Launch of a rocket - its initial displacement and velocity

[Saba]

TL;DR Summary

The process of launching the rocket got a bit confusing for me. The rocket launcher won't work when the rocket is still on the ground is that right? Therefore a rocket needs an external force at the start to shoot the rocket a few meters up, and then that's when the main thrust engine starts working?

so then when we model the velocity of the motion of a rocket (e.g., in a mathematics report), is it reasonable to assume that the initial velocity equals zero? I also apologise for my lack of information if I made a huge mistake in my question

 

[Drakkith]

The rocket motor works perfectly fine while the rocket is on the ground. Nothing else is needed to launch the rocket. Note that the rocket engine is always above the ground or launch pad anyways, as you wouldn't want to block the exhaust nozzle, so there's no difference between the engine firing while the rocket sits on the ground vs while it is a few meters off of the ground.

so then when we model the velocity of the motion of a rocket (e.g., in a mathematics report), is it reasonable to assume that the initial velocity equals zero?

Yes, the initial velocity is zero.

 

[Filip Larsen]

Yes, setting the initial velocity of the rocket relative to the launch site as zero is more or less the only natural choice when modelling a rocket launch from the start: Initial position = launch tower, initial velocity (relative to launch tower) = 0.

Pretty much all rocket systems launched from the ground using a fixed or mobile tower will have the rocket clamped to the tower, possibly with the engine nozzles pointing into a big flame trench for big rockets, and after a valid ignition has been confirmed (e.g. all engines running nominally) the clamps release the rockets which is then free to accelerate upwards. As a force model this means at launch (i.e. at the instant the clamps release) the rocket can be modelled as a freely moving body under the influence of a thrust force (and quickly after launch also aerodynamic forces).
Other launch system, notably submarine launchers and some man-carried missile launchers, have a launch phase where the rocket is first ejected into "free air" before the (single) main engine kicks in.

 

[A.T.]

The rocket launcher won't work when the rocket is still on the ground is that right?

No, who told you that?

Therefore a rocket needs an external force at the start to shoot the rocket a few meters up, and then that's when the main thrust engine starts working?

Are you maybe talking about missiles lunched from tubes/silos? Their main rocket engine often starts with a delay, after being shot out by other means. But that is just to protect the launcher from the rocket exhaust.

 

[PeroK]

This sounds more like an engineering question. I would think the launch parameters and constraints for a real rocket are not inherent in basic kinematics.

 

[cjl]

Pretty much all rocket systems launched from the ground using a fixed or mobile tower will have the rocket clamped to the tower, possibly with the engine nozzles pointing into a big flame trench for big rockets, and after a valid ignition has been confirmed (e.g. all engines running nominally) the clamps release the rockets which is then free to accelerate upwards.

This is true for large orbital launch vehicles, but for smaller rockets like sounding rockets and missiles, it's far more typical for the rocket to just be on a guide rail or in a tube and for motion to start as soon as thrust exceeds weight. The ignition transient of a modern solid rocket motor is fast enough for this to not be an issue though, as you typically achieve full design thrust in a few dozen to hundred milliseconds, depending on motor size.

It is also true that some systems kick the rocket out before igniting the main motor, but that's more because of specific engineering design decisions or usage constraints, not because there's physically any reason you couldn't light the main motor immediately. The Javelin anti-tank missile (as you said) is a good example of this - it gets kicked several meters out in front of the launcher before the main engine ignites, but that's more so the main engine isn't firing as it's leaving a tube right in front of the operator, since that would be hazardous to the person using the missile.

 

[Filip Larsen]

This is true for large orbital launch vehicles, but for smaller rockets like sounding rockets and missiles, it's far more typical for the rocket to just be on a guide rail or in a tube and for motion to start as soon as thrust exceeds weight.

Indeed. I was mainly trying to give the OP some background knowledge by sampling a few of the many different launch mechanisms that are in practical use, since the question was about reasonable initial conditions.

Vertically launched sounding rockets are interesting in that they are very close realizations of the simplest possible one-dimensional mathematical rocket models that students might encounter as math problems initially. Having said that I better for the benefit of the OP also add that the OP problem is it more about math than about providing a realistic model for rocket kinematics.

 

[ebg]

If you want to know how rockets work...get into model rocketry....actually buiding model rockets, you really start to understand all the equations and mathamatics that goes into the flight of a rocket. check out Estes model rocket engine charts and begine from there. when you build a rocket...center of gravity and center of the rockets aerodynamics are important to know....like a rocket rolls in flight because of lift....cool things like that you learn!

 

[PeroK]

If you want to know how rockets work...get into model rocketry....actually buiding model rockets, you really start to understand all the equations and mathamatics that goes into the flight of a rocket. check out Estes model rocket engine charts and begine from there. when you build a rocket...center of gravity and center of the rockets aerodynamics are important to know....like a rocket rolls in flight because of lift....cool things like that you learn!