Help with modified MDRM stability

I have a question about the stability of a Mega Der Red Max that I have modified. It's the one with the fin pods: https://www.rocketryforum.com/showthread.php?64212-REVIVED-Pods-on-Mega-Der-Red-Max

Now that I have extended the fins and added the pods, the CG has sifted aft, and the CP is harder to figure because of the complexity of the pods. Also, the overall weight of the rocket has increased. So I need to check the stability of the new configuration.

I took an OR file for a MDRM that I found in another thread, and I approximated the shape of the fins. I don't think there is a way to add fin pods in OR, so I just added an approximate profile shape of the pod to the edge of the fin. This did shift the CP back slightly from the file I had copied. Then I weighed the rocket, and I balanced it to find the CG --- then did a mass override and CG override for the whole rocket.

In the original file I had copied, the stability was 1.56 without motors, and 1.3 with a G80. In my modified version, the stability is .735 without motors, and .615 with a G80. So the stability has been altered significantly.

Unfortunately, if I add very much nose weight, I'm going to get out of the weight range for G motors and have to move to H motors, and I am not L1 (Maybe I've just built my L1 rocket inadvertently?).

Are there other considerations for the stability of this rocket? Do the fin pods further decrease stability by having weight and drag at the fin tips? Is there base drag to consider? How would I simulate these things or take them in to consideration if they cannot be simulated?

One of the great appeals of the stock MDRM are the awesome slow and stable flights. I'm hoping to be able to still have that and keep the option to fly on G motors, so if I can have it safely stable with minimal nose weight added, that would be great.

Any advice is greatly appreciated!

Here's a screen grab of OR:



I'm attaching the OR file I've modified.

That's surprising. Base drag on these tend to help stability, and you are adding a lot of drag on the fins. I'm not sure but I think the pods should help with stability. You might want to consider not adding foam the the pods and setting them forward enough they don't go past the fins, so they weigh less.

SCrocketfan said:

That's surprising. Base drag on these tend to help stability, and you are adding a lot of drag on the fins. I'm not sure but I think the pods should help with stability. You might want to consider not adding foam the the pods and setting them forward enough they don't go past the fins, so they weigh less.

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I think the issue is that OR doesn't really account for base drag, and I am not able to actually add the true pod shape to the fin edge in OR. All I can do is add a contour to the fin that has the profile of the pod, but that means it is not an actual cylindrical shape like the real pod --- just a flat section of fin in the contour of the pod.

Adding the pod shape and the extra bit of fin do help with stability in the sense that they do move the CP aft. If that was all they did, then that would help. But the pods and the extra fin material, plus the papering layer I added to the fin, all work to move the CG aft as well, because they add weight to the aft section. Unfortunately they move the CG aft more than they move the CP aft, so the margin decreases.

Also, the rocket is now built, so there is no option for changing the design, other than adding nose weight.

So, now the question is, is there something that OR is not taking into account, like base drag, that means the rocket is actually more stable than OR is indicating. If so, what is that factor, and how can I account for it? And if not, is the calculated stability in OR adequate --- would you fly a rocket like this with a stability of around .6 with the motor installed? Or do I just need to add nose weight, and if so, how much?

My cert mini magg is .6 to .7 in rocksim, but that's a design that's proven to have a ton of base drag. Are the pods already filled with foam?

SCrocketfan said:

My cert mini magg is .6 to .7 in rocksim, but that's a design that's proven to have a ton of base drag. Are the pods already filled with foam?

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Yep. I already filled them. I really don't think the foam has added much weight. It is 3 lb density, so 3 lbs per cubic foot. Each pod has about 18 cubic inches. I think it adds up to between 1.5 and 2.0 oz total for the all the foam. Most of the weight is probably the added plywood, balsa and paper used to build out the edges of the fins so they are parallel with the BT of the rocket --- that's a decent amount of material, and ply is one of the heaviest materials in the build. Then the NC's and BT's that are used to make the actual pods --- those add up. The foam is not very heavy, but it certainly did stiffen and strengthen those pods! I'm glad I foamed them, and it gives me a lot more confidence that the pods will survive impact with the ground. In any case, it is already a done deal --- they are installed and foamed.

The Mini Magg has a stability of .6 to .7? Hopefully the MDRM is benefitting from the same kind of base drag, but the MDRM's 4" BT won't generate as much as the mini Magg's larger tube. I air-foiled the fins --- now I wonder if I should have left them squared off for more drag.

I think there is some kind of trick in OR to simulate base drag with a zero-mass cone of some kind on the aft end of the rocket. I'm going to look for info on that.

