Let's discuss short stubbies...rockets that is.

They are not for everyone but I love them. And I know I am not alone. Big Daddy, Cowabunga, Tembo, Lil Goblin and now Mega Magg. Some of my most frequent fliers.

This thread is mostly on the Mega Magg since it is the largest I have and can handle the biggest motors. The larger motors throw up red flags. For the most part it will likely fly on AT 54/1280 motors but it should be able to fly on anything from 54/852 up to 3 grain 75mm. I have a Gorilla clone 75/3600(3500 whichever) and a CTI Pro54 5g. Lengthwise these are pretty close and are the maximum that will fit. Will it every fly on a full blown 3grain 75? Not likely. But if the planets align and there is zero wind, sure why not.
One little caveat to my Mega Magg. I was beveling the fins and had a jig slip, so I had to cut off my bevels and lost about 5/8" off my fins. This has been accounted for in my sims so all is good.

More to follow:
multiple posts to help me organize my thoughts.

Stability:
We all know the rule of thumb of 1 CAL stability. NAR actually requires only .5 CAL if I am not mistaken. Truth is that anything above 0 CAL is stable. Do I fly at say 0.02? Heck no.

Then there is the school of thought that you need 1 CAL stability for every 10 CALs of airframe length. So for my Mega Magg which is 52" long and 7.67" diameter it would need 0.68 CAL to be (rule of thumb) stable.

And there is the rear drag cone (I calling it a) theory. Where you add a zero mass transition behind the rocket that is Pi diameters in length and transitions from nothing at the aft end of the rocket to the diameter of the airframe. I have done this myself on the smaller rockets and it worked well. But not sure I trust that method on this guy.

Example: according to OpenRocket
My 4" Cowabunga on an H148R has .4 CAL stability. I add the rear cone and the stability goes up to 1.04. It flew pretty much straight up.

Now the Mega Magg:
With the J540R (54/1280) it has 0.002 CAL, which I wouldn't feel comfortable with. With the rear cone it goes up to 0.973, which is A LOT better.
If I get crazy and put in the K780R (75/2560) it comes in with -0.255, but .716 with the cone. This is what causes my dilemma and gets me to questioning the rear cone theory. The difference in the two numbers is a tad hard to believe.

I added 29 oz of lead to the nose. This will let me comfortably fly pretty much my full range of 54s but I will have to add another 38oz to fly the 75s. I have the LOC nose weight system so I can easily add nose weight.

Is the rear cone theory sound? Or proven? I am not ashamed to admit I don't fully understand rear drag. Would long skinny rockets still have the same amount of rear drag as short stubbies?

Recovery:
Moving forward, everything I build gets electronics to control apogee deployment. I have had one instance where my electronics failed to pop the nose at apogee. Still don't know what happened. The altimeter survived the ensuing lawn dart and has successfully flown since. I have had multiple rockets get damaged from delays not timed just right and the most recent, my Callisto burnt up when the delay blew before she left the pad. This isn't a thread on delay drilling or delay vs altimeter.

On short stubbies I use an altimeter to pop at apogee and then a JLCR to open the main sometime during descent. And the JLCR brings me to the recovery portion.

My Ethos 2.0 got quite a bit of nose weight when I built her. Like I mentioned earlier I don't fully understand base drag and Ethos has a tail cone which makes things worse, for me anyway. So when she pops her nose cone at apogee via RRC2+, the nose cone comes in like a bullet pulling the fin can behind. I always forget my Alt3 so I don't know the descent rate but it is fast. When the JLCR releases, the main inflates and the fin can falls the length of the harness times two. Hits the end with a jolt and stresses the gear. I have seen it with larger rockets and it is one reason I am against 40' harnesses but that is not what I wish to discuss here. This setup also increases the chance that the fin can will come through the main as it is opening. That would not be good.

The Mega Magg has 29oz of nose weight bringing the nose cone to around 5 lbs. When I stick the 75mm 2 grain in there, I may/will have to add 38 oz to the nose cone. Bringing the weight to 7+ lbs. I don't want it coming in hot like the Ethos does and causing all that stress on the recovery gear. The fin can will weigh ~13 lbs, with a 20 foot harness I would be looking at 433 foot pounds of energy..Is that alot? No clue, I am a tech not an engineer. I honestly don't know if I figured that out right.

Basically, I want to add a guide chute or a drogue to keep the fin can below or out to the side where it belongs. I have little experience with drogues, used one twice and it tangled both times.
Anyone have any experience with nose heavy rockets and using a drogue or guide chute along with the JLCR? I haven't had time to sit and work on it. Busy at work, death in the family, cold and flu, nasty weather, and on and on and on.

