I haven't built an 18mm Mini Fat Boy.
From Rex R on YORF
"A mini fat boy can be made to work with an 18mm motor mount . . . and lost on a D10.
Built mine using a BT60 1" longer than stock, and 0.5 oz nose weight. Borrowed the body tube from a Baby Bertha."
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When upgrading a kit for larger engines, the whole idea is to maintain the Center of Gravity (C/G) location of the original model with the original recommended engine. The C/G is balance point of the rocket with the smaller (in this case 13mm) engine, wadding and recovery device in place.
With a heavier engine installed, weight will have to be added to the nose cone.
How do you know how much nose weight to add?
After the kit is built with a larger engine mount you have no way to weigh the original. The website and face card weights never match my finished models.
I know - I should be looking up the particular model, finding the C/G. Then add nose weight until the C/G is moved forward to match the original model.
This is what has worked for me, shown below:
A 18mm C6-5 engine (largest engine to be used) is .87 oz.
The difference is .58 oz., close to that 1/2 oz.
That's the amount of nose weight I would add. Usually it is clay, pressed into the hollow plastic nose cone.
When a rocket approaches two feet in length, I convert it to 24mm or D12 and E9 engines.TIP: If a model only has one recommended engine (like a C6-3) you probably have a heavy model and will be limited to one engine choice. If converted to a 24mm engine you have more options and could still fly it with a C engine in an adapter.
But - the added nose weight might make the model too heavy for the 18mm C.
A D12-5 weighs 1.6 ozs.
The C6-5 was .87 oz.
The difference is .73 oz.
In this case I'd add 3/4 oz. of clay to the nose.
You can usually look up your kits RockSim file to find the C/G location.
Be warned, these files are submitted and may not be correct.
To see the RockSim library at RocketReviews.com, CLICK HERE
I find that rockets I've designed in OpenRocket never have the CG where the simulation says it will. Partially, this is because OR doesn't take things like glue into account (though in the latest version, you can simulate fillets, but you have to know what the density of the fillet material is). Still, I find that OR often puts the CG ahead of where it is, and I suspect further ahead than can be accounted for by glue.
ReplyDeleteOn those rockets, once I find the actual CG, I do a swing test. If the rocket fails the swing test, I'll use the simulation to find the center of pressure and then measure the distance between the actual CG and CP on the rocket. Sometimes the margin is fine, and the rocket failed the swing test only because it's a long model, and the string was too short. But sometimes I find that the CG/CP relationship on the real rocket is marginal.
If the margin is actually too small, I add a tiny bit of clay at a time until I can achieve stability.
I like doing swing tests. Because you can never see your rocket flying from the side when you're launching it, with a swing test you kind of get to see what it looks like when it's flying. It's kind of fun!
Hi Daniel,
DeleteI've never liked swing tests! Years back I had one fly off the string and hit a fence. I build kits (for the most part) and don't find swing testing necessary. When in doubt on a odd-ball rocket I'll add the 1/2 oz. of clay.
Probably doesn't need mentioning, but the method used -- adding the same amount of nose ballast as the weight difference between larger and smaller motors -- does achieve goal of assuring stability, in all likelihood it adds more than necessary. Since in typical model rocket, the motor ends up being closer to the CG (since it is often the heaviest part) than the nosecone, you generally need to add less nose ballast to re-establish the original CG location (i.e. if we think in terms of moment arm, the additional ballast in the nosecone is likely to have more "leverage" than same amount near the tail). The difference may not be so great on short stubby rocket, but likely to be more pronounced with a longer one.
ReplyDeleteHi Naoto,
DeleteYou are probably right, I'm adding more nose weight than is needed. This is just what has worked for me in the past. Years back I did some double checking of C/G and nose weight. The nose weight needed was pretty close to the weight increase of the larger engine.