3 cold flows, actually. A 'cold flow' is when the system is filled and pressurized akin to the actual launch procedures, with liquid nitrogen in place of lox and isopropanol alcohol substituting for kerosene. This allows us to confirm the system is operating how we expect while discharging propellant at our desired mass flow rates.
Because there are so many factors in a 'blow down' style sounding rocket to consider, we are planning on conducting 3 cold flows over the course of two days, adjusting propellant volumes and ullage pressures to affect the eventual discharge and atomization of the propellants into the rocket engine.
We look at several critical factors in a cold flow:
1. That we run out of lox before kerosene. By having a small volume of kerosene left to discharge after engine shutdown, we ensure that only fuel will be present for the final moments of powered flight. An oxygen-rich shutdown would risk temperature spikes and accidental ignition of components.
2. That we don't dip below an acceptable chamber pressure near the end of our burn due to insufficienct nitrogen ullage pressures. This could cause engine burn instability by poor propellant atomization and greatly reduced mass flow rates.
3. That we obtain and initial thrust at least 3x the weight of the rocket in order to achieve a large enough initial acceleration to reach the speeds required for aerodynamic stabilization by the time the Vulcan reaches the end of its guiding rail.
4. That the drastic thermal gradients involved don't lower the temperature of the kerosene below its pour point or interfere with the operation of any structural or mechanically active components - like actuating valves.
With these aims in mind, we are all working hard to make this coming weekend a success and to clear the way for a launch on May 21st!
Fortunately, we've had a revived effort aimed at making sure our CAD and sims are current and reflective of the actual Vulcan rocket. With the knowledge that we've recently gained from these resources, I can say with some confidence that we are aware of the key weaknesses inherent to our design, such as the proximity of the kerosene to the lox in our Main Valve, shown below.