nose cone weight in shorter fater rockets

[crossbound]

Barley finishing up a build right now and already thinking of the next. A little shorter, and a little thicker. As I am designing in OpenRocket I am kinda amazed how much nose weight is needed in the shorter rockets to get a stability close to 1 caliber. But on the other hand reading through some threads about the Minie-Magg that is flown well on some pretty big motors without added weight. The Magg istructions don't say anything about weight. Then I downloaded the RockSim file from the LOC website and that shows 12oz weight in the nose cone.. what gives.
My current design would be over 13oz in weight added to be around 0.8-1.2 caliber for a range of H motors.
I have tortured myself reading some threads about the base drag hack, but in the end not sure of what to make of it or even use it. There doesn't seem to be a clear consensus when it becomes beneficial to us.

I guess what I'm looking for is some practical advise to be safe without having to go overboard and inadvertently making the rocket to stable.

 

[SolarYellow]

"Calibers of stability" is an error in denominator selection that we have been afflicted with for too many decades now. Go with 8-15% of overall rocket length for stability factor and you should be fine.

 

[rharshberger]

"Calibers of stability" is an error in denominator selection that we have been afflicted with for too many decades now. Go with 8-15% of overall rocket length for stability factor and you should be fine.

Still works out about the same, however short rockets especially fatter ones can use less noseweight than you think. I have a number of short stubby rockets that are perfectly stable and fly great at .3 to .5 calibers of stability. My OrangeCrush rocket is 28" long has s CP of 20.38" and a CG of 18.193" (from tip of rocket) so it has a .7 Calibers of Stability or 7.67% of length (OR shows both by the way) and has 4ozs of noseweight. I use the RocSim Base Drag Hack (aka phantom cone) work around on all or most of my short rockets, and it brings the noseweight requirement down and I have yet to have an unstable rocket using it.

Make your own decisions about which method to use Calibers or Percentage, here is the thread and the 1st post has the links to the relevant Apogee Peak of Flight news letters. Personally the Calibers of Stability is what I will continue to use because its quick and easy to eyeball in the field while doing RSO checks (though it isnt very elegant scientifically).

https://www.rocketryforum.com/threads/lets-put-an-end-to-the-base-drag-hack.184792/

 

[crossbound]

Make your own decisions about which method to use Calibers or Percentage, here is the thread and the 1st post has the links to the relevant Apogee Peak of Flight news letters. Personally the Calibers of Stability is what I will continue to use because its quick and easy to eyeball in the field while doing RSO checks (though it isnt very elegant scientifically).

https://www.rocketryforum.com/threads/lets-put-an-end-to-the-base-drag-hack.184792

Yeah I have been working through that thread. Calibers is what I keep checking as well. It's with this 4in diameter shorter rocket now that I get a little unsure of which way to go.

 

[bjphoenix]

Here is one that I built over 20 years ago. It uses LOC 4" tubing and nose cone, plywood fins, and has a 29mm motor mount. As I recall I've only launched it with G64 motors. Also as I recall it has maybe 2 oz of nose weight. I've never bothered to make a sim file of it.

 

[grapetang]

I see a rounded boattail on the sim render. A blunt base (no boattail) will work but the boattail reduces base drag stability. My gut says to add most/all of the nose weight. Thoughts?

 

[Art Upton]

I see a rounded boattail on the sim render. A blunt base (no boattail) will work but the boattail reduces base drag stability. My gut says to add most/all of the nose weight. Thoughts?

Remove the boat tail

TLDR is in bold
Open Rocket 'can sometimes' give lower CP numbers on many rockets, not just short ones.

RockSim and RASAero can sometimes give better numbers, but even RockSim is the one that had a contributor come up with the base drag hack. Read up on the Base Drag hack and add it.

Next go here and get a free 30 day trial of Rocksim and compare any difference.
https://www.apogeerockets.com/RockSim/Rocksim_Trial

Yes you will need to give an email address, there is a video about why the email right under the form. I did about 3 weeks ago.

For more then 2 decades, I have hated lots of nose weight in certain short HPR rockets. Many times it is un-necessary and makes you use more expensive motors. Who wants to spend more money to launch a shorter rocket to a lower altitude?

Another problem is folks putting lots of epoxy glue weight in the rear end of larger stubby rockets. I have inherited a problem rocket. A past club member needed money and I bought his 7.5" LOC Door Knob Rocket.

