### Re: Dr. Dinosawer answers all your questions on physics

#16In The Expanse books, Gauss cannons are mounted on ships I believe they never mention coilguns. I'm only starting Book #3 so...

In The Expanse books, Gauss cannons are mounted on ships I believe they never mention coilguns. I'm only starting Book #3 so...

Thanks, Dino. That makes perfect sense. You're considerably better at explaining than Wikipedia is with their insane mathematical equations.

Also, I read 5 (I think?) books of The Expanse, and I don't recall anything but railguns in there.

--IronDuke

Also, I read 5 (I think?) books of The Expanse, and I don't recall anything but railguns in there.

--IronDuke

Pretty sure the railguns are long-range ship-ship guns and the Gauss cannons were PDC's.IronDuke wrote: Also, I read 5 (I think?) books of The Expanse, and I don't recall anything but railguns in there.

Ah, I never figured out what the PDCs were exactly. I assumed they were electromagnetic, but wasn't sure.

--IronDuke

--IronDuke

Exactly. They call point defens cannons (PDCs) Gauss cannons sometimes. And railguns are for insane ship to ship hole-doing from far away.0111narwhalz wrote:Pretty sure the railguns are long-range ship-ship guns and the Gauss cannons were PDC's.IronDuke wrote: Also, I read 5 (I think?) books of The Expanse, and I don't recall anything but railguns in there.

Alright, let's have some fun.

Why does carbon fiber have such a bad shear strength vs it's tensile strength?

Why does carbon fiber have such a bad shear strength vs it's tensile strength?

I'd guess because its "glued" together fibers with high tensile strength.

The fibers themself dont give much support in directions that arent pulling on any of them, they just flex. And place all of the stress on the "supplemental" epoxy

The fibers themself dont give much support in directions that arent pulling on any of them, they just flex. And place all of the stress on the "supplemental" epoxy

Do you guys actually draw free body diagrams or do you use computer software?

If you apply a force off of the center of mass of an object, how much angular acceleration does it provide? What's the formula/equation/thingy for this?

E.G. For maneuvering thrusters, I need to know how much force to apply at the thruster location to get a specific angular acceleration. My physics book seems to cover everything except that.

--IronDuke

E.G. For maneuvering thrusters, I need to know how much force to apply at the thruster location to get a specific angular acceleration. My physics book seems to cover everything except that.

--IronDuke

angular acceleration = force * lever lenght (distance from CoM) / moment of inertia

dont forget that that also still applies a linear acceleration as usual if you have unbalanced thrust

edit:

example tables of moments of inertia for a couple of objects

https://en.wikipedia.org/wiki/List_of_m ... of_inertia

dont forget that that also still applies a linear acceleration as usual if you have unbalanced thrust

edit:

example tables of moments of inertia for a couple of objects

https://en.wikipedia.org/wiki/List_of_m ... of_inertia

Last edited by Cornflakes_91 on Wed May 17, 2017 12:52 pm, edited 1 time in total.

And crucially the moment of inertia must be calculated in the plane that the torque is appliedCornflakes_91 wrote:angular acceleration = force * lever lenght (distance from CoM) / moment of inertia

dont forget that that also still applies a linear acceleration as usual if you have unbalanced thrust

Oooh, thanks guys! This is super ~~simple ~~ helpful!

--IronDuke

--IronDuke

you can take variations of the "3D inertia tensors" from my link and adapt them to spaceships (for example the ellipsoid one adapted to the dimensions of the ship)IronDuke wrote:Oooh, thanks guys! This is super~~simple~~helpful!

--IronDuke

and just put your x,y,z torques into those formulas (attention with signs!)

to get a probably passable approximation.

with "only" 7 variables attached to the ship

and to avoid having to calculate accurate moments of inertia for your ships

Last edited by Cornflakes_91 on Wed May 17, 2017 1:12 pm, edited 1 time in total.

Uh dude if you're implementing 3D dynamics that stuff is super complicatedIronDuke wrote:Oooh, thanks guys! This is super~~simple~~helpful!

--IronDuke

The equations of motion get really busy

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