What's the point of Escape Velocity?

sophiecentaur

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

That is what I said?
Well, it is completely different from air drag or similar types of drag.

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

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

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

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

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

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

sophiecentaur

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 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.