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Feb 06 2020

Ascension Mk1 Flight 11 Analysis

Ever since the attack on KSC that damaged the launch pad and destroyed our last Ascension Mk1 rocket we have been working hard towards regaining launch capability. It took 2 months to repair the ground service structures as well as the actual pad surface itself, but coming into the new year and new decade we are once again able to send up rockets from the launch pad. This mission will renew the bid for orbit that began back in 2018, and was then suspended at the start of last year when the rocket proved incapable of flying a trajectory that would allow it to enter a decaying orbit without running out of fuel. After testing new guidance fins on the Progeny Mk7-A, it is now time to scale them up to 1.25m rockets and see whether they can give the additional control authority needed to allow the rocket to pitch over faster. This mission will also see new 1.25m payload fairings based on the 0.625m ones that flew on the Progeny Mk7-A, which will be tested to see how they perform under the heat and pressure of ascent. The payload itself contains another RTG test article so it can be slammed into the ground after the flight and analyzed afterwards to determine whether it successfully remained intact and would have not spilled radioactive material.

The Flight

Due to tightening operational budget constraints, we can no longer afford to fail fast for iteration – in fact we can’t really afford to fail at all. This has led to new policies and stricter launch commit criteria (weather, build process, flight quals, etc) going into effect this year. The result was some early delays for the launch due to weather being outside of acceptable constraints, but thankfully it did not remain uncooperative for long and the countdown was able to begin and run to conclusion with no further issues only a day later than planned.

10 seconds to lift off the guidance fins performed a roll maneuver left and then right, which allowed them all to travel through their full range of motion. This is a simplified version of the check performed by the previous control surfaces and still allows us to determine if hydraulic pressure is nominal. If the fin movements take too long due to low pressure and are still underway by T-6 seconds, the engine will not ignite. This did not occur however and the engine fired up on time, registered nominal chamber pressures and at T-3s performed throttle up to lift-off TWR of 1.2. At precisely 13:40:00.27 local time the engine clamp had released and the rocket was allowed to begin its ascent.

In the last few minutes leading up to the launch, steady winds out of the west began to start gusting near abort speeds, which is when they have the force to push the rocket over while it is only held upright by its own internal structure atop the engine clamp after the service boom detaches and swings away at T-2min. No abort was called prior to the terminal count however and at that point wind speeds are no longer factored into abort decisions because if the rocket is in danger of tipping then we want it off the pad before it is terminated if necessary. Thankfully with winds out of the west the rocket was being pushed away from KSC so it had much more time to recover before the Range Safety Officer would have been forced to activate the Flight Termination System.

Clearing the launch towers at L+3s, still struggling to regain guidance, the main throttle was pushed up to 84%, which was calculated during ascent planning to keep the rocket from accelerating faster than a full-stack heavier Mk2 would. This ensured airflow over the guidance fins would be similar and thus control authority would be similar. Over the course of the ascent, the throttle would be adjusted to maintain velocity close to, if not exactly, how fast the Mk2 would be traveling during its use of this lifter, which would burn out at ~45km.

Despite the early struggles thanks to the gusting winds, by L+40s while passing through 5km ASL the rocket was back on course heading 45° and pitch-over rate was nominal. The rocket remained stable through maximum dynamic pressure of 19.355kPa, traveling through 6km at L+46s before breaking the sound barrier and accelerating past Mach 1 at L+53s. At this point as it flew through the transonic regime and passed 12km it developed “roll shake” turbulence that caused all the control surfaces to fluctuate rapidly, affecting the ability for it to use the two fins that control pitch. However despite this the pitch rate continued to match the planned ascent profile.

The roll shake continued to various extents throughout the remainder of the powered ascent and although the pitch rate remained nominal the time it was taking to pitch over gradually increased over what was planned. At L+2m6s the final throttle command was issued passing through 40km, reducing it to just 40%. At this time velocity was nearing 1.5km/s and heat was beginning to build up at the nose of the rocket, which was also traveling slightly faster than planned for this altitude.

The longer pitch over was cancelled out by the faster velocity and the rocket reached 45km at L+2m15s to within just 0.05ms of when it was planned. Although fuel remained, the engine was shut down to simulate the end of burn for the lifter during a Mk2 ascent. The rocket had also entered an atmospheric region where it was no longer necessary to protect the payload from pressure or heating so the fairing halves were shed, successfully splitting apart and falling away without striking the rocket. Despite the rapidly thinning air, the guidance fins were still able to reduce the rocket’s angle of attack and point towards prograde to reduce drag as it coasted out of the atmosphere.