It was assembled well for sturdiness, but way too much weight in the trunk. He even put RC type aileron stock on the fin to body tube joints with epoxy for fillets. I flew it at NSL 2012. I used a Sky Angle super duty Cert 3 Chute in it to add weight to the front as I was not going to put weight in the nose. I didn't know about Base Drag Hacks yet.

With the added weight from the over build and heavy chute, I had to put a 54mm full J motor in it to simply do a quick pop up flight. Built lite it should have taken a 38mm J for a quick pop up.

Oh and with that huge chute for nose weight? It floated all the way to the edge of the field by the access road at Geneseo Upstate NY.

Build the rocket as lite as possible in the rear, check out the base drag hack, don't use boat tails that remove the base drag. If it still just does not work out, if you don't want to add lots of weight to the front, put an extended body tube and coupler on the rocket to make it longer.

I plan to do that this spring on the 7.5" Door Knob. Lucky I have a 7.5" spare full length tube I can cut down and a coupler in my stash.


I just downloaded the Newest LOC 7.5 Door Knob RockSim file. Guess what?
They got this fin based, base drag hack on the fins in it? I wonder why?


The 2012 launch of the over built Door Knob

 

[crossbound]

Remove the boat tail

TLDR is in bold
Open Rocket 'can sometimes' give lower CP numbers on many rockets, not just short ones.

RockSim and RASAero can sometimes give better numbers, but even RockSim is the one that had a contributor come up with the base drag hack. Read up on the Base Drag hack and add it.

The boat tail was mainly added for visual interest. Wanted something a little different then what's out there. I have been playing around with the BDH and removed the boat tail. Even with that about 5oz of weight are needed in the nose cone to keep some of the I motors around 1 Caliber.

I'll download RockSim later and play around with it.

 

[Art Upton]

The boat tail was mainly added for visual interest. Wanted something a little different then what's out there. I have been playing around with the BDH and removed the boat tail. Even with that about 5oz of weight are needed in the nose cone to keep some of the I motors around 1 Caliber.

I'll download RockSim later and play around with it.

Yea, just export the OpenRocket sim to RKT and then open it in RockSim.


You will have to reload motors and launch them.

Coming from OpenRocket the steps are a bit different.

After you load your design, click the flight simulations tab , it will be empty, the export does not export the sims, only the design. The on the top menu, select 'simulations' -> Prepare to launch
Then a box pops up for you to load a motor, click "Chose engine"



Chose a motor click OK


Chose a delay time, Then click Launch

 

[Art Upton]

Notice in the sim above the guide rail FPS is too low. That is because the default is 3 feet; SO

Highlite the simulation line an click edit


I clicked 'starting state' and put in 12 feet, seems I had set my default to 6 feet already


Then Click Launch

Now we got 48fps off the rail, just the bare minimum [pun] to get it going and pop the chute at 400 feet on an I motor on the heavy short rocket

 

[crossbound]

Yea, just export the OpenRocket sim to RKT and then open it in RockSim.
You will have to reload motors and launch them.

Coming from OpenRocket the steps are a bit different.

Not to difficult to figure out. Have to say though, I think I like the OpenRocket interface better. Will play around a little more with RockSim though.
That said, the difference in stability between RS and OR was only about 0.1 calibers.

 

[Art Upton]

Not to difficult to figure out. Have to say though, I think I like the OpenRocket interface better. Will play around a little more with RockSim though.
That said, the difference in stability between RS and OR was only about 0.1 calibers.

Ok, that is not much. later for grins try it on a more longer , normal rocket and it can be like 4-6 inches

I do agree OR is easier

 

[Art Upton]

Not to difficult to figure out. Have to say though, I think I like the OpenRocket interface better. Will play around a little more with RockSim though.
That said, the difference in stability between RS and OR was only about 0.1 calibers.

I forgot, there are to other settings that control the sim results, one is defaulted to Barrowman so would give you the same result of CP.

First set this to RockSim:



this one makes it a RASP simulation , or more of a Chuck Roger's simulation

 

[Buckeye]

Barley finishing up a build right now and already thinking of the next. A little shorter, and a little thicker. As I am designing in OpenRocket I am kinda amazed how much nose weight is needed in the shorter rockets to get a stability close to 1 caliber. But on the other hand reading through some threads about the Minie-Magg that is flown well on some pretty big motors without added weight. The Magg istructions don't say anything about weight. Then I downloaded the RockSim file from the LOC website and that shows 12oz weight in the nose cone.. what gives.
My current design would be over 13oz in weight added to be around 0.8-1.2 caliber for a range of H motors.
I have tortured myself reading some threads about the base drag hack, but in the end not sure of what to make of it or even use it. There doesn't seem to be a clear consensus when it becomes beneficial to us.

