29mm minimum diameter design questions

[Cooperr]

I'm designing a minimum diameter rocket to go for altitude on a G80-13T. It's going to be for the LDRS41 competition but I haven't built a minimum diameter before. I've had some questions while working in OpenRocket (see pic) and researching:

1) Is it fine to start with low stability (<1) at launch? Making smaller fins has nice performance benefits, and I figure with a rocket going up so fast stability is less of an issue than normal. OR seems to think the rocket will fly fine even starting with a negative stability at launch. (Won't actually do a negative stability of course, thinking of doing at least 0.5)

2) I'm running into an issue where the flight time is too long and the ejection charge would go before apogee. How can I optimize the flight to reach apogee faster but still pretty high up?

3) How thin can I go with carbon fiber fins? On other rockets I've seen fins that look pretty thin. I'm thinking of using something like a 1mm carbon fiber plate.

4) Is a tightly-packed streamer fine for recovery? My worry is it will be coming down too fast. I'd like to stick with just a streamer so that the rocket doesn't drift so far and so that I can keep the body of the rocket as short as possible. (OR is saying a shorter body increases apogee)

5) Are altimeter holes a big consideration in order to get an accurate altitude reading? As I understand it, altimeter holes should normally not be right below the nosecone, however that's pretty unavoidable with this rocket. It seems like this isn't something people put a lot of consideration into, but for an altitude competition I would think it's pretty important.

Thanks for any help!

View attachment 580599

 

[David Schwantz]

I don't do OR. And these are all just IMO. You have a short fat stubby rocket with big fins. Sweep the fins back more, lengthen it for more stability. Keep adding weight to NC in OR till you see a drop in altitude. then add that amount to NC. 1mm fins ok if ridged enough, remember it has to come down with no damage. Streamers are good, I use them lots. When you blow the NC off your alt can get a reading then, but I would have at least one vent hole. Put an Eggtimer Apogee in it to get best apogee height.

 

[Art Upton]

For Altitude you want a longer burning motor then a G80, less drag and make it less stubby.

 

[SolarYellow]

That's for this, right?
https://www.rocketryforum.com/threads/competitions-at-ldrs41.179826/

Altitude Competition #1 -- Objective: To reach the highest altitude using one (1) 29mm single use AeroTech G80-4T, G80-7T, G80-10T, or G80-13T motor.

For the purposes of addressing your questions, I'm going to assume that's true.

1. The "optimum mass" for your design will likely enable you to have more nose mass than you need to meet generally-accepted standards for stability. You don't want to take any chances on something dense and G80-powered skywriting off the pad. Very dangerous.

2. Dual deployment is the solution to this problem. There really isn't any other. Then it also doesn't matter what delay motor you select, since you won't be using the ejection charge.

3. Nobody can tell you that without doing a flutter analysis. Learn how to do a flutter analysis and do it on your own design. Your design will likely be well into the transonic range, if not knocking on the door of Mach1. Avoiding flutter will be crucial. Learn about it, and learn about what kinds of fin designs are more resistant to it.

4. Dual deployment is also the solution to this problem. OpenRocket is nearly useless for predicting streamer descent velocity. You can use a streamer, but you'll probably want a wider one than you've given yourself length for. Make sure the length OR shows you for the motor includes the ejection charge bay at the front, or you'll need to add it on when determining your packaging space. Don't forget wadding, etc.

5. Altimeter holes can be anywhere in the airframe that's several calibers down from the nose cone/body tube junction and above the area blocked by the motor casing. Probably want it above the wadding, if you're using wadding. The good news is you have such a small volume, they don't have to be very big. They must be big enough, though, or your flight control altimeter will deploy the first stage too late.

Other things:
The information above is very, very basic. It's concerning that you are undertaking this and had to ask the questions.

You probably will want a tower to launch from. You can make the tower taller to achieve greater velocity and hence stability when the rocket clears it. Launch lugs and rail buttons will kill your apogee with unnecessary drag and someone launching from a tower will smoke you.

If the rules allow it, you'll probably be flying against "flying motor" configurations where the fins are glued directly to the motor case and everything else is glued to the front. Those rockets will go higher than your conventional tube design.

 

[robopup]

I'm designing a minimum diameter rocket to go for altitude on a G80-13T. It's going to be for the LDRS41 competition but I haven't built a minimum diameter before. I've had some questions while working in OpenRocket (see pic) and researching:

1) Is it fine to start with low stability (<1) at launch? Making smaller fins has nice performance benefits, and I figure with a rocket going up so fast stability is less of an issue than normal. OR seems to think the rocket will fly fine even starting with a negative stability at launch. (Won't actually do a negative stability of course, thinking of doing at least 0.5)

2) I'm running into an issue where the flight time is too long and the ejection charge would go before apogee. How can I optimize the flight to reach apogee faster but still pretty high up?

