May 11 2018

Progeny Mk6 Block I Flight 4 Analysis

The final launch in our latest campaign has been to space and back. This ends our initial exploration of the hazardous radiation zone discovered above the planet, a report on which will be published separately after more data collection and analysis from future Progeny launches. We’ll cover that at the end as usual, for now let’s take a look at this recent flight in detail.

The Flight

Once again the only major change to the ascent profile of the rocket was to launch even further to the south, lifting off from the launch base at precisely 09:02:00.06 local time to head 150° SSE after a delay due to low cloud cover. 67.2kN of thrust from the lower 0.625m booster pushed the rocket at 4Gs and climbed to 68.9kN before beginning to taper off after just 3 seconds to ensure the rocket did not speed through MaxQ too fast, topping out with a dynamic pressure of 77.104kPa passing through 5km traveling at 550m/s at L+18s. Staging occurred at L+35s, 14.9km ASL, after the lower booster expired and was pushed away by the decoupler, its fins shredding via det-cord 1 second later to spoil its lift and send it crashing downrange into the Kerblantic .

Coasting for 7.62 seconds before the nose fell 1.5°, the AFCS kicked off the second stage booster at 18.5km to boost at 14.7kN with a TWR of 3.1. Trajectory remained stable throughout the burn until the booster expired at L+54s, when the stage was dumped and the third booster ignited one second later at full thrust to continue the push for space.

BECO-3 occurred just 56ms before passing through 70km ASL and entering into space, leaving the atmosphere at a speed of 2.053km/s and rising to an apokee of 476.182km. This lower apokee was expected due to the greater inclination of the resulting trajectory (57° vs. 43° the last flight) and using less of Kerbin’s rotational speed to make the trip up to space easier. All systems ran nominally until between 10 and 11 minutes after launch the AFCS encountered a write error in the internal data drive and crashed. This left only ~7 minutes for the team to reboot the computer and upload new instructions just so it could handle the recovery phase of the mission. The new code was uploaded and accepted 3 minutes prior to comms being lost as the rocket approached the atmosphere.

Thankfully the code executed perfectly while the rocket was out of touch with KSC, although MSV Tongjess was in range to send manual commands to the vehicle if the system had crashed again. Splashdown occurred at L+19m29s 347.137km downrange, a new recovery record. Rough seas in the recovery area almost did not allow the crew of the Tongjess to pull the payload out of the water before it lost buoyancy and began to sink, but their skill allowed them to make the save without placing themselves in any serious danger.

Full flight telemetry data. (minus missing data from the crashed period)

Flight Analysis

AFCS Crash

Looking at the data drive for the rocket after recovery has turned up a few small bad sectors that were likely the cause of radiation damage or stray cosmic rays. The logging program happened to attempt to write to a bad sector and was not able to handle the resulting error. We’ll be re-evaluating our hardware/software interface so that a problem like this in the future can be handled more gracefully. There is also talk about moving away from a radiation-hardened system to a radiation-tolerant system. The difference is that instead of adding shielding to try and protect the delicate electronics we make the electronics themselves more capable of handling any damage that might affect their calculations. The best way to do this is have more than one CPU working so that results can be cross-checked for accuracy in addition to providing basic redundancy. We don’t have the ability to cram two CPUs into the Progeny rockets but the team have passed along the suggestion to the Ascension program software engineers.

Overall Trajectory View

Above is a look at the telemetry data recorded for all 5 of the recent launches, with the launch azimuth increasing 15° with each successive flight. It’s worth noting that there is a loss of data at the end of the first two trajectories, but they are close enough to the final resting place of the rocket at this scale. How does a ballistic rocket that can’t steer fly a curving trajectory? You must remember to take into account the projection of the map and the rotation of the planet during the flight. The increasing length of the tracks directly correlates to the decreasing apokee, making for a flatter and longer sub-orbital trajectory. Launching at increasing inclination reduces the ability to use the rotation of Kerbin to help speed along the rocket during its trip to orbit. How much? The last two launches let us know.

Effects of Increased Launch Inclination

The final two MK6 Block-I launches were the exact same mass on launch, using the exact same ascent and booster thrust profiles, allowing us to see the effects of change to only one ascent parameter: inclination. The difference in orbital velocity from the third flight to the fourth flight at first stage separation was 10.5m/s and the rockets ended their final burn with a difference of 14.2m/s. This equated to a difference of ~7km at apokee – we can’t be exact on this figure because the lower trajectory of the fourth flight kept it in the atmosphere a bit longer after engine cutoff to allow drag to lower the apokee more than the third flight. It would be an interesting experiment to fly two identical Ascension Mk1 rockets on trajectories varying between 15° and see if a similar difference shows up in the data.

Another data point that initially surprised engineers was the fourth flight having a higher MaxQ than the third flight, but this quickly made sense when you consider the fourth rocket was traveling lower through the atmosphere and thus through thicker air, even if it was moving slower than the third flight. Luckily the increase wasn’t enough to cause any damage to the rocket, but is something to keep in mind for future inclined flights.

Future Plans

This flight completely missed passing through the radiation field, recording no increase in levels beyond the 0.010rad/hr normally found above the atmosphere. This tells us that not only is this area of space free of harmful radiation, but previous launches that occurred underneath the radiation region were not “shielded” by it in any way. In other words, we should expect 0.010rad/hr of radiation exposure anywhere else in orbit around the planet outside the hazardous zone and still within Kerbin’s protective magnetic field.

Our best bet for figuring out the shape of the radiation region is to launch another Mk6 Block-I with the same mass on a northward azimuth of 45°, which would place it on an opposite inclined trajectory to the third flight of this recent launch campaign (cyan color in the above figure). We choose this trajectory because we know there is radiation there when launching to the south. If we were to launch opposite this flight and also not hit any radiation, we would only be able to assume that the field extends equally in both directions. However if we fly opposite the third launch and encounter no radiation or different levels of radiation we can confirm that the field is lopsided north and south of the equator, where we currently believe the field is thickest. Otherwise if we get nearly identical readings we can confirm the field is uniform. The rocket should be ready to launch on 5/29 @ 13:49 UTC.

We have also placed a parts order for two Mk6 Block II rockets (we had parts for one ready but they have been gradually used up for Block I launches) and plan to send them both up within days of each other next month between launches of the two Ascension Mk1 test vehicles. Building two just allows us to have one for backup in case something goes wrong with the first flight – despite the core of the rocket being a solid well-used design we have never dealt with radial-attached boosters before. Hopefully the first flight is successful in clearing all the way through the hazardous radiation zone over the equator and allows us to try something different with the second flight, like launching inclined to see whether the radiation field extends just as far. Right now these launches are scheduled for NET 6/19.