I Does Escape Velocity depend on a rocket's direction away from Earth?

[Jurgen M]

He explain escape velocity in example where rocket goes straight up,isnt escacpe velocity ,velocity where centrifugal forces and gravity are equal,so refers only when rocket going in circle/orbit?

Can rocket really leave Earth in straight line like he show in video once reach this velocity and turn off engine?

 

[Dale]

isnt escacpe velocity ,velocity where centrifugal forces and gravity are equal,so refers only when rocket going in circle/orbit?

No. The direction is irrelevant. It should properly be called escape speed. It is the speed at which the KE + PE is zero, with PE referenced to infinity. Direction is irrelevant.

 

[PeroK]

isnt escacpe velocity ,velocity where centrifugal forces and gravity are equal,so refers only when rocket going in circle/orbit?

If you are in a bound orbit, then by definition you haven't escaped!

 

[Jurgen M]

No. The direction is irrelevant. It should properly be called escape speed. It is the speed at which the KE + PE is zero, with PE referenced to infinity. Direction is irrelevant.

So if throw stone straight up with escape velocity,stone will leave Earth and never come back?

So orbit velocity has nothing to do with escape velocity?

 

[PeroK]

So if throw stone straight up with escape velocity,stone will leave Earth and never come back?

That's what escape velocity means.

So orbit velocity has nothing to do with escape velocity?

There is a neat mathematical relationship between orbital velocity at a given radius and escape velocity from that radius. But, they are different things. Escape velocity is greater, of course.

 

[Jurgen M]

That's what escape velocity means.

Yes but even if throw rock with escape velocity ,air drag will slow down and rock will fall back to the earth.
Or assumption is that air don't exist?

 

[PeroK]

Yes but even if throw rock with escape velocity ,air drag will slow down and rock will fall back to the earth.
Or assumption is that air don't exist?

For practical reasons, if you want an object to escape to outer space, then it needs the escape velocity as it leaves the Earth's atmosphere. It's the same calculation with a slightly larger starting distance from the centre of the Earth.

 

[Jurgen M]

For practical reasons, if you want an object to escape to outer space, then it needs the escape velocity as it leaves the Earth's atmosphere. It's the same calculation with a slightly larger starting distance from the centre of the Earth.

Yes that make sense.
What is reason why all rockets are lunched with angle not straight up?

 

[PeroK]

Yes that make sense.
What is then reason why all rockets are lunched with angle not straight up?

To take advantage of the rotational speed of the Earth.

 

[Jurgen M]

To take advantage of the rotational speed of the Earth.

So the best way is lunched rocket at equator towards east so it has start speed of 460m/s?

What about Earth revolution,you must lunched rocket at time when Earth tangential velocity point in directon of revolution path so rocket has start speed of 30 000km/s?

 

[PeroK]

So the best way is lunched rocket at equator towards east so it has start speed of 460m/s?

What about Earth revolution,you must lunched rocket at time when Earth tangential velocity point in directon of revolution path so rocket gain speed of 30 000km/s?

With all these scenarios, the actual calculations carried out for real are significantly more complicated than the basic, level 1 physics question of escape velocity using KE and gravitational PE. You have the Sun's gravitation and escape velocity to take into account as well.

 

[Jurgen M]

With all these scenarios, the actual calculations carried out for real are significantly more complicated than the basic, level 1 physics question of escape velocity using KE and gravitational PE. You have the Sun's gravitation and escape velocity to take into account as well.

But rocket has adventage speed of Earth revolution speed?
Did I conclude correct?

 

[PeroK]

But rocket has adventage speed of Earth revolution speed?
Did I conclude correct?

Yes, and if you do an Internet search for "how to escape the solar system" you'll find the information is already freely available,

 

[Filip Larsen]

What is reason why all rockets are lunched with angle not straight up?

The mechanical energy needed to put something in low orbit around the Earth is mostly kinetic energy, so gaining speed is much more important than gaining altitude once you are outside (most of) the atmosphere. Also, any mechanical work done against gravity by any upward component of the acceleration during launch (or in general during any maneuver) is considered gravitational loss to be avoided since lost work is less energy for your orbital mechanical energy.

 

[Astronuc]

What is reason why all rockets are lunched with angle not straight up?

Where did one get the idea that "'all' rockets are launched with angle not straight up"?

