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Adrian.
Adrian.-
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Anonymous
Many flyers more experienced than I have warned about the issues near Mach 2. For my 30K shot last year, I added a significant amount of weight, as I didn’t want to mess w/ the velocities that I was anticipating. Obviously, it can be done – others have done so.
It also stands to reason that you can be too light to get up-and-over mach. There is clearly an optimal mass for a rocket (that would vary with motor, cone shape, drag coefficient, etc.) Bottom line, going as light as possible is likely not the right answer. I’ll go back to my ping pong vs. golf ball example.
I’m surprised we haven’t had more shreds given the extremes we are pushing?
Adrian
Since there is a roll-off in drag above mach, there is an advantage to being light enough to get “over the hump.” Maybe that is both the core of RockSim’s problem and the secret weapon to higher altitude.
My current strategy for mach 1+ rockets (on paper at least) is to select the rocket mass based on a conic nosecone, then build away using a VK or PowerSeries cone. For an I600 shot, that is a dry weight around 700 grams. I have no idea if this is smart or not, but a SWAG beats bad computer code any day.
I like the idea of jettisoned sand as ballast. I was thinking of water actually, but sand may make more sense. Anything to lower the landing forces and enable a faster descent rate. That’s why I tend to go “just below” optimal mass rather than above it.
The Rocksim optimal mass calculator is definitely whacked. But if you run the regular sims yourself with different nose weights you can do it right.
Jettisoned sand is interesting. If there’s a way to hang the heavy nose cone on a really long shock cord, then the more delicate rocket parts should be able to slow down significantly after the nose lands. But it’s probably more volume-efficient to just use a larger chute and dual deployment.
When thinking about drag, don’t forget that the drag force isn’t just Cd but is multiplied by V-squared also. Even when Cd goes down with higher speed, the total drag is still going up. Or in other words, if you’re comparing Mach 2 to Mach 1, the V-squared part of the drag is going up by a factor of 4, while the Cd only goes down by 20%. Gravity losses go down with higher speed, though, so it’s not all bad.
I’m struggling to get the new altimters done with a heavy work schedule at my regular job, but sometime I’ll do my own calculations using my measured drag vs. velocity and measured thrust curve, and play with the mass to see where the optimal point really is.
I’m also having fun looking at multi-stage rockets. I was interested to learn that if I ignite the same rocket that went about 16kft from the ground, at 30,000 feet and some upward velocity, it would add on another 30kft + due to the thin atmosphere up there.
Adrian
Many flyers more experienced than I have warned about the issues near Mach 2. For my 30K shot last year, I added a significant amount of weight, as I didn’t want to mess w/ the velocities that I was anticipating. Obviously, it can be done – others have done so.
It also stands to reason that you can be too light to get up-and-over mach. There is clearly an optimal mass for a rocket (that would vary with motor, cone shape, drag coefficient, etc.) Bottom line, going as light as possible is likely not the right answer. I’ll go back to my ping pong vs. golf ball example.
I’m surprised we haven’t had more shreds given the extremes we are pushing?
The ping pong ball vs. golf ball analogy is a good one.
My last shot just grazed Mach 2 and was unscathed. But there wasn’t much wind shear that day. I can see how a rocket going through different wind layers at Mach 2 could get a lot of bending as the fins try to make it change direction instantly. Here using some unidirectional carbon fibers in the axial direction will make it resistant to those bending loads.
Adrian
Yes, you are right. I remembered the dry weight wrong. My calcs show exactly 500 grams, but that still puts it over what you recommend and what Adrian’s dry weight probably was. I’ll take another look.
For something that is ripping along at Mach 1.7, I kinda stopped worrying about keeping the velocity in check : )
Interesting discussion.
I just spent a couple of hours making a spreadsheet tool to simulate my last I600 flight, using the as-measured Cd vs. velocity curve, and the as-measured thrust curve from the motor. I think I still have some small error, because the sim is showing a couple hundred feet higher than the recorded (and temperature-corrected) altitude. But anyway I think it’s close enough to get an idea of how close I was to optimal mass.
The rocket’s post-burnout mass was 683 grams and its liftoff mass was 1014 grams. My sim shows that getting up to 16,600.5 feet. I tried plugging in some different dry (post-burnout) masses, and it turns out that my rocket was overweight by 10 grams. It could have gone 1.0 foot higher!. Shoot, I was way off. I’d better fix that and fly again! 😆
Oh, and I also confirmed that if I were to launch the same rocket from 30,000 feet (think staged rocket here) it would add another 30,000 feet to the apogee, since the air is so much thinner up there.
I’m thinking that an I1200 to CTI J530 to I600 would make for a fun flight at XPRS. I think I’ll shoot for a 2-stage flight at the July launch.
Anonymous
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