Apr 07 2020

Ascension Mk1 Flight 12 Analysis

Captain Jebediah has waited nearly 5 months for his second chance to fly into space after his first attempt last year was foiled by religious extremists. Extensive training with his backup crew mate Specialist Bob prepared him to handle everything but the wait, which was increased further on his second attempt first by weather delays and then by a faulty engine on launch day. After successful ignition, launch failed due to low thrust issues that caused the engine to not reach a thrust to weight ratio (TWR) of 1.2 for lift off. Trying a second time led to the same result. Although the thrust loss was minimal and still would have been enough to push the rocket off the pad, downrange performance was called into question – it would have been a costly and embarrassing failure if the rocket had been unable to reach space due to thrust issues. Thankfully this additional mission delay was not as long as it could have been since we had another engine that was recently received & tested ready to swap in over the course of a week. After a static fire on the pad the day before launch to make double sure it was capable, Jeb was finally able to ride up into the starry blackness. His mission would be similar to the previous kerbed sub-orbital flights in allowing the gathering of medical data from the 0G environment as well as providing additional flight data on the performance of the capsule, heat shield and crew G tolerances. And as always – amazing pictures!

The Flight

With no issues during prelaunch operations the final countdown led to the terminal countdown led to the ignition of the main engine at 6 seconds prior to lift off. With chamber pressures and pumps nominal the AFCS throttled up the engine to 1.2 TWR (~73%) at 3 seconds prior to L-0. Confirming thrust was good, the engine clamp was released at precisely 12:15:00.76 local time to allow the rocket to begin its ascent. Jeb was obviously gleeful at finally getting off the ground, laughing maniacally as the rocket cleared the tower 3 seconds later, throttling up to nearly full power to begin holding a TWR of 1.5. He was quickly reminded by Bill over CapCom that he needed to start providing call outs from his instruments, which he complied with a few seconds later.

Jeb’s call outs became more important to controllers when the telemetry feed started cutting out every other second. He assured them that it was on their end and his instruments were all reading nominal. Pitching over downrange on a heading of 54° the rocket reached MaxQ passing through 7km ASL, recording 15.388kPa at L+57s shortly before going supersonic, breaching the sound barrier traveling at 300m/s. Minor “roll-shake” buffeted the rocket during the transonic portion of the ascent but the rocket held both course and pitch without issue. As fuel burn-off lightened the rocket and throttle continued to decrease to hold 1.5 TWR, the thinning air and slow speed began to have an affect on control authority as the rocket passed through 30km traveling 602m/s.

Guidance began to falter slightly when the rocket reached and attempted to hold 46° of pitch as it neared 40km at L+2m20s. Just 2 seconds later though it passed through 40km and throttled up to full power. This would ensure the entire tank of remaining fuel would be used up before the rocket reached space and the guidance fins now had increased authority thanks to the faster flow of what little air remained. The kick of acceleration pushed Jeb back into his seat with a force peaking at 5G by the time MECO arrived at L+2m49s, three kilometers short of planned altitude at 61km.

10 seconds later, during which time the rocket entered space, the capsule was jettisoned from the lift stage and the Reaction Control System (RCS) fired the aft thrusters for another 10 seconds to begin separating the two stages. Once the thrust had completed Jeb was supposed to assume manual command and pitch the capsule around so he could spot the stage out the window and confirm good separation with increasing distance. However just 3 seconds later he manually fired the 4 solid rocket push motors at the base of the Launch Escape System (LES) tower, which pushed his apokee from 205km to 244km. This was against flight procedure and caused quite the stir back on the ground in Launch Control, although ultimately it was decided the action posed no threat to the remainder of the mission. The splashdown zone was moved about 25km further east but still within range of the Ockr Relay antenna.

Jeb spent a total time of 10m48s in space, during which he was able to float weightless and get good views out the window to take photos with his camera and watch Mun set behind Kerbin. Like the previous two astronauts to make the journey he reported no adverse symptoms as a result of 0G. Although the LES was spent and could have been detached after use Jeb kept it secured to the capsule to make the RCS less responsive and allow what he felt was better control over orientation.

