Nike X Upscale

[Bruiser]

My Ventris is just about done so I have been giving some thought to what I am going to build next.

I've decided my next rocket will be a Nike X upscaled to a BT60 and built as a two-stage. The booster will be 7 inches long and capable of being powered by a 24mm "D" or "E" motors. The sustainer will be over 23 inches long and powered by 18mm "B" or "C" engines. Total length will be slightly over 30 inches.



I've been working on the OR file today and will finalize it this weekend. My biggest hang up is the fins: 1/8th inch basswood or papered balsa. Such decisions Oh, and I doubt it will be painted black and white

I went to Hobby Lobby today to buy one of the new HL exclusive Vapor kits for it's parts but they did not have one. Luckily they did have a High Flyer XL that was just the right size.

Anyway I think I will be ready to start building my kit on Tuesday. That shouldn't take long as I don't think I'll have to cut out too many pieces/parts. Most will come right from the High Flyer XL.

-Bob

 

[mbeels]

Looks like a fun project

 

[jqavins]

The sustainer fins look really small; with the canards on it as well my mind sim says you might have a stability problem after separation. Have you checked? Of course you've checked, you're not stupid, and I just have to ask because I hear alarm bells.

 

[Bruiser]

Maybe I am I thought the same but this is what OR says. So unless I missed something. I do forget things...



-Bob

 

[jqavins]

I took home a print from your first post and recreated the geometry in RockSim to the best of my ability, and I got pretty darn close. Looking again this morning I think I made the canards a little too big and a little aft of where they should be. The bigger canards would hurt, but there isn't a big difference between your CP and mine; the real difference is the CG.

I did have to make two critical assumptions, those being the mass of the nose cone and the mass of the fins. I went with, (was it 1 mm or 1/2 mm) thick polystyrene for the nose cone and 1/8 balsa for the fins. I got negative about 1/4 caliber. Is your nose cone a bunch heavier than that? Are you using a heavy parachute?

I may be all wet here. My worry is that given how much an engine usually influences a small rocket's CG (because everything else is so light) the CG in your figure just looks really high.

Here's what RS told me.

 

[neil_w]

I believe a BT60 nose cone of that size would run around 1.4 oz plus or minus. What is it in your sims (either Bob or Joe)?

I admit the tiny fins do look iffy to me as well, although with a heavy nose cone and a light motor it doesn't seem impossible.

Regarding the fins: I can think of no reason to use basswood here, unless you really want to. I'd go with 1/8" balsa and save the weight.

 

[Bruiser]

I am not sure what the weight of the nosecone was in OR. That is on my laptop which is at home.

I do have the High Flyer XL here though and my scale is also here. The actual weight of the nose cone is .91 ounces.

I'll have to check what the weight is in OR tonight.

-Bob

 

[mbeels]

Another consideration, while it is possible to obtain a good static margin with small fins and a forward CG, that will likely result in poor dynamic stability (low damping ratio, etc...). So fins that "look" small, may still be too small, even with a good static margin. I'm not sure if Open Rocket can do dynamic stability analysis, but Rocksim can. That would be worth looking into.

 

[BABAR]

Assuming the sustainer is ONLY flown AS a sustainer (not solo), and assuming the booster has enough OOMPH To get the full stack up to speed, can sustainers get by with smaller fins since they should be at a higher speed at separation than they would be coming off the rail? Always a risk making assumptions, of course, but would fin efficiency (effectiveness at altering center of pressure) be much high for a sustainer already at full speed at separation, thus less finnage may still be adequate?

Put another way, for safe flight does the sustainer HAVE to be stable in a single stage configuration to be a stable in a two stage stack configuration?

 

[mbeels]

Put another way, for safe flight does the sustainer HAVE to be stable in a single stage configuration to be a stable in a two stage stack configuration?

That's a good question. I bet you're right that a sustainer can get away with lower dynamic stability, since it doesn't the rapidly changing conditions such as coming up to speed and leaving the rail. After getting up to speed and up to altitude, I would guess that it has relatively smaller perturbations.

 

[jqavins]

I am not sure what the weight of the nosecone was in OR. That is on my laptop which is at home.

Likewise. Tonight I'll not only check my nose cone weight, but update it to 0.91 oz and see what I get.

Put another way, for safe flight does the sustainer HAVE to be stable in a single stage configuration to be a stable in a two stage stack configuration?

Marginal might be OK; I can see some logic to that, but I can't really say. But it certainly has to be stable or else even the slightest of perturbations will start it skywriting.

 

[mbeels]

Marginal might be OK; I can see some logic to that, but I can't really say. But it certainly has to be stable or else even the slightest of perturbations will start it skywriting.

Right, I think BABAR was referring to a sufficient static margin, with less dynamic stability, as would be achieved with smaller fins and nose weight.

 

[jqavins]

Right, I think BABAR was referring to a sufficient static margin, with less dynamic stability, as would be achieved with smaller fins and nose weight.

As I understand it, dynamic stability is really not a good term, since to my knowledge there's really no such thing as dynamically unstable unless you're talking about the CP migrating forward with AoA, which is why we need static margin greater than 0.01 calibers. And there's no single number that constitutes dynamic stability. Things like the damping ratio and so on are good, poor, great, lousy, or some such but there's no stable/unstable dividing line. "Dynamic characteristics" would be a much better term. And yes, small fins plus plenty of nose weight can make for adequate static margin with poor dynamic characteristics, which is probably acceptable. (Well, yes, really bad dynamic characteristics can potentially help the AoA become large enough to migrate the CP into instability. It's complicated. It's rocket science.)

