[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[a.g.]

I have noticed that the new version causes some B9 engines have insane amount of drag: http://i.imgur.com/PGkPCLe.jpg. After a number of experiments in reverting parts of the changes, I found that it is caused by the propeller fix, which when applied to these engines somehow manages to remove the whole engine from consideration, and thus confuses the surface area math. Checking the names like this seems to fix it:

                propellerName = (string)propeller.GetType().GetField("blade1").GetValue(propeller);
                if (propellerName != "") {
                    tmp2 = p.FindModelComponents<Transform>(propellerName);
                    tmp1 = tmp1.Concat(tmp2).ToArray();
                }

[Starwaster]

Also less Atmosphere. Use mechjeb's "Limit acceleration" function. Set it to about 20m/s^2. That should prevent any sudden unplanned disassembly.

^^^ THIS ^^^

So important. For MJ users with large unwieldy rockets, it's about as important as the 'prevent overheating' option. You should have those set even if you're flying on manual control.

[asmi]

2G is too low imho. Soyuz rocket pulls up to about 4G, and that's manned rocket.

[KerbMav]

The thrust is the same, but the acceleration is higher because there's less fuel, meaning less mass. Same thrust + less mass = more acceleration.

Also less Atmosphere. Use mechjeb's "Limit acceleration" function. Set it to about 20m/s^2. That should prevent any sudden unplanned disassembly.

There would be the same amount of fuel and atmosphere, there actually is while flying horizontally, which is what I am doing (to play with the game engine)

I turn the rocket 180 an suddenly the thrust seems to crush the tank - accelerating to 500m/s should be the same as slowing down, it is the same change in momentum.

[jrandom]

There would be the same amount of fuel and atmosphere, there actually is while flying horizontally, which is what I am doing (to play with the game engine)

I turn the rocket 180 an suddenly the thrust seems to crush the tank - accelerating to 500m/s should be the same as slowing down, it is the same change in momentum.

At what altitude does this happen? If in the atmosphere, accelerating forwards is dampened by the atmosphere (thrust - drag) whereas aiming retrograde has thrust and drag working in the same direction (thrust + drag). That might account for the difference.

[ferram4]

Alright, the 0.9.6.2 revision is out, fixing the isShielded bug and the problems with my attempted fixes to Firespitter compatibility. Everything should function properly now; sorry about the bugs guys. Give a big thanks to a.g., who is apparently better at bugfixing my code than I am.

[BubbaWilkins]

Quote Originally Posted by Eric S View Post

The thrust is the same, but the acceleration is higher because there's less fuel, meaning less mass. Same thrust + less mass = more acceleration.

Quote Originally Posted by Torminator View Post

Also less Atmosphere. Use mechjeb's "Limit acceleration" function. Set it to about 20m/s^2. That should prevent any sudden unplanned disassembly.

There would be the same amount of fuel and atmosphere, there actually is while flying horizontally, which is what I am doing (to play with the game engine)

I turn the rocket 180 an suddenly the thrust seems to crush the tank - accelerating to 500m/s should be the same as slowing down, it is the same change in momentum.

It's not the same because momentum is not a static value, it's the product of mass and velocity and has a direction. Flipping the craft over puts your engine thrust in direct opposition to the momentous forces of the craft. Therefore, the stress being applied at the structural connection of the engine is much higher decelerating than it is accelerating. The net change in velocity is the same, but the stresses on the structure are much higher when it is moving in a direction different than the one you are trying to accelerate in.

[jrandom]

It's not the same because momentum is not a static value, it's the product of mass and velocity and has a direction. Flipping the craft over puts your engine thrust in direct opposition to the momentous forces of the craft. Therefore, the stress being applied at the structural connection of the engine is much higher decelerating than it is accelerating. The net change in velocity is the same, but the stresses on the structure are much higher when it is moving in a direction different than the one you are trying to accelerate in.

That doesn't seem quite right... Put a rocket in space and use that as your frame of reference and you can consider the rocket to be at rest with the rest of the universe moving past it. In a vacuum you should get the same kind of force regardless of what direction you point the engine.

[taniwha]

DRE's g-force damage is nothing but bogus. It completely ignores the m in F=ma. Parts sandwiched between a mainsail and a heavy ship undergo much higher stress than the same part between a LV-T45 and a light probe body, yet because of the acceleration explode. Parts on a high TWR rocket are ok until you stage, but explode due to "g-force" damage when you stage as they rapid decelerate from drag... while experiencing less total force than when they were still attached.

With its g-force damage, DRE2 is just as bogus, if not more so, than DRE1 was. I have disabled the g-force damage in my local copy, though I do plan on putting it back once I figure out how to fix it (if nothing else, make it affect only Kerbals).

[Torminator]

Momentum is a product of force times mass. While you're correct about the direction, you have to take into account that the net force is going to be Thrust + Drag, while when burning prograde, it's Thrust - Drag, so the Net Force is greater when burning retrograde.

[BubbaWilkins]

If you don't like it, take it up with Sir Isaac Newton.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

[jrandom]

If you don't like it, take it up with Sir Isaac Newton.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

If you're in a weightless environment in a vacuum and not under acceleration, there is no experiment you can do to determine if you are moving nor in what direction. Applying thrust in any direction will net the same amount of g-force. The only time you're going to get a difference is if you're in an atmosphere and have to contend with drag.

[taniwha]

There are two net forces: that which results in acceleration, and that which results in compression. With 10kN-><-10kN, the Kerbal will not accelerate at all (net accelerative force is 0kN), but it will be squished by net 20kN of compressive force.

