Nov 19 2019

Progeny Mk6 Block I Flight 12 Analysis

Announced back in August, the Progeny Mk6 Block I was chosen to carry student-built experiments up into space two at a time. This will be the first time we are collaborating so directly with the various higher educational institutions around Kerbin and participation levels indicate the next generation of kerbs are eager to make their mark in space exploration! This mission was delayed from its original Nov 5th launch date due to the deadly attack on KSC last month, then further delayed due to hazardous weather in the recovery zone. Set up in the North Field due to damages done to the launch pad, the launch site allowed for condensed launch procedures thanks to being far enough away the rocket could remain fueled after the wet dress rehearsal on the day prior to lift off.

The Flight

The rocket left the launch base at precisely 12:05:00.08 local time under command of the AFCS after a smooth preflight with no holds. Ascent was nominal and followed the standard profile through the first stage burn, second stage coast and second stage burn. Upon completion of the second stage burn (MECO-2) at L+52s the booster was discarded a second later and the third stage engine ignited a second after that. During that time a pitch change was registered that saw the rocket’s nose drop from 61° to 58° in less than half a second – a relatively large movement.

The rocket continued up towards space while oscillating between 2-3° of pitch during the third stage burn, which ended at L+1m25s. The drop in pitch may have been minor overall but it still contributed to an exceptionally low apokee of barely 300km, much lower than planned for even with a heavier combined payload mass than past flights. Thankfully while the time spent in space would be shorter than expected, high altitude was not a requirement of either the onboard experiments and the mission could still be carried out and called a success if the re-entry went well.

The rocket fell back to the atmosphere at L+12m56s and despite the precession caused by the second stage decoupling it remained “upright” and began to encounter the rarefied air with its engine pointing along the direction of travel to take the brunt of re-entry heat. Due to the shallow ascent angle the rocket traveled much further downrange than anticipated and contact was lost shortly afterwards at L+13m13s, possibly at that point by heated plasma but eventually the horizon.

It took ~4 hours for the recovery ship MSV Aldeny to sail to the new splashdown location and in the past we’ve lost payloads that have been left on the ocean surface for too long. Thankfully as in a previous case of flying too far downrange the payload landed close to shore and washed up on the beach so it wasn’t carried away by currents or sea creatures.

Telemetry Data

Flight Analysis

Improper Launch Orientation

An unforeseen consequence of leaving the rocket fueled and sitting out on the North Field through a day of launch readiness and a day of launch delay was that the launch base ended up sinking into the soft ground of the launch site, listing slightly forward and to the side. It was not visually noticeable by the ground crew and the launch crew failed to also take notice of the fact that prior to launch it was pitched at 83.35° instead of 85° and pointing 85.85° ENE instead of 90° E. Flight Director Lanalye has admitted to being embarrassed by this oversight. While we have launched before from this location the rocket was not left fueled on the launch base and also the launch base itself was the original lighter variant before being beefed up to handle the larger Block II.

Third Stage Pitch Bump

Review of tracking footage shows that the second stage booster did not fall away cleanly at separation and instead appeared to remain close to the third stage prior to ignition. The angle of the ground footage from almost directly behind the rocket, which is still pointing up at nearly 60°, makes it hard to see if the second stage bumps into the third stage but that is the most likely explanation. Examination of the recovered third stage hasn’t provided conclusive evidence due to the damage the engine also receives from re-entry, but it does appear to have been struck lightly.

Looking back at all previous Mk6-I ascent telemetry has revealed that the second stage booster has never fallen away at an angle less than 66° and this flight it detached at 61° due to the already shallow ascent angle from the tilted launch base. This could have been enough of a lateral release to negate some of the decoupler force and not push the booster far enough away to escape the slipstream of the rocket.

Re-Entry Tumble

Further analysis of telemetry data shows that upon re-entry the rocket took an uncharacteristic tumble as it passed through 30km. Thankfully this occurred just after Max Q and despite rotating lateral to the airstream several times the rocket held together under forces as high as 15.8G as airbrakes were deployed. At this time it’s uncertain whether the unstable re-entry was a direct result of the precession it picked up from the second stage impact or also had something to do with the payload placement causing an unbalanced center of mass.

Had the rocket been returning from the expected height of +400km it’s likely this tumble could have ripped the engine and fuel tanks off, carrying with them the air brakes, and the payload would not have had the drag to slow to a safe landing speed even after chute deployment.

Future Plans

There’s not enough time between flight analysis and launch of the next rocket to make any major changes, so it will fly constructed similar to this one. Measures will be taken to ensure that the launch base remains level and if it is not that it will be picked up on during preflight. If the second stage impact and re-entry tumble repeat themselves on the next mission then we will have time before the next two launches to attempt corrective measures with the rocket itself.

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