What's the point of Escape Velocity?

[QuantumPion]

Escape velocity is a property of position only, I think. A rocket in LEO still needs ~11km/s as escape velocity - it is just easier to reach it.

A rocket in LEO still needs 11 km/s to escape but it is already going 7 km/s due to its orbital velocity. Otherwise it wouldn't be in orbit.

Actually, a rocket needs more energy to escape - if you could burn all the fuel in a second (and neglecting drag), the rocket would be more efficient. This is easy to see in the limit of no rocket acceleration: You have to keep burning fuel to counter gravity, but you don't increase the velocity of the rocket.

Yes, gravity is considered as a type of drag in this sense.

 

[sophiecentaur]

Is it not true to say that no spacecraft has ever been given the full 'escape velocity' with its motors? I think that the craft that have enough energy to escape from the Solar System have all relied on pinching energy, using a Slingshot orbit.

 

[QuantumPion]

Is it not true to say that no spacecraft has ever been given the full 'escape velocity' with its motors? I think that the craft that have enough energy to escape from the Solar System have all relied on pinching energy, using a Slingshot orbit.

New Horizons was launched directly into solar escape trajectory. As far as Earth escape velocity is concerned, any probe leaving the Earth-moon system would need the full escape velocity (unless it possibly received a gravity assist from the moon, although I don't know if that is typically used or not).

 

[rcgldr]

A rocket accelerates much more slowly than a cannon and is fighting against drag and gravity as it is accelerating.

To avoid fighting against gravity, a rockets thrust can be directed so that it's always perpendicular to gravity so that all of the thrust goes into increasing velocity of the rocket, but this can't be done until outside the atmosphere and close to low Earth orbit.

 

[sophiecentaur]

New Horizons was launched directly into solar escape trajectory. As far as Earth escape velocity is concerned, any probe leaving the Earth-moon system would need the full escape velocity (unless it possibly received a gravity assist from the moon, although I don't know if that is typically used or not).

Interesting link. This was the highest launch speed so far, apparently and well above escape velocity - but Wiki says it still made a gravity-assist pass of Jupiter. Well, I suppose they didn't want to hang about. You know - places to go, people to see. . . .

 

[mfb]

Escape velocity is a property of position only, I think. A rocket in LEO still needs ~11km/s as escape velocity - it is just easier to reach it.

A rocket in LEO still needs 11 km/s to escape but it is already going 7 km/s due to its orbital velocity. Otherwise it wouldn't be in orbit.

That is what I said?

Yes, gravity is considered as a type of drag in this sense.

Well, it is completely different from air drag or similar types of drag.

Is it not true to say that no spacecraft has ever been given the full 'escape velocity' with its motors?

Starting from what? Every interplanetary mission uses the velocity of Earth (~30km/s), and most rockets use the rotation of Earth (up to ~500m/s) to begin the mission.

 

[QuantumPion]

Well, it is completely different from air drag or similar types of drag.

Actually gravity drag it is very analogous to air drag in the context of rockets. Both are forces which oppose the acceleration of the rocket during its launch. The difference is air drag is proportional to speed, while gravity drag is constant but only in the vertical direction.

 

[sophiecentaur]

Actually gravity drag it is very analogous to air drag in the context of rockets. Both are forces which oppose the acceleration of the rocket during its launch. The difference is air drag is proportional to speed, while gravity drag is constant but only in the vertical direction.

That is a strange way of looking at it. When you increase your GPE, that energy is of some use to you (it's half of the energy involved in your final orbit). Energy lost because of drag is of no use at all.
I understand the message in that link but it strikes me that it's not much more than an alternative explanation for the process of carrying fuel and extra rocket stages with you in a launch. It is taking two values of 'wasted energy' and lumping them together. There is no 'gravity drag' on the bit of the vehicle that is finally up there and in orbit. It still possesses the same amounts of GPE and KE it was given.

 

[QuantumPion]

That is a strange way of looking at it. When you increase your GPE, that energy is of some use to you (it's half of the energy involved in your final orbit). Energy lost because of drag is of no use at all.
I understand the message in that link but it strikes me that it's not much more than an alternative explanation for the process of carrying fuel and extra rocket stages with you in a launch. It is taking two values of 'wasted energy' and lumping them together. There is no 'gravity drag' on the bit of the vehicle that is finally up there and in orbit. It still possesses the same amounts of GPE and KE it was given.

Correct, energy lost to gravity drag is of no use at all. As MFP said, if you had a rocket burning at full thrust just to hover, you have 100% of your energy lost to gravity drag. There is no air drag on the vehicle that finally gets to orbit either. Gravity drag and air drag are terms used for determining how much extra delta-V over the theoretical minimum you need to reach orbit. If you had a spaceplane that could fly to the very upper reaches of the atmosphere using jet engines, and then ignite rocket engines to accelerate horizontally to orbital velocity, losses due to gravity drag would be very low for example.

 

[mfb]

Actually gravity drag it is very analogous to air drag in the context of rockets. Both are forces which oppose the acceleration of the rocket during its launch. The difference is air drag is proportional to speed, while gravity drag is constant but only in the vertical direction.

In the limit of burning all the fuel at once, gravity still acts on the rocket, but there is no energy loss due to gravity. Air drag, on the other hand, is always there if the rocket is in motion, just its amount is variable.

Who is MFP?

 

[QuantumPion]

In the limit of burning all the fuel at once, gravity still acts on the rocket, but there is no energy loss due to gravity. Air drag, on the other hand, is always there if the rocket is in motion, just its amount is variable.

Who is MFP?

I meant mfb not P, sorry. But yes you are correct.

 

[sophiecentaur]

Who is MFP?

Your long lost dyslexic brother, perhaps.

I really don't like the term Gravity Drag but, if it's used by people in the know, I think we have to go along with it. After all, it is Rocket Science.

 

[Travis_King]

I just want to chime in and say that drag should should be used for forces acting in the opposite direction as the velocity. Typically drag is defined as:
From Merriam-Webster

the retarding force acting on a body (as an airplane) moving through a fluid (as air) parallel and opposite to the direction of motion

So while a component of gravity may be acting as a "drag" force, certainly when the craft is pointed directly opposite the direction of Earth's gravitational acceleration, I don't think it's at all useful to call it gravity drag...

In the upper atmosphere, the craft must still overcome gravitational acceleration to continue climbing, but due to it's thrust vector and velocity, it's using very little energy overcoming the "drag" component of that force.