Dec 20 2017

Progeny Mk5 Block I Flight 3 Analysis

Progenitor team members have spent the last few days reviewing telemetry data from the third launch of the Mk5 Block I in order to determine the overall performance of the rocket and what happened to cause two anomalies that occurred during the ascent into space. Both have been tracked down to their root cause and we will cover them after review of the flight.

The Flight

No issues preceded the launch, with perfect weather deciding to show up for launch day although we had plans in place to push into the weekend if necessary to get the rocket launched before the end of the operational year. The lower 0.625m solid rocket booser lit off at 12:30:00.03 local time to push the rocket away from the launch base at 4Gs with 146.142kN of thrust to establish the rocket in its initial climb and not allow lift at the nose to flip it vertical or over backwards. Launching from 85° It stood up to 86.8° before beginning to pitch back over towards the east and away from KSC. After 5 seconds the new lower stage began to reduce its base-level thrust output although given the rocket was climbing into decreasing air pressure the overall thrust continued to increase. 10 seconds after lift off it had reached Mach 1 and 8 seconds later dynamic pressure began to fall as it passed through a Max Q of 78.373kPa, within our planned range of 60-80kPa. It burned out after 33.71s with a thrust of 172.596kN, pushing the rocket to a velocity of 653.520m/s at an altitude of 14.009km.

After a clean stage separation with the assistance of a slightly more powerful decoupler the rocket coasted for 15 seconds before its pitch fell 1.5° and the second stage booster was triggered at T+48s, producing an initial thrust of 9.979kN at 23.098km for a TWR of 2.21. Its velocity had fallen to 505.213m/s but the second stage pushed it up to 771.382m/s by the time it burned out 19.46s later with a thrust of 9.997kN. Video footage shows staging happened simultaneously with the booster’s fins being shredded, possibly having an adverse affect on the coast of the third stage, which began to pick up a growing precession that after 2 seconds violated the 1° AoA constraint and caused the AFCS to return ignition control to the launch team on the ground.

A quick consultation among controllers brought about the decision to wait until near space to fire off the third stage to see how well it recovers under thrust. 70s of coasting later at T+2:21 and 68.177km ASL the third booster was ignited, throttle set to produce an initial TWR of 2. Because space was so close and the atmosphere already basically non-existent the AFCS began to throttle up to full power only a second later – however the throttle stopped increasing once it hit 76%. After burning for 42.7s the booster flamed out at 97.055km with a velocity of 1.378km/s and an apokee of 252.137km.

The coast through space was without issue, battery levels remained nominal and the rocket was pitched as much as 50° off its velocity vector but nowhere near as bad as the last launch – it would still re-enter the atmosphere predominantly engine-first.

Re-entry occurred at T+11:48, with signal to the rocket lost shortly afterwards at T+12:07 due to the heat of re-entry. By this time it was 144.291km downrange and so quickly fell below the horizon, not allowing us to regain a connection beforehand. Chute deployment occurred at T+12:41 when the rocket passed through 2.5km ASL traveling at 409.970m/s which was just barely still supersonic at the time. Thankfully the chute stayed attached and fully deployed just 170.765m above the water at T+13:02 with splashdown occurring shortly afterwards at T+13:23 154.898km downrange with 46% power remaining. The location was roughly 41km from where our recovery ship was stationed, which arrived at the location nearly an hour later to find the rocket still floating on the surface.

Anomaly Analysis

Stuck Throttle

Review of telemetry data clearly showed that the throttle for the third stage became “stuck” as soon as the rocket reached space. This caused the outside pressure to reach and maintain a value of 0 for the remainder of the burn. Looking at the code that controls the engine throttle-up we see that if pressure were to equal 0 and maintain 0 then the lack of difference would cause no throttle action to occur. The team will be correcting this so that throttle-up can continue to occur if the rocket exits the atmosphere.

Second Stage Decoupling

This issue took a bit more effort to track down given that the booster is under several kilometers of water and not easily retrievable for analysis. However we always knew this would be the case in the event of problems which is why the VAB keeps detailed records of how things are assembled. Digging into the paperwork revealed all the proper steps were taken when putting together the second stage booster, which means a control fault must have occurred. Several tests were carried out with the small control unit that commands the fins to explode 1 second after decoupling and it was found that sub-par solder was being used and prone to failure. We’ll be cutting ties with that manufacturer and seeking a new one.

