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Jan 22 2021

Kerbin II Mission Analysis

In the months following our first orbital mission, which came very close to failing, lots of work was done on the Ascension Mk2 to make it more capable of reaching orbit. The result was a success on our second attempt, which placed the Kerbin II satellite into a stable and nearly circular orbit 191x228km@26° above Kerbin. Now that the hardest part of the mission was done, it was time to begin our first long-term operation of a spacecraft on orbit.

The Mission

The main goals of the mission in addition to telecom testing was just to see how the spacecraft fared over several weeks in space, in regards to things such as wear on its equipment to the stability of its orbit. It was equipped with two main antennas that each by themselves could get a strong signal to the ground for the transmission of science & telecom data and a tertiary backup antenna which could get enough of a signal to send & receive commands. This meant that potential loss of the satellite due to communications issues was unlikely. The probe core was constructed better than that of Kerbin I so although it remained a single point of failure it was at least robust. Whether the science instruments would last several weeks of space radiation exposure was also a question. Finally, scientists were still unsure if the 70km boundary to the atmosphere was “hard” or “fuzzy” – could an orbit remain stable or was there enough drag to gradually bring a spacecraft back down into the more well-defined atmosphere?

The mission was conducted over a period of 12 weeks and has been well documented in Ops Summaries and the complete mission timeline.

Mission Analysis

Orbital Stability

We are going to cover this mission aspect first as it was absolutely vital in allowing the spacecraft to remain on orbit for as long as it did. The amount of cold gas it had to expend was directly related to how stable its orbit was. If there was any drag from the exosphere bringing it down it may have eventually needed to perform a boost maneuver with its cold gas engine to keep it from re-entering prematurely. As it turned out, the only time the cold gas engine was needed was to perform the planned de-orbit maneuver.

What is the exosphere? It’s where the density of gas diffuses so much that although the particles are still bound to Kerbin’s gravity they no longer collide with each other. They can still collide with spacecraft however and thus produce drag to slow it down slightly. The exact nature of this portion of the atmosphere, largely theorized, is still a complete mystery to us.

While its orbit did not remain precisely the same throughout the mission due to perturbations mainly from maneuvering with its RCS thrusters to take photos and remain properly oriented for thermal control, no major overall change was noted. It first entered orbit at 190.804×227.936km and was de-orbited from 191.181×227.691km. Its SMA was as low as 809.1km and as high as 809.4km but fluctuated up and down, it did not exhibit a downward trend.

Spacecraft Health

Overall the spacecraft remained healthy and perfectly operational throughout its mission. The primary comm antenna remained active and functional as well as all the science instruments. No component failures were recorded and only two minor faults occurred during the mission.

The first incident was triggered when a bad command was sent up to the probe (you can learn more about how we command spacecraft in this twitter thread). The flight computer crashed into safe mode and as designed it was able to be successfully recovered by the operations team on the ground. A new boot file was uploaded with a fix for the issue that brought on the event and no further troubles were encountered.

The second incident was due to an EC overdraw during peak science operations that caused the power regulating routine to shut down some of the instruments so that the emergency batteries would not be activated. The problem was found and fixed with only a few days of delay in science data collection, after which continuous collection proceeded with no further interruptions.

Science Operations

In total the mission returned 4,409.0756MB of data from all its instruments. The largest data set came from measurements of the kerbolar wind, which showed the most variance during the spacecraft’s time on orbit. The science data analysis from the mission remains ongoing and several papers are expected to be published over the coming months in various journals.

The only real challenge to science observations during the mission was that they were a secondary objective. The main goal was to prepare for deploying communications satellites based off this spacecraft, the science instruments were just ride sharing to make the most of the mission. So the main antenna was mostly used for telecom testing and did not leave much bandwidth available, initially, for science data transmission. Having to balance these needs meant that not as much data was collected as there could have been if the science objectives were primary.

