How Marginal Is Too Marginal?

[SteveA]

This is a design I've been working on and I decided last night to re-design it last night and made the boosters live powered by B6-0s. Of course, the stability dropped to .21. Yet, in all the sims it shows no indication of an unstable flight and that, even, in 18 MPH winds. So what do you think? is .21 too marginal and the sim does not take this into account when ran?

Thanks for your time
Steve

[AlphaHybrids]

That is a landshark! I'd bump it up to at least 2.5, especially since the boosters are powered.

Edward

[SteveA]

Any suggestions? More nose weight? I have 2.5 oz in the nose now.

I really hope that I can get this to where I can build and fly it. I think it would make for a spectacular launch!

[azgreg]

Make the fins a little larger?

[SteveA]

Make the fins a little larger?

Yeah, I've considered that, but there's going to be a big trade-off with increased drag. I guess I may have to bite the bullet and live with the decrease in altitude.

[SteveA]

The addition of smaller fins and larger main fins really knocked-down the velocity. The max altitude, not so much.

[AlphaHybrids]

Getting all those lined up correctly will be a challenge. Can you lengthen the rocket at all? It would get your nose weight further in the right direction. What if you go to 3oz? I'd make the fins larger, also.

[SteveA]

Getting all those lined up correctly will be a challenge. Can you lengthen the rocket at all? It would get your nose weight further in the right direction. What if you go to 3oz? I'd make the fins larger, also.

I probably could lengthen the rocket a little more, it couldn't hurt. I sort of cringe at the idea of enlarging the fins much more, ( I enlarged them almost double their original size) a max velocity of 197mph seems kinda slow...to me anyway (maybe I'm full of beans too). I did go ahead and added a total of 3oz in the nose. Surprisingly, the change in stability was minor. I guess I should expect that, there is a lot of weight hanging on the back end of this. As far as the alignment goes, yeah, you're probably right and this looks better in the plans than it will in application.

Thanks for the input!
Steve

Addendum I lengthened the BT 80 main tube by 2 inches and stabilized it to 2.32.

[plano-doug]

This is a design I've been working on and I decided last night to re-design it last night and made the boosters live powered by B6-0s. Of course, the stability dropped to .21. Yet, in all the sims it shows no indication of an unstable flight and that, even, in 18 MPH winds. So what do you think? is .21 too marginal and the sim does not take this into account when ran?

Here are some thoughts I've had on this. You might play with them in a simulator. I know they won't all work. But here are some things to consider.

1. Right now, you have a fat-skinny-fat profile that tends to move the CP forward. Consider making both forward sections skinnier. Right now, it's 2.6"-2"?-2.6". How about making it 2"-1.6"-2.6". That will shift the CP aft some and improve the stability margin while preserving the fat-skinny-fat theme.

2. Consider making the aft outboards larger diameter and/or add more of them. That will increase the aft area thus moving the CP farther back. My L2 was built with a very wide back section and almost vestigial fins, but had a very good CP location thanks to the fat aft section.

3. As for altitude and velocity, I'd be more concerned about deployment velocity. With such a complex rocket, getting in a good, damage-free flight is a much higher priority than performance. So I'd be focused more on motor delay combos (altho right now, your DV numbers look very good).

4. As for margin, rather than looking at calibers, I like to think in terms of fractions of overall length, for which I like 10%. So, for a 41" long rocket, I'd want ~4.1" of margin (which is about 1.5 calibers).

HTH.

Doug

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[SteveA]

Doug,

I did work on another revision using your suggestions. I made the forward sections smaller in diameter and increased the diameter of the boosters as well as enlarged all the fins. It did, as you stated it would, increase the stability margin significantly. I also increased the overall length by about five inches. The best I could get with the margin was something like 3.95, which I thought was pretty close to your 4.1. but in the end, I didn't like how it turned out. The forward and aft portion looked fine but the enlarged boosters changed the symmetry to much for me. So, I returned to original version, enlarged the all the fins considerably and again lengthened the overall rocket and came up with the below version. I also ran the sim using C6-0s for the boosters.

