The rocket that does its own math
Relay is the successor to the Echo program. Just like Scout and Echo, Relay is built with propulsive landing in mind - flying with all new hardware, avionics, and ground support systems. Running the Signal flight software, Relay uses the exact apogee measured in flight to determine the optimal altitude to begin the retro-propulsive burn. Offline simulations are helpful, but cannot predict real life performance exactly, making this feature critical to Relay's chances of success.
The first of the Relay program - the flight was a partial success. Flying with all new thrust vectoring hardware, the vehicle remained stable and upright through roughly half the flight. After reaching roughly 50ft AGL, the vehicle tipped to one side slowly. Though it continued up and wasn't entirely unstable, there was clearly an anomaly. This has been determined to be caused by a communication cable between the thrust vectoring mount and the flight computer. The cable had too much slack, and got caught in one axis of the system, obstructing it on only that axis. All future Relay vehicles have much less slack left in their comm lines.
After reaching an apogee of 120ft AGL, Relay failed to deploy all three of it's external systems - drag fins, parachutes, and legs. The vehicle fell from apogee, slamming mostly horizontally into the ground. Relay sustained little damage on impact, most being purely cosmetic. The cause of this anomaly was initially thought to be due to a wiring/connectivity error on the 7.4v nichrome trigger rail. After further investigation into the computer and in-flight data, the cause was found to be a software bug. The issue boiled down to two conflicting sections of code in the same control loop. One section correctly identified and send commands to trigger the drag fins, chutes, and legs to deploy. A second section later in the control loop had a faulty time limiter on it, restricting all activity on the 7.4v rail until after the limit had been passed, along with a few other criteria. The issue is slightly complicated to describe via text, but has since been fixed, and the two sections of code have been merged into one, eleminating the possibility for this error to occur again.
Mission objectives - 6:09
T - 15 Seconds - 9:59
Relay F2 was the first successful in-flight test of the drag fin system, developed to keep the rocket passively stable during descent. The system largely failed the test. After drag fin deployment, the rocket behaved more like a pendulum than a stable rocket in flight. Ideally on the way down, Relay would become stable using the same basic principals as a traditional model rocket - center of pressure behind the center of mass. The cause of this pendulum-like behavior involves a few explanations, but it boils down to the fins simply obstructing the flow of air, instead of creating drag and a restoring force, as a traditional model rocket fin does.
During the flight, the rocket had the ability to decide if attempting a propulsive landing was safe. There are a few conflicting bits of data from the flight, and it is unclear whether the rocket tried to land using the retro motor at this time. The in-flight data recorder shows that the rocket calculated the correct time to burn the retro motor, and sent the command to light it at the correct altitude. One of the requirements to initiate the burn include the angular orientation of the rocket being close to upright. At the burn altitude, the rocket was close to upright, but rotating fast. Rotation will be considered in future flights as a safety measure.
The rocket certainly sent the command to light the retro motor, but it clearly didn't burn at the correct time. Two errors are at play here. The first was likely a poor set of alligator clips. In the post flight inspection, one of the clips was found to be slightly broken. The leads for the retro motor were still in contact with the clips, but were somewhat loose. The second error was that upon impact with the ground, the relay used to trigger the retro motor experienced enough shock to trigger accidentally. To avoid this issue, the relay had been mechanically isolated from the airframe with a significant amount of soft foam. Obviously this wasn't sufficient, and this issue poses a significant safety threat. All future BPS rockets will use exclusively solid state electronics to trigger dangerous onboard events like this.