Minimum diameter rockets?

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JimByrne

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What qualifies a rocket to be called a minimum diameter rocket?
I see them mentioned all the time but don't know what makes them different from others.
 

tsmith1315

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Short version: when the airframe is the motor mount. For example, if you have a 54mm motor, the minimum diameter tube you can use use for the airframe is 54mm. Stick a 54-38mm motor adapter in there and it's no longer minimum diameter.

With sub-minimum diameter, you forego the tube and just use the motor as the airframe.
 

heada

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Another example is the 38mm LaserLOC by LOC

 

manixFan

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The biggest issue is the fins are basically mounted directly to the surface of the airframe, which requires good construction techniques. Also the limited space relative to a typical rocket makes recovery a bit more challenging. Finally, motor retention can be a bit of a challenge.


Tony
 

manixFan

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Good comments, so to summarize, you don't have with the motor class ....... can I build a minimum diameter for a D motor?
Yes. If you are using a 24mm D motor, the body diameter would be 24mm. For a composite D motor in an 18mm case, obviously the body tube would be 18mm. The reason why minimum diameter is a thing is you are faced with several building challenges, and recovery is often a challenge due to room constraints.

If built short and light, MD rockets can go very fast, which will really test your building techniques. My 38mm and 54mm MD rockets routinely exceed Mach 2, which requires good fin design and construction.


Tony
 

Kelly

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Just to add to this, the point of a minimum diameter (MD) rocket is generally to get the highest impulse engine into the smallest drag rocket, in order to get the highest performance. Drag is largely dependent on diameter, so one of the best ways to minimize drag is to put your motor into the smallest diameter rocket that will hold it.
 

Alan15578

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Great, I ask why I would want to build a model to reach Mach 1.
As a beginners subject, you do not want to do that. As you become more advanced, you want to demonstrate skills such as staging and clustering, and flying a competition style boost glider. If your launch site is adequate you should demonstrate high altitude and high speed rocket flight. Mach 1 is a historic challenge that can be easily achieved, but accurately measuring the speed is difficult. As an expert rocketeer you should be looking to improve the state of the art in some way.
 

Rocketjunkie

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These are sub-minimum diameter rockets. 18 mm 80 N-s F13-10 motors with 10" propellant. Fins epoxied to motor case.Rolled tape upper sections and 3D printed nose cones (thanks Chuck!).
18mm rockets 4.jpg
 

dhbarr

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An 18mm Mach1 flight is just barely theoretically possible with the discontinued d21. However, it's a bit difficult to prove you broke the sound barrier if you don't get it back....
 

Rocketjunkie

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Mach is easy with Rx motors like my 18 mm F13. I can also make core burners with 60 N-s and a burn time of .8 sec.
 

6inchmonster

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Kinda facetious :)

I think the real question is why we say minimum diameter when it aint.
 

manixFan

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Kinda facetious :)

I think the real question is why we say minimum diameter when it aint.
Can you expound on what you mean by that? When someone tells me they are working on a minimum diameter rocket, I take that to mean the airframe is the same size as the motor mount. And that seems to be the common understanding of most everyone in this hobby. What does minimum diameter mean to you? I’m interested to hear the difference to see if a more precise set of terms could be used.

Tony
 

6inchmonster

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Can you expound on what you mean by that? When someone tells me they are working on a minimum diameter rocket, I take that to mean the airframe is the same size as the motor mount. And that seems to be the common understanding of most everyone in this hobby. What does minimum diameter mean to you? I’m interested to hear the difference to see if a more precise set of terms could be used.

Tony
Oh I understand, the hobby community takes that as the definition and I am totally fine with that. We also typically say tip to tip to mean 1/3 & 2/3 coverage for most of the fin, motor to mean AP reload and engine to mean BP disposable. All A-OK by me, its communal jargon.

But specifically/scientifically speaking, minimum has a fairly concrete definition. If you can remove something from "minimum", then it was not minimum to begin with. Right?
 

neil_w

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Oh I understand, the hobby community takes that as the definition and I am totally fine with that. We also typically say tip to tip to mean 1/3 & 2/3 coverage for most of the fin, motor to mean AP reload and engine to mean BP disposable. All A-OK by me, its communal jargon.

But specifically/scientifically speaking, minimum has a fairly concrete definition. If you can remove something from "minimum", then it was not minimum to begin with. Right?
Maybe true, but you're flirting with pedantry here. MD refers to a rocket of traditional construction, i.e. an airframe with a motor inserted inside. Depart from traditional construction and you can go sub-minimum. That is a totally reasonable definition as far as I'm concerned.
 

Rocketjunkie

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what does sub-minimum mean
Minimum diameter would be BT-20. These are the diameter of the motor case. Fins are glued to the motor case so they are smaller than BT-20. These are not reusable as the motors are fiberglass cased single use.
 

manixFan

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Oh I understand, the hobby community takes that as the definition and I am totally fine with that. We also typically say tip to tip to mean 1/3 & 2/3 coverage for most of the fin, motor to mean AP reload and engine to mean BP disposable. All A-OK by me, its communal jargon.

But specifically/scientifically speaking, minimum has a fairly concrete definition. If you can remove something from "minimum", then it was not minimum to begin with. Right?
What can you remove from a minimum diameter rocket and still have a rocket? You need something to attach the fins to, some how to hold the nose cone, and somewhere to put the recovery gear. We’re talking model rockets here, to start bringing in definitions from other areas would not make any sense, and worse, they’d lose the contextual meaning that we as a hobby rely on exactly in these kinds of discussions, which you acknowledge.

Can you give an example of what you mean by a minimum diameter rocket not being ‘minimum’? I’m not sure I’m following your logic here.

Tony
 

6inchmonster

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What can you remove from a minimum diameter rocket and still have a rocket? You need something to attach the fins to, some how to hold the nose cone, and somewhere to put the recovery gear. We’re talking model rockets here, to start bringing in definitions from other areas would not make any sense, and worse, they’d lose the contextual meaning that we as a hobby rely on exactly in these kinds of discussions, which you acknowledge.

Can you give an example of what you mean by a minimum diameter rocket not being ‘minimum’? I’m not sure I’m following your logic here.

Tony
Shave the body tube, paper/glass the case/liner, or remove it entirely. There are dozens of "flying case" examples out there, and I myself have done a paper wrap of a 29/40-120. I think yall are overthinking this, it is not complicated.

No, I am not trying to make everyone use new terms. We all know what it means. Its fine. Was only waxing philosophic.
 

CoyoteNumber2

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6inchmonster

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Yeah, fair. I thought the anti-kate rant was going to be more unpopular than this, guess I was wrong.
 
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