The tower was finally decoupled after Jeb oriented the capsule retrograde for re-entry into the atmosphere, hitting the upper-most layer of thin air at L+13m45s. In addition, he put the capsule into a slow roll to make sure heat was more evenly distributed around the sides in order to prevent the two radial backup chutes near the nose from becoming too hot and possibly burning up. A 29-second plasma blackout ensued as the capsule plummeted back through the atmosphere at supersonic speeds, with G-forces peaking at 11.3 (10+ for 6 seconds). The main chutes were deployed in a reefed state shortly after comms were re-established at L+14m38s, passing down through 11.5km traveling at just over 400m/s. Full chute deployment occurred 597.6m ASL at L+15m59s, slowing the capsule to a relatively leisurely 6m/s rate of descent.

Seconds before splashdown the heat shield was released to allow the flotation collar to deploy. The capsule hit the water at L+17m40s, 684.7km downrange within sight and reception of the Ockr Relay tower. MSV Aldeny was less than half an hour away already making best speed for the recovery site and able to get a great shot of the capsule descending under chute. Both the capsule and heat shield were recovered from the water and Jeb clambered out of the capsule under his own power once it was on the deck. He has become the third kerbal ever to fly up into space and return!

Flight Telemetry Data

Flight Analysis

Rocket Design Changes

This was the first Mk1 to fly with two significant changes to its original design. The first was a modification that was made to the upper liquid fuel bulkhead, which required us to also move up the pressurized nitrogen gas tanks. This caused them to stick up through the cross-brace of the decoupler and rest against the heat shield of the capsule. An interstage shroud was added to cradle the heat shield above the decoupler and allow for better spacing again.

The second change was the addition of two radial chutes to the top of the capsule, near the main chute. They actually provide quadruple redundancy. The nose cone chute has redundancy built in as it can land the capsule fine if any one of its 3 main chutes fails to properly deploy. Each of the radial chutes is large enough to land the capsule safely on its own, albeit with a fair amount of damage. Ideally for a proper landing both would be used in the event of a complete main chute failure. There are also two simply to keep the capsule balanced.

Both these new additions did not change the ascent profile much other than increasing the overall mass of the rocket slightly.

Improper Rocket Mass Calculation

While the shortened ascent was originally attributed to control issues due to lack of authority from the slow travel speed, full analysis of the performance data shows that the rocket massed as much as 53kg more than expected. We can’t weigh a rocket when it is fully assembled – although this is mostly due to having no means in place to do so currently. The rocket mass is a combined total of parts and propellants that is figured late in the mission design process and locked in once planning begins. Due to the long delay in the mission and the changes made to the rocket described above in the time since, proper oversight of the mass discrepancy was not carried into the re-planning of the mission.

The slow speed of the rocket causing it to slightly deviate from the proper pitch profile past 30km did still play a minor role. Click for full size

Control Authority Reduced to Prevent Roll Shake

The guidance fin control surfaces were restricted on this mission to the vertical fins only being able to control roll and yaw while the horizontal fins were only able to control pitch. While the pitch controls retained full range of movement the roll/yaw controls were restricted to half their maximum deflection. This is the first time the rocket has flown with such limitations to its guidance system. As a result some minor roll-shake began at Mach 1.2 with the rocket stabilizing again along the roll axis by Mach 1.6. Although we see roll deflection bouncing back and forth between max deflection during the worst period, both pitch and heading remained within mission constraints during this time. This was a good result because even though the more severe roll-shake on previous missions also did not greatly affect the guidance of the rocket, the extra vibrations generated by it were a cause for concern. Dampening these vibrations will ensure better ride comfort for crew and sensitive payloads.

It is still not entirely clear whether this phenomenon is due to guidance system oscillation or aerodynamic forces – it is likely caused in some parts by both and at least we currently have a good deal of control over one of these effects.

Crew LES Activation

Post-flight debriefing confirmed that Jeb’s decision to trigger the LES was a futile effort to boost his apokee higher than Commander Val’s record, something he has always been looking forward to doing. With the initial mission plan coming up short of a new record, he had decided months ago to fire off the LES without prior authorization from Flight Director Lanalye, who was quite taken aback by his audacious behavior during the mission. Had the rocket made an initial ascent as high as planned his actions would have led to a new kerbed space altitude record. Coming up short on the ascent he wasn’t able to take the time to calculate whether or not the LES motors would take him higher than Val and every second wasted was a further decrease in velocity.