Telecom Operations

The primary goal of the mission was a resounding success, with conglomerate CommStar proving that their system was ready for service. After an initial period of signals testing the first satellite radio call was made between our Operations Director Drew Kerman from KSC to the Presider above ground at Sheltered Rock. Before this only science or telemetry data was transmitted from a spacecraft to the ground and picked up at a ground station before being sent along cables to a receiver. Here a signal was being sent directly to the spacecraft from one receiver to another.

The ability to communicate via satellite was tested on a KerBalloon expedition out to the far reaches of the Badlands. Not only could the expedition contact their home base via satellite, but they could stay in touch with various ground teams spread throughout the area. This provided a whole new level of expedition support and allowed for a much faster response to be mounted in the event of any emergencies, allowing for less supplies needed to be packed out to such a remote location.

Spacecraft Recovery

The cold gas engine proved the worth of its simplicity by activating with no hassle to de-orbit the spacecraft after spending the entire duration of the mission unused. Thanks to good fuel management in only re-orienting to take a photo once a week, enough fuel remained for us to plan a trajectory that would place it over the grasslands near the Kongo river basin. This was the largest expanse of relatively flat terrain anywhere along its orbit to give us the best chance of coming down over land if the spacecraft flew short or long on re-entry.

While the spacecraft ended up coming down well short of where we had hoped it would, MSV Aldeny was already deployed and ready to undertake the recovery just in case. Unfortunately while it recovered the probe core intact after it parachuted into the water the RTG was missing. During re-entry the RTG, the heaviest part on the craft, remained deployed so it could act as a fin – if the probe core was unstable during re-entry and the reaction wheels could not hold it steady, the RTG would swing out into the air stream and be dragged back behind to keep things stable.

It would seem however based on analysis of what remains of its supports that the heat cavity behind the spacecraft was smaller than anticipated and subjected the RTG to extreme heating that weakened its supports and eventually tore it away. We initially estimated when this occurred based on how long the probe lasted on backup battery power and sent the Aldeny out to search for it via the sonar ping locator that would have activated after it hit the water. The first attempt turned up nothing however and the second search attempt is still ongoing as of the time of this writing. We assume now the RTG did not come off the same time power was disrupted – the wiring leading to the spacecraft likely burned away first to cut the power and the RTG separated later during the descent.

We have no doubt the RTG will eventually be located and that it will be intact – we have already taken the effort over several missions to ensure that the casing can survive an unassisted re-entry and impact even over land.

Despite the loss of the RTG the probe core at least survived re-entry as planned which has allowed us to examine it closely to see how re-entry affected it. This adds data to our plans to re-use spacecraft components and also allows us to properly decide how much ablative material should be placed on a heat shield, which can save mass.

Future Plans

Seeing no sign of any orbital decay is potentially great news for future missions, which could either carry less fuel or plan to do more with it. We say “potentially” because this was still only our first mission lasting longer than a few days and we can’t say for sure if a more noticeable effect will become apparent after several months or years. In addition the drag may only affect craft in noticeable time spans of months or years if they are larger than a certain size. We also recognize that the space environment is a dynamic one, and it could just be that conditions (what ones and how many we are still not sure) were very good during this mission to keep the amount of excess drag to a minimum.

Lack of failures has given us greater confidence in the quality of the components we are sending up into space. However, while 3 months was a significant amount of time compared to any previous missions it’s still not anywhere near the length of time some probes will reside in such a harsh environment. We have to manage our expectations accordingly and still plan as much redundancy as possible into spacecraft that we plan to remain in operation for years.

This mission was the first real orbital operations test for the Automated Flight Control System and led to several improvements that will only make it more capable for tackling future objectives.

CommStar has confirmed that no major changes will be needed for the satellites it plans to deploy later this year with the Ascension Mk3. We will be finalizing the design in the coming months and plan to begin launches in the middle of this year if no problems arise with the Mk3.

When it comes to satellite recovery, our confidence took a big hit when we failed to come even remotely close to where we intended to land. Kerbin I at least came down within a respectable error margin of its intended landing area. Thankfully a closer look at how we went about planning the return trajectory exposed the flaws in our design and retooling things properly showed that we can be within a few km when it is done right.