The rocket entered space at L+3m13s, coasting up towards an apokee of just 79.96km, which was only 725m higher than planned due to the slight excess velocity during ascent. Shortly afterwards the fuel valves were opened and the tanks over-pressurized to begin draining them so that the lifter would not explode on impact and start a fire on the surface. Before that was completed however (it continued unaided) the rocket reached apokee at L+4m20s and the payload was detached so that its smaller size would create less drag and allow it to maintain velocity as it fell back to Kerbin.

The rocket’s slower-than-planned pitch over also sent it roughly 44km further downrange than expected, which put it outside the higher terrain we had hoped to impact the RTG into, as the sooner it reaches ground the less time it has to slow down. Re-entry began at L+5m32s, with communications lost shortly afterwards due to plasma blackout at L+6m43s. The antenna was not designed to withstand the heat of re-entry and when we failed to regain contact it was determined that it had burnt off as expected. The last-known position was sent to the airship searching for the impact point and within an hour the location had been found, 423km downrange. The RTG casing was successfully recovered for analysis.

Flight Telemetry Data

Flight Analysis

Wind Gusts at Launch

While this is still something that is being looked into by meteorologists on Lead Scientist Cheranne’s team she has at least been able to give us a theory: mountain waves. These are strong turbulent bundles of air that can sweep down from mountains if wind speeds over top of them are high enough. Lacking any significant elevation change downwind, the effects can travel several dozen or even over 100 kilometers. KSC lies less than 50km from the tallest peaks in the mountains to the west. The phenomenon is known but not well-studied. New wind stations have been setup out in the grasslands to the west for scientists to monitor winds and there will be several airship flights to track winds aloft

RTG Casing Report

Analysis of the casing that protects the radioactive material of the RTG showed significant damage after recovery but it was all within what was expected for the impact velocity the casing suffered. It is estimated to have impacted between 100-130m/s, which is still quite close to the ~180m/s terminal velocity it would experience before crashing into some of the highest terrain on Kerbin. This is the second time we have let a test RTG freefall from space and have it survive. The Kerbin I probe launching atop the Ascension Mk2 will provide the third and final impact test needed to certify the RTG for LKO operations. Above that, a separate certification will be needed because the casing has not been tested against the heat and pressures of a high-speed re-entry from higher orbital altitudes.

Roll Shake

This issue has been plaguing the Ascension Mk1 ever since the first launch and we have also confirmed it occurred during both Progenitor Mk7-A flights so it is not a problem inherent to Ascension but rather one specific to controlled flight since all previous uncontrolled Progenitor rockets were spin-stabilized so they were already rolling. The shake adds extra stress on the rocket, extra drag due to all 4 control surfaces deflecting to the air stream (rather than just the two pitch fins most of the time) and lowers the effectiveness of the pitch control since those same guidance fins are being used to adjust for roll. So we have good reasons to want to eliminate it, but we are still not sure exactly what is causing it. There is now a decent flight history to the Ascension Mk1 so let’s run a light analysis of the previous flights.

  • Flight 1 – roll shake is induced at 1.35M (Mach) while traveling through 7.9km ASL with a velocity of 417m/s, massing 7.59t and throttle set to 100%. It ceased at 2.54M while traveling through 24.9km
  • Flight 2 – no roll shake is detected, however the rocket’s control surfaces did not make any roll corrections on this flight after initial ascent past the towers. The cause was never determined but it does potentially imply the unstable airflow itself is not able to cause the shakiness but rather the constant minute control corrections are creating an oscillation in the steering system
  • Flight 3 – no roll shake is detected on ascent. The rocket passes 1.4M while traveling through 8.7km with a velocity of 425m/s, massing 6.76t and throttle set to 100%
  • Flight 4 – roll shake is induced at 1.46M while traveling through 8.7km with a velocity of 440m/s, massing 7.71t and throttle set to 100%. It ceased at 3.06M while traveling through 26.8km
  • Flight 5 – no roll shake is detected on ascent. The rocket passes 1.4M while traveling through 8.7km. massing 7.71t and throttle set to 100%
  • Flight 6 – roll shake is induced at 1.57M while traveling through 10km with a velocity of 466m/s, massing 7.50t and throttle set to 100%. It ceased at 6.50M while traveling through 58.3km
  • Flight 7 – no roll shake is detected on ascent. The rocket passes 1.4M while traveling through 14.5km, massing 7.25t and throttle set to 58%
  • Flight 8 – roll shake is induced at 1.63M while traveling through 10.8km with a velocity of 478m/s, massing 7.39t and throttle set to 91%. It ceased at 2.52M while traveling through 24.6km
  • Flight 9 – roll shake is induced at 1.35M while traveling through 12.6km with a velocity of 387m/s, massing 7.49t and throttle set to 60%. It ceased at 2.35M while traveling through 36.6km
  • Flight 10 – did not get off the ground
  • Flight 11 – roll shake was induced at 1.34M while traveling through 11.9km with a velocity of 387m/s, massing 6.50t and throttle set to 61%. It eases but does not cease throughout the entire ascent