3) How thin can I go with carbon fiber fins? On other rockets I've seen fins that look pretty thin. I'm thinking of using something like a 1mm carbon fiber plate.

4) Is a tightly-packed streamer fine for recovery? My worry is it will be coming down too fast. I'd like to stick with just a streamer so that the rocket doesn't drift so far and so that I can keep the body of the rocket as short as possible. (OR is saying a shorter body increases apogee)

5) Are altimeter holes a big consideration in order to get an accurate altitude reading? As I understand it, altimeter holes should normally not be right below the nosecone, however that's pretty unavoidable with this rocket. It seems like this isn't something people put a lot of consideration into, but for an altitude competition I would think it's pretty important.

Thanks for any help!

View attachment 580599

Sounds cool! Here are a few comments:

1) In general, NO!! If a lot goes right, you *may* luck out. But I've seen a lot of minimum diameter (and tried 1 or two myself back in the day) rockets with ~1 caliber which went unstable. I've also found, particularly if you only have 1 or 2 attempts, that you get more bang for buck with a straight flight on a rocket with fins which are a bit bigger than you do with a touchy, swirly low stability rocket anyway. I usually aim for ~2 calibers through all speeds for my minimum diameter rockets. It's also important with min diameter rockets when you're designing fins to moniter stability as a function of speed (Cp can move) and flight time (Cg is moving due to burning propellant and Cp is moving due to speed) - OR has this ability. Finally, looking at the picture in your sim, those fins look pretty dinky tiny too small (a technical term) to me...

2) Yes, my buddies and I all see the same thing on our "fun" no altimeter motor eject min diameter rockets. In general, if you're trying to separate at apogee, use electronic deployment.

3) Very thin if you have good quality material, a good bond and straight fins. For reference, I've flown 0.045 in thick uni prepreg carbon fiber (T700) [-45/0/45/90]s on a 38mm J570 and 54mm K250 without issue. For a G80, 1mm should be plenty.

4) I don't see a problem with a streamer. I find if I sew my own chutes with thin 0.75oz material, I can pack smaller with a chute than a streamer anyway though.

5) It depends on your altimeter, your mounting location the sensor(s) it uses to determine apogee and any data filtering it applies. The rocket is (ideally) moving slowly at apogee so you're usually going to get good static pressure in that phase of flight. It's the fast portions you have to worry about if you're measuring pressure. Below is data from a flight I did last fall which compares altitude calculated from raw baro data, from accelerometer data and the final kalman filtered result I actually use to determine flight events. Note the baro altitude even went negative(!) during boost, which shows why it's important to consider mounting location and altimeter logic. Also attaching a picture showing where the holes were in the nosecone - these were several inches ahead of the start of full diameter tubing.

 

[bad_idea]

To add to the chorus, consider electronic deployment. I would use a single-deploy altimeter with a small-ish reflective streamer to bring the rocket down quickly, based simply from what I hear from others about recovery at Bong. An Eggtimer Apogee is about 10.5g with battery. Add a couple of grams for an ematch and charge.

Of course from what I hear about recovery at Bong, you may also wish to consider the smallest feasible tracker you can find, but that might be mass-prohibitive for what you're trying to do. Use OpenRocket's optimization feature on a nose weight and see what your mass budget actually is.

For a subsonic rocket, I would say that a rocket with an aspect ration of less than 10:1 can easily get away with less than one caliber of stability, but since you will almost certainly go supersonic (your sim says you will) or at least deep into the transonic region, I'd try to get stability close to two calibers. Check your final design from OpenRocket in RasAero and adjust as needed.

To add to another chorus, consider a launch tower if you're not already planning one.

A quick glance at the rules indicates that as @SolarYellow says, you'll probably be up against flying cases, so you might want to reconsider your entire design.

 

[Cooperr]

For Altitude you want a longer burning motor then a G80, less drag and make it less stubby.

Makes sense, for the competition I specifically have to use a G80.

The rocket is indeed for the competition that a few mentioned. I'm going to try and get in contact with the organizers about the rules to see if a flying case is allowed. I'll go for that instead if it is, but I wasn't so sure if they would be happy with that.
In general for a competition like this, would it be fine to file off the thrust ring on the motor and remove the ejection charge, or do they want motors to be completely unmodified?