[BubbaWilkins]

If you're in a weightless environment in a vacuum and not under acceleration, there is no experiment you can do to determine if you are moving nor in what direction. Applying thrust in any direction will net the same amount of g-force. The only time you're going to get a difference is if you're in an atmosphere and have to contend with drag.

Which is no reflection of the internal stresses on the craft.

[jrandom]

Which is no reflection of the internal stresses on the craft.

It is if you only have one engine (eg, no other thrust to create compressive forces). Aiming your thrust forward, backward, or sideways to the vector of movement will result in exactly the same internal stresses on the spacecraft (again, in a vacuum), because moving through space is no different than being stationary with space moving past the spacecraft. The two frames of reference are equivalent.

[Torminator]

If you don't like it, take it up with Sir Isaac Newton.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

Okay, I will.

Newton's First Law: ΣF=ma

That's the sum of all forces acting on an object is equal to the object's mass times that object's acceleration.

We'll also define positive X to be the prograde horizontal direction, and negative X to be the retrograde horizontal direction, both where thrust is perfectly perpendicular to gravity.

Now, if we look at a rocket burning prograde in atmosphere, you have the force of thrust, which we'll call Ft, the force of gravity, Fg, and the force of drag, Fd. As gravity is perpendicular to the axis we are interested in, it can be ignored without much impact, so the two primary forces we're looking at are Ft and Fd. I will also avoid going deeper into aerodynamic forces, as that is beyond my knowledge, and isn't strictly relevant for a hypothetical case where we're only considering two orientations.

Recall that Newton's First Law describes the sum of all forces on an object, so we take those two forces, and look at their influence on our object.

In the prograde case, Ft is acting in the +x, and Fd is acting in the -x. This means that the net force is Ft-Fd(or Ft + -Fd if you prefer). In the Retrograde case, both vectors are aligned, and so the net force is Ft+Fd

Therefore we have:

prograde acceleration: ap=(Ft-Fd)/m

retrograde acceleration: ar=(Ft+Fd)/m

Now, our mass is constant in each case, since you mentioned fuel level didn't change (or at least not enough to be significant.)

Therefore, we can combine these equations by setting m=m, and get (Ft-Fd)/ap=(Ft+Fd)/ar, then simplify to ar=(Ft+Fd)*ap/(Ft-Fd). For the sake of math, let's say both thrust is 10, and drag is 1. Then, you have ar=11ap/9, or in plain words, the acceleration, which is proportional to the force felt by the tank, during retrograde is, in this case, 11/9 the acceleration during prograde. Even if we make drag 10 and thrust 1, that merely reverses the sign, indicating that our acceleration is in the opposite direction from what we assumed.

Please feel free to point out any mistakes I made in my analysis. Classical Mechanics is a few years behind me, and I was doing all the work in my head. Still, it's a very simple exercise.

Keep in mind that DR only looks at acceleration, not forces. In Taniwha's example, the kerbal is under great compressive stress, but no acceleration, and so DR would be perfectly happy. Momentum and impulse are likewise irrelevant, as those only come into play when looking at a system over time. As we are only concerned with the peak acceleration, we can treat this as a static problem. This also avoids having to deal with all the nonsense of elasticity, exponential drag, etc...

Edited September 5, 2013 by Torminator

[KerbMav]

Alright, the 0.9.6.2 revision is out, fixing the isShielded bug and the problems with my attempted fixes to Firespitter compatibility. Everything should function properly now; sorry about the bugs guys. Give a big thanks to a.g., who is apparently better at bugfixing my code than I am.

Thank you, my friend!

And sorry for ... whatever stone I got rolling in your thread ...

[jrandom]

We just like arguing to pass the time between releases.

[KerbMav]

Which made you miss the new release!

[BubbaWilkins]

Jrandom,

Sometimes.

Torm,

I don't claim to know the inner workings of DE, but that might be a question to pose there. I do know that structural stresses even in the stock game are calculated individually and exceeding the load on a given node results in failure. Given that all forces are in fact calculated individually on each part, and that momentum is part of the equation, than it only stands to reason that deceleration from a high velocity is going to induce more compression stress on a component then acceleration. Clearly this in line with the observed behavior in game. Either way, the topic is really better taken elsewhere at this point.

Ferram,

About to test it on my Firespitter craft! I'll post my observations on the props and such.

UPDATE!:

Multi engine bomber flies real nice now! Single engine planes also fly nicely although feel a little on the slow side, but that may just be in the engine/mass department now as much as anything.

Edited September 5, 2013 by BubbaWilkins

[jrandom]

9.6.2 is looking mighty good! (B9, Procedural Wings)

[Torminator]

That's quite a nice A-10-alike you made there. Very nice

And yes, sorry for getting all nerdy there. I tend to get carried away with physics.

[jrandom]

That's quite a nice A-10-alike you made there. Very nice

Thanks! It's my second one, the first being a 1.25-meter design using a Firespitter cockpit and fuselage. This wing/engine layout makes for a great low-speed, low-altitude exploring flyer. Super stable and easy to work with. Everyone should build one! Remember to pitch the tail fins down one click to help offset the off-center engines!

Here's the small guy:

[NathanKell]

Yay! Problem solved.

Thank you, ferram! As always, obviously, but a bit especially right now.

[KerbMav]

I did not check yesterday and now I wont see my Kerbals till Sunday - was the CoL displacing bug squashed too?

And if not I would like to repeat my question: Is it just a display issue in SPH or will the craft actually use this CoL in the game?