Stability - "temporary" nose weight

[Faroutspacenut]

Hi Everyone, I have all but a couple coats of paint left on the 2.6" Madcow DX3 I'll be launching this weekend. (my first serious mid-power rocket) My one lingering concern is that I'm a little close to the edge on stability - ranging from about 1.6 to 1 caliber depending on the size of the motor. 2 ounces of weight in the NC will buy me another .5 cal roughly, but I'm not really keen on epoxy and led shot. Can anyone suggest a more "temporary"/adjustable form of ballast? Considering going with good ol' modeling clay. I'm already pretty heavy at 26 oz, so not sure I want to go much more than that as I'm also right on the line with my speed off the rail.

 

[cbrarick]

bbs and spray foam. Stick a baggy in first, then the bbs and the foam. dissolve the foam away with acetone when you want it out. use the cheap stuff from home depot as I know that dissolves with acetone.

 

[terryg]

Use a larger quicklink to connect the shockcord to the nose.

 

[dhbarr]

What's the nose setup currently look like?

 

[Faroutspacenut]

What's the nose setup currently look like?

Nose cone (blow-molded plastic) with the attachment ring offset - very small hole in center - would need to be enlarged to fill. Kit is designed to allow for DD but for me I have the upper section empty payload. E-Bay in coupler. Not a big deal to put some "dead weight" in the upper tube, but I'd like to take advantage of getting the weight as far forward as possible so I can shift the CG with as little weight as possible.

I'm thinking also maybe some threaded stock with a couple of nuts on the end I could have running up the center of the NC, attached at the opening with some washers and a wing nut. This way I can add or remove nuts to change the weight as needed.

Intrigued by the spray foam idea, but would that stay put if it's inside a plastic bag? [edit: I supposed it wouldn't matter if you filled it up all the way]

 

[rstaff3]

If you want to kludge it a little...

One time I cut a hole in the middle of the cone to fit a 24mm tube (I ended up using non-rocket scrap tubing). I moved the cord attachment off to the side (a loop of kevlar). I chunked in a few rocks and screwed the tube in place (may not be as easy on your cone). To my amazement, this held up perfectly well...until the non-eject event.

This was a field mod and I don't recommend rocks as nose weight. I offer it just to get you thinking out of the box.

 

[AfterBurners]

why are you concerned. Figure your CG and CP based on the largest motor you will fly and everything else will be fine. 1.6 - 1 sounds perfect. I would worry if you are below 1.0 and you have something like .45 etc. I'd leave the weight alone. I would use BB's, epoxy and fill the gap with expanding foam. Just make sure you compensate for all 3 items

 

[rharshberger]

1 caliber of Stability is pretty much ideal for a sport flier rocket unless you are planning on flying Mach 1+ then a 1.6 or 2 would possibly better as there is a CP shift involved with going supersonic or through transonic (can't remember which at the moment). Most of my rockets are targeted for 1 caliber of stability as I usually don't fly above transonic speeds.

 

[rstaff3]

1 caliber of Stability is pretty much ideal for a sport flier rocket unless you are planning on flying Mach 1+ then a 1.6 or 2 would possibly better as there is a CP shift involved with going supersonic or through transonic (can't remember which at the moment). Most of my rockets are targeted for 1 caliber of stability as I usually don't fly above transonic speeds.

This!

 

[Faroutspacenut]

1 caliber of Stability is pretty much ideal for a sport flier rocket unless you are planning on flying Mach 1+ then a 1.6 or 2 would possibly better as there is a CP shift involved with going supersonic or through transonic (can't remember which at the moment). Most of my rockets are targeted for 1 caliber of stability as I usually don't fly above transonic speeds.

Ok this puts my mind at ease. I've been seeing lots of different theories on this. This is not a huge rocket, but it's not a tiny sport flyer either. Everything up to the highest G I could use is 1 caliber or over. What I did not mention however is if the rocket checks out and I'm feeling up for it, I'd like to try for my level 1 cert. with it on an H128W. The simulations seem almost perfect but it's pretty fast off the rail (75.1 ft/s) and just a fraction under 1cal. of stability. Max velocity is only .43 mach.

 

[T-Rex]

Sounds like you are wanting to add a skoch of weight when/if you fly that H128. As has already been said, a larger quick link will add some. You might also look in the fishing section of your local store and grab some weights with loops. They can be tied in place with a loop of wire, kevlar, paracord guts, etc when needed and removed when not. You will just have to figure out a way to tie them in. For that matter, tie in anything weighty that has a hole.... (hex nuts, washers, etc)

Since it is a plastic nosecone, most likely nothing will stick to it well. I usually drill 2 sets of 1/4" holes across each other (give or take 90 degrees) and insert a piece of dowel through each. You can loop some kevlar or thin cable around the dowels, then fill the end of the nose with epoxy to lock it all together and in place. You can lock a piece of all thread in the same way, just make a 'J' or 'U' shape so it gets caught by the dowels. (difficult to explain, and I don't have a picture available, I'm sorry)
I have seen recommendations for placing the end of the nose in water to keep it from melting when the epoxy kicks. If you do that, you will need to seal the holes to keep the water out of the epoxy.... I have never had it be an issue with LOC nosecones, but they are thicker and a different plastic.