Here's a thread that mentions adding the massless cone to the tail to simulate base drag in short stubby rockets in Rocksim, and it has a link to an Apogee article about it. https://www.rocketryforum.com/showthread.php?58407-Still-obsessing-Gizmo-stability-and-(ACK!)-math!

I added the cone to my OR file, and the stability moves to just over .9 without the motor and just over .8 with. So I'm wondering if this is a legitimate trick in OR, or is it just for Rocksim? And does the MDRM count as a short stubby rocket, or is this technique not appropriate for the MDRM?

ChrisAttebery mentioned it in a thread a few weeks ago (I was wondering how much weight in the nose I'd need for the Magg) that it works for both RS and OR. It would be 12 inches long tapering from 0 to 4 inches for the MDRM. I think the MDRM is stubby enough for it to matter, and the reason few mention it is a stock built one is very stable. What's the speed off the launch rod? You may be able to add a bit of weight and still fly fine on a G. What motor did you decide on for this?

Does anyone know if .6 cal is typically a safe margin?

I would think the pods, making the rocket so wide and short, might require increased stability, although mostly if not aligned perfectly. The amount of nose weight needed to offset their weight would only be a small fraction of their weight, so it's more of a question if you're pushing it already.

As to the subject of base drag, I think the added cone thing is only appropriate to the degree the rocket type has empirically been determined to be more stable. It seems like a logical fudge, but it's still fudge. I designed a finless rocket with a large base that OR considered far more stable than actual. If it had also given credit for base drag, the error would have been even higher. I think "base drag" may be a myth, used to compensate for the extra stability of flat conical saucer shapes and for when a rocket is so short and fat that the margin can be small compared to the diameter.

bill_s said:

I would think the pods, making the rocket so wide and short, might require increased stability, although mostly if not aligned perfectly. The amount of nose weight needed to offset their weight would only be a small fraction of their weight, so it's more of a question if you're pushing it already.

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I've never had to do these kind of calculations before, and it has been a long long time since any kind of physics class for me! Do you know how to calculate what kind of mass is needed in the nose? Here is my stab at it.

The CP is at 32.188
The diameter is 4
So the New CG should be less than 32.188 - 4 = 28.188 in order to have 1 caliber of stability

The mass with motors is 48.7 oz
The CG is 29.729

Mass x CG = New Mass x New CG

So 48.7 x 29.729 = New Mass x 28.188

New Mass = 51.366

Nose weight = 51.366 - 48.7 = 2.666 oz

If I add 2.666 oz or more of nose weight, the rocket should have 1 cal or more of stability and the weight of the rocket with motors will be 51.366 ounces or more.

Does that sound about right?

^^-- yeah. The simplest calculation would be relative: example added 5 oz., 5" behind orig. CG = 25. This would require 1 oz., 25" in front of orig. CG to offset.

Another way is use OR. First, had to take out motor and total mass override. Wow, it's heavier. Overrode NC and fins weight. Found it difficult so took off an ounce, added a 1 oz. mass to slide around and match CG. Now added nose weight. This time I came up with 3 oz. ... It does take a lot to shift the total CG much, since it's a fraction of the heavy total. Bad thing about 3 oz. added is you hit 3.3 lbs. total with 4.8 oz. of motor. Maybe should try swing test or something.

View attachment MDRM-P-2_bill.ork

I think you can fly more than 3.3 lbs at moffet, and in CA I think we also have a non-waiver limit of 1500 grams only. 3 ounces extra nose weight is not that bad.

bill_s said:

^^-- yeah. The simplest calculation would be relative: example added 5 oz., 5" behind orig. CG = 25. This would require 1 oz., 25" in front of orig. CG to offset.

Another way is use OR. First, had to take out motor and total mass override. Wow, it's heavier. Overrode NC and fins weight. Found it difficult so took off an ounce, added a 1 oz. mass to slide around and match CG. Now added nose weight. This time I came up with 3 oz. ... It does take a lot to shift the total CG much, since it's a fraction of the heavy total. Bad thing about 3 oz. added is you hit 3.3 lbs. total with 4.8 oz. of motor. Maybe should try swing test or something.

View attachment 184464

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Thanks, Bill. I kind of wish I had tried harder to sim the thing BEFORE building it so I could see how heavy it was going to get, but the materials were pretty complex with the way the fins and pods are made, so I didn't see OR helping much at that point.

