Another stability question.

[Senior Space Cadet]

I'm working on a design I hope will get up to around 2,000 ft. without resorting to a minimum body tube design.
Looking for a low cost, low hassle design.
24mm body tube with 18mm motor.
What would be a good stability number for an appropriate motor? Most likely a D.
In the past, I think my rockets have had shorter tubes and larger fins than optimal.
Any thoughts on what a good length would be?

 

[Antares JS]

1-2 calibers of stability is standard for most rockets.

The length that will get you that number depends on your fin area and how heavy your nose is. Use OpenRocket to figure it out.

 

[Senior Space Cadet]

Let's say I want a stability of 1.5. Well, there are an infinite number of possible combinations of body tube length, fin shape and size, nose cone profile, etc. to achieve that.
I could spend hours and hours, even days and weeks on OR trying different combinations.
Maybe there are features of OR, I don't know how to use, that would shorten the process.
If someone were to make a suggestion, about tube length, that would point me in the right direction, that would shorten the process considerably.
A stability of between 1 and 2 is quite a large range. What would be the lowest you'd go?
There isn't an official NAR record for a rocket with a 24mm body tube and 18mm motor.
I'd like to set the unofficial record or, at least, reach an altitude that would be considered very impressive.
Any idiot can design a stable rocket. I'm living proof of that. I've never built a rocket that didn't fly well.
I'm trying to do a lot more than just build a stable rocket.

 

[Antares JS]

Let's say I want a stability of 1.5. Well, there are an infinite number of possible combinations of body tube length, fin shape and size, nose cone profile, etc. to achieve that.

This is exactly the problem with trying to offer a suggested tube length. You can take almost any tube length and make it stable with the addition of appropriate fins.

Maybe there are features of OR, I don't know how to use, that would shorten the process.
If someone were to make a suggestion, about tube length, that would point me in the right direction, that would shorten the process considerably.

I can't speak for sure with OR because I have RockSim, which is not free, but I've had it since before OR existed and have stuck with it. RockSim will update your CG/CP and stability margin in real time with every change made. I imagine OR probably does the same thing. If your stability margin is too low, increase the size of your fins or make the rocket longer. If it's too high, make the rocket shorter or decrease the size of your fins.

A stability of between 1 and 2 is quite a large range. What would be the lowest you'd go?

1.

There are special cases where you can get away with less than 1 or would want higher than 2, but neither is likely to apply to your design goal.

I'd like to set the unofficial record or, at least, reach an altitude that would be considered very impressive.

Then you want to build the rocket as small and lightweight as possible within your given BT-50 w/ BT-20 motor mount design constraint. That's probably the best starting point I can give you. What is the minimum tube length needed to accomodate the motor, wadding, recovery device, nose shoulder, and onboard altimeter, if any? If you are out to maximize altitude, start there and lengthen as needed to reach the needed stability.

Be warned that building a small rocket designed to reach high altitudes makes you more likely to lose it. Brightly-colored paint and streamer recovery are highly advised.

 

[Steve Shannon]

Let's say I want a stability of 1.5. Well, there are an infinite number of possible combinations of body tube length, fin shape and size, nose cone profile, etc. to achieve that.
I could spend hours and hours, even days and weeks on OR trying different combinations.
Maybe there are features of OR, I don't know how to use, that would shorten the process.
If someone were to make a suggestion, about tube length, that would point me in the right direction, that would shorten the process considerably.
A stability of between 1 and 2 is quite a large range. What would be the lowest you'd go?
There isn't an official NAR record for a rocket with a 24mm body tube and 18mm motor.
I'd like to set the unofficial record or, at least, reach an altitude that would be considered very impressive.
Any idiot can design a stable rocket. I'm living proof of that. I've never built a rocket that didn't fly well.
I'm trying to do a lot more than just build a stable rocket.

Use an elliptical nose cone, clipped delta fins, a 180mm long body tube, and a tailcone. That’s just a starting place. The real work is the optimization.

 

[BigMacDaddy]

I do not have much practice trying to hit high altitudes but have played around with maximum speed and height in OpenRocket just for the heck of it. It is not too difficult to simulate something that goes over 2000' w/ BT-50 tube and 18mm engine (flight realities might be very different).

Keep your drag as low as possible (i.e., 3 fins instead of 4, fins not too tall, smooth paint, shorter body tube, etc...). Also you will need some noseweight to optimize momentum and altitude during ejection delay. Also I think that anything above 1cal of stability will be less efficient (fins too large, too much noseweight or body tube length). If you can live with less stability and a longer launch rail that will help. Ah, don't forget your launch lugs since those are draggy (I forgot them here).

I think a D21 is the highest impulse engine in the 18mm list provided by OpenRocket. Load a D21-7 engine in there and play with everything else. I did a 9" BT-50 tube here and .5oz of noseweight. This sim is with 2mm balsa fins. Might need through-wall fins due to excessive speed and might need to use 1.5mm plywood. This is also with a mylar streamer and Kevlar shock chord.



