Sunday, July 19, 2009

Machbuster on a G80

I've decided to go for buying a G80-13T for the Machnum Force for reasons:

1) It's more powerful. The G80 has a hair more average thrust and about 30% more impulse, spread out over a longer time period, in the same physical case. It is the single most powerful model rocket engine available*. This means Mach 1.23 versus Mach 1.08, which is a huge difference given that I don't know the Cd of it within a reasonable period, nor do I know the conditions of the launch or the exact margin of stability (more speed = more stable (while subsonic) = safer), and I won't know the weight till it gets checked before putting it on the pad.

2) It's only 3g heavier. That means I don't have to rebalance it.

3) I'd like to use the G78G and its beautiful green propellant in a lower-altitude rocket like the Nike-Apache (after my cert); the G80T has no such nice flame.

4) The G78, as mentioned in a previous post, is missing the forward insulator disk and while the Nike-Apache or Mozzie can probably survive a CATO, the Machnum Force would be utterly destroyed by a CATO at Mach 1.1.

5) I can conveniently buy a G80 at NERRF along with the H128 load for the Nike-Apache.

6) The 7-second delay on the G78, while good for the lighter machbuster I'd originally planned, is far too short for the heavier rocket, which needs the 13-second delay to have a low-speed ejection. The 7-second delay, however, is perfect for a mid-power rocket like the Nike-Apache.

It's gonna be fun. I'll bring this tiny rocket out to the RSO table and he'll ask something like, "Nice little rocket. What're you flying it on, a C6" since that's a reasonable assumption. Handing him a rocket weighting 10oz loaded and saying, "Nope. G80" will be a moment to remember.



* The G69N (38/120) is also a 137 Ns model rocket motor but is 38mm, reloadable, and not nearly as convenient since it doesn't have an ejection charge. The G75J is 155 Ns and under 80N average thrust but has more than 62.5g of propellant. The G104 and G339 are both higher average thrust, but both reloadable and due to their high thrust are high power motors and less impulse than the G80 anyway, plus the G339 has no ejection charge because it's Warp-9 propellant.

2 comments:

Dick said...

I'd be shocked if the rso at a major launch hasn't seen (or even built) several of these :) They may want to see if you know where the cp is.

The G80 is a good choice due to the additional oomph and the 13 sec delay. A major failure mode with these rockets is ejection at high speeds. Even if your shock cord can take it, the chute can't. As you mentioned before, pull the cap and remove some of the charge. (see I might even remember this next time ;) )

The EGE said...

I imagine most people are smart enough, though, to use a longer and more stable body. I simply used the longest (13") piece of 29mm tubing I had so I wouldn't risk having the airframe fail midflight.

I'll have a niceified version of the CP calculations, just in case. I've actually done a swing test with it loaded with the G78 and it's very stable - about 0.8 calibers stability.

I think I only used about 2/3 of the full charge, and I might reduce that to even less.