 

[scsager]

Hi Everyone, I have all but a couple coats of paint left on the 2.6" Madcow DX3 I'll be launching this weekend. (my first serious mid-power rocket) My one lingering concern is that I'm a little close to the edge on stability - ranging from about 1.6 to 1 caliber depending on the size of the motor. 2 ounces of weight in the NC will buy me another .5 cal roughly, but I'm not really keen on epoxy and led shot. Can anyone suggest a more "temporary"/adjustable form of ballast? Considering going with good ol' modeling clay. I'm already pretty heavy at 26 oz, so not sure I want to go much more than that as I'm also right on the line with my speed off the rail.

Hi Rob!

I understand your concerns, and It's good to get your first big rocket right.

The 2.6" DX3 should weight around 15 oz. I'm not sure why yours comes in at 26 oz. That seems way too heavy - even for an "overbuilt" model.

Also the instructions are really clear about the CG. Measure from the tip of the nosecone 31 inches down the rocket and make a mark. Balance the rocket in "ready-to-fly" condition (including the motor). It must balance in front of that mark (closer to the nosecone) IF it balances behind that mark, you must add nose weight.

 

[rharshberger]

Hi Rob!

I understand your concerns, and It's good to get your first big rocket right.

The 2.6" DX3 should weight around 15 oz. I'm not sure why yours comes in at 26 oz. That seems way too heavy - even for an "overbuilt" model.

Also the instructions are really clear about the CG. Measure from the tip of the nosecone 31 inches down the rocket and make a mark. Balance the rocket in "ready-to-fly" condition (including the motor). It must balance in front of that mark (closer to the nosecone) IF it balances behind that mark, you must add nose weight.

There are three versions of the 2.6" DX3 a fiberglass, a carbon fibre, and a cardboard.
He did mention a plastic NC so it is most likely the cardboard.

Was that 26oz rtf, if so that sounds pretty good.

 

[Faroutspacenut]

There are three versions of the 2.6" DX3 a fiberglass, a carbon fibre, and a cardboard.
He did mention a plastic NC so it is most likely the cardboard.

Was that 26oz rtf, if so that sounds pretty good.

It's the cardboard kit. This was my weight primed, not painted but otherwise ready to fly. My main goal for this build was to get more experience with some of the construction techniques I would need for high power.

It's definitely not the stock kit aside from the included nomex blanket. Larger chute (because of weight and hard playa landings), rail buttons with weld nuts, altimeter bay with a JL Altimeter 3 and the snap-in holder, Aeropack retainer, kevlar harness with sewed loops, swivels and quick links. I used no epoxy except for the motor retainer but was quite liberal with the Tightbond 2 when mounting the fins. I used elmers wood filler to fill my tube spirals and three applications of tightbond trim and moulding for the fin fillets. They weren't all that pretty so a couple of swipes of elmers filler to even them out. 3 coats of sandable primer with a LOT of sanding (and filling) in between coats. So yeah, all those little bits and pieces add up to a heavy rocket. I just put my second coat of finish paint on this morning and I'm really happy with how it's looking.

I've had a ton of fun (over) building it and am excited to see it fly. Just want to make sure it does so safely. This forum has been a tremendous help to me, so thanks to you all for your suggestions. There is often the desire to get things perfect from the start, and I've certainly fallen into that trap here, but I'm also a firm believer in learning from failure and I expect to have plenty as I gain experience and skills along the way.

 

[rharshberger]

It's the cardboard kit. This was my weight primed, not painted but otherwise ready to fly. My main goal for this build was to get more experience with some of the construction techniques I would need for high power.

It's definitely not the stock kit aside from the included nomex blanket. Larger chute (because of weight and hard playa landings), rail buttons with weld nuts, altimeter bay with a JL Altimeter 3 and the snap-in holder, Aeropack retainer, kevlar harness with sewed loops, swivels and quick links. I used no epoxy except for the motor retainer but was quite liberal with the Tightbond 2 when mounting the fins. I used elmers wood filler to fill my tube spirals and three applications of tightbond trim and moulding for the fin fillets. They weren't all that pretty so a couple of swipes of elmers filler to even them out. 3 coats of sandable primer with a LOT of sanding (and filling) in between coats. So yeah, all those little bits and pieces add up to a heavy rocket. I just put my second coat of finish paint on this morning and I'm really happy with how it's looking.

I've had a ton of fun (over) building it and am excited to see it fly. Just want to make sure it does so safely. This forum has been a tremendous help to me, so thanks to you all for your suggestions. There is often the desire to get things perfect from the start, and I've certainly fallen into that trap here, but I'm also a firm believer in learning from failure and I expect to have plenty as I gain experience and skills along the way.

Sounds like a well built rocket and with the proper accessories as well, recovery using a oversize chute for hard surfaces seems well thought out. I can see 26ozs rtf.

