Doubts about minimum diameter rockets

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Tolstoi28

New Member
Joined
Jun 21, 2024
Messages
2
Reaction score
1
Location
Brazil
Hello!

I'm new to this forum and to rocketry in general. I recently joined a university group about amateur rocketry and we're going to a competition (LASC) where we'll make a 1km apogee rocket. A uni veteran told us that we should try to make a minimun diameter rocket.

Firstly, I wanted to ask about the diameter itself. We were planning on using a 74mm diameter motor, however when searching online I only see people talking about MD rockets with at most 54mm. Is our motor to big for an MD rocket? Or does it not matter that much?

Also I wanted to know if there are any special concerns we should have regarding the rocket being MD. As in, are there any problems that could happen that wouldn't happen if it was not MD? Considering that we're aiming at a 1km apogee.

Thanks!
 
What is the motor you are using, and what is the payload? The lowest-impulse 75mm motor available here in the US is a K motor that could typically loft considerable payload to 1km.

Disadvantages of minimum diameter rockets are 1) not being able to brace the fins against both the body and an interior motor tube, only against the body and 2) having fewer options for motor retention, especially in the case where the motor has an ejection charge.

I tend to think both of these disadvantages are overstated by many folk. At least for composite airframes, fins surface-mounted on the body are generally more than strong enough, and there are plenty of ways to retain a motor in a minimum diameter rocket.
 
What are your project requirements? Reaching 1 km is not that hard, especially with a 75mm motor. A minimum diameter rocket with a motor that big is likely to far exceed 1 km unless it's built comically heavily. Reaching 1 km on a 29mm motor would be more of a challenge, but still not that hard.
 
I went to the organization's web site. At a cursory review, it looks like you need to lift an 800 gm payload. I'd start doing sims of really simple 4FNC minimum diameter designs based around an AeroTech 38/720 motor case as a starting point, just to see where that is. Without having actually looked at it, my gut says that's about the right zone.
 
Hey, sorry i didn't specify everything correctly:

By 1km I mean we need to reach as close to 1km as possible, not go over 1km.
The motor we're planning to use is KNSB (fine) (just like richard nakka's) with 74mm diameter.
We have to carry at least 800g of payload (competitions rules).

I read online that it's recommended to reinforce the fins with fibreglass or carbon fiber, but is this really needed considering our motor burns for a very short time?
 
I would not recommend a new, inexperienced team try to build a minimum diameter rocket unless absolutely necessary. Once you've test fired your motor and have a thrust curve for it, you can use OpenRocket or Rocksim to see what you need for a rocket design to lift your payload to the required altitude. I'd try to make it sim to a little above 1km, as rockets tend to be heavier when built than pre construction estimates, and it's also easier to modify a rocket after it's been built to go lower than higher.

You will definitely need some sort of fillets to help with fin rigidity and attachment; if you're using a non minimum diameter airframe and through the wall fins this can be little more than a bead of glue; if minimum diameter you will need to glass near the fin root.

Good luck!
 
Last edited:
The motor we're planning to use is KNSB (fine) (just like richard nakka's) with 74mm diameter.

How was this determined? Do you have performance data for it? Have they been burned on a test stand?

I quickly skimmed the rulebook. You're allowed to control orientation electronically. It was explicitly mentioned that this system can include airbrakes, which means it can be used for apogee control. You'll be going up against teams that do that. If you don't do it, you'll lose to them because motors simply aren't consistent enough to beat an electronic control system driving to the objective altitude. Also, launch conditions are variable, and the contest can change your launch parameters with little to no warning, and it's impossible to make motor adjustments to adapt to those factors. So you have to do electronic flight control.

I would focus on two areas: absolute reliability and consistency of your motors, and the electronic orientation control. Keep everything else as simple and cheap as possible.

If you're for sure doing a 74mm motor, consider a 4-inch airframe. That will give you room to have controls built into the fins.

Go back to your intro to engineering classes. Build a top-down requirements tree. In the beginning, it will look like a graphical reorganization of the rulebook. You will flesh out details from there. Work out all the interdependencies. Turn that into a flow chart and plan for how to engineer this machine. Then schedule it out.
 