Rockets, e.g., Saturn rockets and Space Shuttle were launched vertically. At an angle, the rocket structure would be subject to tremendous loads. The rockets are launched vertically and gradually turn (rotate) with respect to the Earth's surface (or away from the radial direction), and usually toward the east (some rockets may be launced north or south into a polar orbit). As the angle (and speed) increases, the rocket gets closer to horizontal, or parallel with the Earth's surface, in the upper atmosphere.

Launching vertically gets the rocket up into the thinner portion of the atmosphere so as to limit the aerodynamic forces on the structure, which is why the Space Shuttle would throttle down briefly before throttling up. At higher velocities, the craft would be subject to aerodynamic heating, which is why craft returning from orbit experience high temperatures on the leading edge as they slow down in the atmosphere.

Ballistic projectiles, or short range missiles, may be fired at an angle from vertical, but they will be traveling well below a velocity necessary to achieve orbit. In fact, projectile fires from a 'gun' will lose thrust one they leave the 'barrel', unless they have a small rocket motor, and missiles generally have limited propellant, and lose thrust one the propellant is expended.

A rocket could leave the Earth in a straight line, if it had sufficient thrust and duration of thrust, which means a sufficient amount of propellant and thermal energy. The problem for large rockets is that they must accelerate the mass of propellant with the rocket. The reduce propellant one would need a different mode to accelerate a mass against gravity, e.g., magnetic launch system. Then one has to balance the acceleration involved, especially if living creatures are being launched.

 

[Filip Larsen]

Rockets, e.g., Saturn rockets and Space Shuttle were launched vertically.

For orbital launches, the pure vertical phase is only very short compared to the whole launch trajectory and is quickly replaced with a gravity turn ascent initiated with a pitch-over maneuver. In fact, the vertical phase is so short its hardly visible on the usual Falcon 9 night shots, like for example:

 

[Janus]

Yes that make sense.
What is reason why all rockets are lunched with angle not straight up?

While rockets are initially launched straight up, they will begin to tilt over afterwards. Remember, most of the time, the objective is to put something in orbit around the Earth. This means that once it reaches orbital altitude, it needs to be moving at over 7 km/sec parallel to the Earth's surface. If you launched it straight up the whole time, when it reaches that altitude it would still be moving straight up. It would shoot past, eventually lose speed ( since it would have escape velocity) and fall back to Earth. (Technically, it would still be "in orbit", just an highly elliptical one that intersects with the Earth)
So, instead, they use a trajectory called a "gravity turn", where the rocket adjusts its angle as it climbs so that it is headed in the right direction with the proper speed when it reaches orbital altitude. It is the most efficient trajectory that achieves this purpose.
To make use of the Earth's rotation, your final orbital direction needs to be at least somewhat the same as Earth's rotation. ( There are cases when this is not desirable, such as Polar orbits, which orbit in a North-South direction. For these, the Earth's rotation is a complication to be adjusted for not an aide)

 

[gmax137]

View attachment 292343

Wow that's a pretty picture. What's the exposure duration? The star trails are pretty long.

EDIT: my simple measure shows the trails are about 8 degrees; that's close to 30 minutes

 

[Filip Larsen]

Wow that's a pretty picture. What's the exposure duration? The star trails are pretty long.

It is an old shot, to see some newer ones just search for "falcon 9 night launch"

Regarding the exposure time, a quick measurement of the star trails gives me around 15 min exposure time, which also more or less fits with a total burn time of first and second Falcon 9 stage of around 9 min.

 

[Astronuc]

For orbital launches, the pure vertical phase is only very short compared to the whole launch trajectory and is quickly replaced with a gravity turn ascent initiated with a pitch-over maneuver.

Yes, I should added a comment about the pitch.

I remember watching the Apollo launches and some STS missions. I have some profiles stored somewhere. I believe the Saturn 5 remained vertical longer, but I can't find the detailed profiles.

I did find = https://spaceflight101.com/falcon-9-ses-10/flight-profile/ (pitch starts around 16 s).
and a simple profile for Saturn V - http://home.kpn.nl/panhu001/Saturn_V/Saturn_V_info/SatV-Apollo_perform_char.html

I was looking for the launch profiles from NASA, specifically the time or altitude of the pitchover.
According to one document, pitch and roll initiation occur around 12 seconds from first motion.
See Figure 2-1 in https://history.nasa.gov/afj/ap12fj/pdf/a12_sa507-flightmanual.pdf
Figure 2-2 pitch angle (from vertical) during boost to EPO as function of time in minutes.

