My Sci Fi space fighter needs heavy weapons (literally- 25g would do nicely)

According to OR, this will fly assuming those "laser cannon" thingys in front weigh 25g a piece.


I was thinking of like launch lugs filled w/ clay but that's not enough, maybe filled w/ a section of a carpenter's nail.

But I was wondering if anyone had a better idea.
Maybe some metal war model I could kit-bash. Something from model trains or planes I could re-purpose?

AHA TIA

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EDIT: CONCLUSION/TLDR:
This was a terrible idea. Guys toss around a few ideas on this in the thread below. Some tell me not to do it. I did some further work on the subject;


1. Its ridiculously dangerous.
No matter what you use as ballast (putty, metal and epoxy, whatever) by the time you pack it into an aerodynamic shape (a straw, a launch lug, a cone) and cover that in glue, and glue that to the point of a balsa fin, cover that in three coats of enamel, and fly that at 150 mph … if that’s not bullet, it is an arrow. Actually two of them!


2. And it will never fly:
According to OR, with a C engine, this thing gets 12 Gs of acceleration before it’s off the rod. Which means these 25g weights become the equivalent of 300g weights! And while they do combine to have a common CoM in the center of the rocket, individually they do not. So they are going to be pulled downward and outward (at 300 grams) during launch. If that doesn’t just rip the fins off the body tube or snap them in half, then it *must* bend them drastically, and that’s going to be steering the rocket some random direction – WITH TWO ARROWS ON THE FRONT OF IT. This was a terrible, terrible idea.


So please do not attempt.


That said, I decided to put all the ballast in the nosecone, and bring the wings back a bit. There is still some disagreement over whether that will fly. But I’m trying it anyway.
In this thread

Lead cast in the shape of your Lascannon would be cool! should easily be the weight you need, sand casting or lost wax method would work.

BrainlordMesomorph said:

... maybe filled w/ a section of a carpenter's nail...


AHA TIA

Click to expand...

Pretty sure that's a safety code violation.

try this stuff

https://www.pinewoodpro.com/pinewood-derby-tungsten-putty-weight.html

widely available at hobby shops.

8thPaladin said:

Lead cast in the shape of your Lascannon would be cool! should easily be the weight you need, sand casting or lost wax method would work.

Click to expand...

hmmm......

interesting idea, thank you...

I actually have experience w/ molding and casting. but lead is soft, and poisonous.

Use tungsten powder mixed into epoxy inside a launch lug tube.

jlabrasca said:

Pretty sure that's a safety code violation.

Click to expand...

I wasn't going to put nails on it. I was thinking filing down cylindrical sections of nail putting them inside launch lugs filled w/ glue.

Isn't putting 50g of anything out there going to be about the same saftey-wise?

rharshberger said:

Use tungsten powder mixed into epoxy inside a launch lug tube.

Click to expand...

well thanks, but isn't that stuff $1 a gram?

$50 worth of dead weight?

(the putty would cost $20, and that's too much)

And either way don't I end up with a hard cylindrical thing of the same mass and volume? (i,e the same risk?)

And BTW guys, by moving the engine an inch forward, they're down to 20g a piece

BrainlordMesomorph said:

I wasn't going to put nails on it. I was thinking filing down cylindrical sections of nail putting them inside launch lugs filled w/ glue.

Isn't putting 50g of anything out there going to be about the same saftey-wise?

Click to expand...

NAR Saftey code, item #1

https://www.nar.org/safety-information/model-rocket-safety-code/

The "lightweight" part is open to interpretation -- since dense substances used for ballast are not lightweight. A cut-off nail wrapped in paper and mounted at the front of the rocket is is a little harder to wink at.


I see tungsten putty for as little as $7/oz. 50 grams is just less than 2 oz. Question is how wide a tube do you want to mount on the front of the wing?

Not sure but you might have other issues than just weight. Forward swept fins like to twist which might lead to issues with angle of attack ( could be you getting attacked&#128512. Make sure and test it from a long distance for the first flight.

