I Why do rockets turn horizontally so soon after launch?

[poor mystic]

Possibly, the fact that the Earth is rotating away from the rocket's launch point makes it seem to turn to the horizontal, when it's actually moving in a straight line.

 

[jbriggs444]

Possibly, the fact that the Earth is rotating away from the rocket's launch point makes it seem to turn to the horizontal, when it's actually moving in a straight line.

Except for the fact that this effect makes it tip upward relative to the local surface, not downward. That would be at a rate of only one rotation per approximately 90 minutes. Less until orbital velocity is achieved.

 

[russ_watters]

Possibly, the fact that the Earth is rotating away from the rocket's launch point makes it seem to turn to the horizontal, when it's actually moving in a straight line.

The Earth is rotating west to east; in the direction of flight, not away from it.

 

[Steelwolf]

Rockets tip shortly after launch so as to gain the benefit of Earth's rotation added to final orbital speed., so they are accelerating horizontally, with the Earth's spin, as they climb, an eventual 'permanent' horizontal flight is what is the desired end (orbit).

I did have another thought: Military missiles, typically cruise missiles, launch vertically for a short distance and then attitude jets rapidly kick it horizontal, or close to it. Those critters also tend to have wings which deploy, so they are, in effect, launching a jet airplane vertical, tipping it over after launch and deploying wings for long flight. If THIS kind of rocket is what the kid asked about, then it is a whole different critter than orbital mechanics of launch as it is an 'in atmosphere' weapons launch instead.

 

[A.T.]

Rockets tip shortly after launch so as to gain the benefit of Earth's rotation added to final orbital speed.

They have that benefit already on the platform.

 

[jbriggs444]

They have that benefit already on the platform.

True, though in order to fully utilize the pre-existing velocity, one needs to thrust parallel to it rather than perpendicular.

 

[Khashishi]

There's a game you can get on smart phones called SimpleRockets. It's a simplified clone of Kerbal Space Program. I don't know enough rocket science to attest to the accuracy but you can learn some things playing it. You do launch vertical and start turning to the side once you pass much of the thicker atmosphere.

 

[CWatters]

Rockets tip shortly after launch so as to gain the benefit of Earth's rotation added to final orbital speed.

Perhaps I missunderstand your answer but rockets get that benefit just standing on the pad. It's and advantage of launching nearer the equator than the poles.

I don't see how turning horizontally increases that.

 

[jbriggs444]

Perhaps I missunderstand your answer but rockets get that benefit just standing on the pad. It's and advantage of launching nearer the equator than the poles.

I don't see how turning horizontally increases that.

Vector addition of velocities. You get the biggest increase if the added delta v is parallel to the existing v.

 

[sophiecentaur]

Vector addition of velocities. You get the biggest increase if the added delta v is parallel to the existing v.

Why does the order of applying velocities affect the final velocity? Is this a rocket / reaction engine thing to do with efficiency?

 

[jbriggs444]

Why does the order of applying velocities affect the final velocity? Is this a rocket / reaction engine thing to do with efficiency?

The comparison is between applying a delta v parallel to velocity versus applying a delta v perpendicular to velocity. Nothing specific to rockets. Nothing to do with sequencing. Purely a geometrical effect.

If I am coasting at 3 meters per second eastward and apply a 4 meters per second delta v northward, I get a new speed of 5 meters per second.
If I am coasting at 3 meters per second eastward and apply a 4 meters per second delta v eastward, I get a new speed of 7 meters per second.
If I am coasting at 3 meters per second eastward and apply a 4 meters per second delta v westward, I get a new speed of 1 meter per second.

Direction matters.

 

[sophiecentaur]

Of course the azimuth setting should be Eastwards. My issue is that you already have your Original horizontal velocity at take off. That is the same when you are in orbit and the engines provide the rest. If the Earth were a bit less massive and not spinning at all, would the answer to the OP be the same? Would they still go horizontal 'soon after lunch'? (Spelling in purpose)

 

[jbriggs444]

Of course the azimuth setting should be Eastwards. My issue is that you already have your Original horizontal velocity at take off. That is the same when you are in orbit and the engines provide the rest. If the Earth were a bit less massive and not spinning at all, would the answer to the OP be the same? Would they still go horizontal 'soon after lunch'? (Spelling in purpose)

Velocity is a vector. You use vector addition rules. The direction that you apply the delta v matters. This is not a complicated idea.