 

[SolarYellow]

@AeroTech (Gary) has said he doesn't consider removing the thrust ring to be an experimental modification. He's on the BOD of Tripoli, as well as being the manufacturer of the motors.

https://www.rocketryforum.com/threads/aerotech-open-thread.125657/page-125#post-2044762
I have PDF'ed the post and keep it on file in case I ever need to make that argument.

--------------------

ETA:
This rule logically precludes the use of finned cases.
https://ldrs41.com/contests

A rocket that is unable to sustain a safe future flight will be disqualified.

A finned single-use motor case can't "sustain a safe future flight."

If I was going to LDRS, I would probably just use the Apogee thin-wall 29mm tubing and nose cones, at least as a default, "easy button" starting point. I have some in my rocket parts inventory, and they are pretty decent, quick and cheap. Depending on sim work, that might morph to Chinese c.f. tube and a resin-printed Von Karmann nose cone.

I really like the idea of this competition. I hope it becomes more widespread, as it's in the sweet spot of rockets I consider most interesting.

 

[Art Upton]

Same with removing the ejection charge, it's ok.

 

[Cooperr]

A finned single-use motor case can't "sustain a safe future flight."

Right that would make sense. When I read the rules I interpreted them only as the rocket recovering safely and not sustaining damage.

I've emailed LOC Precision, who are organizing the LDRS competitions, to double check about the kind of rocket designs allowed and what they would consider acceptable for motor modifications.

Thank you SolarYellow and Art Upton!

 

[tfish]

@AeroTech (Gary) has said he doesn't consider removing the thrust ring to be an experimental modification. He's on the BOD of Tripoli, as well as being the manufacturer of the motors.

https://www.rocketryforum.com/threads/aerotech-open-thread.125657/page-125#post-2044762
I have PDF'ed the post and keep it on file in case I ever need to make that argument.

--------------------

ETA:
This rule logically precludes the use of finned cases.
https://ldrs41.com/contests

A finned single-use motor case can't "sustain a safe future flight."

If I was going to LDRS, I would probably just use the Apogee thin-wall 29mm tubing and nose cones, at least as a default, "easy button" starting point. I have some in my rocket parts inventory, and they are pretty decent, quick and cheap. Depending on sim work, that might morph to Chinese c.f. tube and a resin-printed Von Karmann nose cone.

I really like the idea of this competition. I hope it becomes more widespread, as it's in the sweet spot of rockets I consider most interesting.

@Cooperr
@AeroTech

I'm not sure Aerotech had the G80 ...phenolic motor cases... in mind when they approved removing the thrust rings.

Tony

 

[SolarYellow]

The more I have thought about this competition, the more I like it! It's the perfect sweet spot, with Mach-busting HPR performance but MPR costs and PITA-ness. Anyone can play because you don't have to be L1 and you don't even need a waiver to launch.

The long burn motors (i.e., E6, F10, G12, H13 & H14) are basically a lifting contest. Finished builds are almost inevitably going to come out above optimum weight for the motor, so the goal is just to make them as light as possible, consistent with as low drag as possible. That constrains the choice of electronics, etc.

By requiring everyone to use the same motor and picking one with some real sauce, the speeds are high enough that optimum mass comes out well above any reasonable build. (I picked a sim from my OR library that I'd already done that was basically suitable for this and tweaked it a little. Ended up with >3 oz. of ballast over and above a Quark/EF Mini combo and still not to optimum mass, per OR's apogee calc.) So there's a ton of room to mess with different electronics, deployment strategies, materials, construction and finishing techniques, mass distribution, etc. And there are so many different curves to optimize. Do you want to put all the mass in the nose and make it as stable as possible? Do you want a more balanced distribution? Do you want to make it a barbell for maximum rotational inertia? Do you keep piling on the grams because your sim says it drives the apogee a few feet higher, or do you prioritize getting off the rod/rail/tower with more velocity to better resist any disturbances at the beginning that screw up the whole rest of the flight? There are all the Mach busting technology issues.

A lot of rule sets clearly drive everything in one direction very quickly. The goal is obvious, and it's just about different methods for getting there, or tiny differences here and there. This concept has tons of room for experimentation and trading off one element versus another on multiple fronts. It's small enough that material costs are low and most working grownups can afford to build a bunch of them and iterate designs quickly. By far the most expensive parts, assuming you are successful (and lucky, perhaps) with recovery and don't lose electronics frequently, will be the motors and the building time.

At the club/friends level, there's room for simplifying constraints, such as specifying a body tube (e.g., the obvious choice of standard 1.14 ID/1.21 OD 29mm MMT) and maybe fin material (e.g., some kind of wood). That simplifies the design element to fins and overall construction and finishing techniques, while leaving open the mass distribution, packaging, electronics, recovery, etc. (I would strongly advocate not doing such a thing if it became a standard competition at the national/international level.)

