I worked on Kerbal yesterday and today, and just now got the whole rocket stacked together with virtually all the components (av-bay, parachutes, shock cords, etc. – everything but a motor) for the first time. The main snag I finally discovered was that the outside diameter of the forward av-bay bulkplate was ever so slightly larger than the diameter of the coupler tube, which was causing it to get bound up inside the nose cone. Once I sanded down that edge, everything fit. I’ll go back over it and sand everything again when I finish things up.
This allowed me to get an actual weight and balance so I could update my flight simulation and see if the rocket was going to need extra nose weight. It doesn’t – this is excellent news because it would have been really difficult to find space for it. This rocket uses “head-end ejection” which means that the main parachute and its shock cord live inside the nose cone. At the moment I have it loaded with a 52″ SkyAngle Cert II parachute that I had originally bought to use for Black Widow (and don’t use for Black Widow anymore, but that’s another story). It takes up almost all of the space inside the nose cone; I may end up finding something different for it between now and Airfest, but this was good enough to let me weigh & balance.
Numbers so far: total weight is 10 lb 5 oz, and the no-motor CG is 33.25 inches from the nose. (I still haven’t been able to break the bad habit of designing in pounds and inches.) Using the base drag modeling method described here, I end up with a stability margin of about 1.35 on a three-grain motor, which should be just right. (Without the extra base drag, the margin is down to around 0.52, but still positive.) The J360 sparky motor that I had picked out for it might be a little bit too wimpy for typical Kansas conditions, so I might have to get a new motor for it at Airfest (J449 might be just about ideal) and fly the J360 at a local launch when it’s calm.
I’ll also add an update about SwampGas, although this is really conceptual. The idea all along was that SwampGas was going to be a saucer rocket with a string of neopixels around the outer edge. The string I have is a meter long, so I’ve been visualizing this as a fairly large rocket.
[See above for how I still haven’t internalized the metric system.] It turns out that a one-meter circumference circle is about the size of a dinner plate. I actually got to the stage of cutting fins and putting an airframe together before I really realized this, and that realization kind of took the wind out of my sails. I immediately started thinking, “well, after I finish this one, I’ll make another one that’s bigger.” But I kept not having motivation to keep at it, because a dinner-plate sized rocket was really not very exciting to me.
So what I’ve decided to do is just skip ahead to the upscaling part and scrap the airframe I started. I still have plenty of materials left to do that – I just need to get more neopixels. I also have to figure out how many neopixels I can actually power with the battery setup I have in mind. I think two meters of 60 pixels/meter should be doable, but I need some napkin math to convince me that’ll be OK. (And I may just buy a whole spool of neopixels and work it out practically, since power consumption isn’t really straightforward – it depends on how many pixels are lit at the same time, how bright they are, and even what color they are, since RGB only light one LED and secondary colors use two or three.) But anyway, stay tuned for SwampGas 2.0.