I will get to it, just wanted to hear some thoughts.

MickeyDSlagle,
You’re smart to move to electronic deployment. The TMT Chair, Alan Whitmore, published data last year in the Tripoli Report showing how variable delays really are, especially in motors where the delay is burning while the motor is burning (which is everything other than end burners I think).
I’m very sorry about the death in your family.

Woo! Rockets that don't have 10:1 L/D ratio!

I aim for a static margin of 12% vehicle length (regardless of body diameter(s)) and simulate to see if the stability shifts outside of 8-15% vehicle length (old sounding rocket and artillery parameters).
Don't really mind if it goes over 15, but like to keep it above the 8.

My 4" patriot wants to nose drag when I use the Chute release too. Try simulating an 18 or 24" drogue at apogee and seeing what it does to the descent (those new windsock looking guide chutes are interesting)

I can't remember where it is, but I recall a thread about drogueless booster drag during the freefall period. Something like: [tube area (LxW) plus planform area of fin (x2 for 4fin, x1.5 for 3fin)] divided by 2 gives an area that can be used as a simulated sudo-chute in OR that gets triggered at apogee separation.

If you get that number, you could try using a drogue that's just a tad larger, so that it has more drag than the booster (making it hang down), but not so much that it removes the dual deployment benefit.

Experimentation is fun!

Mikey,
My 2 cents (just about what it's worth). Move the main with JLCR toward the fin can end of the harness. Attach a small drogue (streamer or chute) to the nose cone. Size the drogue to match the fin can and nose cone rate of descent or bias it so the nose cone is a bit lower under drogue.
Straight flights, Pat.

Stability is tricky. I think it is on a sliding scale. People have varyiing ways of deciding if a short/stubby is stable, including tricks in OR/RS, etc. But I have personally seen short/stubby rockets that show very tiny stability margins in OR fly straight as an arrow. Something short/stubby and base drag that seems to violate the accepted rules.

Only trouble is, the only way to find out it is NOT stable is to crash it. Not a good approach. But I don't think anyone has nailed down a formula. Please correct me if I'm wrong.

Steve Shannon said:

MickeyDSlagle,
You’re smart to move to electronic deployment. The TMT Chair, Alan Whitmore, published data last year in the Tripoli Report showing how variable delays really are, especially in motors where the delay is burning while the motor is burning (which is everything other than end burners I think).
I’m very sorry about the death in your family.

Click to expand...

Thank you for the condolences.
I have been migrating toward electronic deployment for a while now. It started when my Wildman Sport popped the chute early and did a bit of damage to the airframe, almost a zipper. I retrofitted it to hold a RRC2+ in the nose cone. I recently finished up a 2.6" "pancake" style bay that will fit in the airframe of my Lil Goblin and my 2.6" Wildman V2. Had to make changes halfway through but it fits in both. That was why I was asking about parts compatibility in an earlier thread. Now I won't be so scared to fly my V2...well that I added 12 oz of ballast to the nose cone. Like I said, I am not good with tailcones.

Nytrunner said:

Woo! Rockets that don't have 10:1 L/D ratio!

I aim for a static margin of 12% vehicle length (regardless of body diameter(s)) and simulate to see if the stability shifts outside of 8-15% vehicle length (old sounding rocket and artillery parameters).
Don't really mind if it goes over 15, but like to keep it above the 8.

My 4" patriot wants to nose drag when I use the Chute release too. Try simulating an 18 or 24" drogue at apogee and seeing what it does to the descent (those new windsock looking guide chutes are interesting)

I can't remember where it is, but I recall a thread about drogueless booster drag during the freefall period. Something like: [tube area (LxW) plus planform area of fin (x2 for 4fin, x1.5 for 3fin)] divided by 2 gives an area that can be used as a simulated sudo-chute in OR that gets triggered at apogee separation.

If you get that number, you could try using a drogue that's just a tad larger, so that it has more drag than the booster (making it hang down), but not so much that it removes the dual deployment benefit.

Experimentation is fun!

Click to expand...

So for the Mega Magg, 12% of 52" is 6.24. I'm guessing this number is in inches? 6.24" forward the CP which would translate .81 CAL. And 8% would be .54 CAL. Am I right on your method? Not a bad way to do it. 10:1 is 10% after all, which is what I would say is the universally accepted stability. With an L1390G (3grain 75) I get .52 CAL with the added 39 oz, .135 CAL without it.

This would be something to learn, but right now it is giving me a headache. LOL Is that the formula?

