Scout is a concept testing rocket, roughly 1.2m (4ft) in height, depending on the build version. The rocket is designed to use an active thrust vectoring system to ascend to a low apogee, begin descending, perform a second burn, and land upright on the ground. Scout’s flight computer, Fetch, controls the vectoring system, primary data logging, drag fins, secondary ignition, and landing legs onboard the vehicle.
Since the altitude and descent rate are relatively unchanging, the second burn is triggered by altitude alone. However, later builds of the vehicle (V0.9 and V1.0) carried code onboard with multiple flight plan scenarios. This allowed for a retro-burn triggered at the correct height based on whatever apogee was achieved, within a realistic range.
The Scout program never accomplished its primary goal of landing the rocket upright, or achieving a fully stable thrust vectoring system. However the year-long program was an effective exercise in persistence, stress management, and the value of high speed footage :)
The pioneer of the Scout vehicle program, Scout V 0.1 failed after the liftoff thrust of the main booster overloaded one of the TVC servos. Inertial measurements were also inaccurate, but results showing this could not be obtained or observed due to the mechanical failure of the TVC system, and lack of an onboard data logger.
Scout V 0.2 featured much stronger TVC servos, and an improved TVC mount. At liftoff, the system was able to maintain its position, but the inertial measurement system was still faulty. The onboard computer only used an accelerometer for inertial measurement, which fails under thrust, so future builds used a complimentary filter to read from a gyroscope and accelerometer for increased accuracy.
Scout V 0.3 was the first in the Scout program to correctly measure the position of the vehicle during flight. Due to overcorrection in the TVC system, the rocket lost vertical thrust and fell to the ground shortly after liftoff.
After this test, propulsion for the main booster was upgraded from an Estes E9 motor to an Apogee F10 motor, allowing for longer burn times, and slightly higher average thrust.
The first Scout vehicle to use generally successful thrust vectoring, and deployment of the drag fins, Scout V 0.4 was a milestone in the Scout program. Ascending to roughly 20-30 ft, Scout V 0.4 was able to correct for deviations in its orientation before descending due to lack of vertical thrust. Once descending, the rocket anticipated ignition of the retro-burner on board, and recalibrated the vectoring system, resulting in a loss of stability.
Scout V 0.5 failed due to excessive play in the internal gears of the thrust vectoring servos onboard. While stabilization of the vehicle was still active and functional, the "zero-position" of the servos ended up with a deviation of ±2 degrees, which was enough to set the vehicle off on a poor trajectory, especially considering that the computer's stabilization was directly reactive to its measured position at this point, and not anticipatory, such as in a PID control system.
Two attempts were made to launch Scout V 0.6 - one on March 6th, 2016, and a second on April 13th, 2016. Both attempts were unsuccessful. The first due to issues perceived to originate in the onboard accelerometer (later determined to be cause by residual voltage between shut down and startup of the flight computer).
The second launch was scrubbed on the pad after 3 failed ignition attempts. Ignitions failed due to the main booster e-matches lacking sufficient pyrogen. The Apogee F10 APCP motor is significantly harder to ignite than conventional APCP or BP motors and requires igniters that are coated with a large amount of pyrogen. AeroTech Copperhead and First Fire Jr. igniters are sufficient, but not First Fire Mini's which were used.
In a similar fashion to Scout V 0.5, this rocket failed due to play in the TVC system. The last rocket to use this TVC mount design, it was the nail in the coffin for a vectoring design which used both non-axial thrust for the vehicle, and relied directly on stiffness of the internal gearing of one servo to remain zeroed out.
With an entirely new thrust vectoring system, Scout V 0.8 corrected well for its orientation with two caveats. The first being that due to errors within a computer simulation of the flight, the liftoff weight was much too heavy to allow for a suitable apogee. The second being that the onboard PID system was running for roughly 30 seconds before liftoff, and too much error was accumulated during that time to maintain a zero-position in the TVC mount.
After starting the PID system roughly 1 second before liftoff in order to reduce error accumulation, the code onboard failed to account for a discrepancy in timing within the complimentary filter, resulting in a massive initial drift of ± 50 degrees on each axis before zeroing out after 3 seconds, which was much too late in this case. The rocket lifted off with poor TVC orientation, damaged the PID starting gantry, and knocked over the articulating support tower, before spinning out of control and crashing to the ground. This accidentally tripped the onboard relay to trigger the retro-rockets.
After modifying the code to seemly remedy the onboard time management issue, it was revealed during the flight of Scout V 1.0 that the complimentary filter was still not functioning properly. The IMU measured changes in orientation at a rate that suggests that only the accelerometer was used in the filter. This would, and did result in a similar flight profile to Scout V 0.2. The rocket lifted off, arced over, and slammed into the ground since the IMU was not being interpreted correctly, and the vehicle made no corrections in the vectoring mount.
After the flight of Scout V 1.0, the Scout program was put on hiatus indefinitley in order to focus on a simpler vehicle, Echo. Using more powerful propulsion, and a smaller, more lightweight design, Echo builds on the knowledge and lessons learned from the Scout program. Specifically, all systems of Echo will be tested individually, then incrementally added together, as opposed to the full-up testing style of the Scout program.