Solid Rocket Motors 1.5: OpenMotor Tips

hi rocketeers it's Charlie Garcia back with the short appendix to episode 1 of solid rocket motors when you're running your open motor simulations there's a variety of things that can happen some are good some are bad and some just are I'm quickly going to run over some of the software conditions that may occur and what you can do to change your design to make them go away first we want to make a quick review of important terms to know for this video KN is the ratio of propellant surface area to throat area the port is the area of the grain that gas can flow through so on a bait screen that is a cylindrical grain it's the size of the perforation down the middle of the grain whereas on say an end burner grain it's the entire face of the grain the throat is the smallest part of the nozzle this is where choked flow occurs alright rocket tears sometimes call this Ocado or catastrophe a take-off if the motor pressure exceeds the configured limit the software will give you a warning upon completing the simulation to resolve this condition acknowledge the warning and then lower your kN you can see your KN listed here and you can see here our peak KN is 552 which gives us a peak pressure of 1600 and 44 psi which is above our configured limit of 1500 psi the most straightforward way to lower the kn is to increase your throat size increasing the throat size will decrease the pressure that the motor runs at if we run the simulation again you can see that our peak can has now decreased as well as our peak pressure the other way you can decrease your kN is by shortening the grain so in this case if we shorten it by a few inches if you're deliberately going for a high-pressure burn you can change your configuration options here by increasing the maximum allowed pressure you just need to make sure that you can design your rocket motor case to handle these high pressures it's also worth noting that most repellents are only characterized for a certain regime of K ends and you exceed this KN the propellant may not perform as expected situation to erosive burning erosive burning is the term we use to describe the burning of propellant when it is affected by the gas flow conditions in the port normally the conversion of solid propellant into gas is driven by convective and radiative heat transfer into wall in a well-ordered propellant service if there is a fast flow of gas over the surface however shear action can mechanically remove propellant from the surface either by removing entire crystals of ammonium perchlorate or aluminum at once or by removing entire chunks of propellant binder and all either of these will result in a significant increase in the surface area of the propellant and therefore the KN of the motor the simulation can't model this behavior so instead we have to have the simulation check for conditions that can indicate erosive burning might occur erosive burning tends to happen when the flow in the port of the motor is above Mach 0.7 we can check for this condition by measuring the mass flux through a grain once again when running a simulation where the peak mass flow SiC is exceeded you'll see a warning pop-up upon completion of the simulation you can see the peak mass flux down here in the bottom corner it will tell you what the peak mass flux is as well as what grain it is occurring at mass flux is a mass flow which we already have which is normalized by the area of the port we typically use 2.0 pounds per foot squared as the concern threshold and 2.7 pounds per square foot as the do not exceed threshold you can see what your maximum mass flux is down here or you can put it on the graph and see it as your burn progresses you do that by checking this box here to get the scaling right you'll need to uncheck the other boxes mass flux is per grain so you can see in this one grain motor it's only measuring the mass flux at the bottom of the first grain if we were to split this into multiple grains it would show you the mass flux at the bottom of each of the grains here you can see that the mass flux only exceeds our threshold for a short period of time and erosive burning will likely end as soon as the mass flux dips below this threshold experience rocket motor designers sometimes use erosive burning to increase the efficiency of their rocket motors but this is playing with fire and could possibly lead to unintentional explosions to fix this condition you'll want to increase the size of the port on whatever grain has the highest max flux until all the grains are below 2 pounds per square foot situation 3 port 2 throat area ratio having too little area isn't the only condition that can cause a ruse of burn if you remember from the liquid rocket engines Nelson's video nozzles work by choking the flow of gas and choked flow occurs wherever the smallest constriction is in the nozzle whenever the pressure drop is high enough between the upstream condition and the downstream condition normally this would be at your throat in your nozzle but if your throat is too large it could instead happen at the bottom of your last grain the software will once again warn us about this condition to fix this issue either make your nozzle smaller or your port larger this condition tends to pop up on long small rocket motors designed for high cans with simple cylindrical ports this condition also only occurs at startups since the grains will rapidly regress to increase the port size in relation to the throat situation for unsupportive propellant propellant is a rubbery deformable substance you really don't think about it much just handling the stuff but at several thousand degrees several hundred psi and sometimes several tens of G's of acceleration propellant can really move around on you if you have beit's grains that are too short rod and tube grains finish those with negative fin lengths or other custom geometries there's a possibility that a large amount of propellant can be liberated when its connection to the rocket motor burns through or fails mechanically the easiest way to check with for this problem is to go to the grains tab and scrub through the slider to check the entire duration of the motor and see if any chunks of repent will become liberated during its burn the situation is unlikely to occur in normal rocket design but could be something to worry about it if you're working with more interesting geometries situation 5 unburned propellant you expect one of the key safety features of a PCP propellant is that it has a self extinguishing tendency if the rocket motor suddenly loses pressure combustion will often stop leaving behind unburned rocket fuel in the event of a failure this is a good thing but if the cane of your motor suddenly changes the propellant can extinguish itself each propellant has a different threshold for extinguishing itself the propellants with catalysts may not exhibit this property propellants with a lot of aluminum potassium nitrate or ammonium nitrate or chlorides or ox amides exhibit this tendency to a greater degree open motor can detect this problem in some cases but not all cases and this is an active area of development the suburb will not currently give you a warning but if the pressure drop is too low to continue combustion the simulation will end with propellant left this is visible on the grains tab you can see if we scrub all the way to the end of the rocket motor there's a large amount of propellant left in the middle of this motor design that is because the motor does not have a KN sufficient to continue combustion leaving 0.14 pounds of propellant unburned at the end of this motor if there is a rapid change in combustion pressure but the lower pressure can still support combustion the simulation will continue it is left to the user to try and avoid to designing a motor that could experience this problem a rule of thumb is to try to avoid having a lot of grains burn out all at the same time so that pressure changes more gradually another way to reduce the pressure drop all at once is to use grains that burn out slower for example thinnest oat grains burn out over a longer period of time than Bates grains do as their burning surface area slowly decreases as opposed to a Bates grain which burns out all at once I hope this was a helpful review of all the things that could go wrong when designing the geometry of a rocket motor and simulating it an open motor a lot of the concepts we learned here will be important during the rest of the motor build process for example too high of a KN will over pressurize the motor but even a correctly designed motor could over pressurize if the bubble and the propellant causes the KN to suddenly increases the propellant burns just another disclaimer rocketry is a dangerous hobby and you're taking responsibility for your own actions by participating in it no simulation software can capture all of the complex minutia that occur in a real rocket motor and the farther you diverge from conventional designs the higher the risk will be conventional tools fail conventional tools can fail even on normal designs for example if your motor displays a behavior like this one you've rediscovered a bug in our maths flow smoothing filter and you should file an issue ticket on github so make sure to apply a sanity check filter on all of your results the takeaway from this video should be to use your head start simple and build your experience and when in doubt remember this advice from astronautics magazine in October of 1937 a good rule for rocketeers to follow is this always assume that it is going to explode I'll see you in the next episode until then good luck and Godspeed