Stratospheric Projectile Experiment of Entry Dynamics (SPEED) Avionics and Instrumentation

The Stratospheric Projectile Experiment of Entry Dynamics (SPEED) project is the first demonstration of a novel approach for capturing free-flight dynamics data for entry capsules. Traditional methods to characterize dynamic stability of entry vehicles include ballistic range testing at supersonic speeds, free or forced oscillation testing at transonic speeds, and wind tunnel and drop testing at subsonic speeds. However, the limited number of operational facilities in the U.S., coupled with the complexities of data collection and analysis, leads to high uncertainties in calculating stability parameters like pitch damping coefficient. Computational methods provide an alternative, but they frequently suffer from insufficient validation due to the scarcity of flight-like experimental data.

SPEED offers a flexible, data-rich testing approach, enabling free-flight experiments in environments that closely match flight conditions. The experiment involves subscale entry capsules housed within high ballistic-coefficient projectiles, carried to the stratosphere by a high-altitude balloon. Once the target altitude is reached, projectiles are released and rapidly accelerate. Upon detection of desired flight conditions, which is set to ensure dynamic similitude to flight, capsules are ejected from the back of the projectile and decelerate through transonic and subsonic regimes until reaching terminal velocity. Fig. 1 shows the concept of operations. A robust avionics system and comprehensive instrumentation package are critical to releasing entry capsules at the right time and capturing high-fidelity data throughout flight.

The first SPEED flight is set for March 2025. A stratospheric balloon will carry the payload to White Sands Missile Range, over which projectiles will be released. For this test, eight projectiles are secured within a cage-like drop platform, each held by a trapdoor mechanism fastened with Vectran cord and a hot-wire cutter. The first of three avionics systems receives a signal from the balloon and subsequently activates the first hot-wire cutter, allowing current from lithium-ion batteries to sever the cord and release the projectile. The system then waits ten seconds before triggering sequential release of each projectile.

The second avionics system, housed within the projectile, senses onset of free fall and ejects the entry capsule after it has been in free fall for a specified duration, corresponding to reaching the appropriate altitude. Ejection is achieved by a hot-wire cutter severing the cord that keeps the springs, atop which the capsule sits, compressed.

The final avionics system, housed within the entry capsule, collects data throughout the test. Capsules are equipped with accelerometers along with five pressure sensors arranged in a cruciform on the forebody. In addition to recording data from these sensors, once free-fall begins the avionics system activates a horizon camera, which records video during descent. Finally, capsules contain standalone IMUs, which record accelerometer, gyroscope, and magnetometer data to internal SD cards. Fig. 2 shows a schematic of this system.

The avionics systems have undergone extensive functional and environmental testing, but their behavior during flight is still unproven. This presentation will exhibit preliminary analysis of flight data for a first look into what worked and what can be improved upon for subsequent flights.