Jul 27 2017

Progeny Mk4 Launch 3 Analysis

We did it. We made it. SPACE! At last! Of course our accomplishment was somewhat overshadowed by the Monolith Incident that occurred during the ascent but nonetheless we have finally achieved the #1 primary goal of the Progenitor Program after 16 launches – flying above the atmosphere of Kerbin. You can relive all the major moments in the flight by visiting its vessel page on our Flight Tracker and using the Prev dropdown list to look back. You can also view detailed telemetry data from the flight for a closer look at what the rocket was doing for the eight minutes and twenty-nine seconds of its journey up to 135km and back down to Kerbin. The ascent up to MECO can be watched at the bottom.

Let’s dig into the flight analysis. The rocket launched with its lower booster solid core reduced to 81% thrust, which produced 106.6kN or 6.08Gs at the time of ignition, climbing to 110.4kN or 10.106Gs by the time the booster flamed out and was discarded at L+6 seconds while traveling at a velocity of 377m/s. All three fins were shredded a second later, fouling the booster’s aerodynamics. It impacted 1.3 km downrange. The second stage booster was lit off at L+14 seconds after the rocket’s pitch had dropped roughly 1.5° during the coast. At 52% thrust, it began the push with 22.2kN producing 0.944Gs and ended at L+19 seconds with 23kN pushing the rocket up to 4.97Gs, now traveling at a velocity of 426.3m/s. The second stage impacted only 847m east of the launch site after having its fins shredded as well, falling back nearly along its ascent trajectory as its spin kept it upright, engine pointing back towards the pad.

The third stage still picked up a very, very minor precession after separation but it was small enough that flight controllers were still able to use the pitch readout in order to ignite the LF/O engine after roughly 1.5° of change was seen at L+29 seconds. Starting off at 50% thrust, 5.7kN generated 0.475Gs. By pure chance, the rocket happened to be passing through MaxQ at the time the 3rd stage was ignited. After confirming dynamic pressure was falling, the engine was gradually throttled up to 100% by L+51 seconds. It expired from fuel exhaustion at L+1m09s while outputting 12kN and pushing the rocket up to 5.908Gs and a record velocity of 1.293km/s. At this point the rocket was 32.244km ASL. By the time it reached 70km at L+1m44s it was traveling 1.108km/s with a final apokee of 134.534km. Passing through apokee at L+4m07s it began to fall back to Kerbin. Because the third stage fins were shredded at L+1m39s the rocket remained pointing engine-first when it re-entered the atmosphere at L+6m30s, which allowed the most heat-resistant part to take up the brunt of the friction while passing through the denser and denser air as it continued to fall, reaching a maximum speed of 1.277km/s at L+7m10s. Although the rocket lost contact with KSC at 70km when the Monolith activated, we calculated that at L+7m23s it would have fallen below the horizon and out of sight of KSC. Chutes were set to a pressure sensor and pre-deployed at 6km but the rocket was falling much faster than anticipated and damaged the chute on opening, causing the rocket to splash down at L+8m29s traveling still at 26m/s rather than the intended 6-10m/s. The impact was taken up by the engine and the empty fuel tanks provided a crush core that also absorbed the majority of the energy, saving the payload. Thankfully the fairings remained intact and not enough water leaked in to sink the rocket before the recovery ship was able to reach it, 135km ENE downrange from KSC.

Re-entry was the real test of this mission – we had no idea if the rocket would behave as expected but it all went exactly as planned and it survived even after striking the water faster than intended. We are also very happy that the command was sent to shred the fins before the rocket reached 70km and all contact was lost, otherwise the re-entry would have played out very differently as the fins would have tipped the rocket nose-first into the atmosphere. The decision to shred prior to leaving the atmosphere was already part of the flight plan, done so the debris would not follow the rocket all the way to apokee and potentially get in its way during re-entry. In regards to the recovery, the next mission will have its chute deployment altitude lowered to 4km to allow some additional time for the air to slow the rocket. We still surpassed 40kPA of dynamic pressure during this ascent, reaching a maximum of 73.6kPa during the first stage boost, but we will continue to lower the thrust in the next two launches to see if we can move slower through the thicker lower atmosphere without sacrificing the stability of the rocket. 135km was a great first leap, but we are looking to see just how high ~2km/s of Δv will get us