According to the Wikipedia article on Saturn V, "As of 2021, the Saturn V remains the only launch vehicle to carry humans beyond low Earth orbit, as well as the tallest, heaviest, and most powerful (highest total impulse) rocket ever brought to operational status. It holds records for the heaviest payload launched and largest payload capacity to low Earth orbit (LEO) of 310,000 lb (140,000 kg), which included the third stage and unburned propellant needed to send the Apollo command and service module and Lunar Module to the Moon."

Some other information of F9 launches. Payloads are rather light.
https://spacelaunchreport.com/falcon9ft.html

Some other trivia - https://www.cnbc.com/2018/08/28/elo...ed-rocket-scientist-and-it-led-to-spacex.html

 

[Filip Larsen]

I believe the Saturn 5 remained vertical longer, but I can't find the detailed profiles.

Indeed. While there are several of the old Nasa reports around, it is far more difficult finding a nice, accurate and modern (link-friendly) plot.

Going for one of the original ones, the 1968 Nasa document General Apollo Saturn V CSM Launch Abort Analysis has a nominal altitude/down-range profile in figure 3. And as I understand it from various other descriptions, the roll and pitch program started at a height of around 130 m.

 

[Jurgen M]

Rockets, e.g., Saturn rockets and Space Shuttle were launched vertically.

Vertically up just at start or all the time?
So all rocket that will not orbit around Earth are going straight up and rocket that going to orbit goes first vertically up and then turn?

 

[anorlunda]

So all rocket that will not orbit around Earth are going straight up and rocket that going to orbit goes first vertically up and then turn?

If you want to shoot the rocket "straight up" , then you must launce at the precise second that the rocket on the launch stand points directly at the target. You must also locate the launch stand at the precise latitude that points at the target. Since those conditions are not practical, real life missions must make use of curved trajectories.

 

[Filip Larsen]

If you want to shoot the rocket "straight up" , then you must launce at the precise second that the rocket on the launch stand points directly at the target.

This does not really make practical sense, for multiple reasons.

As already mentioned, due to gravitational losses no one in their right mind would launch any orbital rocket using a fully vertical trajectory, not even if you want to put something on an escape trajectory. Fully vertical trajectories are only "interesting" for sub-orbital flights where the goal is to get some altitude (e.g. for high altitude measurements or tests) and then return close to the launch site.

And if you really did launch with a fully vertical trajectory at the instant some orbital target passes overhead the trajectory would surely not intersect the target anyway.

 

[Filip Larsen]

So all rocket that will not orbit around Earth are going straight up and rocket that going to orbit goes first vertically up and then turn?

Rockets that launch vertically (like most launch systems do) are only moving vertically for a few hundred meters or so, then they make a small pitch (tilting) maneuver that will put them on what is called a gravity turn trajectory which is a curved trajectory where thrusting is done along the velocity vector (i.e. zero angle of attack) at all times and where gravity over time will turn the velocity vector more and more away from the vertical.

I found the following (old) plot someone posted on reddit that pertains to show the first stage profile and other instersting data for several Falcon 9 missions (it is nice because both the altitude and down range axis has same unit and scale):

 

[Astronuc]

Vertically up just at start or all the time?

Vertically at the start, then gradually turn (from vertical) or pitch over gradually.

I don't know about all launches, but launches to low Earth orbit (LEO) gradually turn over. As far as I know, mankind has not a launched a rocket straight up out of the atmosphere. Manned missions traveled to LEO.

Project HARP did fire projectile to high altitude, but as far as I know, they did not achieve orbit.

 

[hutchphd]

No one has mentioned this explicitly so I will. For a vacuum planet, the most efficient use of a rocket to achieve escape velocity would be to use all your fuel in the first millisecond (at very high thrust and ignoring all the concommitent problems...i.e. a canon). Pointed in the tangential rotation direction.
In particular the energy used carrying the yet-to-be-burned propellent against gravity is wasted. Fuel burned at 10 km altitude will give the same $\Delta v$ as fuel burned at liftoff, but you have carried it up to altitude and that work is not recovered. Of course you always need to bring the remaining fuel onboard up to speed regardless.
So the optimal trajectory is determined by getting "out" of the atmosphere and then burning the remaining fuel at as low an altitude as is possible. This is the Hohman Transfer to an parabolic orbit I guess.

 

[Filip Larsen]

In particular the energy used carrying the yet-to-be-burned propellent against gravity is wasted.

And just to make sure the OP is not confusing this as something new, this is what is meant by gravitational loss mentioned earlier in the thread.