 

[CWatters]

Nothing to do with sequencing.

but sequencing was at the heart of the OP... why do they turn horizontal almost immediately and not after they clear the atmosphere?

 

[jbriggs444]

but sequencing was at the heart of the OP... why do they turn horizontal almost immediately and not after they clear the atmosphere?

Your response seemed to imply that they should never turn horizontal at all, that the eastward velocity of the launch pad was an advantage that would apply regardless. Unfortunately, that is incorrect. The advantage of the eastward velocity of the launch pad is squandered if the [much larger] delta v is applied vertically rather than parallel to the existing velocity.

Yes, one needs to get clear of the atmosphere. But one also wants to avoid squandering that free delta v from the eastward motion of the pad.

 

[CWatters]

Your response seemed to imply that they should never turn horizontal at all, that the eastward velocity of the launch pad was an advantage that would apply regardless. Unfortunately, that is incorrect. The advantage of the eastward velocity of the launch pad is squandered if the [much larger] delta v is applied vertically rather than parallel to the existing velocity.

No I can see your point about vector addition. The issue I have is only with the ordering. For example..

3 East + 4 North + 7 East
vs
3 East + 7 East + 4 North

give same result overall.

Except in the second case the velocity after the first addition is higher (7 vs 5). So the effect of turning horizontal earlier appears to make the velocity in the atmosphere higher without affecting the final velocity.

 

[sophiecentaur]

Your response seemed to imply that they should never turn horizontal at all, that the eastward velocity of the launch pad was an advantage that would apply regardless.

How could you have come to that conclusion? They, of course, need to turn eastwards to bring their tangential velocity to what's required. The eastwards component from the Earth is there all the time. Apart from the fact that it always gives you an advantage, you have given no reason why it affects the best time at which the rocket drops its nose.

The direction that you apply the delta v matters. This is not a complicated idea.

It actually is a complicated idea, in detail, which is why we are trying to think out the precise reason for using the available tangential deltaV so early. ~The reason must be to do with efficiency and the notion of Gravity Drag is at the bottom of it. Near the start of a vertical trajectory, the engines are doing very little work - only providing some GPE. Allowing the rocket to accelerate (in any direction) is good value and improved efficiency by having the engines working at low speeds for as short a time as possible.

 

[jbriggs444]

How could you have come to that conclusion? They, of course, need to turn eastwards to bring their tangential velocity to what's required. The eastwards component from the Earth is there all the time. Apart from the fact that it always gives you an advantage, you have given no reason why it affects the best time at which the rocket drops its nose.

It actually is a complicated idea, in detail, which is why we are trying to think out the precise reason for using the available tangential deltaV so early. ~The reason must be to do with efficiency and the notion of Gravity Drag is at the bottom of it. Near the start of a vertical trajectory, the engines are doing very little work - only providing some GPE. Allowing the rocket to accelerate (in any direction) is good value and improved efficiency by having the engines working at low speeds for as short a time as possible.

You are defending a claim that is different from the one that was made: that the launch pad's eastward speed is a free bonus that applies regardless. It is not.

 

[sophiecentaur]

You are defending a claim that is different from the one that was made: that the launch pad's eastward speed is a free bonus that applies regardless. It is not.

Why not?

 

[jbriggs444]

Why not?

Because it can be squandered. E.g. if the launch is purely vertical.

 

[DaveC426913]

Because it can be squandered. E.g. if the launch is purely vertical.

It seems to me, there is an issue of defining "vertical". To-wit: is that relative to Earth FoR or a remote FoR?

If the rocket maintains a vertical ascent wrt Earth's FoR (i.e. it remains over the same geographic location), then it carries with it the advantage of the Space Centre's rotation of 14 miles per minute East.

If the rocket maintains its vertical ascent wrt to a non-rotating FoR (say, the Earth as a whole, or just its centre), then it is true that the Earth's rotational boost is squandered.

(Of course, for the former to be true, it would have to start vectoring eastward just so as not to fall behind.)

 

[russ_watters]

Because it can be squandered. E.g. if the launch is purely vertical.

I guess I'm not following either. Perhaps the cases are getting mixed-up. I'll try to be more descriptive in defining them...