Unfortunately there is no clear picture painted here that would lead engineers to spot the reason the roll shake comes about. Despite it not happening a few times, in nearly similar instances for some cases it does show up, for other cases the velocity, altitude, mass or throttle settings do not seem to determine whether it appears or not. All rockets have had aerodynamic profiles similar enough that engineers do not believe changes between rockets (slightly different fins, pointier noses) have had an effect either.

Engineers are developing new instruments to install and monitor rocket behavior during flight and hope to see some difference in behavior for the longer Ascension Mk2, which will place the fins farther back from the nose of the rocket.

Payload Fairings

The larger 1.25m fairings that were used for the first time on this flight and will also fly on the first Mk2 performed extremely well. The dynamic pressure of the ascent only reached 19.355kPa and the skin temperature of the fairing half facing downwards during the ascent reached a peak of 576.2K (577.49°F/303.05°C) – both well within designed tolerances. Upon separation the halves broke apart cleanly and were pushed far enough away to not impact the rocket. They will be tested under similar circumstance for the Ascension Mk2 to confirm these findings and allow for additional experimentation.

Guidance Fin Performance

This was the big test of the mission, since it was the main reason the Mk1 could not reach orbit:- how well can the new fins with their larger surface area affect the pitch of the rocket? The mission was carefully designed to match the velocity of the Ascension Mk2 by continually throttling back the lighter and therefore faster Mk1 during the ascent. This made sure increased airflow over the guidance fins from traveling faster wouldn’t allow for more control authority than the Mk2 would have. This does not take into account the extra length of the Mk2 providing a stronger pitch torque, but that’s okay because knowing the Mk2 will have more authority means if this rocket can do it within a reasonable margin, the Mk2 definitely can.

kOS represents the logged data from the flight computer. LVD is the planned Mk1 profile, exactly matching the Mk2’s ascent. Click for full size

And the results are conclusive: the fins can do the job. Despite the thinner and thinner air as the rocket ascended the pitch angle was always nearly right on the money with control deflection never exceeding 75% during the powered ascent phase up to 45km, which is the entire duration of their use on the Mk2 since the lifter is staged shortly afterwards for ignition of the Viklun vacuum engine, which then uses RCS to maintain attitude. Just to see what they were capable of, after MECO the rocket was commanded to hold prograde, which was nearly 10° off its current pitch. It made the adjustment and managed to hold it nearly all the way up into space. It drifted off a 0° angle of attack at 68.7km to enter into a slow nose-up rotation.

Coefficient of drag over altitude in meters. Click for full size

With onboard monitoring equipment from Ferram Aerospace Research we were able to compute the drag coefficient (Cd) throughout the flight and you can actually see the rocket making larger and larger attempts to stay pointed as the air thinned. Each double peak late in the flight is the pitch fins moving down, centering and moving up to re-adjust orientation. The earlier increase in drag was from the payload fairing separation and the large pitch-up motion to return to 0° AoA. We have smoothed out the Cd so we can use it for future ascent planning with the payload fairings.

Future Plans

The new wind sensors will allow us to track incoming mountain waves for future launches and take necessary precautions, like holding the launch sequence if possible prior to terminal count so the rocket remains attached to the service structure and is more resistant to high winds. Although the gusts began prior to terminal count, they were relatively light to begin with and we had no idea or expectation as to how violent they would shortly become.

The payload fairings have already been delivered for the Ascension Mk2 but moving forward we plan to experiment with different conical and parabolic shapes to the top of the fairings as well as getting a good idea of what thickness is appropriate for what amount of pressure and temperature on ascent so that we can save mass and decrease drag.

With the fins proving capable of performing the Mk2 orbital ascent the only thing left to do now is update the original Mk2 ascent plan that this Mk1 mission was based off of with new drag information and other flight dynamics data from the actual Mk1 ascent. Assuming there are no major changes that need to be addressed and so long as the VAB stays on schedule we should be able to schedule a launch date next week!

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