Regarding the 3.3 lbs, what is the significance of that? Is everything over that 3.3-lb limit considered HPR regardless of motor? I can never keep track of these regulations, but the fact is, if I'm not flying small LPR rockets, I'm flying at a club launch, so I follow their rules and assume they have the legalities covered. I'm going to check with the club where I am hoping to fly it first and see if they have a weight limit. If so, then I'll see if I can keep it under 3.3 lbs, but still feel comfortable with the stability. If it has to go over the limit to be stable, then I'll just have to get my L1 and fly it at the HPR launches!

About the swing test, I hung the rocket from some parachord in my garage and used a fan to blow air at it. Without any weight, the rocket did mostly eventually track into the wind stream, but was very slow to correct. Then I taped about 2.7 oz weight to the tip of the nose (a bunch of steel nuts in a small bag taped outside the NC on the tip) and tried it again. It seemed like it corrected faster, but it also tended to overshoot the center and oscillate back and forth before stabilizing. Part of the problem is probably the weak turbulent airflow from the fan, and I also think having the small bag of weight taped to the tip of the NC messes up the airflow and moves the CP forward from what it would be if the weight were inside the NC.

SCrocketfan said:

I think you can fly more than 3.3 lbs at moffet, and in CA I think we also have a non-waiver limit of 1500 grams only. 3 ounces extra nose weight is not that bad.

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I'll check with LUNAR about the weight limit, but I have never seen them mention one. I'm thinking I will probably end up over that 3.3 lb mark.

Off topic --- I did check with LUNAR about the MA3 drag race, and they said they can do it. Keeping the number of rockets to 6 or fewer works best, because each bank of pads has 6 pads. If my group can all make it to the launch and you bring yours, we will have exactly 6. But if we could find ANOTHER 6, maybe we could get 2 banks of pads...

EDIT: Changed 4 to 6. Makes a big difference! We can have 6 rockets in a race.

3.3 lbs needs a waiver in mosts states, and I think 500 grams needs a waiver in california. Lunar has said waiver at moffet for sure, I've flown stuff well over 500 grams. I will check about the MA3 drag race at the club meeting on thursday.

SCrocketfan said:

3.3 lbs needs a waiver in mosts states, and I think 500 grams needs a waiver in california. Lunar has said waiver at moffet for sure, I've flown stuff well over 500 grams. I will check about the MA3 drag race at the club meeting on thursday.

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I'll be OK with it needing a waiver, because I don't have any plans to fly it outside of club events. As you said, you need some kind of additional permission to fly even a 500+ gram rocket in CA --- I think it may be a pyro license --- sp club launches are really the only practical way to go anyway.

Check at the club meeting on Thursday about the MA3 drag race to see if ther are any other details. I sent the club president an email using teh address on teh website and got back a response:

Eric,
Yes, we can do drag races at Ames. I would prefer no more than 6 rockets in
a drag race, as that uses a complete rack of launch pads.
David

Click to expand...

So it sounds like we are in the clear for a 6 rocket race, and that would be great! If we can find others who want to join in, meybe we can convince them to give us 2 banks of pads. Or maybe we'll just have to do 2 races...

EDIT: It looks like I typed 4 rockets in my earlier post about the drag race. We can have 6, not 4. I went back and fixed it. Sorry about the mixup. 6 will be perfect for the group we already have planned.

My larger parachute, shock cord and nomex blanket just arrived, and they weigh 3 oz more than the stock recovery that I was using for my weight estimates. So I am definitely going over the 3.3 lbs limit with motors installed. I've got paint on the rocket, so I can't handle it for awhile, but I'm close to being able to put everything together in final form and weigh and balance it, then I'll figure out the nose weight.

What does OpenRocket use as a threshold for warnings about things like stability, speed off the rail, deployment speed, and ground hit velocity.

Here's a screenshot of some of the simulations of different motors with my rocket. (This might mot be the final weight --- I'm still messing around with it.)



Simulation 1 shows a warning, and the reason is the super-high deployment speed with the long delay. All of the other flights seem like OR thinks they are OK, even with a pretty low Velocity Off Rod for the G40. Also, before I started messing with the weights and the CG, the stability with motors was about .6, and all of the flights were green --- velocity off the rod was higher before nose weight, but stability was lower.

Rule of thumb would say you need more speed off the rod than some of these sims are showing and also that you need a better stability margin than .6. Does OR provide warnings for low velocity off the rod and low stability? I assume it does, but apparently it doesn't use the typical rules of thumb. What are the thresholds it will use?

I'm trying to dial this nose weight in for a safe flight on as many G-class single-use motors as I can, but it's a trade-off between speed and stability. And with all the flights showing in the green, it is hard to say what is the better trade off. What would you do?