OK,OK, OK, it is all the same risk: too much!


By moving the engine forward another half an inch, and adding another 5g I can put all the weight in the nosecone

problem solved


EDIT: having weight on the tips of the fins like that would really make then bend! But empty launch lugs should be ok, right?

BrainlordMesomorph said:

OK,OK, OK, it is all the same risk: too much!

By moving the engine forward another half an inch, and adding another 5g I can put all the weight in the nosecone

problem solved

Click to expand...

Well yeah, but then you're risking the Krushnic Effect... <smile>

BrainlordMesomorph said:

EDIT: having weight on the tips of the fins like that would really make then bend! But empty launch lugs should be ok, right?

Click to expand...

Empty launch lugs are fine. Launch lugs filled with stuff that won't be shrapnel if the rocket disintegrates should be fine too.

Its a very cool design. If OR calls it stable off the rod, then go with it. Paper the wings so that they don't bend so much, put the ballast in the cannons, and call a "heads up" flight when you launch.

I did a boiler plate model of a tie defender and it ended up spinning upon launch, no matter how the weight was distributed. It took the addition of front towing pod/droid to get it stable.

https://www.starwarsmodels.com/images/models/tiedf001.jpg

My brainsim has a very hard time imagining that thing to be flyable, regardless of what OR says. Definitely want to see some video if you give it a shot.

jlabrasca said:

Well yeah, but then you're risking the Krushnic Effect... <smile>

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I get that your joking but I don't get the joke.

/I paper all my balsa

jlabrasca said:

Well yeah, but then you're risking the Krushnic Effect... <smile>

Empty launch lugs are fine. Launch lugs filled with stuff that won't be shrapnel if the rocket disintegrates should be fine too.

Its a very cool design. If OR calls it stable off the rod, then go with it. Paper the wings so that they don't bend so much, put the ballast in the cannons, and call a "heads up" flight when you launch.

Click to expand...

No, Now I'm thinking that any ballast in the cannons is a really bad idea. Whether its epoxy and tungsten, or whatever, anything that is the shape I want and the density I need would be basically a nail, and shrapnel.

no. clay weight goes in the nosecone.

version 1B
45g of nose weight in the nosecone



Mainly, she's an excuse to go totally nuts with the inkjet decals.
I'm talking ILM qulaity, high-detail, dirt, weathering, a girl riding a rocket on the side of the nosecone (with appropriate paint chipping).

That's the fun part!

The question is: is it from the Tau Ceti Colonial Militia, or The Republic of Mars Orbital Defense Force?

EDIT: BTW; rear blow! I figure the nosecone would get stabbed by those wings!

Very cool project. Looks like a Baby Bertha with some modifications. 18mm composite motors, nose cone filled with lead shot. Long launch rod, good conditions at launch.

BrainlordMesomorph said:

No, Now I'm thinking that any ballast in the cannons is a really bad idea. Whether its epoxy and tungsten, or whatever, anything that is the shape I want and the density I need would be basically a nail, and shrapnel.

no. clay weight goes in the nosecone.

Click to expand...

Tungsten putties have a densities between 7 and 10 grams/cm³ This stuff has about the harness of silly putty and is very adhesive.
https://www.derbydust.com/shop/tp7-tungsten-putty/
Steel has a density of about 8 grams/cm³. It has the hardness of steel.

BrainlordMesomorph said:

I get that your joking but I don't get the joke.

Click to expand...

Yeah, it wasn't that funny.

https://www.rocketreviews.com/krushnic-effect.html

Can you move c.p. farther aft? Mount the fins at the base of the boat tail, or extend their chord length aft. Fins over the boat tail might look cool too. You don&#8217;t have to rely only on moving the c.g., unless you really don&#8217;t want to change the appearance.


P.s. you might consider building up those fins from multiple parts with various grain directions as appropriate so those tips don&#8217;t break off. Papering might be sufficient though.

