minimum diameter Q

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Well-Known Member
Sep 7, 2004
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what is a good diameter to length ratio for a minimum diameter rocket
The answer is:
It depends.
It depends on what you are going to do with it, what size motor, fin design, nosecone design, ect...
For the most part, especially during design phase, you want the BT long enough, with all things considered, to make the rocket stable.
Go to the Apogee website and download the free trial version of Rocsim and play around with your design.
Originally posted by bachsta
what is a good diameter to length ratio for a minimum diameter rocket

If it's less than about 10:1, it'd be real difficult to get 1 caliber stability without using a heavy nose and fins so light (or of an excessively swept design) that making them strong would be impossible without ruining stability.

Many minimum diameter high performance designs are longer than the appear to need to be because the weight of the components themselves is so low. These tend to be built for speed more than for altitude. Speed depends on behavoir during thrust. Altitude comes from trading speed for coast time and involves weight (too little being as bad as too much) and speed.

There are too many other variables related to your question as well as things not being considered in order for there to be an easy answer.

I'll bet that 95% of high performance rockets built are built around what the builder thinks looks good, or on the length of the parts that are available. And that includes high performance kits. I doubt more than a few will ever say to take their stock size tube and cut it at such and such a length for the optimal design.

My cluster airframe birds are built for performance. The main tubes are 34" long because that's how they came. They could be half as long, but that'd save only 1 ounce on a 10 ounce rocket, which is already so light it probably doesn't coast as high as it could. Unfortunately one of the major components can't be simulated in any software available to me, so I'm stuck with trial and error. With all the other variables I have to work with, length is way down the list of priorities.
Man, you scared me with a 'minimum diameter Q' post......thought you were talking extreme altitude!

IMHO, 7:1 - 10:1 ratios look the best.
bachsta ,

you can download a trial version of Rocksim from Apogee.
with it, you can try out different body ratios ,nose shapes, motors ,fin designs...obviously you have alot of questions, and I think with the aid of rocksim you will be able to narrow them down somewhat more specificly.

I'm assuming you're planning on launching these out in the desert, since the only other club launch in our area is at Fiesta Island, where extreme is NOT the name of the game (small area for launch/recovery!).

I'd recommend going out to Plaster Blaster at the end of this month to get some more ideas. There'll be quite a few min-diameter rockets out there that you can look at and see what you like.

In the meantime, build some small fun stuff, work up your skills, and fly 'em out at Fiesta!

Also, if you want to get past the limitations of L1 Certification, there are a lot of folks out at the monthly Plaster City launches who can offer you some guidance on getting cert birds built and flown. Both DART and Tripoli SD are very friendly about that.

alright cool and of course i would only launch the min diameter ones at plaster valley. theres a high enough chance id lose em there don want to make the likelihood of one coming back any lower:) .
Originally posted by bachsta
so basically its builder's preference and convenience?

More often than us rocket scientists care to admit probably.

If I could sim my cluster airframe and came up with a different length that was optimal, I doubt it'd make much of a difference, and I doubt I'd change it. A rocket that's all a cluster of three 24mm tubes sitting on fins just looks cool being over 3 feet long.
Originally posted by bachsta
what is a good diameter to length ratio for a minimum diameter rocket

Long enough to get in whatever you need. Motor, recovery, and altimeter if used. Any more would be just for appearance if you like long rockets. A shorter rocket will have a slightly lower drag all other things being equal.
in addition to rocket junkie, wouldn't the lenth of the rocket have something to do with the CP or CG of the rocket and affect the stabilit?
I thought this post was about a minimum diameter Q rocket. That sounds crazy but just having returned from BALLS, I can say it isnt. I believe it was Frank Kosdon that launched a Q motor at BALLS I believe it was min. diameter as well. Very impressive. I may post pics later if I can.
Originally posted by bachsta
so basically its builder's preference and convenience?

I'll agree with much of what Dyan posted earlier. between 7:1 and 10:1 seem to work best for general high speed models. for Atitiude throw weight (optimal mass) is as or more important then length to diameter. Keeping in mind that every single thing attached or added to the exterior of the model will have a drastic effect on performance. For instance a single dot (1mm dia x .04mm high) of excess glue on the exterior airframe will cause such disturbance/turbulance to the boundary layer as to cause as much as an 8% decrease in attained altitude.
11 years of study on cluster altitude designs and performance model testing have confirmed with observations, Optimal mass has much more effect on alttiude performance then minimum diameter streamlining. Not to mention the many other factors that play important roles; Finish, transition treatments, drag reduction strategies etc.
Here's a group pic of 4 winning cluster alt designs. Note that not all are super long but as the motor size and mass increase the length goes up eleminating the need for additional nose weight. hope this helps, but I'm really not sure if it answers your question;)