At this point in the mission the LES was only capable of serving as a backup means to decelerate during re-entry and with the extra chute redundancy this wasn’t a big loss to mission safety. Still, while the capsule commander does have a large degree of latitude in the actions they are allowed to take on their own volition during a mission this one was not critical to the overall success and was done mainly for personal gain. As such, a reprimand was handed down barring Jeb from being allowed to become the first kerbal to orbit Kerbin. Since Bill was first in space, this leaves the honor to either Val or Bob.

FD Lanalye did at least recognize that Jeb wasn’t so vain in his attempt that he went the step further to expend cold gas to continue to raise his apokee, which is something that could have impacted the mission. If he had run out of maneuvering fuel before assuming the proper retrograde re-entry orientation it could have cooked him inside the capsule.

Engine Thrust Failure

After a close look inside the engine that failed to produce proper lift off thrust it was discovered that a fuel flow sensor had gone bad. This meant that the engine was simply not receiving the proper amount of propellants to reach the required thrust level. Although the engine was tested after delivery as usual, in the time between that and launch the sensor became unreliable. It has been returned to the manufacturer for a complete analysis and we’ve told them to source better quality components if they want to retain the contract. It will be replaced so the engine can be used for Specialist Bob’s upcoming mission.

New Communication Protocols

An upgrade to the comm system software aboard the capsule was made to comply with changes to our protocols that went into effect at the start of this year. The new protocols will allow for better communication between multiple spacecraft, which will start to become a major factor once orbital communication and other satellites come into operation.

Fixed Alternator Power Circuit

In the previous mission, power failed to get from the engine alternator to the payload. This was determined to be due to an issue with the connector that runs through the decoupler. Because we needed a new design with the addition of the interstage, we were able to tackle this problem directly and the new electrical bus properly supplied power to the capsule on this flight.

Log Timing Issue

During the initial ascent there was an issue with telemetry data dropping in and out until the rocket passed through Max-Q, which was at the time attributed to signal problems. Upon review of the logged telemetry data however it became apparent that the problem was in the flight computer itself since the log also showed missing intervals. The published log data shows intervals of 2 seconds instead of 1 second from the time the rocket cleared the tower until it passed through Max-Q.

While we have found the problem in the code, we still don’t understand why it’s causing an issue since it’s the same code we’ve been using since at least 2018 to check for Max-Q. Most likely it’s due to a hardware upgrade that went into effect earlier this year after the previous Mk1 flight and we will be working with the kOS engineers to see if the problem can be found and corrected. Regardless, this isn’t the only way we can detect Max-Q so the problem will not affect future missions.

Part Reuse

For this mission we switched to the backup capsule and heat shield, which will keep the usage rate of our two kerbed capsules below that of the unkerbed one. This is to make sure any failures with the unkerbed capsule due to reuse make themselves apparent well before the kerbed ones – in theory anyways. The fresh heat shield burned off the normal amount of ablator and after it is resurfaced should have ~180 units remaining out of the 200 it began with. All 4 aft-facing RCS thrusters were brand new since they receive too much damage during re-entry to be used again but the roll thrusters on the side of the capsule were re-used for a second time and functioned without issue. They will continue to be refurbished and re-used.

Future Plans

While we are using the current operational suspension to take a close look and revamp procedures involving rocket construction, which should hopefully prevent future occurrences of things like mass discrepancy, we also plan to add sensors to the load-bearing parts of the Mobile Launch Platform and other rocket carry vehicles. This will allow us to determine the mass of the rocket and double-check that it is the same value used in mission planning. If not, then a delay will be caused in order to change plans as necessary.

There was mention of perhaps using an alternate land-based landing site in the event that weather at sea is not favorable for recovery, however for sub-orbital missions we will continue to rely on a single point of recovery for the mission and delay launch as necessary. For future orbital missions however we will need to have an alternate landing site or two because delaying the return of the capsule could potentially jeopardize the lives of the crew. It is also a lot easier to plan in an alternate landing location for an orbital mission than a sub-orbital one, which would require an entirely different ascent profile and this is the most complicated part of mission planning.