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Apr 21 2020

Kerbin I Mission Analysis

Originally planned to launch aboard an Ascension Mk1 in 2019, when that rocket was deemed unable to make orbit the payload was delayed and eventually committed to the Ascension Mk2, which was not able to fly until early 2020. The purpose of our first orbital satellite was to place our mission control team in charge of actual orbital operations, test the new Archtagon Aerospace RB-8 cold gas engine, test the new Probodobodyne OKTO common probe core and perform one final re-entry/impact test of the radioisotope thermoelectric generator (RTG). The probe carried a massive bank of batteries because the RTG remained non-functional, allowing for the probe to stay up in orbit for several days with the use of hibernation to save power. The mission got off to an uncertain start when the Viklun upper stage placed the satellite into an unstable orbit, leaving the team on the ground with the challenge of recovering the situation to still carry out the previously-stated mission objectives.

The Mission

Deployment to begin the mission was carried out once the Viklun stage came back into communications range of the ground via its more powerful 1.5Mm antenna. Having run out of power about an hour prior to this, it had been leeching electrical charge from Kerbin I’s battery banks to keep itself alive. Although Kerbin I only carried a 500km antenna, deployment was planned for now because afterwards Kerbin I could go into hibernation, conserving energy over the time it would have spent coming within its own comms range. The Viklun stage had enough residual power after separation it could continue to relay signal while Kerbin I transitioned into hibernation, after which the Viklun became derelict. How it fared after this can be read about in the Ascension Mk2 Flight 1 analysis.

Kerbin I was now flying free and able to begin to attempt the completion of its mission objectives, the full story of which can be reviewed in this tweet collection.

Mission Analysis

Areobrake Trajectory Modification

This was an unintended benefit of the unstable orbit the probe initially found itself in, which ironically was also our original plan for the first orbital attempts. Being able to watch the probe’s orbit decay with each passage through the upper reaches of the atmosphere allowed us to collect valuable data and also learn how to interpret and use that data. This not only allowed us to plan ahead for inserting the probe into a stable orbit but gives us experience that can be used for future interplanetary missions to other bodies with atmospheres.

One surprise from the aerobraking was just how much the thin air near space can affect the spacecraft. Dipping down less than 10km into the atmosphere with a perikee of 64km allowed the air to push the spacecraft around a lot when the reaction control system (RCS) was not used to hold orientation. Tumbling around like that varies the drag profile and makes trajectory predictions post-areobrake less accurate.

Thanks to the data collected during the mission we were able to make a strong correlation to help us predict the outcomes of aerobrakes where the space craft was not holding a specific attitude. Applying this to the orbital decay of the Viklun stage (as described in the flight analysis linked earlier) showed that it is not effectual over long periods of time, but in the short-term it did allow enough accuracy to tell us where Kerbin I would be after 2-3 passes.

KSO Re-Entry & Impact

Another unintended benefit of the unplanned orbit gave us the opportunity to test the RTG casing in a more extreme environment by dropping it from the altitude of keosynchronous orbit, which is the highest altitude the casing was designed to withstand. Nominally, decommissioned satellites would be brought down into a lower orbit before re-entry but in emergency cases where not enough fuel remained or a speedy return was required, a direct descent would be carried out. This test was not planned to occur until later this year or next because no satellites were planned for KSO operations anytime soon (this remains the case, however the opportunity was too good to pass up as no extra fuel was needed to reach the higher altitude than planned for the mission thanks to aerobraking).

Although we did our best to predict where the probe would come down, we should have anticipated that it would maintain a fairly streamlined profile due to aerodynamic forces and thus over-estimated the drag coefficient in our planning. The actual landing site was ~130km long but we targeted the middle of the largest landmass on the planet to make sure we still struck the ground and not water. The impact site was ~85km northeast of the Kongo River research station.

Post-impact analysis of all the debris recovered at the site has shown that the probe came down completely intact – bits from every piece on the probe was found, and although the radial fuel tanks were made of the same stuff, their mounts were still all found bolted to the remains of the central fuel tanks. The entire battery bank was once fused chunk of melted metal compounds. Coming down engine-first helped absorb as much of the impact as possible before the RTG slammed through the probe and into the ground, it was dug out last.

The casing of the RTG was cracked and warped mainly from the heat and stress of impact however the simulated radioactive pellets inside were all intact. Had this been a functioning RTG there would have been no spillage of radioactive materials to contaminate the ground. The design has now been certified for space flight so that future probes can stay on orbit for even longer periods without the huge extra mass of batteries.

Probe Core Fault Tolerance

There were two instances during the mission that forced the kOS core into safe mode. This is a protective measure which suspends all operations and places the probe into a configuration that allows it to send/receive data but not process anything on its own until it is instructed to do so. It is essentially “paused” waiting for new instructions from mission control. It is triggered when the spacecraft is unable to properly execute instructions or enters into a process that does not ever finish executing, rendering it unresponsive. Obviously we can’t send someone out to switch it off and back on again so it has to know how to safeguard itself against lock-ups.

