Friday, June 26, 2009

Machnum Force update - advice needed

I did some more work on the Machnum Force today.

I found, alarmingly, that the CG was almost an inch behind the CP. Not good. I needed some weight in the nose to make it fly straight. Fortunately, as long as I keep it under 7 ounces unloaded it'll still break Mach.

I found a 300-count can of air gun pellets - 4.5mm lead pellets that pack pretty well - at Wally World for 3 bucks. I hollowed out over half the volume of my 29mm balsa ogive ( from BMS) with a drill clamped in place, which left a lot of volume. (Enough for about $2.30 worth of dimes, and then some). I'm glad for the wood glue coating - extra protection against splintering. I also made a solid, heavy attachment point, inspired by the way Madcow does their nose cone weighting. It's a long-screwed eybolt, plus a large, inch-diameter washer and three nuts. I poured a little shot in, the put on top a piece of pen tube for the long bolt. I poured shot around it for weight and to fix it in plac, then wood glue into it, to fill the space between the shot and for weight. Once I put the eyebolt - washer contraption on and epoxied it into place, I had an unbreakable, very solid 3.5-ounce nosecone with a solid anchor. Suitable for G power, or even a SU H if such a thing exists.

Then, to make sure, I actually did a swing test with it. Yes, with a 9-ounce rocket. I have a long roll of nice synthetic thread, of which I wrapped about three feet all around the fins and body tube so it would hold. I masking taped it so it's held right at the CG, and I swung (swing?) tested it. At low speeds, under about 8-10 m/s, it's not stable and tends to fly backwards, but above that it is stable, albeit with only half-caliber stability.

Now, the part I need advice with.

I assembled tonight the G78-7G loadable motor for the Machnum Force. Unfortunately, I forgot the forward insulator disk as I assembled it. Everything still fit together perfectly - Nothing's sliding around or anything like that. My question is, will it still fly okay, or will it CATO due to the missing disk? It's okay if it blows up during the delay or ejection - it just needs to survive 1.5 seconds of thrust to get to Mach.

*Sleep, goes to bed.*


Anonymous said...

hello... hapi blogging... have a nice day! just visiting here....

Sascha Grant said...

The forward insulator disk will be to reduce/stop the chances of combustion gasses getting past the delay. I believe that that there is also a delay insulator as well, also designed to reduce/stop combustion gasses getting past.

If you do decide to fly it, I imagine that one of two things will happen - it may blow the eject charge too soon or it will fly and operate correctly.

I would expect that with everything well packed as you said, the odds are in favor of everthing working ok!

You may wish to talk to the RSO about this before launch and, if possible, place it on an away pad if possible.

Good luck!!

Dick said...

Sascha is correct that one operational risk is blow-by and early ejection.

Another risk is a heavy nose section coming in by itself, not the direct effects of a CATO. A .9 Mach separation will likely shred off any recovery system. I assume if it ejects early it will at least not be ballistic. However a lead filled nose coming in from 1000' wouldn't be fun. Using a farther away pad may or may not help, depending on the wind, crowd location, trajectory, etc.

Since I've never seen any of my mach busters again, I say push the button.

The EGE said...

@Hapi: Glad to have another visitor. Just wanted to let you know - you've got a Javascript error somewhere in your blog that's causing IE8 to refuse to open it fully. This site might have the reason why.

@Sascha: Thanks for the reassurance. I plan to launch it off a 1/4" rod, or a tower or piston if anyone has one, well away from the crowd.

@Dick: I built the recovery system extra strong - it's got a streamer rather than a chute, and everything is reinforced. I designed it to survive ejection at up to 300 fps because I have no idea what the Cd is, and at the speeds it goes to, a 0.2 difference in Cd can mean the difference between ejection at 20 fps and 200 fps.

Also, if anything fails, I would expect it to be the not-quite-perfect epoxy joint for the eyebolt. That would mean I'd have a balsa casing and about 120 airgun pellets coming down, but not a full 3.5oz streamilned nose.

I also put in only 2/3 of the normal ejection charge, both for that reason and because it's such a small body for a G motor to pressurize.

Dick said...

Excellent idea on the ejection charge. I often adjust them up or down. Occasionally I don't, when I also should have.

A tower or piston would be mucho better than a big draggy 1/4" lug...

Get some colored construction clalk or other tracking powder!