I quickly skimmed the rulebook. You're allowed to control orientation electronically. It was explicitly mentioned that this system can include airbrakes, which means it can be used for apogee control. You'll be going up against teams that do that. If you don't do it, you'll lose to them because motors simply aren't consistent enough to beat an electronic control system driving to the objective altitude. Also, launch conditions are variable, and the contest can change your launch parameters with little to no warning, and it's impossible to make motor adjustments to adapt to those factors. So you have to do electronic flight control.

I would focus on two areas: absolute reliability and consistency of your motors, and the electronic orientation control. Keep everything else as simple and cheap as possible.
I don't agree. It's a new team. The goal for the first year should simply be a successful flight, not winning the competition. Once they've accomplished that, they can look into active controls next year.
 
We get a fair number of university teams at our club launches. Their designs are overly complex, and they don't test enough before the launch. They often have recovery issues as they concentrate on all the other aspects and regard recovery as an afterthought. As we often say, the up part is easy, the down part is hard.
 
Hello!

I'm new to this forum and to rocketry in general. I recently joined a university group about amateur rocketry and we're going to a competition (LASC) where we'll make a 1km apogee rocket. A uni veteran told us that we should try to make a minimun diameter rocket.

Firstly, I wanted to ask about the diameter itself. We were planning on using a 74mm diameter motor, however when searching online I only see people talking about MD rockets with at most 54mm. Is our motor to big for an MD rocket? Or does it not matter that much?

Also I wanted to know if there are any special concerns we should have regarding the rocket being MD. As in, are there any problems that could happen that wouldn't happen if it was not MD? Considering that we're aiming at a 1km apogee.

Thanks!
Do you have access to a simulation program like ROCKSIM or Open Rocket?
Also try Thrustcurve.org. put in rocket specs and it will tell you which motors will work and which will not work as well as altitude, velocity, delay times, and a lot of other useful information. Free, Fun.
 
Any reason that the motor has to be homemade? I just did an Openrocket sim with a "Cesaroni H53-12A Mellow Rocket Motor", 29 mm min. dia design, weight with payload but no motor 1,000 g. Nothing fancy. Openrocket thinks it gets to 1111 meters, max. velocity about the same as an Estes Alpha with a C6-7. At 900 meters, it's only going about 130 mph, so maybe there could just be a little canard to put it a bit sideways for braking without breaking anything.
 
For reference, I sent a 38mm min dia rocket over 3km without too big a motor (J500G iirc).
 
You don't *need* minimum diameter rocket to get 1km on a 75mm KNSB. Nearly all MD rockets with a 75mm KNSB will exceed 1km. You can build a 75mm MD but you'll have to fight to keep it near 1km. Making the rocket MD adds completely when it isn't needed.

Look at commercial rocket kits that have 75mm motors. Run those rockets in simulator programs and see how they do. Most will exceed 1km by a far margin. I can get a 54mm MD to hit near 9km and mach 2 with a commercial rocket kit. LOC LaserLOC223 with Loki L1040

I'd suggest you look at rockets near 150mm diameter. They'll be big enough provide internal space for the 800g payload and all electronics you can think of and still targrt 1km with ease.

And as others have said, the up part is easy. The down part is hard.
 
Use OpenRocket. It's free. Build your rocket there first. It won't get you the whole design, but it's by far the fastest way I know to iterate changes to get to any particular objective or optimization.

Are you able to actually launch anywhere prior to the actual competition? Testing at the race is usually the most expensive way to do it.
 
He lives in Brazil. He probably doesn't have access to a lot of rocketry stuff we can get our hands on quite easily. Probably the main reason for the sugar motor.
 
He lives in Brazil. He probably doesn't have access to a lot of rocketry stuff we can get our hands on quite easily. Probably the main reason for the sugar motor.
I was about to say the same thing. No clue what commercial motors are available in Brazil.

I'm wondering what experience levels the rest of the team has. If they have a relatively consistent motor design in place, then design the rocket around that. Determine how big the rocket needs to be to hit the desired altitude with the payload...Adjust the diameter and length of the rocket to fit the motor - doesn't need to be minimum diameter.
 
Back
Top