You're right, I do not want to trade stability for performance. My concern with the velocity was coming off the rod. I thought maybe the speeds were kinda slow even though it was 3mph faster than minimal speed for stable flight. I just thought I should have a little bigger margin.

Thanks for all your help so far, it was useful and made me re-think a lot of things with this design. I really appreciate that!

Best wishes
Steve

[plano-doug]

I also increased the overall length by about five inches. The best I could get with the margin was something like 3.95, which I thought was pretty close to your 4.1. but in the end, I didn't like how it turned out.

Glad I could help, Steve.

Was that 3.95 calibers? Or inches? I I was pushing for 4.1" or about 1.5 calibers, to be clear.

It looks like you have ~8" of margin, so it should be plenty stable as you have it now. You might could even drop a little nose mass.
...
On a different note, are you using one launch lug or two? With a long rocket and only one lug, a little cross wind at the pad can cause the rocket to bind on the rod thus scrubbing off precious impulse as it climbs during liftoff. I prefer two short lugs - one at the aft end, and, in this case, one about 12" ahead of that. By spacing them apart, there is very little binding that can be imparted.

BTW, how long is the launch rod? A longer launch guide is another way of improving rod speed, FWIW.


Doug

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[GDJ]

Just my 2¢...

What's really important is the stability cal off the launch rod. I usually aim for no less than 0.95 off of a 5' launch rod. More is better of course. Anything less and it gets really touchy when it comes to side winds, rod angle, whether or not the rocket gods passed gas, etc. and that's no fun at all.

Your second screen shot I would be completely happy with (2.0 cal is plenty fine usually) as long as it has good cluster firing.

ps. I want pics of this bad boy when you fly it. With this many engines, it's gonna be spectacular!!

[SteveA]

Your second screen shot I would be completely happy with (2.0 cal is plenty fine usually) as long as it has good cluster firing.

ps. I want pics of this bad boy when you fly it. With this many engines, it's gonna be spectacular!!

Great! That's what I was shooting for. Oh, believe me, there will be pictures. I think it will look pretty spectacular as well. Lots of smoke and fire!

[SteveA]

Glad I could help, Steve.

Was that 3.95 calibers? Or inches? I I was pushing for 4.1" or about 1.5 calibers, to be clear.

It looks like you have ~8" of margin, so it should be plenty stable as you have it now. You might could even drop a little nose mass.
...
On a different note, are you using one launch lug or two? With a long rocket and only one lug, a little cross wind at the pad can cause the rocket to bind on the rod thus scrubbing off precious impulse as it climbs during liftoff. I prefer two short lugs - one at the aft end, and, in this case, one about 12" ahead of that. By spacing them apart, there is very little binding that can be imparted.

BTW, how long is the launch rod? A longer launch guide is another way of improving rod speed, FWIW.


Doug

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Doug, It's inches. I'll drop the nose weight since you mentioned it. I really didn't like that much weight hanging over the end like that. There are two launch lugs and both are an inch long. I have the aft lug tucked along side the booster. I need to go back through Rocksim and see how to change the inches to calibers. If, I am at around 8, then I would really like to knock off some length. Living here on the high plains seldom affords us days of less than 5mph winds and I can see this thing weather-cocking like crazy. I'm glad that I have this ironed-out, I'm really wanting to build this puppy and maybe even have a chance to fly it before it gets too cold. Now all it needs to do is rain for about a week...It's a little parched here. To put it mildly.

Thanks Doug and everyone else for the input!

Steve

[SteveA]

Here it is Doug and anyone else who wants to play with it or tweak it.


Steve

It will fly on three motors in the event of a misfire. Worse case scenario was flight on one main and two outboards. (D12-7 mains and C6-5 outboards) In 8-14 MPH winds...That said, it was dicey, but successful. I suspect that it might have shredded the recovery package.

[mikemech]

You might consider keeping nthe longer length and reducing the fin size.

[markkoelsch]

I would be concerned about getting all the motors lit.

Velocity off the rail/rod is your friend...replace the D12 with CTI E-75 :-)