Yeah but the rocket is not being dropped from a height at t=0, it’s stationary. Acceleration should be zero. I’m going to have to play with this.Gravity's acceleration is -9.8m/s² so this is what is reported.
Well, mine certainly are.Yeah but the rocket is not being dropped from a height at t=0, it’s stationary.
That's what I expected. I was wondering if I was doing something wrong — which might mean that the other sim results (e.g. velocity at top of launch rod) won't be useful.Acceleration should be zero.
Attached. The "simple model rocket" example included in the download has the same "feature".
That only makes sense if the rocket is in freefall at T=0. It is not. It's stationary — sitting on a launch pad. While sitting on the launch pad before ignition, the force due to gravity is counteracted by an upward force from the launch pad.Gravity's acceleration is -9.8m/s² so this is what is reported.
Yeah but the rocket is not being dropped from a height at t=0, it’s stationary. Acceleration should be zero. I’m going to have to play with this.
In this case acceleration is a constant and motion is constrained until the thrust exceeds the gravitational force. Velocity and height will then start to change.That only makes sense if the rocket is in freefall at T=0. It is not. It's stationary — sitting on a launch pad. While sitting on the launch pad before ignition, the force due to gravity is counteracted by an upward force from the launch pad.
If the rocket's acceleration is -9.8m/s² at T=0, and velocity is 0 at T=0, then velocity should be trending negative and the height should be dropping below 0. It is not (according to the plot).
If you create a free body diagram, it is correct. You have gravity pulling down at 9.8m/s and no rocket motor thrust.
It is a transition problem. As soon as the rocket starts moving the pad force vanishes. You could model that but then you have to decide when acceleration from motor thrust exceeds 1G so you can drop the pad force. Worse, you have to ignore the motor thrust until it would result in more than 1G of acceleration.But you also have the launch pad pushing up with the same force (9.8m/s * the mass of the rocket) thus the FBD shows 0 net force on the rocket, and thus there should be zero acceleration.
It looks like the programmers took a shortcut. The equal and opposite force will always be whatever it takes to prevent downward travel due to gravity. So, as long as the thrust doesn’t exceed the full weight of the rocket they just left it out of the acceleration value shown on the graph.But you also have the launch pad pushing up with the same force (9.8m/s * the mass of the rocket) thus the FBD shows 0 net force on the rocket, and thus there should be zero acceleration.
The definition of acceleration is \( \delta v/\delta t \), or \( \delta^2 x/ \delta t^2 \). Neither the velocity nor the position of this rocket is changing, so the acceleration is 0.
I now understand exactly what you're saying.It is a transition problem. As soon as the rocket starts moving the pad force vanishes. You could model that but then you have to decide when acceleration from motor thrust exceeds 1G so you can drop the pad force. Worse, you have to ignore the motor thrust until it would result in more than 1G of acceleration.
And until the rocket starts moving, that constant is 0 because the net force on the rocket is 0.In this case acceleration is a constant
Correct. That happens when the net force on the rocket is non-zero and the acceleration is therefore non-zero.and motion is constrained until the thrust exceeds the gravitational force. Velocity and height will then start to change.
No, accelleration is dv/dt. If velocity is a constant 0, then acceleration is, by definition also 0.If the programmer locks the initial velocity and altitude at zero then acceleration should be -9.8 m/s at ignition and increase with the thrust curve of the engine.
The simulation of velocity and altitude is correct. The simulation of acceleration after liftoff is correct. The simulation of acceleration before liftoff is incorrect, but everybody agrees it doesn't really matter.It may take a time step or two for thrust to exceed weight, but as long as velocity and altitude remain at zero until that point the simulation is correct.
+9.8m/s/s2- If you have an Accerometer chip in your rocket then it will measure an acceration of -9.8m/s/s when the rocket is sitting on the pad. Another proof this thinking is correct.
Acceleration is dv/dt or F/m (the two are defined to be the same in classical physics). In the frame of reference we're using (the launchpad), velocity is 0 and unchanging before liftoff. Therefore acceleration is 0. Prior to liftoff, the net force on the rocket is 0, therefor acceleration is 0. Claiming that acceleration (within a given frame of reference) is non-zero when velocity is unchanging (within that frame of reference) is wrong.1- The rocket's weight is its Mass times the Gravitational pull (-9.8m/s/s at Earth's surface). This makes the acceleration of -9.8m/s/s correct even when fixed on the ground in the pure calculations of Physics.
Prior to liftoff, acceleration is zero because the launch pad exerts an upward force equal to the weight of the rocket minus the engine's thrust. Before liftoff, net force is zero, therefor acceleration is zero, and and velocity is unchanging. The OR simulation engine appears to ignore that upward force and applies constraints to position/velocity to try to hide that omission without also constraining acceleration.The Velocity is constrained to not be negative.
That is measuring the force of gravity, and the sign of that value is positive (the same as the acceleration produced by the engine thrust). The "extra" acceleration value shown by OR has a negative sign. In the frame of reference we are using (the launchpad), what is measured by the accelerometer chip when at rest is not acceleration — though it is indistinguishable from acceleration from the accelerometer's frame of reference. That value should be zeroed out before flight if you want to know actual acceleration in the launchpad frame of reference.2- If you have an Accerometer chip in your rocket then it will measure an acceration of -9.8m/s/s when the rocket is sitting on the pad. Another proof this thinking is correct.
Your simulation appears to be ignoring the upward force exerted by the launchpad before liftoff. If you do that, you end up with pre-liftoff downward acceleration as shown by OR. Displaying non-zero acceleration that doesn't produce any ∆v makes no sense. We know why it does that, and we know it doesn't affect post-liftoff results, so I'm happy to ignore it. But I'm not going to admit that it's "right".3- Calculating the rockets motion one starts with the Thrust as applied to the Weight (Mass times Gravity) of the rocket. Once Thrust exceeds weight then there is motion and Velocity can be calculated. NOT the other way around (Acc = dv/dt is not done although mathematically correct).
Agreement is not the same thing as correctness.Yes, I have written rocket simulation code and after finding Open Rocket I compare the results are the same in both.
Huh?Model rocket simulations are driven by the forcing function - the engine thrust curve. In these simulations velocity is the integral of acceleration NOT the derivative of velocity.
F = ma is always true in non-relativistic physics.Also, Newton said force on an object was equal to rate of change of momentum, F=ma only if mass is constant, which is not true during thrust for a rocket.
Right. The force on the accelerometer's weight due to gravity has the same sign as the force due to upward acceleration of the rocket. In the coordinate system used by OR, they are both positive. The "raw" output of an accelerometer attached to the rocket would start out at +9.8m/s² before liftoff, and then increase from that value as the rocket accelerates upward.The tricky thing is that a MEMS accelerometer is like a weight between two equally loaded springs, connected to a fixed frame.[...]So, we actually have to subtract the acceleration due to gravity from the total displacement-derived acceleration to get the "up" acceleration of the frame (i.e. rocket).
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