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Nov 19 2020

Progeny Mk7-B Flight 4 Analysis

With all three previous missions failing for different reasons, the fourth and final planned mission for the Mk7-B received new objectives in the hope that this mission would be successful at last. The first change was to swap out the new booster for one that had been refurbished from a previous flight, which necessitated a launch delay that pushed the date back from 10/20 to 10/29. This booster was from the second Mk7-B flight back in late August and would be the first time we attempt to re-fly an engine. On the opposite end of the rocket we swapped out the parachute nose cone for a payload fairing that encapsulated a new Luciole smallsat prototype. This would be deployed onto the sub-orbital trajectory to test its systems and reaction wheels. Recovery would be of only the first stage booster and RTG casing from the payload, which did not contain any radioactive material for this mission. Other than the delay for booster integration, no problems occurred in the lead-up to launch.

The Flight

After good retraction of the support arms at T-5s the Boostertron II solid rocket motor successfully ignited for an on-time launch at 13:45 local, pushing the rocket upwards off the pad with an initial force of 3.3Gs while the fins actuated to begin rolling the rocket from 90° towards 34°. Less than 2 seconds after launch it had spun enough to allow pitch-over to begin as well, with the flight computer guiding it along a gradual and constant change rate throughout the ascent. By L+10s the rocket had locked onto its heading and thrust from the SRB had already begun to taper off from its max 74kN to reduce loads on the rocket as it approached Mach 1.

Going supersonic at L+14s the rocket passed through Max Q 6s later with a pressure of 55.301kPa while traveling through 4.6km ASL. At this time, passing through Mach 1.3, fin flutter began to shake the rocket violently at first, but this quickly died down somewhat as thrust continued to decrease along with dynamic pressure and outside air pressure as the climb continued towards the upper atmosphere. Despite the shakiness, guidance remained nominal enough for controllers to call the rocket as “on course”.

L+46s and just entering the upper atmosphere past 18km the booster engine expended all solid propellants and expired. Guidance fins continued to pitch the nose over as the rocket coasted upwards, losing velocity but remaining stable and gradually losing the last effects of fin flutter. After shedding 149m/s during the coast up to 25km, the first stage was decoupled, the interstage fairing split into two halves to expose the Ospray vacuum engine which ignited to resume the push towards space with a thrust of 13.4kN or 2.7 TWR.

Thrust vector control allowed the rocket to continue its pitch-over and the next major step in the flight occurred at L+1m24s when the rocket’s apokee was pushed out of the atmosphere, guaranteeing our trip up into space. Thrust and guidance remained nominal through L+1m39s when the rocket passed through 60km and the payload fairings were split to fall away, exposing the smallsat to the thin air of the outer atmosphere. With no goal of restarting the engine it was allowed to burn itself out, with MECO arriving at L+1m54s, 66.8km, just 4s before the rocket blasted out of the atmosphere at 1.7km/s and into the void of space.

After taking a minute to confirm the rocket’s status and trajectory ground controllers began to activate the “Bot” smallsat for deployment. The RTG boom was extended along with the high-gain antenna and the onboard batteries were connected to proved independent power. The satellite was deployed at L+3m35s and since its reaction wheels were too small to affect the rocket, it continued on a slow pitch-over that allowed the booster camera to eventually see the probe over the payload adapter to confirm clean separation. At this point the Progeny Mk7-B had successfully completed its part of the mission.

With the probe now free of its ride to space, reaction wheels were activated and the Stability Assist System brought online to orient the probe prograde. Once this was achieved a series of commands was begun to work the Bot through all primary points of orientation – normal, anti-normal, radial-in, radial-out, retrograde. On average the probe took 1min to move between major orientation (radial-in to radial-out), with less time needed for the minor orientations (radial-out to normal). After it was pointing retrograde it was allowed to hold that orientation until re-entry so the reaction wheels could prove they can also maintain stability.

Both the Bot and the rocket passed through an apokee of 219.9km at L+6m27s, about halfway through the Bot’s pointing maneuvers. The rocket had plenty of remaining battery power so continued to transmit its telemetry, now primarily being received by the Sheltered Rock aerial. After 1m41s of holding retrograde orientation SAS was switched off on the Bot as the probe and rocket both encountered the atmosphere at L+10m57s. Neither was expected to survive the re-entry and indeed contact was lost with the Bot probe at L+11m18s and never regained.

The rocket itself went into plasma blackout at the same time but enough of it survived to regain contact through Sheltered Rock at L+11m41s and continue to transmit until near-impact when terrain occluded the signal at L+12m53s. This was a great help as it allowed for a better position for the recovery airship to begin searching for the RTG casing, which was found and recovered a few hours later. The impact site of the rocket was also located nearby although there was nothing worth recovering from it.

While the space mission was underway, after achieving an apokee of 33.8km the first stage booster deployed two good chutes on its way back down and was recovered 16km downrange from the bay north of KSC. This marks the first and only fully successful mission of the Mk7-B.

Flight Telemetry Data

Future Plans

With no major anomalies on this mission and no further flights planned for the Mk7-B, not much analysis needed to be done – at least not pertaining to this flight. Moving forward the data from this and previous Mk7-B missions will be integral in the planning and execution of Mk8 missions.

Our experience with Launch Vehicle Designer (LVD) continues to grow, and we will continue to use flight data from these missions to work with ArrowStar Tech to improve the software’s accuracy. We had planned for the rocket to come down over an expanse of grassland north of Sheltered Rock and Ockr for ease of recovery. The actual re-entry point was off our target but still over the area we wanted.

The move from fins to thrust vector control (TVC) should greatly aid in trajectory accuracy. The fins, although cheap and less complicated, are not a stable means of guidance at high speeds and their deflection adds significant drag to the rocket’s flight that is not possible to work into the LVD trajectory model.

An extensive search was performed of the area where the RTG casing and rocket impact site were found to attempt to locate any other debris from the Bot probe. None was found, which means the entire thing burned up on re-entry, as planned. This was done to prove smallsat disposal would not litter Kerbin’s surface with debris from orbital missions. The RTG casing itself was closely examined and found to have properly protected its simulated payload of radioactive pellets. Despite being the same design as the active RTG currently certified for use on probes like Kerbin II, the new casing form factor had to be checked again for design integrity.

The recovered booster will be refurbished once more and static fired later this month to ensure that another flight might have been possible. Although static fire tells us the engine will work again, it does not simulate the loads an actual flight also places on the booster casing. So if the static fire is successful we will know after two flights we can re-light and run the engine for a third one, but will it actually survive during the ascent? Even if this booster survives its static fire, it will be retired to the Rocket Garden at KSC so all visitors can gaze upon the first engine hardware to be re-used for flight.

While the Mk7-B is officially retired, its service life is not over. At some point after multiple re-use the Boosterton II is likely to fail due to over-stress of the casing. We can guess when this will be with careful examination of the casing itself, but we’d also like to know for sure. The Ascension Mk3 first launch will use 4 brand-new SRBs and after recovery they will be refurbished and one will be selected to re-fly to destruction. We will build several more Mk7-B rockets with a dummy 2nd stage to test the SRB on multiple flights until it fails.