Some questions about OR flight sim data

[Banzai88]

So I’ve been mind simming a mach/mile project for a while now and just started an Open Rocket sim to validate some of my ideas, and came to a realization that I don’t know how to use some of the flight sim data that it provides to me.

Specifically, the whole correlation between Max Velocity and Max Acceleration (if there is one). I even went back into some of my other sims of rockets that I’ve built and I’m only now becoming aware of the fact that, while certain motors may be aerodynamically stable due to the CP/CG relationship, I may be WAY underbuilt for the acceleration loads that certain Vmax and others may provide! :jaw:

I’m taking it that Max Velocity is the highest predicted speed, which I’m shooting for something over mach-ish. oint:

It’s Max Acceleration where I get fuzzy. What is this actually telling me? Can I correlate this to how many G’s the vehicle can expect to endure? As in “a real kick in the pants” vs “slow, steady push”?

Also, isn’t material strength going to interact with the Max Acceleration figures? I’ve seen a few flight failures where the airframe tubes crumpled at the mach transition (and I understand that), and a few that failed way earlier than that (presumably from too many G’s?).

How do you guys approach analyzing if your airframe is capable of ‘taking the hit’ from a certain motor? Sim? Gut check? Seat of the pants? Hope for the best?

Or am I over analyzing it?

 

[blackbrandt]

I tend to do a gut check and a look at what other people have done. For the size motors i launch, the G-forces aren't incredibly important. I'm working on a rocket designed around the 38/1200 with a propellant called Ferric fudge. It should be able to do about 220g off of the pad. For this, I'd be using CF and FG for the airframe.

What type of g's are you looking at? And post your file.

 

[bclark989]

... It’s Max Acceleration where I get fuzzy. What is this actually telling me?

Also, isn’t material strength going to interact with the Max Acceleration figures?

For the physics part of your question, acceleration is simply change in velocity over time. Example: You drive down the road at 60 miles per hour. If you speed up (accelerate) to 70 miles per hour over the course of 1 hour, your acceleration is 10 miles per hour per hour. Stated another way, the unit of measure for acceleration is distance per time per time, or even distance per time squared.

G-force is a specific implementation of acceleration in relation to gravity, which is a constant measure of acceleration. Gravity is not a "force" on its own (g-force is sort of a misnomer). You have to add mass to the equation. "Standard gravity" is 9.80665 meters per second squared, which equates to 9.80665 Newtons of actual force per 1kg of mass.

Once you use OpenRocket to determine that acceleration, you could look at the material properties for your construction materials and adhesives (as well as factoring in the density of the air you are flying in) to see how they will handle those sorts of stresses. Unfortunately, the amount of force exerted on different elements of the vehicle vary a lot, making it difficult to do the math to determine if a certain feature will survive. Read the Wikipedia page on dynamic pressure and you'll see what I mean. Q is the symbol for dynamic pressure, which is where the term "Max-Q" comes from in rocketry. You are asking one of the most fundamental questions of rocketry: Will the rocket survive Max-Q? In our world, Max-Q is basically peak acceleration. In commercial rocketry (real rockets launching into space), there is a tipping point where the atmosphere gets thin enough that dynamic pressure begins to taper off despite continued positive "jerk" (increase in acceleration... change in velocity per time per time per time, lol).

In the real world, most people just go by experience. Many people report flying supersonic using cardboard tubes, wood fins, plastic nose cones, and no fiberglass overlays. That being said, your components better be in good shape. A chain is only as strong as its weakest link and all that...

That was little more run-on than I meant it to be. Hope that helped =/

BC

 

[kcobbva]

That was little more run-on than I meant it to be. Hope that helped =/

BC

Actually it was excellent! Thanks.