Message to my Pacific Spaceflight group today, describing the events at Copenhagen Suborbitals as I understand them (photo of an intact and burst 'burst disk'):
The rocket uses alcohol as fuel and liquid oxygen (LOX) as the oxidizer. Inject them together, from separate tanks, into the rocket combustion chamber and you get an explosion that is directed out the nozzle, creating thrust in the opposite direction. Cool physics courtesy of a certain Isaac Newton!
The rocket consists largely of two cylindrical tanks, 600kg each of LOX and alcohol. LOX tank above the alcohol tank. Valves open to allow flow of LOX and alcohol to combustion chamber. Rather than pumps or turbines, these fluids were to be injected into the chamber by pressurizing the unfilled volumes of each chamber with inert nitrogen and helium (can't remember which tank was pressurized with which gas).
OK so procedure before firing is to fill tanks partially with alcohol and LOX; that went fine, leaving an unfilled volume in the tanks. Into this volume the gasses were pumped to raise tank pressures to about 18-20 bar (1bar = 1 atmosphere = 14.7psi; so their tanks were to be pressurized to 261 PSI and 290 PSI (can't remember which pressure for which tank). Compare to our scuba tanks at 3k psi.
So, tank filling was fine, about 30 min before proposed ignition.
Now followed tank pressurization. Everyone (mostly) well clear of the pad because if the tanks rupture you have a bomb and only the most dedicated 'pad rats' were still on-site to monitor these last phases. Alcohol tank pressurization was fine up to its specified about 20bar. LOX tank pressurization, monitored by a remote digital gauge, went fine, up to 8bar, but then a moment later the gauge read 0.8bar. HMMM???. A glance at the rocket showed no LOX leak or anything else awry. As the crew tried to determine the problem, a jet of LOX was seen to shoot out of the side of the rocket. This was instantly identified by Peter M. on the radio as a 'burst disk', and while I muttered damn he instantly said "LOX burst disk, engine firing is scrubbed for today".
This is because while the team were thinking over the issue, LOX tank pressure continued to increase even without pumping in new inert pressurization gas! That is because LOX has to be stored at -369F; if the temperature rises, it begins to boil, increasing pressure in the LOX tank. There is no provision to keep the LOX cool once it's in the rocket, that is heavy machinery, so once fuelled you have to pressurize and then get outa dodge! In just a few minutes, though, that pressure in the LOX tank increased to the point (I don't know the PSI) at which the LOX tank pressure relief valve (AKA 'burst disk') cracked, as it was supposed to do, to dump pressure from the tank. This prevents the tank from pressure rupture. So, why not just reset the pressure relief valve (burst disk) and start over? Because a burst disk is not resettable or reversible; it is literally a patch of metal on the pressure vessel (LOX tank) weaker by a known degree than the rest of the tank. So a 'burst disk' rupture means there is now a hole in the tank, and that's that for the day.
So, a faulty pressure gauge, that gave a bad pressure reading (from 8bar to 0.8 bar over the course of a second) caused the team to spend some time thinking over the issue (backup gauge had been unplugged from the rocket a bit earlier), and in that time the 'passive pressurization' of the LOX tank, by warming LOX, blew the burst disk.
Such is an experimental rocket program. I am sure they're having a glum night at CS tonight. I am sure they are also tracking down the LOX tank pressure gauge fault, and looking at records to see when it was last tested. It worked fine, remember, until 8bar, but then in a moment the gauge died. A backup digital gauge might have helped, but of course you can go down the road of 'backups to backups' and never come out.
The silver lining to me is that the burst disk worked. It did its job of preventing a larger rupture of the LOX tank, wich might easily have burst the alcohol tank, thus making a bomb of the whole rocket. So, the rocket was saved by the correct functioning of the burst disk!
Food for thought RE our own sensors, procedures, checklists, pre-flight tests, and attention to detail.
Regards CMS
Well I was up all night watching the live stream on YouTube, but no engine ignition for the static test today. Here is why, which is very instructive.
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