 

[Faroutspacenut]

Also the instructions are really clear about the CG. Measure from the tip of the nosecone 31 inches down the rocket and make a mark. Balance the rocket in "ready-to-fly" condition (including the motor). It must balance in front of that mark (closer to the nosecone) IF it balances behind that mark, you must add nose weight.

This is an excellent point and for the benefit of anyone contemplating a similar build or having a similar question: 31 inches from the tip of the nosecone is in fact the exact center of pressure (also confirmed in Openrocket). That might seem obvious to some but it's not explicitly stated in the instructions.

 

[Faroutspacenut]

Two successful flights this weekend. The first to 1040 feet on a G74W. The decision to use a larger chute turned out to be a bad idea. It was windy. VERY windy. Nice soft landing but the rocket was dragged more than a half a mile across the lakebed. If not for some low scrub on the OTHER side of the highway it would have kept on going. My cherry paint job was wrecked and the body tube took some damage where it meets the coupler. Stiffened it up with some thin CA and a quick sanding and I was back in action, now with it christened "Road Rash". For the second flight I used a smaller chute and decided to try out my chute release for the first time. This was on a G79W and it flew to 1451 feet. By this time the winds had died down a little, so that and with the smaller chute it didn't try to run off on me again. Oddly the cute popped at deployment. I'll be posting about that in the Recovery forum. Thanks to all for the advice. Looking forward to flying it again soon.

 

[jqavins]

OK, I know I'm late to the party, but I can't hold back my 2 cents on temporary nose weight techniques.

1. I believe this was mentioned, but I second it, and perhaps offer a variant: Tie some nuts or washers onto a loop of sturdy line and simply add that loop into what's already on the existing quick link.

2. Drive a lag screw into the nose cone. Either use the very small center hole or punch a new hole near it. Twist the crew in, and you can twist it right back out later. Or twist in a bigger one when you've got a heavier motor.

3. Think heavy thoughts. :wink:

Incidentally, if you need a little weight for stability (I like about 1.25 calculated just to be sure I've really got 1.0 or better despite errors) but your speed off the rail is a little on the low side, then what you obviously need is more thrust. In other words, go for that level 1!

 

[John Beans]

For the second flight I used a smaller chute and decided to try out my chute release for the first time. This was on a G79W and it flew to 1451 feet. By this time the winds had died down a little, so that and with the smaller chute it didn't try to run off on me again. Oddly the cute popped at deployment. I'll be posting about that in the Recovery forum. Thanks to all for the advice. Looking forward to flying it again soon.

Rob,
Add a good sturdy "shake test" right before flight to make sure your bundle won't fall open at ejection, and that your Chute Release is holding it securely and that it won't just slip out when it is ejected.

 

[Faroutspacenut]

Rob,
Add a good sturdy "shake test" right before flight to make sure your bundle won't fall open at ejection, and that your Chute Release is holding it securely and that it won't just slip out when it is ejected.

Thanks John. And now for the stupid but obvious question: what am I holding when I shake? The chute bundle itself? At the swivel? All of the above?

 

[Faroutspacenut]

In other words, go for that level 1!

Yes! Building a LOC Vulcanite for this very purpose.

 

[bill_s]

A bolt threaded into the rear of the nosecone. For example, if the hole is exactly 1/4", use a 7 mm machine bolt as a 1/4" will be loose. Or bore out the hole.

Note that weights that are not solidly attached will not fully add stability and may even destabilize the rocket. At the least, such weights should be removed before checking CG, not used to improve CG.

 

[John Beans]

Thanks John. And now for the stupid but obvious question: what am I holding when I shake? The chute bundle itself? At the swivel? All of the above?

The shock cord. You're trying to simulate the bundle getting ejected and yanked to a stop by the cord. You also never want to see tension on the CR's tether. Grab the shock cord and shale it around and see if the CR still holds the chute well.

 

[jqavins]

Yes! Building a LOC Vulcanite for this very purpose.

That's great, and best of luck! But it's not what I meant.

You're adding weight to the DX3 for stability, and then you're concerned about speed off the rail. The solution is more thrust. The two motors you used are of different length, so I assume there is no engine block. You're rocket should be able to handle a small H motor such as an Aerotech H128W or H97J, or Cesaroni H133. Then you'd do you're L1 flight with the rocket you've got in hand.

 

[Faroutspacenut]

That's great, and best of luck! But it's not what I meant.

You're adding weight to the DX3 for stability, and then you're concerned about speed off the rail. The solution is more thrust. The two motors you used are of different length, so I assume there is no engine block. You're rocket should be able to handle a small H motor such as an Aerotech H128W or H97J, or Cesaroni H133. Then you'd do you're L1 flight with the rocket you've got in hand.

You are absolutely correct, and in fact I was hoping to do a L1 flight on that same launch day with it - just ran out of time. This particular rocket seems to be well suited for H motors given all the factors. I could still use it for my cert flight next time for sure (and might still since I know it flies), but I'm itching to build something new.