher will press the firing button. This button must be pressed and held for 5 seconds to activate the rocket igniter system. 

Flight Profile

At T=0 the rocket is ignited and lift-off occurs. With 1,3 metric tons of thrust the rocket blasts off into the empty sky above Lappland, at an acceleration that is 20 times higher than Earth’s own gravity. 1.7 seconds into the flight the launch crew in the ground control station will loose sight of the rocket.  EXPLORE is now relying on the telemetry data sent from the rocket to check on the status of the experiment. After only 26 seconds, burn-out of the Improved Orion rocket motor occurs. At this point the rocket has already reached an altitude of 25 km, which is more than twice the altitude at which a commercial airliner cruises on a transatlantic trip. With a velocity of over 1000 m/s REXUS 9 is rocketing towards space. After burn-out of the motor, the rocket will continue moving upwards and its speed will decrease, similar to a ball which is thrown into the air.

During the first phase of the ascent the rocket is spinning around its own axis for stability reasons. 70 seconds into the flight the Yo-Yo-Despin systems is activated, which reduces spinning frequency almost to zero. At this point of flight the rocket is high enough (~60 km) so that the drag caused by the residual atmosphere is negligible. Therefore, after de-spinning is completed, the microgravity environment is established.  Seven seconds later the nosecone is ejected and the experiments separate from the rocket motor. Shortly afterwards EXPLORE starts filling its test chambers under microgravity conditions. 

139 seconds after launch the rocket reaches the highest point of the trajectory, the so called apogee. At this point EXPLORE has completed filling of 4 of its 6 test chambers. In the following minutes, while REXUS 9 is falling back to earth and accelerating again, a variety of other student experiments is performed. Finally, Earth’s atmosphere becomes dense enough again during descent to decelerate the rocket. At an altitude of 16 km the rocket's velocity drops below the speed of sound. 4.6 km above ground the rocket’s parachute systems are deployed, which leads to a significant reduction in velocity.  9 minutes and 10 seconds into the flight the onboard cameras of EXPLORE are turned off, followed by a power down for all experiments. A few hundred meters above ground the recovery beacon activates, allowing the recovery crew to locate the rocket’s position after landing in the vast snowy planes of northern Sweden.

After landing of REXUS 9 has been confirmed, the recovery helicopter will head out to safely return EXPLORE to the rocket base. A few hours later we will have confirmation if all the hard work that the team has put into EXPLORE over the past year has paid off.

Overview of the time plan