Aerotech G-Force strength

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Frank Hermes - Father of the RockeTiltometerTM
Jan 21, 2009
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San Diego
Anybody have any idea of what or how to determine the max velocity or max accceleration that a stock exterior G-Force (with epoxy-filleted thin plastic standard fins) can handle? When I built it, I used a 38mm MMT rather than the supplied 29mm MMT, so I can obviously use the larger motors. I am new to all this stuff, so I don't even know if the plastic fins supplied are inherently weaker or stronger than balsa or G10. So, rather than stick too big of a motor in, is there any information or software out there that could give me a sense of how powerful I could go. I have Rocksim and therefore can run the sims, but don't know how to judge just what outputed factors are safe to try. Any thoughts? Thanks.:confused:
Well just from personal experience, I built one also with the 38mm motor mount. The biggest motor it's flown on so far is an AMW I 315 skid [640 case]. Went 4200 something. I've flown it on H-165, H123, I-218, I-211. H-180.

I did use the coupler for DD.and tubular nylon for shock cord. Other than that it's stock, even the chute. Built with CA. I did put a section of coupler in the rear end, to help keep the tube from denting on landing, due to the added weight, when using big motors.
After 3-4 flights one of the fin tips broke when it stuck on landing. I just ground off the other 2 to match with a belt sander.

It's all beat up, but still one of my favorite fliers. It does well on anything from a G to an I.
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I don't have a G-Force, but from my experience, most kits, if build as per the instuctions, are a lot stronger then most people believe. As long as you stay below 75% Mach, most of your damage will come from landing and transport.

From what blackjack said, I would think you could use most of the 38mm H and I motors. I might stay away from anything above about 300N average, at least until I had a successful flight on a I1299N in the 38/480 case, then anything goes, even the J's.
I'd say stay below .8 mach or so, stay below about 40G, and you're pretty much guaranteed safe.
I appreciate the advice...I guess where I was really going with this was whether I could use it for a Level 2 cert...from what I am hearing, it sounds possible, pending its survival on some intermediate flights, which I will do...if your responses had been more negative, I would have ordered a kit now that I know would work and start building it - this way, I will fly the modified G-Force and hope I get lucky and it survives to that point! Thanks again, Frank
I feel like such a wimp! ;)

I've only flown my G-Force on G77R, G78G & G80. :rolleyes:

I DO have some G142s to use next time I fly my G-Force out at a desert launch. :D

G-Force Liftoff.jpg
I understand where you're coming from when you want to use the G-Force for a L2 cert. If it flies and survives on a 600Ns I435T, it will almost surely survive a 650Ns J420R.

Personally, I did things differently. I had two rockets I flew MPR that could have easily been used for a L1 cert flight. Instead I bought a 38mm MMT, TN for recovery, and some hardware for the Alt bay. I use 1/8" wall mailing tubes, 6mm 5 ply wood for fins and made a nose cone from foam and epoxy. I did all this because I wanted to learn more about designing and building HPR. I had also never made a nose cone before so that was also an incentive. It turned out great, I learned a huge amount and had a successful L1 flight on a I motor with dual deployment on a home made drogue and main chutes.

The second flight was on a I1299N and it turned out great too. I have absolutely no doubt I can fly any 38mm J motor in this rocket and have a successful L2 flight.

Instead, I'm building another rocket for my L2 cert. Again it is because I want to learn more about building HPR. The is the first rocket I ever fiberglassed. It has two layers of 6 oz glass that turned out to be much easier to do then I thought it would be. This is my first HPR zipperless design and I'm still undecided about buying a conical fiberglass nose cone or building one of my own. Either way I'm leaning toward pushing the limits of what I learned on this rocket for it's cert flight and using a large K or L motor for the cert and trying to bust Mach and 10K ft.

I see each cert level as a opportunity to use larger motors and because of this, learn more about building bigger and stronger HPR rockets. You can use the same rocket for your L1 and L2 cert flights and there's nothing wrong with that. But I think you pass up the chance to learn more about building HPR between the L1 and L2.

When you build a L1 rocket that can also handle the small J motors for an L2 flight, you should be congratulated on how well you build your L1 rocket. My question is, did you learn enough to make your next build that K & L sized rocket? Only you can answer that. For me, I want to answer that question with my cert flight.

Whatever you decide, good luck!
I see each cert level as a opportunity to use larger motors and because of this, learn more about building bigger and stronger HPR rockets. You can use the same rocket for your L1 and L2 cert flights and there's nothing wrong with that. But I think you pass up the chance to learn more about building HPR between the L1 and L2.

