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Old 09-24-2012, 10:44 AM
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CPMcGraw CPMcGraw is offline
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Join Date: Mar 2005
Location: Mobile, Alabama
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Quote:
Originally Posted by Brain
I was hoping to see a new design of yours here again (for a change), and here we are.
You definitely have a different eye than I do for design details. But I like!


I've been in a dry spell for nearly two years. The itch to do something is really just getting started again.

What I've been trying to do with my more recent stuff is to design for low-Dv. Simply put, to achieve a low velocity at the apogee so that the parachute(s) don't open "explosively". I've got a few like that where you think the chute is going to rip itself off; the sound of the chute opening (even 300-400 feet up!) is as loud as a gunshot. If I design to avoid such, then the end result is usually a better-flying rocket overall, even if I have to sacrifice some top-end performance to get it.

Quote:
A question from looking at the RS schematic: When building a rocket in that prog (or even ORK), does it behoove the designer to think about things they will encounter when trying to build one of their designs?


If you do enough of it over time, it starts to become "second-nature"; you don't think of it so much, it's just the way you work. You can get longer tubes; the 18" lengths are easier for the model companies to bag up. Longer tubes have a greater chance of getting folded. I think "TotallyTubular" and maybe BMS sells up to 36" lengths, but when you place an order for those, you need to order enough to fill a decent-sized shipping box so that they tend to protect each other.

OTOH, there are times when you have to think about the assembly sequence, like if you have a long BT-20/ST-7 tube that is also the engine tube. It helps to break that tube into two smaller lengths, like placing a 2.75" or 3" length at the rear with the coupler acting as both a coupler and an engine block. Double-duty. Or, as you've seen, you separate what actually remains a single tube into smaller sections for working within the program (like hidden lengths inside a transition, for example). Such tricks become part of your kit-bag after awhile.

Quote:
For example, generally-speaking I've only seen 18" total tube lengths for sale and I wonder if I wouldn't want to keep that in mind in the design phase and actually edit a tube's total length to that 18" and then add a tube coupler & more tube length to get what would be in the real world... I suspect the addition of a coupler in RS or ORK would bring you closer to what you'd really get during a flight sim... True? Unimportant consideration?


I've used random-length tubes in some designs, placing a coupler at known tube lengths to keep the mass correct, but not breaking the long tube down otherwise. From the POV of the program, it doesn't care. It calculates aerodynamics on the total exposed length(s), not the individual lengths that may be internal. It calculates the mass on the total of all the parts based on their area totals and the mass values per unit in the database; not on any specific part. We've run into issues with the database on this already; some component materials have bogus entries, or values that are at best whacked. It catches us occasionally...
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Craig McGraw

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