Be advised: The preceding is based on theory read and understood but not on experience.

Marginal might be OK; I can see some logic to that, but I can't really say. But it certainly has to be stable or else even the slightest of perturbations will start it skywriting.

I've been thinking about that since I wrote it. Personally, I wouldn't build a sustainer that can only be safely flown with its booster. Whatever one intends, sooner or later someone will try flying it on its own; that someone may well be me on a good day for bad decisions.

(It's similar to why I will never buy a coffee table with spindly legs. It's at a good height for sitting and you can be sure that eventually some will sit on it.)

On the other hand, Bruiser's OR sim shows the static margin of the sustainer alone is more than ample. So unless OR is wrong there's really no safety issue.

 

[Bruiser]

I picked up my laptop at lunch and overrode the weight of the nose cone. Old weight of the sustainer was 4.42 ounces and the new weight was 3.8 so it is now lighter. Old stability was 2.46 and new is 1.86.

This is based on C6-5 motor, Estes bt60 tube, 1/8th bass fins, 3" 18mm engine tube with engine block and cardboard centering rings, 18" plastic parachute (Might want to rethink that to a 12). I haven't added an engine hook yet but it will have one. Also thinking of an ejection baffle if specs permit... I just changed from an 18 to 12" parachute and the numbers did not change.



So Joe, it'll be interesting to see what RS says with the weight updated.

-Bob

 

[Mugs914]

The C of G still looks kinda suspect to me. You have something showing at the tip of the nose (the black dashed line) that could be biasing the C of G toward the front. Might be a stray "mass object" or something.

 

[Bruiser]

The mass object was a screw eye that weighed .348 ounces. I removed it and the weight dropped to 3.46 ounces and the stability went to 1.06 calibers.

I decided to enlarge the fins slightly and see what happens. I made them 1/2 inch longer and the weight went to 3.54 ounces and stability went to 2.65 calibers.


So then I wondered how the slightly larger fins would affect the look of the whole rocket. I don't think it hurts the look at all.


Here's the original


-Bob

 

[mbeels]

As I understand it, dynamic stability is really not a good term, since to my knowledge there's really no such thing as dynamically unstable unless you're talking about the CP migrating forward with AoA, which is why we need static margin greater than 0.01 calibers. And there's no single number that constitutes dynamic stability. Things like the damping ratio and so on are good, poor, great, lousy, or some such but there's no stable/unstable dividing line. "Dynamic characteristics" would be a much better term. And yes, small fins plus plenty of nose weight can make for adequate static margin with poor dynamic characteristics, which is probably acceptable. (Well, yes, really bad dynamic characteristics can potentially help the AoA become large enough to migrate the CP into instability. It's complicated. It's rocket science.)

Oh yes, I totally agree, "dynamic characteristics" is a much better term. Poor dynamic characteristics can lead to underdamped oscillations which can result in the fins stalling out if the angle of attack is too large. Probably the most likely times for that to happen would be coming off the rail, while accelerating quickly, or very large windshear.

 

[neil_w]

I decided to enlarge the fins slightly and see what happens. I made them 1/2 inch longer and the weight went to 3.54 ounces and stability went to 2.65 calibers.

Big impact on stability, negligible impact on appearance. Seems like a good change.

 

[jqavins]

Meanwhile, back at RockSim, I increased the weight of the nose cone, which helped a lot. I also made a correction to the canards, shrinking them a little and moving forward a little, which made very little difference. I didn't have that screw eye, which move would move the CG forward a little more than I have it. I also changed the fin material to basswood to match your design (though I think you'd be fine with balsa).

The nose cone weight made a huge difference in the CP, a lot more that I expected. I also switch RS from using the straight Barrowman equations to using its improved equations (something I often forget to do) and that mad a really surprisingly big difference to the CP. I get about 1 1/4 calibers, so we're in pretty good agreement.

This time yesterday, if I recall, you reported over two calibers, which I could only get to by removing the engine. But then there's the screw eye, so it's not unbelievable. And I surely have some other details left out.

It seems that my false alarm kicked off a 15 post long tempest in a teapot.

 

[Bruiser]

Oh, I wouldn't say it was a false alarm at all. It pointed out what might have been a serious problem with the design. I'll have to admit that when I first put the rocket into OR I thought the fins looked entirely too small. Then OR said they where fine and that got me thinking about some scale rockets like the Pershing and Saturn V. Those fins look too small too.

It also re-emphasize the practice of weighing your parts or built assemblies and inputting that data into OR for the most accurate sim data. That's something that I am still trying to teach my brain to remember to do. The old noggin just ain't what it use to be

I think I will stick with the larger fin just to be on the safe side. I'll decide for sure once I add the ejection baffle in the sim and see the results. It should shift the cg forward a little more but I don't think it will be much. I also need to weigh an engine hook and add it as a mass object to the rear.

Anyway, I had not started cutting parts yet because I wasn't sure the design would fly but I think it's safe for me to start cutting up some parts now

Thanks,
-Bob