Advanced Rockets Corporation ARC Scratch Build

jqavins

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Not sure how this one is going to go until I get it all put into Rocksim, but this will eventually be one of my next scratch builds. I've got all my to scale measurements, but I need to finalize components.

View attachment 569321
You mean that's not finished? Because it looks finished. Except for finishing.
 

jqavins

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Those side tubes present surface to the wind, so would act as fins at least a little. The diameter expansion at the bottom, the "reverse boat tail" as it were, brings the CP back as well.

The first part is difficult at best to simulate. Body tubes are not counted in computing the CP, and they can't count as tube fins since they're closed off (and probably too long to do any good anyway). But the second part is easy; transitions are included in the CP calculation, and an expanding one at the base will have just the effect it should have.

Will the transition and side tubes together be sufficient? Well, I'm not saying they will. Neil is probably right. But, since it does look finished except for finishing, you could make sure all the pieces are snugly friction fitted and give it a swing test. Make sure there's a mass stand-in for the motor if the motor mount is not there.

Also, As I'm thinking about it more, I can see you're going to need some sort of connection near the tops of the side tubes to the main tube or they'll shake right off. For a preliminary swing test, lumps of fun-tack should do.
 

COSTransplant

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Thank you all for taking the time to share some really great information -- I really do appreciate it. So after some pondering:

I guess I could make the boosters (even a little bit) hollow so that the air flowed through them and they acted like tube fins?
My son took a look at the rocket last night and said -- Hey, there is nothing in here! -- and he was correct. I've just fit the main pieces together, so it will be a bit before I get everything in place. That said, a spin test is in order!
Asthetics are primary for me, but of course asthetics have to go out the window if the thing does not fly! So I do probably have to come up with something to connect the top of the boosters to the main BT -- or strengthen them internally somehow without adding alot of weight.

And don't laugh -- I'm just spitballing here. But for the booster tubes, if I was to make them hollow like fins, would there be a way to baffle them or "rifle" the inside of the tubes that would help with the location of the CP?

Just wondering.
 

neil_w

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Generally, very long skinny tubes like your boosters do not behave as tube fins, they act like plugged tubes. So I don’t think they’re going to help you out. Putting more stuff in the tubes will only increase the friction and reduce the airflow,

Not sure why you couldn’t do something with regular old fins and make it look great.
 

neil_w

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I still may add fins -- as I did on my Relativity Terran build -- but I can't give up on to scale aesthetics w/o trying! :)
Actually I apologize... despite the thread title, it hadn't really registered that this was a model of a real rocket. I was unfamiliar with ARC and just looked them up. Scale models of any modern orbital rockets are challenging because none of them rely on fins for stability. You generally need to come up with *some* type of modification or addition to make them stable.
 

jqavins

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Actually I apologize... despite the thread title, it hadn't really registered that this was a model of a real rocket. I was unfamiliar with ARC and just looked them up. Scale models of any modern orbital rockets are challenging because none of them rely on fins for stability. You generally need to come up with *some* type of modification or addition to make them stable.
Oh, wow, same here. I was going to comment:
Asthetics are primary for me, but of course asthetics have to go out the window if the thing does not fly!
Not "out the window", but aesthetics must often be compromised to some degree. Finding a compromise that keeps essential elements of your original vision while making it flyable is part of the challenge. Now, knowing that it's a model of a real orbital launch vehicle, that is even harder.

That's the challenge that often ends with clear fins, which nobody likes, but which work to make a rocket stable. But let's try to find a better way. First, to repeat myself, swing test it as is. Swing test it with as much nose weight as you feel you can tolerate given the motor mount you intend to use. You haven't said what mount size you intend, but you might want to consider lots of nose weight and 24 mm composite motors. But that only works if the CP altering features I mentioned above actually work worth a damn. And working against that is the fact that the bottom in the big picture on Advanced Rockets Corporation's landing page seems to show a ball, not a cone. A ball down there won't help your CP as a cone would.

And here are two more thoughts on compromises that you might consider to have acceptably small effects on the look:
  • Expand the bottom cone to get more CP help from it. How much bigger would it need to be? :dontknow: Maybe little enough. (But remember that even having it shaped as a cone is a compromise in the first place.)
  • Open the tops of the tubes and shorten them so they do act as tube fins. There are a couple of issues with that, but it may be a possibility. Taking a second look at the big picture, I think you've got the side tubes too long to begin with.
1679400721761.png
As for tying the tops of the side tubes to the sides of the main tubes, it looks like gluing them to the strakes is very much in line with the picture.
 

COSTransplant

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Actually I apologize... despite the thread title, it hadn't really registered that this was a model of a real rocket. I was unfamiliar with ARC and just looked them up. Scale models of any modern orbital rockets are challenging because none of them rely on fins for stability. You generally need to come up with *some* type of modification or addition to make them stable.
Oh gosh no worries! I think it is fantastic that people are willing to take the time to chime in -- the interaction and exchange of ideas is what makes this forum so much fun! Keep the thoughts coming!
 

lakeroadster

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Update... See Post #18

I just threw together a quick and dirty simulation... and it's crazy unstable, there's really nothing there to give this rocket any realistic stability.

But
it's a beautiful rocket.

This one is going to be a challenge to get a nice stable flight with.

1679410690430.png
 
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jqavins

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I just threw together a quick and dirty simulation... and it's crazy unstable, there's really nothing there to give this rocket any realistic stability.

But it's a beautiful rocket.

This one is going to be a challenge to get a nice stable flight with.

View attachment 570122
Do us a favor an try doubling the aft diameter of the tail, like this:
1679415915640.png
I've done designs (only in silico) that used nothing but a base flare (without any base drag hack) for stability, and they come out fine.
 

lakeroadster

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Do us a favor an try doubling the aft diameter of the tail, like this:
View attachment 570134
I've done designs (only in silico) that used nothing but a base flare (without any base drag hack) for stability, and they come out fine.

Well, it appears that the boat tail I had on the original simulation is what was making the rocket show a ridiculous negative stability. All I had to do was delete it and then the rocket shows a stability caliber of 1.76.

Weird, right? I mean it was a 1/2" long boat tail.

Keep in mind the entire nose cone and transition in this simulation is a made from solid redwood. That's why the rocket weighs nearly a pound without a motor.

Jimmy Durante "the schnozzola" to the rescue.

1679422754765.png

1679422950924.png
 

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