Before anyone asks: I am metric with everything except for noseweight since I use fishing weights in my noses and Walmart sells fishing weights in ounces and fractional ounces.

 

[86mustang408w]

I do not have much practice trying to hit high altitudes but have played around with maximum speed and height in OpenRocket just for the heck of it. It is not too difficult to simulate something that goes over 2000' w/ BT-50 tube and 18mm engine (flight realities might be very different).

Keep your drag as low as possible (i.e., 3 fins instead of 4, fins not too tall, smooth paint, shorter body tube, etc...). Also you will need some noseweight to optimize momentum and altitude during ejection delay. Also I think that anything above 1cal of stability will be less efficient (fins too large, too much noseweight or body tube length). If you can live with less stability and a longer launch rail that will help. Ah, don't forget your launch lugs since those are draggy (I forgot them here).

I think a D21 is the highest impulse engine in the 18mm list provided by OpenRocket. Load a D21-7 engine in there and play with everything else. I did a 9" BT-50 tube here and .5oz of noseweight. This sim is with 2mm balsa fins. Might need through-wall fins due to excessive speed and might need to use 1.5mm plywood. This is also with a mylar streamer and Kevlar shock chord.

View attachment 602764

Before anyone asks: I am metric with everything except for noseweight since I use fishing weights in my noses and Walmart sells fishing weights in ounces and fractional ounces.

That pretty much says it all right there.

Your going to have a hard time hitting 2000 feet with a 18mm motor. Like said above.

 

[SolarYellow]

The 18/20 reloadable case with D12 or D24 loads makes 2000' in a minimalist BT-50 highly doable. An Estes D12 can sim to ~2200' with a Lucerne launch elevation and the D12/24 have ~15% more impulse.

The closest to elliptical NC that's available COTS AFAIK is the Alpha III. Play with fin size/thickness/sweep. With a D24, you should end up below optimum weight for a basic build, so adding NC weight is no problem. TTW weight shouldn't be an issue, either. I'd go 0.020 G10 for the fins and use epoxy to assemble them. Minimal fillets done just for mechanical strength, not aerodynamics, as the "rule of thumb" that's usually repeated is wrong when you chase down all the way to the original source materials (or close enough to them). If you play enough in OR, you'll find that an optimized four fin design will actually sim to ~1-1.5% higher apogee vs. three fins for the same stability factor. I target 10-12% of overall rocket length for a stability factor, rather than calibers. Insert your motor as high up into the airframe as you can practically retain it. Build yourself a mini tower to launch with no launch lug. I've had a heck of a time with fly away launch lugs screwing up little rockets like this.

 

[Back_at_it]

With 18mm reloads you could possibly get there but I've never tried.

I can tell you that you don't need to try real hard to get to 2000ft using a D12-7 motor. I've built and flown a standard Alpha with .25oz of nose weight and it sim'd to 2140ft.

I switched to the hollow Alpha III nose cone and flew it with a cheap Estes Altimeter on board and recorded 2021ft.

As for the stability question. I shoot for at least 1 Cal using OR but I've successfully flown hundreds of rockets that were in the .90 range without any issues.

 

[SolarYellow]

As for the stability question. I shoot for at least 1 Cal using OR but I've successfully flown hundreds of rockets that were in the .90 range without any issues.

It's about safety margin. In theory, if everything goes right, a stability factor of 0.1 cal should be OK. But it would be expected to be disrupted by very small disturbances. 0.9 is probably fine, but as you push the number down, the susceptibility to disruption increases. A little wind at launch, and wind-caused instability could have a rocket skywriting that would do a dozen flights just fine in still air.

 

[BigMacDaddy]

It's about safety margin. In theory, if everything goes right, a stability factor of 0.1 cal should be OK. But it would be expected to be disrupted by very small disturbances. 0.9 is probably fine, but as you push the number down, the susceptibility to disruption increases. A little wind at launch, and wind-caused instability could have a rocket skywriting that would do a dozen flights just fine in still air.

[I think] the length / stability of launch rod/rail makes a big difference in the OP's scenario since the speed the rocket is moving upward should make it more stable and resistant to things like cross breezes... If he can launch with a 1.5m Makerbeam rail that would be better than a 3' rod (also makerbeam buttons are much smaller than straw-style lugs). An 2-3m 1010 rail would get it to an even higher speed off the rail but increased drag on rail as well as drag caused by larger launch lugs might offset any gains.

 

[Spacedog49Krell]

Don't forget about the base drag. A boat tail will reduce base drag. The motor/motor mount will extend ~0.7" from the end of the BT-50. The CG will also shift aft, so more nose weight.

3-D printers are great for these parts. In the past I used polymer oven bake clays for these parts, but there are issues with shrinkage and mold release which require some experience to get correct.