The first instance was due to a sensor error during an atmospheric pass, which fed corrupted data into the logging system. After returning the space craft to normal operations tests were done with the sensor and the software was updated to prevent out of bounds data from attempting to be logged. No future issues occurred during aerobraking.

The second instance was due to an operational oversight in how we expected the probe to behave. Every time it connects with KSC, one of the things it automatically does is re-upload certain core files from our archives, which allows it to update any changes that may have been made. The maneuver code made use of a recent function addition to one of these core files, but the changes never made it onto the probe as expected. When the probe awoke from hibernation and accessed the “wake file” it was instructed to run after boot, the code for the maneuver failed to find the function it was looking for and caused an error. The onboard fault protection attempted to reboot to clear the issue but the error occurred before the wake file was purged so every time it would boot up it would crash, eventually triggering safe mode.

The reason the file was never updated was because the act of updating the files only occurs if the probe has a connection to KSC at the same time it is undergoing the boot up process. Coming out of hibernation boots up the probe core and then activates the antenna. Activating the antenna prior to boot when coming out of hibernation is not an automatic process because the probe will not always wake up within range of KSC and it is better in these instances to leave the antenna off to save power. So it must be instructed by the wake file to turn on if needed. So the conditions for a file update were never met during the most recent pass. This could have been caught if controllers had thought to check the size of the file onboard the probe against the one here at KSC, but the AFCS is only able to send down a list of file names in the probe’s storage, not their sizes.

Reaction Control & Stability Assist Systems

The cold gas thrusters making up the RCS that each produce 0.02kN of force were adequate in allowing the probe to change its orientation as needed and maintain proper pointing, which was vital for carrying out the several maneuvers required over the course of the mission using the cold gas engine, which also performed as expected. The SAS made use of reaction wheels inside the common OKTO probe core to allow the space craft to maintain a given orientation at the expense of some electrical charge rather than propellant via the RCS.

One problem that was noted in using these two systems is that they did not function independently as designed. While the RCS was active in making large changes to the space craft’s orientation the SAS was also using its reaction wheels to enact finer control over the entire maneuver. Apparently some memos did not make it through the design process properly as SAS is only supposed to kick in after RCS has done the heavy lifting. Fortunately enough excess battery power meant that the extra use of the SAS did not have a detrimental impact on the mission.

Radiation Damage

An unfortunate loss in being unable to recover the probe intact was losing the ability to take a close look at the circuitry in the probe core after it had made several dozen passes through the outer radiation belt. While we have only briefly explored this region, we understand the effects space radiation can have on unshielded electronics. Because this mission was never meant to go anywhere near the belts, to save mass there was no shielding over the onboard electronics beyond what was needed to protect it from the general radiation environment found in space. While the main systems remained functioning, there were glitches and data errors noted over the course of the mission that were likely due to increasing damage from the radiation exposure.

What was fortunate for the mission though is that the inclined orbit took the space craft both above and below the torus of the inner belt, whose much higher radiation levels would have done a much faster job of frying the onboard electronics.

Thermal Considerations

Space is cold, but the energy from the sun is greater. Without air, heat transfer is not efficient. Placing an object in direct sunlight in space therefore means that object will continue to absorb energy and heat up if that energy is not dissipated. It’s even worse when the object is also generating its own heat thanks to batteries and internal electronics. Kerbin I was designed for a circular orbit that would spend almost equal time in sunlight and shadow, allowing for the probe to cool off. The orbit it ended up in was highly elliptical with perikee over the night side so it spent very little time in darkness as opposed to sunlight. While the RTG was non-functional and not producing heat, it was still designed to radiate heat, which helped to keep the space craft cool in addition to controllers putting it into a slow roll so all sides received light and shade. The aerobrakes also helped shed heat as the air friction at that high altitude was not enough to generate more heat than the probe carried.

Had thermal conditions worsened, the mission control team was ready to expend additional cold gas to orient the probe so that the engine faced the sun and the RTG and its radiator fins were in shade. This would have been done at various periods, not just continuously, because then the heat would have damaged the engine, which was still needed for orbital maneuvers.

Future Plans

Probes that plan to perform aerobraking will need to have stronger RCS thrusters or an aerodynamic shape to ensure that can maintain proper attitude while skimming through the atmosphere.

The AFCS has already received upgrades to better handle file checking and onboard software updates. Its fault-protection system is now more robust than ever thanks to the experiences from this mission.

Increased redundancy and radiation protection will go into future missions. Kerbin I only had a single antenna – if anything had happened to it that would have been the end of the mission. Expecting benign conditions led the design team to not bother with a backup system and it was a foolish oversight.

The RC/SA systems will be redesigned to operate as originally designed and ensure that neither propellant or electrical charge is unnecessarily wasted while orienting the space craft.