GlenP said:

Can you move c.p. farther aft? Mount the fins at the base of the boat tail, or extend their chord length aft. Fins over the boat tail might look cool too. You don&#8217;t have to rely only on moving the c.g., unless you really don&#8217;t want to change the appearance.

P.s. you might consider building up those fins from multiple parts with various grain directions as appropriate so those tips don&#8217;t break off. Papering might be sufficient though.

Click to expand...

1 you can't put fins on a transition in OR but I goosed those fin designs around for hours! Putting any more balsa any farther back actually pulls the CM rearward more than the CP, the same is true w/ the rear points on the wings. I'm planning 1/4" balsa for the wings, 1/8 for the fins, (I paper everything).

Originally it had two wings and two fins, adding the third fin helped a lot.

So I just tried adding a 4th:
Stability went from 0.3 to 0.51, loses 1 mph, cuts 20 feet off the apogee. (but looks cool)

EDIT: Adding the 4th fin means I can reduce the noseweight!
35g not 45. I still have stability of 0.38, but that adds 40 feet to Ap, and almost 20 mph!

Of course the right way to fix it would be adding a inch or two of body tube, but that's the one thing I don't want to do.

BrainlordMesomorph said:

1 you can't put fins on a transition in OR ...

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Create a "phantom" inner tube the same diameter as the diameter of the transition at the point of attachment-- that is create a tube and set the mass to zero. Attach the inner/aft corner of the fin to the end of the phantom tube. It gets fiddly and frustrating as you conform the inner edge of the fin to the transition, so that that OR doesn't error out.

jlabrasca said:

Create a "phantom" inner tube the same diameter as the diameter of the transition at the point of attachment-- that is create a tube and set the mass to zero. Attach the inner/aft corner of the fin to the end of the phantom tube. It gets fiddly and frustrating as you conform the inner edge of the fin to the transition, so that that OR doesn't error out.

Click to expand...

Best way to do this quickly is to zoom into the side view so the joint between the transition and the fin is nice and big in your face. Then edit in the freeform editor to align the root edge with the transition. Takes only a few seconds.

Depending on the fin, I’ve also just let a fin tab take up the gap. Does the visual trick.

But while it makes the sim appear right, none of those tricks affect Cp - so it’s up to the OP if they want to go through the effort.

Thanks guys.

But like I said balsa back there moves the CM as much as the CP.

Also the boattail is a paper cone, totally non-structural (and its part of the rear blow section).

And I couldn't get OR to do a fin w/ and angled base, or one w/ any point smaller than the base diameter. (i.e the foward points couldn't be narrower than the main body tube)

Cool design, but it looks like the kind where the sims are going to have a problem predicting how it will actually fly. Make sure to take a video of the first flight!

BrainlordMesomorph said:

And I couldn't get OR to do a fin w/ and angled base, or one w/ any point smaller than the base diameter. (i.e the foward points couldn't be narrower than the main body tube)

Click to expand...

This may or may not end up being useful for your current build, but it's a common and useful OR technique so worth getting sorted out:


So:

1. Phantom BT has zero length (I usually also make it zero wall thickness), matched diameter to trailing edge of transition
2. Fin is mounted to phantom BT
3. Two root edge points of freeform fin are placed right next to each other at the back of the fin
4. Remaining points are filled out to desired shape, including edge hugging the surface of the transition. So the edge along the transition is *not* the root edge, as far as the simulation goes.

ooh I see.

That OR CP is way up there so you can't get any more conservative and still fly it. Power and nose weight will solve your problems on this one.

neil_w said:

Best way to do this quickly is to zoom into the side view so the joint between the transition and the fin is nice and big in your face. Then edit in the freeform editor to align the root edge with the transition. Takes only a few seconds.

Click to expand...

Indeed, you are correct. I should like to amend and amplify my previous. It gets fiddly and frustrating when trying to conform a fin to a transition with something different from a conical cross-section. I am really hoping that you will now correct me on this too.