Additional Concerns

The two main issues with this flight were thankfully not major problems in the end, but there were some additional minor things that occurred which engineers are looking to address in the next launch.

Spin Rates

We changed up the fin configuration on this rocket to only pitch the lower-booster fins and attach the second stage fins straight on. This got the rocket up to about 163RPM by the time the first booster quit, which was less than the roughly 225RPM spin rate of the previous Mk5 Block I flight. The plan moving forward is to decrease the spin rate to around 100RPM to see how that affects overall stability. The first stage booster fins are currently angled at 1.5° and will be reduced to 1° for the next launch.

Angle of Attack Deviation

After the initial nose-up at launch, under thrust from the first stage booster the rocket stayed under 0.1° AoA for the majority of the burn. During the coast phase the AoA slowly increased and almost violated the 0.5° AoA constraint that would have returned ignition control to the launch team before the second stage booster was fired off by the AFCS. During the boost AoA continued to gradually increase and shot upwards after stage separation, although we don’t attribute this entirely to the anomaly of the fins being shredded at the same time.

By comparison, the previous flight of the Mk5 Block I, which suffered a much more serious staging anomaly that imparted a significant wobble to the rocket’s ascent, showed a decrease in AoA under thrust of the second stage before coasting brought it back up.

The second stage booster for the previous flight was set to produce nearly 2x as much thrust as the one for this flight, so we will be looking to increase the TWR on ignition up from 2 to 3.5 to help re-stabilize the rocket after its coast period. We also know now that second stage booster ignition will occur higher than the 15km we originally thought it would.

The long coast period of the third stage on this flight was done so we could see what affect the boost would have on stabilizing the rocket and the results are very conclusive:

The rocket was well off its prograde vector but the boost brought it nearly back on track (and recall this was only at 76% throttle). It’s notable that the effects were somewhat reduced when the rocket left the atmosphere. These results play directly into our next concern.

Lack of Third Stage Pitch-Over

Had the AoA constraint not been violated during the third stage coast the ascent profile would have been nearly the same as what occurred anyways because the rocket failed to continue to pitch over once the second stage decoupled, which means the AFCS would have waited for its backup ignition trigger of vertical velocity falling below 100m/s. This event would have occurred shortly after controllers manually ignited the third stage engine (vertical velocity at that point was 170m/s).

Looking through data from past Mk5 flights (all of them, not just the Block I) we can’t see any behavior similar to this, which all have the nose dropping through the third stage coast and boost. This leads us to the conclusion that the lack of fins is playing a role here. It’s most likely that the spin of the rocket allowed them to produce enough lift to overcome the low center of mass the rocket has when flying with just the third stage.

Yellow ball is the wet CoM, red ball is the dry CoM

Being bottom-heavy and having no lift down there anymore to compensate, the rocket really wants to stand upright as soon as the second stage is detached, leading to an increase in pitch as well as an increase in AoA thanks to the nose rising. To counter this, the 3rd stage will no longer coast but light off immediately after staging. The ability to throttle the engine will allow us to maintain a reasonable ascent rate through the upper atmosphere. For the next flight we will bump up the initial TWR from 2 to 3, keeping in mind that the lack of thrust on the second stage this launch did not help stabilize the rocket.

Progeny Kicking Off 2018

Since no major changes are needed for the rocket we will be ordering new parts and boosters this week for delivery at the start of next year, with a launch date no earlier than 1/15. Although we had hoped by now to be in a regular launch cadence the need to continue to “debug” the Mk5 Block I means we have to do at least one more “launch-and-see” before committing to a mass ordering of parts with a solid design in place. Engineers are hopeful however that all minor issues on this launch should be suitably solved with the next one. Because the Block II core is based on this design with the only additions being strap-on boosters and a larger LF/O capacity, its important we nail down the Block I before attempting a launch of the Block II, which we now expect to fly no earlier than February.