I guess what I'm looking for is some practical advise to be safe without having to go overboard and inadvertently making the rocket to stable.

I flew my Minie-Maggs with and without nose weight. Could not tell the difference. The kit was never sold as needing nose weight, AFAIK. There are a lot bad RockSim files on the internet, including those from the manufacturer. Apogee is more diligent than most when posting files, so look at the one on their site. Many years ago, an old timer told me that as long as the CG was in front of the fins on the Mini-Magg, it will be fine.

If you get through all 344 posts of the Hack thread, you will find aero theory and data vs. anecdotes. I like data. This post in particular pretty much sums up your situation:

https://www.rocketryforum.com/threads/lets-put-an-end-to-the-base-drag-hack.184792/post-2560183

Your model has a tail cone simulated in OpenRocket, which may be exacerbating problem. More data and flight tests starting here:

https://www.rocketryforum.com/threads/cop-simulation-comparison-including-cfd.184722/post-2652750

This is the danger of the phony cone behind your rocket. It pulls the CP too far back. I highly doubt that the CP is behind the fin leading edge in reality.

 

[Art Upton]

Next is CD, unlike OR, you can plot and track CD thru transonic and deploy.
Depending on the rocket, especially Dart type rockets, Rocksim produces higher altitudes that come closer to real.
Even on an Estes Nike Smoke

So I've been back to using both. I do start it out in OR, and then check some things in RS

 

[lr64]

crossbound: Just make it look like the rocket in your avatar.

 

[Art Upton]

CP and Caliber Differences in all 4 ways

See CP Inches here, I have this FG in my shed and still have not flow it yet
First Rocksim calculation method CP 53.1 / 1.8 cal



RS Barrowman - OR method CP 50.4 / 1.1 Cal



And Cardboard Cut out for Grins CP 41.3 / -1.2 Cal



Next OpenRocket Barrowman CP 50.42 / Cal 1.07

 

[crossbound]

I flew my Minie-Maggs with and without nose weight. Could not tell the difference. The kit was never sold as needing nose weight, AFAIK. There are a lot bad RockSim files on the internet, including those from the manufacturer. Apogee is more diligent than most when posting files, so look at the one on their site. Many years ago, an old timer told me that as long as the CG was in front of the fins on the Mini-Magg, it will be fine.

If you get through all 344 posts of the Hack thread, you will find aero theory and data vs. anecdotes. I like data. This post in particular pretty much sums up your situation:

https://www.rocketryforum.com/threads/lets-put-an-end-to-the-base-drag-hack.184792/post-2560183

Your model has a tail cone simulated in OpenRocket, which may be exacerbating problem. More data and flight tests starting here:

https://www.rocketryforum.com/threads/cop-simulation-comparison-including-cfd.184722/post-2652750

This is the danger of the phony cone behind your rocket. It pulls the CP too far back. I highly doubt that the CP is behind the fin leading edge in reality.

View attachment 682521

I totally get it. Using the cone is a hack at best. What I'm looking for is some way, as of this moment in time, to let me determine if the rocket will be stable.
I'm looking forward to seeing actual data from doing flight test, especially the guy that is planning to add sensors to get base of his rocket.

 

[rharshberger]

I see a rounded boattail on the sim render. A blunt base (no boattail) will work but the boattail reduces base drag stability. My gut says to add most/all of the nose weight. Thoughts?

At the velocities most of our rockets tend to fly the boattail will have some effect but not as much as at higher velocities. I sim all of my V2 builds (six rockets ranging from BT50 to 4") with the Base Drag Hack and they have been stable on every motor I have used in them (the V2 is my second favorite rocket behind the Cherokee D)

 

[crossbound]

crossbound: Just make it look like the rocket in your avatar.

That's when I digged the Baby Bertha.. haha

 

[sr205347d]

I totally get it. Using the cone is a hack at best. What I'm looking for is some way, as of this moment in time, to let me determine if the rocket will be stable.
I'm looking forward to seeing actual data from doing flight test, especially the guy that is planning to add sensors to get base of his rocket.

Short and fat rockets do not need as large a margin. Here is a flight test showing stable flight with the CG 1mm forward of the Rocksim CP.

Post in thread 'Let's put an end to the "Base Drag Hack"'
https://www.rocketryforum.com/threads/lets-put-an-end-to-the-base-drag-hack.184792/post-2599843

 

[nickt.mrc]

Remember that as soon as you ignite the motor it is burning weight off the back end. It's stability margin will be quite different, even as it leaves the rail, than the static figure. Getting it to accelerate quickly, for me means favouring larger fins over added nose weight. The quicker it leaves the rail the better. As others have said, you can use percentages, rather than calibres, although I would say 12 to 20, rather than 8 to 15.

 

[SolarYellow]

8 to 15 is often cited in the hobby. I've gone down the rabbit hole trying to find an original source in aero research papers and so far have not. But it seems to work reasonably well.

Overall, the percent of rocket length rule is consistent with short-fat rockets being safe with less than one caliber and long-skinny rockets needing several calibers to be safe in any kind of breeze. The problem with the caliber rule is that when you change your rocket in one direction, the rule requires adjusting in one direction, and when you change your rocket in the opposite direction, the rule also requires adjusting in that opposite direction. The percent rule takes care of that for you.

 

[Buckeye]

What I'm looking for is some way, as of this moment in time, to let me determine if the rocket will be stable.

1. Use the Rocksim CP method. The boattail throws OR's CP even more out of whack, for some reason. Don't add hack cones.

2. Strive for 8%-15% margin. If you like calibers, then 0.5 caliber is probably good enough.

 

[hball55]

Barley finishing up a build right now and already thinking of the next. A little shorter, and a little thicker. As I am designing in OpenRocket I am kinda amazed how much nose weight is needed in the shorter rockets to get a stability close to 1 caliber. But on the other hand reading through some threads about the Minie-Magg that is flown well on some pretty big motors without added weight. The Magg istructions don't say anything about weight. Then I downloaded the RockSim file from the LOC website and that shows 12oz weight in the nose cone.. what gives.
My current design would be over 13oz in weight added to be around 0.8-1.2 caliber for a range of H motors.
I have tortured myself reading some threads about the base drag hack, but in the end not sure of what to make of it or even use it. There doesn't seem to be a clear consensus when it becomes beneficial to us.

I guess what I'm looking for is some practical advise to be safe without having to go overboard and inadvertently making the rocket to stable.

View attachment 682301View attachment 682302

There’s a formula for figuring this out, but I have forgotten it. People in here can probably tell you. I have used the formula in Rocksim, in Rocksim it will result in a triangle behind your rocket, just check your caliber of stability after creating that triangle using the formula I have forgotten.

 

[crossbound]

1. Use the Rocksim CP method. The boattail throws OR's CP even more out of whack, for some reason. Don't add hack cones.

2. Strive for 8%-15% margin. If you like calibers, then 0.5 caliber is probably good enough.

I figured that's what it will have to be. I am however also down scaling this design to a BT-60 tube and use a slightly shorter nose cone and do some test flights as well to see how it reacts with minimal to no margins.

 

[OzHybrid]

Use the volume of the nosecone. Put a bulkhead with holes up against the nosecone shoulder. Put all your laundry above that in the nosecone. You've now got weight where you need it without adding any nosecone weight. Bonus... The laundry cannot shift backward under acceleration. Bonus 2, the laundry is much further away from the motor ejection charge. Bonus 3 the rocket is lighter and will fly higher.

 

[Sir_Launchalot]

Please read the attached file from the Apogee website.

It is gold.

There are are even wilder deviations from the Barrowman parameters than mentioned in the article but this is not the time nor place.

 

[Reid]

My bit of advice have flown and crashed a Minnie-magg with marginal stability and added nose weight.

Don't forget the base drag only kicks in as speed increases. Do no fly a motor that is slow off the rail. Because that added weight is not your friend in building speed.

I used an H100 my magg climbed 150'-200' made a right angle turn and flew horizontally off into the sunset... literally

 

[bjphoenix]

I am however also down scaling this design to a BT-60 tube

Here is one that I built as an experiment using BT-60 tubing (on the right). My goal was to build something as light as I could to fly on B6 motors. I didn't intend to make it short but making it short saved some weight, and I carefully adjusted fin dimensions to minimize their weight. It flies better than my ESAM, almost as well as a Baby Bertha, with respect to altitude.