This has amazingly strong legs. I could see it growing to become the MPR/HPR world's Spec Miata. Would be awesome if the volume on G80s increased to the point that AT could drop the price. Or maybe make a -P version to cut some cost out of it and drop the price.

 

[AeroTech]

@Cooperr
@AeroTech

I'm not sure Aerotech had the G80 ...phenolic motor cases... in mind when they approved removing the thrust rings.

Tony

The G80 has been through several design and specifically casing design iterations since it was released. Paper phenolic, straight molded and finally molded with the integral thrust ring. The current thrust ring cannot be removed without potentially affecting casing integrity. Fiberglass-cased DMS motors are another story.

 

[Cooperr]

and finally molded with the integral thrust ring.

That's what I have on the motor I bought, it's molded straight into the plastic case. It looks like I probably can't go with a single use case type of rocket anyways (still waiting on a response about the competition rules), so it shouldn't be a problem to leave the thrust ring on. Thanks!

 

[mynameisfinn02]

I'm designing a minimum diameter rocket to go for altitude on a G80-13T. It's going to be for the LDRS41 competition but I haven't built a minimum diameter before. I've had some questions while working in OpenRocket (see pic) and researching:

1) Is it fine to start with low stability (<1) at launch? Making smaller fins has nice performance benefits, and I figure with a rocket going up so fast stability is less of an issue than normal. OR seems to think the rocket will fly fine even starting with a negative stability at launch. (Won't actually do a negative stability of course, thinking of doing at least 0.5)

2) I'm running into an issue where the flight time is too long and the ejection charge would go before apogee. How can I optimize the flight to reach apogee faster but still pretty high up?

3) How thin can I go with carbon fiber fins? On other rockets I've seen fins that look pretty thin. I'm thinking of using something like a 1mm carbon fiber plate.

4) Is a tightly-packed streamer fine for recovery? My worry is it will be coming down too fast. I'd like to stick with just a streamer so that the rocket doesn't drift so far and so that I can keep the body of the rocket as short as possible. (OR is saying a shorter body increases apogee)

5) Are altimeter holes a big consideration in order to get an accurate altitude reading? As I understand it, altimeter holes should normally not be right below the nosecone, however that's pretty unavoidable with this rocket. It seems like this isn't something people put a lot of consideration into, but for an altitude competition I would think it's pretty important.

Thanks for any help!

View attachment 580599

I did something similar, sub minimum diameter on an AT G80. About 6k feet, but the rocket was suuupppper dodgy, I'm talking no altimeter, no tracking just a tiny parachute in a tiny fibreglass tube, a nosecone and motor deploy. Never saw it again.

No big booms though. Single use, 4mm 3D printed fins, reinforced with fibreglass and bonded right to the motor. LEAVE ENOUGH ROOM FOR THE CHUTE(S). The whole design was a little sketchy but it took 2 weeks to make and only cost ~$100, so I was prepared to lose the thing.

I plan on doing it again and applying the knowledge I've gained by doing a sub MD rocket to the new one, such as: Radio tracking (you won't see it coming back down when it's that small, trust me), longer airframe (for the electronics and so that your parachute doesn't have to be so tightly packed) and finally, thinner fins. Make sure to fibreglass reinforce your fins.

It would be badass if you did a sub minimum rocket. Reinvent the competition.

I'm sorry but I don't have much else to offer in the way of advice. Good luck!

 

[SolarYellow]

Still waiting to hear a call from LOC or whoever is responsible regarding this issue:

ETA:
This rule logically precludes the use of finned cases.

Contests | LDRS 41​

ldrs41.com

A rocket that is unable to sustain a safe future flight will be disqualified.

A finned single-use motor case can't "sustain a safe future flight."

 

[mynameisfinn02]

Still waiting to hear a call from LOC or whoever is responsible regarding this issue:

Oof, That's a shame but still solvable. Short piece of 29mm ID tube that slides over the motor with fins attached, simple friction fit.

Sorted.

 

[SolarYellow]

Anyone else building one of these rockets? Build threads? Photos?

 

[Cooperr]

Anyone else building one of these rockets? Build threads? Photos?

I ended up being busy with other things this summer and abandoned the project sadly, but I'll fly the G80 in something else at LDRS.

Never did get a clarification about the rules either. Dennis Pallen is the one to contact, feel free to DM me if you want his email.

 

[Glasspack]

So what was the final winning altitude for the 29mm altitude contest at LDRS41 ???