Bat-mite said:

Stability is tricky. I think it is on a sliding scale. People have varyiing ways of deciding if a short/stubby is stable, including tricks in OR/RS, etc. But I have personally seen short/stubby rockets that show very tiny stability margins in OR fly straight as an arrow. Something short/stubby and base drag that seems to violate the accepted rules.

Only trouble is, the only way to find out it is NOT stable is to crash it. Not a good approach. But I don't think anyone has nailed down a formula. Please correct me if I'm wrong.

Click to expand...

My Big Daddy is a regular flyer at .38 CAL. Straight up. We could always swing test these things? 100 foot of rope to decrease angle of attack? LOL That would be interesting.

So I ask any of you who have acted as RSO, and Steve being TRA President, if I came to the table with a 21lb rocket including the L1390G and I say the stability is .419...Would either of you turn me away?

PatD said:

Mikey,
My 2 cents (just about what it's worth). Move the main with JLCR toward the fin can end of the harness. Attach a small drogue (streamer or chute) to the nose cone. Size the drogue to match the fin can and nose cone rate of descent or bias it so the nose cone is a bit lower under drogue.
Straight flights, Pat.

Click to expand...

Yes I looked at this a bit. Ideally the entire stack would fall flat. Which is what I am trying to achieve via drogue and leaving the main attached toward the nose cone.

I have also considered letting it fall nose first but have the bundle trailing behind the stack as it falls, attached close to the fin can, like you suggest. One problem may be the bundle not getting pulled from the airframe because the harness didn't fully extend. There are ways around this of course, but I see it being prone to tangling.

Having the bundle close to the nose as it is now, and adding a drogue seems to be the most practical, ideally having the bundle falling above the drogue. But that seems prone to tangling as well. I'll have to do some scribbling on this.

This is something I can experiment with on my Tembo, it has a good deal of nose weight. It is cheaper to fly than Ethos and especially Mega Magg. I may need to build a nose cone heavy bird that can fly in my yard to play with.

It's good to have folks with different perspectives.

MikeyDSlagle said:

So for the Mega Magg, 12% of 52" is 6.24. I'm guessing this number is in inches? 6.24" forward the CP which would translate .81 CAL. And 8% would be .54 CAL. Am I right on your method? Not a bad way to do it. 10:1 is 10% after all, which is what I would say is the universally accepted stability. With an L1390G (3grain 75) I get .52 CAL with the added 39 oz, .135 CAL without it.

This would be something to learn, but right now it is giving me a headache. LOL Is that the formula?

Click to expand...

Sounds like you have as good a grasp on it as I do Never hurts to choose motors that will give it a good kick in the pants either

Do you have an ork file? I can look at it and give you a couple drogue size suggestions

MikeyDSlagle said:

Thank you for the condolences.
I have been migrating toward electronic deployment for a while now. It started when my Wildman Sport popped the chute early and did a bit of damage to the airframe, almost a zipper. I retrofitted it to hold a RRC2+ in the nose cone. I recently finished up a 2.6" "pancake" style bay that will fit in the airframe of my Lil Goblin and my 2.6" Wildman V2. Had to make changes halfway through but it fits in both. That was why I was asking about parts compatibility in an earlier thread. Now I won't be so scared to fly my V2...well that I added 12 oz of ballast to the nose cone. Like I said, I am not good with tailcones.



So for the Mega Magg, 12% of 52" is 6.24. I'm guessing this number is in inches? 6.24" forward the CP which would translate .81 CAL. And 8% would be .54 CAL. Am I right on your method? Not a bad way to do it. 10:1 is 10% after all, which is what I would say is the universally accepted stability. With an L1390G (3grain 75) I get .52 CAL with the added 39 oz, .135 CAL without it.

This would be something to learn, but right now it is giving me a headache. LOL Is that the formula?



My Big Daddy is a regular flyer at .38 CAL. Straight up. We could always swing test these things? 100 foot of rope to decrease angle of attack? LOL That would be interesting.

So I ask any of you who have acted as RSO, and Steve being TRA President, if I came to the table with a 21lb rocket including the L1390G and I say the stability is .419...Would either of you turn me away?

Click to expand...

"So I ask any of you who have acted as RSO, and Steve being TRA President, if I came to the table with a 21lb rocket including the L1390G and I say the stability is .419...Would either of you turn me away?"

If you know enough to be able tell me it has .419 cal of stability, and be able to tell me how you arrived at that number ( ex.sim with or without phantom cone), and the rocket is properly built and "feels" right then it would get a pad for the first flight at the away pads. Once the rocket has " proven" itself on that motor combo to be stable then its next flight could be from the closer pads. If you change motors for the second flight it would depend on what motor and how it compares to the previous successfully flown motor (a 2 grain 75, might be less stable than a 6 grain 54 due to the concentration of mass of the 75 being toward the rear and the long 54 having significant mass towards the nose).

