Edit: This question might be best posed to folks with experience or knowledge flying over 40-50K feet.
I'm working on a two stage project - both stages are 75mm minimum diameter, using a 4-grain motor in the booster (M1297) and 6-grain in stage two (M685).
I built a single stage 75mm min diameter (M1350) last year and flew successfully at Airfest - I have good correlation between the OpenRocket and RASAero models from that project and the actual results (~20K altitude and ~1.9mach).
However, looking at the modelled altitude prediction for this two stage design there is a big difference between OR and RASAero - on the order of 55K vs 85K respectively (and max velocity ~2.1mach vs ~2.5mach) . From going through threads I know there's a methodology difference between RASAero, which calculates thrust at different altitudes (using nozzle exit diameter) versus OR which holds thrust constant, but 55K v 85K seems like a pretty big difference.
Initially I assumed my RASAero model had some sort of mistake (and it could) but I've spent quite a bit of time looking for errors, and thought it might be a good time to throw the question out to the forum as a sanity check - is this a glaring mistake in my model, and keep looking for errors? Or a reasonable difference based on higher altitude and the software methodology?
Also worth noting I'm not an engineer, just a guy who likes to build HPR and push each project a little further. Meaning I've spent a good amount of time learning and using RASAero over the last couple years, but I'm no expert on the software.
Thanks for any informed feedback!
I'm working on a two stage project - both stages are 75mm minimum diameter, using a 4-grain motor in the booster (M1297) and 6-grain in stage two (M685).
I built a single stage 75mm min diameter (M1350) last year and flew successfully at Airfest - I have good correlation between the OpenRocket and RASAero models from that project and the actual results (~20K altitude and ~1.9mach).
However, looking at the modelled altitude prediction for this two stage design there is a big difference between OR and RASAero - on the order of 55K vs 85K respectively (and max velocity ~2.1mach vs ~2.5mach) . From going through threads I know there's a methodology difference between RASAero, which calculates thrust at different altitudes (using nozzle exit diameter) versus OR which holds thrust constant, but 55K v 85K seems like a pretty big difference.
Initially I assumed my RASAero model had some sort of mistake (and it could) but I've spent quite a bit of time looking for errors, and thought it might be a good time to throw the question out to the forum as a sanity check - is this a glaring mistake in my model, and keep looking for errors? Or a reasonable difference based on higher altitude and the software methodology?
Also worth noting I'm not an engineer, just a guy who likes to build HPR and push each project a little further. Meaning I've spent a good amount of time learning and using RASAero over the last couple years, but I'm no expert on the software.
Thanks for any informed feedback!
Last edited: