Optimal mass?

[Anonymous]

Guys, I’ll post this here rather than tack onto other threads. I’m seeing an interesting trend of late – both w/ Adrian’s recent I600 blast, as well as with Curt Von D’s TRA “M” record. Both rockets were insanely light (the M rocket weighed about 4# and had nearly 8,000NS of propulsion).

Is the Rocksim optimal mass that whacked out? I’ve personally added mass many times… I typically use sand that gets dumped at apogee. Now I wonder if that hurt my altitude?

Adrian, your I600 shot was similarly light given the materials used and the mass you reported. I added mass to my I600 boost, and I have typically added mass to the J570 boosts.

I may have been approaching this all wrong. That said, anyone who has ever tossed a golf ball and a ping pong ball knows there is indeed a LOT to optimal mass, especially on fast-burning motors. Thoughts?

[Bruce R. Schaefer]

I know, JW. Now, I’m confused. I always laughed at those who said that lighter flies higher… since optimum mass flies farther, in theory, during coast phase (essentially the same drag but greater momentum, like the golf ball). Now I don’t know what to think… sounds like an experiment is in order to put this to rest. One rocket, built light, flown… optimum mass calculated and added, then flown again. ❓

[Anonymous]

Also keep in mind that optimal mass varies with the motor. In other words, the same rocket would have very different optimal mass calculations if flown on a K1100 vs a K185 – both of which are 1500NS motors.

[Bruce R. Schaefer]

And, was Adrian’s rocket AT optimal mass with that motor? Too many variables… my head is beginning to hurt… 🙄

[AdrianParticipant]

The optimal mass varies a lot with the rocket drag. With a draggier rocket it helps to have more mass, and less mass for a lower-drag rocket. I left mine relatively light because there was so much variability in the Rocksim nosecone drag estimates and I didn’t know how this new nosecone shape would come out. I figured I could add more later. With the nosecone shape that I’m using, there’s not as much of a penalty for going fast, so I’m glad I have the lighter weight. It was 1014 grams sitting on the pad.

After I tweak my sim based on my measured data, I’ll be better able to optimize the mass for different motors.

[Bruce R. Schaefer]

Adrian, keep posting as you progress so we can see if the optimum mass “theory” is really true; and if so, HOW true, being that there really are numerous variables involved: drag, NC shape vs. expected velocity (i.e., conical Mach 1 and above), mass, airframe length, on and on. 😉

[sserell]

I like the idea of doing controlled experiments – I’ve got a Carbon Fiber Mongoose 38 on order – its a minimum diameter high performance bird that should be a great kit. I built and flew my Mongoose 54 to over 22k at MHM. If someone else is interested we could build 2 identical rockets and then carry out experiments varying mass and maybe nosecones to if we can generate any significant data. the 38mm hopefully wont be too expensive or go too high to prevent getting several flights in a day. Anyone interested? -Sean

[Chad]

I doubt I’ll get around this summer to building a I600 shot to go up against Adrian, but I have messed around with RockSim enough to know that the optimal mass calcs are wacky. First, there is some hysteresis in the weight function, which doesn’t make sense as I presume it is an iterative calculation. Occasionally rockets produce unrealistic weight-altitude plots. Second, the pressure drag above mach isn’t right. We have all noticed that a conical nose cone in RockSim beats a VK, and all the NACA test reports say the opposite for a transsonic bird. If the program can’t nail down the drag, then the weight would be off. In particular, it seems like it is adding too much nose-pressure drag at and above mach. Only drag calcs like what a Parrot altimeter can do will answer this question definitively.

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.

[Anonymous]

One wild card for everyone here – I truly believe the older Rocksim versions were far more accurate (on a number of levels). I threw away the newer versions and have used v4.0 for a long time. That was largely written by someone else, is “ancient”, not supported by Van Milligan – and I daresay a lot more accurate than most would think. It was old when I started flying in the 90’s.

Anyway — according to that version of RS, a dry weight of 700g would be well over the optimal mass. By way of example, my I600 rocket was well below 500g, and that was with added weight. FWIW, my I600 rocket was phenolic glassed with 6 oz. cloth. I used a conical cone because I haven’t gotten around to making a VK (and if I ever do, I probably wouldn’t want to make it for phenolic, anyway) and there was a lot of lead in the nose.

Bottom line, Chad – I think 700g is probably more than enough.

Interestingly, on many of these type boosts there is a very flat curve for OM – a large “sweet spot”, if you will. If that is indeed the case, I’d much rather go a wee bit heavier because you can always dump weight to pare back, and it keeps velocity in check.

[Chad]

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.

[Author]

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