Nytrunner said:

Sounds like you have as good a grasp on it as I do Never hurts to choose motors that will give it a good kick in the pants either

Do you have an ork file? I can look at it and give you a couple drogue size suggestions

Click to expand...

My sims contains proprietary information, items being tested and what not. I can not post it here.

Psyche... here ya go.

I currently have a 6ft Rocketman that will likely fly in it. The descent rate in OR does not match the descent rates on the Rocketman site. And the two times I have flown Rocketman chutes, they came in way hotter than the sims or, to a lesser extent, the website suggested.

rharshberger said:

"So I ask any of you who have acted as RSO, and Steve being TRA President, if I came to the table with a 21lb rocket including the L1390G and I say the stability is .419...Would either of you turn me away?"

If you know enough to be able tell me it has .419 cal of stability, and be able to tell me how you arrived at that number ( ex.sim with or without phantom cone), and the rocket is properly built and "feels" right then it would get a pad for the first flight at the away pads. Once the rocket has " proven" itself on that motor combo to be stable then its next flight could be from the closer pads. If you change motors for the second flight it would depend on what motor and how it compares to the previous successfully flown motor (a 2 grain 75, might be less stable than a 6 grain 54 due to the concentration of mass of the 75 being toward the rear and the long 54 having significant mass towards the nose).

Click to expand...

Right right. We hear "Michael Dixon with his x-rocket. This is a heads up flight." often enough. LOL. Yeah the 2G 75 will take quite a bit more nose weight than the 6G 54.

MikeyDSlagle said:

My sims contains proprietary information, items being tested and what not. I can not post it here.

Click to expand...

Now you're just ASCing for trouble with that shtick

Nytrunner said:

Now you're just ASCing for trouble with that shtick

Click to expand...

Only thing missing with that one Nytrunner was a rimshot! Nicely played both of you.

rharshberger said:

"So I ask any of you who have acted as RSO, and Steve being TRA President, if I came to the table with a 21lb rocket including the L1390G and I say the stability is .419...Would either of you turn me away?"

If you know enough to be able tell me it has .419 cal of stability, and be able to tell me how you arrived at that number ( ex.sim with or without phantom cone), and the rocket is properly built and "feels" right then it would get a pad for the first flight at the away pads. Once the rocket has " proven" itself on that motor combo to be stable then its next flight could be from the closer pads. If you change motors for the second flight it would depend on what motor and how it compares to the previous successfully flown motor (a 2 grain 75, might be less stable than a 6 grain 54 due to the concentration of mass of the 75 being toward the rear and the long 54 having significant mass towards the nose).

Click to expand...

I would also have it announced as a "heads up" flight. Get the spectators on their feet and paying attention. A long rail and a good bit of thrust go a long way.

Bat-mite said:

I would also have it announced as a "heads up" flight. Get the spectators on their feet and paying attention. A long rail and a good bit of thrust go a long way.

Click to expand...

I did forget that point, our club always announces first flights as heads up flights.

rharshberger said:

I did forget that point, our club always announces first flights as heads up flights.

Click to expand...

I would. This is especially true then you put a bugger motor in it such as the fat boy on a 29 mm motor vs the stock 24. If you do not add nose weight, it tends to fly a little wonky.

Aight,

Tube area times a factor for cylinder drag ~=115 in^2
Fin area times ~1.5 for 3 fin config ~= 66 in^2

That adds up to the fin can behaving(ish) like a 15" flat chute

Stick a 12" drogue on the nose and it should slow it down a bit

Nytrunner said:

Aight,

Tube area times a factor for cylinder drag ~=115 in^2
Fin area times ~1.5 for 3 fin config ~= 66 in^2

That adds up to the fin can behaving(ish) like a 15" flat chute

Stick a 12" drogue on the nose and it should slow it down a bit

Click to expand...

Very cool. Thanks. I'll see about getting a sturdy chute and try that.

I do have a 18" Ultra X made by Gary not long before he sold TFR. That should work well in place of a 12" flat sheet. But this is the one that is 2 for 2 on tangling.

Just a random thought here. Rocketman and Sphereachutes have a loop in the top. So what about using a 12" Rocketman Pro-x and tying the main, bundled with the JLCR, to that top loop? I am thinking of a few reasons why it likely won't work. One being the drogue will be used as part of the recovery harness. Another being the bundled main may not stay above the drogue which would present a few more issues. Not thinking all that clearly..exhausted and received more bad news. I lost another Aunt, a great aunt actually...my Father's aunt. She was one of the few remaining of that generation. Ho hum.