Thomas Pederson of Copenhagen Suborbitals offers an insight into why Nexø II did not reach its expected altitude of 12 km.
(Translated with google translate)
The first thing to hit is the low engine pressure. We start around 14 bar but reach T + 15 sec below 13 bar. Then it rises up to 14 bar again. The same type of course is seen on the pressure in the LOX tank and the pressure in the LOX side of the injector. It is therefore clear that the pressure in the engine has been too low due to the low pressure in the LOX tank.
(Referring to graphs in the original post,)
…Here we see both the pressure drops above the injector, but also the error between the injector pressure and the desired injector pressure. It can be seen that on the fuel side the error is below 1 bar throughout the trip, but on the LOX page the error is between 2 and 4 bar. It is quite violent. Thus, the LOX flow has been too low, which automatically produces a higher flow of fuel and thus low OF ratio. In addition, it means that fuel is consumed too quickly and we see that at T + 39.
The low pressure and thus the low thrust give rise to a lower acceleration than expected. The acceleration (vertical) is plotted at the bottom right along with the expected acceleration. It is seen that we lie a little all the way. Overall, it means a little lower acceleration and the premature MECO that we only reach approx. 6,500 meters over 12,000 meters.
It is therefore clear that around the T-18 something happens with the LOX tank pressure setting. Either we have a mistake on its pressure relief valve or we have a leak in the tank, so hell is coming in, but it is lost somewhere else. However, it seems strange with such a leak, we have had the system up at work pressure previously without problems. However, for the time being it is difficult to determine more precisely.
Great to see they have figured out why the rocket did not reach its mark and the possible cause of the problem. It was great to see the rocket fly after the delays of last year, looking forward to seeing what is next on their agenda.
Joe from BPS continues on his quest to propulsively land a model rocket, as seen in his latest test, Joe is well on his way to making it happen. You can buy the exact same Signal flight computer and try it yourself!
Video Caption: This is the first real test of model rocket retro-propulsion! At 50m AGL, Signal(the flight computer) commanded release and began computing the optimal retro-burn altitude to “land” the rocket at 14m AGL. Signal missed its landing target by a whopping factor of 2, landing instead at 30m AGL. There are several factors involved, but the most likely candidate is that the pre-flight simulations used to generate the landing math were out of date. Now with real flight data, we can dial in landing simulations more reliably. As with SpaceX and Blue Origin, getting this well-tuned will take several tries.
Stability was quite poor during this test, I believe this was caused by excessive roll on the vehicle. At this time, the current theory for roll accumulation is abnormal motor nozzle geometry. Post-flight motor inspection confirms this as well, and the flight data shows a steady increase in Z axis roll as the burn progresses. This is generally rare, and I’ve only noticed it occasionally with these motors. Aerodynamic sources seem unlikely as roll continues to accumulate at low airspeeds. The TVC servos have been characterized to handle roughly 180 degrees per second of roll before they can no longer keep up, and this holds true in the flight data as well. More tests like this will certainly be performed, but before that we need to review a lot of data!
Of note, this was the second largest university built rocket motor and the largest rocket flown by the MIT team!
Video Caption: MIT Rocket Team flew Hermes 1 on July 21, 2018 at the Friends of Amateur Rocketry site in California. Hermes 1 weighed 121 lbs at liftoff, and flew on an student developed O4300 that delivered the vehicle to 32,400 ft above the Mojave Desert. A newly developed piston based recovery system deployed a disk-gap-band parachute we based on the design from the Viking landers. At 2,000 ft above the desert floor the main parachute was extracted by the drogue, allowing for a safe landing 1.8 miles from the launch site. The student build carbon composite fin can fared well during the flight. The rocket flew a payload developed at the University of Victoria as part of an inter-team collaboration to study DNA repair mechanisms in microorganisms.
The team is grateful to our many industry sponsors, the many mentors we’ve had along the way, and our peers at other universities for their insight and friendship during this launch campaign, and we look forward to returning to IREC in 2019.
The team at Copenhagen Suborbitals had a successful launch and recovery of their Nexø II this past weekend. Launching from the Baltic Sea on Saturday, August 4th, the 6.7m long, 0.3m diameter rocket ascended to an altitude of 6.5km (21,325ft), before coming down, first on a ballute and then its main chute. The liquid oxygen/ethanol fueled engine provided a nominal thrust of 5000N (1124N) and was intended to power the rocket to between 8-12km in altitude.
The flight was obviously a shortfall in the altitude expected but a success in every other way. It will be interesting to read the post-mortem as it becomes available.
In the meantime, check out the raw footage of the flight below.
Congrats to the team for a successful flight!
Video Caption: Rockey, ULA’s mascot, makes his dream of launch come true with the help of some ULA interns.