The OP was asking about a slow ascent to orbital altitude, then turning downrange and increasing speed to orbit. Let's assume for that case the vertical speed when you reach orbital altitude would be zero. (Case 1) Other cases would be:
2. Turning almost horizontal, at the right inclination almost immediately after the launch (implied but not necessarily intended by the OP to be "actual").
3. Rotating to the right inclination and pitching over gradually after launch (actual path).

To me it seems like the vector addition of increasing the speed and changing the direction *downrange* should be exactly the same in all three cases. And it should look like this:

Starting: 900 mph at 90 degrees (east)
Needed: 17500 mph at 135 degrees (angle guessed)
Assist: 636 mph
Wasted: 900-636=264mph
Delta-V: 16,863 @ 137 degrees

How is this math different for one case vs the others? The only difference I see between the three cases is in the gravity and aerodynamic drags, not the delta-V/vector addition.

 

[russ_watters]

It seems to me, there is an issue of defining "vertical". To-wit: is that relative to Earth FoR or a remote FoR?

I guess it is possible, but I would hope not, since we kind of already discussed this. If the rocket thrusts perpendicular to the Earth's surface, the tangential velocity is preserved forever and you eventually reach an altitude where the tangential velocity is above orbital velocity (albeit not at the right inclination). As far as I can tell, the portion of the tangential velocity that helps you is never lost regardless of how you get to orbit (unless you purposely angle your thrust against it).

A trajectory where you remain above the same point (again, inclination notwithstanding) requires pitching-over, so it *is* the real trajectory rockets. It's geosynchronous, only differing in altitude of the orbit/magnitude of the pitchover from LEO we've been discussing to this point.

 

[sophiecentaur]

Because it can be squandered. E.g. if the launch is purely vertical.

"Squandered" means it doesn't follow Newton's First Law then? We all know that throwing a ball vertical will result in it landing back in the hand. It's horizontal momentum has not been "squandered". Amazingly, it has been conserved, as will the momentum of the rocket.

 

[jbriggs444]

If the rocket thrusts perpendicular to the Earth's surface, the tangential velocity is preserved forever and you eventually reach an altitude where the tangential velocity is above orbital velocity (albeit not at the right inclination). As far as I can tell, the portion of the tangential velocity that helps you is never lost regardless of how you get to orbit (unless you purposely angle your thrust against it).

Yes, for a purely vertical thrust, the tangential velocity would be there, certainly. But as an increment to the resulting speed it would be small.

 

[jbriggs444]

"Squandered" means it doesn't follow Newton's First Law then? We all know that throwing a ball vertical will result in it landing back in the hand. It's horizontal momentum has not been "squandered". Amazingly, it has been conserved, as will the momentum of the rocket.

The relevant metric is not momentum. It is energy.

 

[sophiecentaur]

The relevant metric is not momentum. It is energy.

We must be talking at cross purposes. I am sure you know your Newtonian Mechanics. Are you actually saying that the horizontal momentum it started with is not there, once it reaches an orbital position? (The Earth's curvature could be a second order issue) The only thing that could change it would be the difference in the g vector angle.

 

[russ_watters]

Yes, for a purely vertical thrust, the tangential velocity would be there, certainly. But as an increment to the resulting speed it would be small.

What resulting speed? Could you please be more specific because it feels to me like this lack of specificity is the entire problem here.

My interpretation is that if you thrust vertically [perpendicular to Earth's surface], your tangential speed will start at and forever be about 900 mph and is totally decoupled from your vertical speed. Do you agree/disagree?

 

[jbriggs444]

We must be talking at cross purposes. I am sure you know your Newtonian Mechanics. Are you actually saying that the horizontal momentum it started with is not there, once it reaches an orbital position? (The Earth's curvature could be a second order issue) The only thing that could change it would be the difference in the g vector angle.

I agree that we must be talking at cross purposes.

The horizontal momentum is still there, certainly. It does not go away. But if you add a horizontal momentum of 3 units to a vertical momentum of 4 units, you get a diagonal momentum of only 5 units, not 7 units. The magnitudes of momenta do not add.

 

[jbriggs444]

My interpretation is that if you thrust vertically [perpendicular to Earth's surface], your tangential speed will start at and forever be about 900 mph and is totally decoupled from your vertical speed. Do you agree/disagree?

I agree.