Saturday, November 28, 2009

Thoughts on the Mach Goon

I've finally gotten to the point where I'll actually be building the Mach Goon soon. Most of the design is settled; however, there are still 2 things I need to figure out.

First and foremost is motor size. I have 4 options: Cesaroni Pro38-1G or 2G, or AT 38/120 or /240. Simulations using a variety of motors are below; Cesaroni motor data is estimated and not exact. Parameters: 3oz, 1.65" diameter, Cd = 0.6.
SU | G80T: 3066 ft, Mach 1.11. (137 Ns)
38/120 | G61W: 3276 ft, Mach 0.88. (120 Ns)
38/120 | G67R: 2985 ft, Mach 0.85. (110 Ns)
38/120 | G69N: 3486 ft, Mach 1.02. (137 Ns)
38/120 | G339N: 2959 ft, Mach 1.29. (110 Ns)
38/240 | H123W: 5256 ft, Mach 1.30. (230 Ns)
38/240 | H148R: 4368 ft, Mach 1.27. (220 Ns)
38/240 | H242T: 4554 ft, Mach 1.62. (230 Ns)
38/240 | H669N: 3790 ft, Mach 1.72. (220 Ns)
Pro38-1G | G115WT: 3401 ft, Mach 1.15. (141 Ns)
Pro38-1G | G185VM: 2908 ft, Mach 1.19. (128 Ns)
Pro38-2G | H152BS: 4813 ft, Mach 1.45. (279 Ns)
Pro38-2G | H400VM: 4327 ft, Mach 1.93. (255 Ns)

Several things are clearly obvious. First, the 38/120 case is right out - the only 2 motors in it that break Mach are Warp-Nine reloads, which I can't use because they can't have delay grains or ejection charges, and there's no room for a timer.

Second, that most of the two-grain reloads - AT 38/240 and CTI Pro38-2G - would not be good, because many of them would be pushing Mach 1.5, and a few even get past 2000 fps. At those speeds, there's no way I could ever manage to hold fins on without an absolutely perfect epoxy job. Hence, the best choice will be the Cesaroni Pro38-1G case with a G115 White Thunder or G185 Vmax reload. The G115 was the advantages of being far easier to track and offering slightly gentler acceleration; the G185 brings higher speed and a lower altitude - within reason, even - to recover from.

Third, note that a 29mm G motor can, theoretically, break the sound barrier. However, an complete Aerotech 29/120 case is actually more expensive than a 38/120 casing, as is a CTI Pro29-2G system including the rear closure, which is not needed on their 38mm reloads. The G80 is then within reason, but I'd prefer the 1G case since, if I do get it back, then I can fly G69 sparkies in it!


The second issue, the fins, I'll deal with later today.


Motor data from Thrustcurve.

2 comments:

DTH Rocket said...

The fins aren't as bad as I used to think. It all depends on what resourses you have available to you. A simple fiberglass layup should work, shouldn't it?

(I love mach rockets! I hope to build another next summer!)

The EGE said...

Eh, I'm not nearly skilled enough to do fiberglass. I will coat the entire body and fins with 2-3 coats of wood glue, and that should be enough. The same worked for the Machnum Force.

And if you do build another machbuster, don't do what I have done so far. Build it long and stable, with small fins. Build it out of Blue Tube if possible. Build it so it'll go past mach on the larger available SU motor, rather than a reloadable motor that you risk losing the case. That means a 24mm F32T, 29mm G80T, or 38mm I350R. Don't attempt to visually track the rocket; just put in lots of tracking powder, launch it on a calm day with a smallish streamer, and just follow the direction of the cloud of tracking powder to try and find it.