Also, for the flights above, would you feel comfortable with all of these, knowing there is at least 1 cal of stability, but not much more? Or are some of these speeds too low?

I personally would be worried about the G40W and also the G79. 40-45 ft/s is usually ok. The G40 also scares me because of the altitude, you really want to get a rocket this size well over 400 feet in case something goes wrong (chute takes a few seconds to open, deployment failure) but if you want to keep it low it could work. I would try a big motor first just to make sure it goes fast enough. Rocksim does not have the check/exclamation mark that openrocket has, and I'm not sure if 35 ft/s is safe. It's safe on the decaffinator, but that's a very light and stable rocket that is literally 1/10th the weight of most rockets it's length.

One last thing I noticed is ground hit velocity, it's really fast. Is that the new chute or the stock one? Last month I broke a fin off my MDRM (I used white glue for the fins they now have epoxy) and crumpled a bit of the BT on my partizon. For this weekend, I think I'll put a 30" madcow chute in my partizon or leviathan and the 42" chute setup from my G-force in my MDRM.

Just wondering, what glue did you use to attach the fins?

That's what I thought as well, start with the G80 and see how it goes to get a feel for the rocket. Then work down the list in order of takeoff speed and apogee if I want to try different motors. Unfortunately, I like the reds and whites at the bottom of the list, and I already have s bunch of G80's from the Fry's sale! I haven't actually bought a motor for the rocket yet. I am supposed to meet Mr. G from the forum at Moffett, and he has a variety of G motors in different delays that he is willing to part with, so we will see what is available (I'm going to guess his selection of G motors is NOT why he is called Mr. G). Hopefully, he has a G80-4. If not, I may have to wait on flying this rocket.

Those sims were not for the new chute. I've plugged in the new 45" chute that came today, and the ground hit velocity is now 19 ft/s --- that's still higher than I expected when I ordered the chute, because I didn't expect it to end up so heavy! I was aiming for a gentler landing.

I had similar experiences at Moffett with landing on the concrete. My first time there, I borrowed the 30" chute out of this MDRM kit and used it on my Leviathan for several launches and landed with only a minor crunch on one fin tip --- no big deal. The next time, it was a bit more windy, so I used the stock Leviathan chute, and had some bad damage to the fin and a bit of damage to the BT as well.

For the fins, I used Titebond Molding and Trim Glue. I'm pretty confident in them. I left the aft centering ring off and filleted every single joint inside before gluing the CR into place. Everything that touches anything is glued with a thick fillet, and the way the fin tab is cut with a slot that interlocks with a slot in the middle centering ring makes it seem pretty bulletproof. I'm more concerned about damage to one of the pods than having the fin break loose, but I guess until I see it actually fly and land a few times, it's hard to say what kinds of stresses it might have.

My first ever midpower flight was at moffet a few months ago:

Flew great on an F50-4,but it landed behind the fence around the hanger and the chute dragged at around 50 feet. Fortunately it ended up right next the the fence so I was able to get it out. For one flight total, it's a really beat up rocket and it has a new nose cone now (from my big daddy that CATOd) because the old one was scratched.

Sounds like you anchored the fins really well, should not have problems then. I built mine before I realized how to add internal fillets with the CR off. If you can't get a G80-4, you'll probably be able to fly it on a 77R-4 or 78G-4.

SCrocketfan said:

My first ever midpower flight was at moffet a few months ago:
View attachment 184508
Flew great on an F50-4,but it landed behind the fence around the hanger and the chute dragged at around 50 feet. Fortunately it ended up right next the the fence so I was able to get it out. For one flight total, it's a really beat up rocket and it has a new nose cone now (from my big daddy that CATOd) because the old one was scratched.

Sounds like you anchored the fins really well, should not have problems then. I built mine before I realized how to add internal fillets with the CR off. If you can't get a G80-4, you'll probably be able to fly it on a 77R-4 or 78G-4.

Click to expand...

Moffett is brutal! I've only been to a few launches there, but several times I've overheard people comment, "That's a beautiful rocket. I can't believe you're going to fly that here! At Moffett! On concrete!"

This is the first time I've tried leaving off the aft CRa and doing the internal fillets. I think it worked really well. I tried a lot of new techniques on this rocket, and I am pretty happy with it --- just a bit annoyed it turned out so much heavier than expected. I guess that's just another part of the learning experience.

If Mr. G does not have a suitable G motor for me, then I am going to try to contain my Go Fever, and just wait for a better, softer field and order an appropriate motor. Now that this rocket has turned out so heavy, it seems more like an HPR than an MPR, and maybe I'll take it to October Skies and try to get my Level 1 with it. I probably would have chosen to build a more conventional rocket for that, but now I have this, so maybe I'll give it a shot.