I see the same. I may have to wait 4 more years to do my L2, but when I do I will probably use airstarting as part of my design.
Here's a couple of shots of my old beat up G-force. After crashing it and a Sumo. I hobbled all the parts together to get my "Gumo"

Nose cone and 1 fin from the Sumo and payload and 2 fins from the G-Force. New tube for fincan and assorted other parts to assemble rocketry's Frankenstein.
These shots were on a G-64. I still love how easy it is to fly. Pop a motor in, stuff the chute and GO!

It has nice slow lift offs and the chute always comes out right at apogee. Flies low enough you can watch everything easily. It's always been a crowd pleaser.

It was my first BIG kit and I'm glad it's still around after flying 6yrs. When this one R.I.P.'s I'll build another with wood fins and a 54mm mount.

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Blackjack - I too like the notion of slow liftoffs and seeing it all - that is what lead me to the G-Force in the first place, so, I think as long as I don't RIP it or lose it, it will always be around for that! Unless I can figure out something bigger and slower! But, the altitude bug has bitten me a might - see below.

Handeman - it's not that I don't want to learn - I do! I have been sorta non-stop since I first re-discovered rockets about a month ago, but I must say that I seem to be more interested in the avionics and telemetry - gadget- side of things than the model building aspects themselves. At least at this point, but like I said, I am VERY new.

I find the certification process somewhat annoying so that is why I was wondering if I can shortcut getting to the point where I can have easy access to the bigger motors. That's all.

My immediate goals now seem to be to get my LI and L2 out of the way and to get the G-Force to 5K'...the G-Force is sitting in the garage in need of a final sanding/painting/personal decal application...hope to fly at Plaster City at the end of the month...

In addition to the MMT mod, I also modified the innards a bit so as to include a removable avionics bay that right now houses a beeper locator and an elementary altimeter - I figure if my first try is with a G77 and the rocket weighs out at 56 oz. dry, it ain't gonna get too far away from me!

Tomorrow, I am supposed to get a new Yaesu VX-8R HT radio so that when my Big Red Bee 70cm GPS and tracker shows up next week, I will be able to see the APRS info (my call is K6FBH)...I figure if I can get it up to 5K', I might need more than the beeper! Hopefully somewhere during all that, I will be able to load a low thrust J and get my L2...

Then, the next target will be to do a new one, use DD and get to 10K' - I would also like to use staging for that, so I am considering the CPR version of the PML Quantum Leap...but it looks as though trying to get more gear inside that one may be a pain, so I may forget it and go to a glass-finned Eclipse. I too have never done any FG, so that should be interesting. I would also like to see about putting a camera/xmtr on board, so using the bigger body might be better for that aspect too. The Eagle Tree stuff looks like fun conceptually (all the realtime display and such), but it's expensive.

Then, I would like to collaborate with someone and get to 25K'.

Thats' it - my ulitimate goal at this point is 25K' - full of cool gadgets and such! But I still like the notion of slow and seeing all of the flight, so I suspect the G-Force or similar will always be in my arsenal!

You're right in the sense that I will need to learn a bunch along the way. And I want to do that. From my perspective, the L2 cert thing is just an obstacle rather than a goal, so if I get it with my mod'd G-Force I can move on to the other more quickly.

Thanks again to all for the input! Frank:D

P.S. Sorry to be so long, but I really enjoy this stuff
No need to apologize for a long post. If you really work at it, you might be able to top some of the essays I've put together over the years.

I understand where you're at about the certifications. It's a slightly different look at it then what I have, but that's why it's such a great hobby. We can each get the enjoyment out of it concentrating on the aspects we enjoy the most. It's all about what trips your trigger!

Keep us up to date on that road to 25K and I definately want to see the onboard video from that flight!
Back to the specific point of the original post:

Going on the basis that my G-Force with stuff in the e-bay is about 55 oz. dry (I Rocsim'd it and actually weighed it and they basically agree), and, using cjl's recommendation to stay below .8 Mach and 40 G's, there are a few J engines that could work (though I realize that with restricted mail order access I will be subject to whatever the range may have available to me)...

Just as an FYI, the best sim altitude is around 4.5K' with the J350, of course assuming decent construction, which may be a stretch considering it's my first "real" is now so heavy (stock kit weight is supposed to be about 36 oz.) that the G38 and G40 that I originally bought with it don't really sim out, but the G77 should barely be OK - I will probably start there and work up and see if it holds together thru a small J...

I will report...thanks again all, Frank
I have a 55 oz rocket I flew on an I1299N and it simmed to 80g. The H999N actually simmed to 100g, so it really does depend on the actual thrust curve.

Just keep running those sims. you mean a G-Force with the raw plastic fins or was it something you had that you had glassed?