Safe velocity off rod varies. It is lowest when the aerodynamic profile is large and also aided by a high moment of inertia. The fins look huge and a lot of weight is in the ends, also out in long fins, so the moment of inertia is high. So I'd say the low rule of thumb of 30 fps would be totally adequate in this case. And then there was the guy that claimed to fly a MDRM on a F15....

For calculating the stability of the rocket as pictured above, see https://my.execpc.com/~culp/rockets/Barrowman.html Representing the pods as additional fin area is optimistic in my opinion and will set the CP further aft than it actually is.

I prefer the rule of "CP-CG distance should be 10-15% of rocket length" over the 1 caliber rule as it directly relates the inertia of the rocket to the correcting torque that the fins apply.

Hope this helps.

Oliver

bill_s said:

Safe velocity off rod varies. It is lowest when the aerodynamic profile is large and also aided by a high moment of inertia. The fins look huge and a lot of weight is in the ends, also out in long fins, so the moment of inertia is high. So I'd say the low rule of thumb of 30 fps would be totally adequate in this case. And then there was the guy that claimed to fly a MDRM on a F15....

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Well, that would definitely be good news if the lower 30 fps would work. I think I'll do the first flight on a higher thrust motor like the G80, and see how things look as I work my way down the list of motors toward the lower thrust end.

I have a G77-4R and G78-4G I'll bring and would be willing to sell or trade if you can't get a motor from Mr. G.

Personally if I was RSO, I would be hesitant to allow that rocket leaving a rod/ rail at 30fps. I generally would look for 40fps on a rocket that size. While it doesn't sound like a lot, percentage wise it is and when you factor in tolerance of thrust ( manufacturing tolerance) and any wind that may be occurring, I think you are closer to failure than sucess.
If I was gonna let it go, it would be out at rods/ rails for "k and l impulse motors ( just moving it farther away from everyone) and definitely it would be a "heads up" flight.

Raketenolli said:

For calculating the stability of the rocket as pictured above, see https://my.execpc.com/~culp/rockets/Barrowman.html Representing the pods as additional fin area is optimistic in my opinion and will set the CP further aft than it actually is.

I prefer the rule of "CP-CG distance should be 10-15% of rocket length" over the 1 caliber rule as it directly relates the inertia of the rocket to the correcting torque that the fins apply.

Hope this helps.

Oliver

Click to expand...

Oliver, thanks for the info.

When using this method, would I just omit the fin pods altogether for calculating the CP? As an intermediate step when adapting the original OR file, I did save a version that did not have the pod profile added as fin area. It did have the extra fin material I had added to the rocket to make the outside edge of the fin parallel to the BT, but not the pod profile. OR calculates the CP to be .5" further forward when you omit the pod profile. Also the overall length of the rocket is 43" and diameter is 4", so 10% of length would be 4.3", instead of 4" for 1 cal.

If the CP without the pods is 31.7", and 10% of length is 4.3", then I am looking for the CG to be at 27.4" or less, correct?

Thanks for your help.

SCrocketfan said:

I have a G77-4R and G78-4G I'll bring and would be willing to sell or trade if you can't get a motor from Mr. G.

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Thank you! That would be great.

Rrwjr said:

Personally if I was RSO, I would be hesitant to allow that rocket leaving a rod/ rail at 30fps. I generally would look for 40fps on a rocket that size. While it doesn't sound like a lot, percentage wise it is and when you factor in tolerance of thrust ( manufacturing tolerance) and any wind that may be occurring, I think you are closer to failure than sucess.
If I was gonna let it go, it would be out at rods/ rails for "k and l impulse motors ( just moving it farther away from everyone) and definitely it would be a "heads up" flight.

Click to expand...

Thanks for your thoughts on this. I intend to pick a motor that will get it moving at least 40 fps, probably more. When you say, "I think you are closer to failure than success," do you mean at 30 fps, or do you mean with the design in general? What would be your recommendation for making the safest flight with this rocket?

ThirstyBarbarian said:

Thanks for your thoughts on this. I intend to pick a motor that will get it moving at least 40 fps, probably more. When you say, "I think you are closer to failure than success," do you mean at 30 fps, or do you mean with the design in general? What would be your recommendation for making the safest flight with this rocket?

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I meant by the 30fps. Design in general no issues. I also have a tendency to overbuild everything , so I found I can't use a lot of recommended motors at times.