# B Canceling Orbital Motion

#### metastable

I think metastable has asked several different questions.
My original question is about efficiency... does such a trajectory improve the energy efficiency of the rocket defined as change in velocity from a fixed point (trapped electron?) on earth?

#### PeterDonis

Mentor
does such a trajectory improve the energy efficiency of the rocket defined as change in velocity from a fixed point (trapped electron?) on earth?
"Improve the energy efficiency of the rocket" is a strange way to look at it when you're just using the rocket for a single burn (to negate the 215 km/s velocity of the solar system relative to the galactic barycenter) and then the rocket is done and everything else is just free fall.

#### metastable

"Improve the energy efficiency of the rocket" is a strange way to look at it when you're just using the rocket for a single burn (to negate the 215 km/s velocity of the solar system relative to the galactic barycenter) and then the rocket is done and everything else is just free fall.
Perhaps, but my meaning is efficiency compared to other potential trajectories. For example, suppose I have a 30,000km/s target relative to trapped electron on earth, the implications of the original question are: will any other trajectories get me there faster with less impulse energy?

#### russ_watters

Mentor
Would accelerating a rocket in a vector that "cancels" earth's orbital motion relative to the center of the milky way in some cases improve the energy efficiency of a rocket for a given amount of change in velocity relative to the earth?
If what you are really asking is what is the most efficient way to thrust to get from orbit around a the center of a gravity well, such as the galaxy center, to falling straight into it, then your answer is correct: thrust against your orbital velocity, not toward the object/gravity well.

#### russ_watters

Mentor
I wondered if it would be the most efficient method for a craft to reach arbitrarily high velocities in terms of propellant energy expended to reach a given change in velocity relative to a fixed point on the earth's surface.
This is the part that's confusing: it doesn't just matter where you are going from, it also matters where you are going to. Unless you are actually talking about travel to the galaxy center, this technique doesn't do anything for you. It doesn't help you get to Mars, for example. Your description of what you want is unclear enough that could be hidden in it...

#### metastable

This is the part that's confusing: it doesn't just matter where you are going from, it also matters where you are going to.
The destination I had in mind: anywhere that eventually gets me to (for example) 30,000km/s relative to a trapped electron on earth, with the least amount of propellant.

#### russ_watters

Mentor
Perhaps, but my meaning is efficiency compared to other potential trajectories. For example, suppose I have a 30,000km/s target relative to trapped electron on earth, the implications of the original question are: will any other trajectories get me there faster with less impulse energy?
Get you where? Versus what alternative trajectory?

#### russ_watters

Mentor
The destination I had in mind: anywhere that eventually gets me to (for example) 30,000km/s relative to a trapped electron on earth, with the least amount of propellant.
That's a speed, not a destination.

#### metastable

Get you where? Versus what alternative trajectory?
Any rest frame >30,000km/s with respect to trapped electron on Earth with the least possible propellant.

#### russ_watters

Mentor
Any rest frame >30,000km/s with respect to trapped electron on Earth with the least possible propellant.
I would say yes, in only that one direction (toward the galactic center). Now what? What can you do with that other than getting pulled apart by a black hole?

#### DaveC426913

Gold Member
If what you are really asking is what is the most efficient way to thrust to get from orbit around a the center of a gravity well, such as the galaxy center, to falling straight into it, then your answer is correct: thrust against your orbital velocity, not toward the object/gravity well.
Ah. Like the shuttle, deorbiting, does a burn directly opposed to orbital motion.

#### PeterDonis

Mentor
Any rest frame >30,000km/s with respect to trapped electron on Earth with the least possible propellant
In other words, you don't actually care about your speed relative to the galactic barycenter, or relative to anything else except Earth?

In that case, pretty much everything said so far in this thread has been a waste of time.

#### metastable

I would say yes, in only that one direction (toward the galactic center). Now what? What can you do with that other than getting pulled apart by a black hole?
To potentially reach other destinations along that vector.

#### russ_watters

Mentor
To potentially reach other destinations along that vector.
No, it doesn't help you do that except for an extremely short flyby or spectacular crash.

#### metastable

No, it doesn't help you do that except for an extremely short flyby or spectacular crash.
I envisioned a very long duration mission with a series of flybys, with potential for gravity assist maneuvers along the way.

Mentor

#### russ_watters

Mentor
I envisioned a very long duration mission with a series of flybys, with potential for gravity assist maneuvers along the way.
Like the Voyager probes - sure.

#### metastable

A mission where?
As far from earth as possible in the least time with the least energy.

#### russ_watters

Mentor
As far from earth as possible in the least time with the least energy.
Please note that those are competing parameters that need to be specified in order for the answer to be meaningful. Most distance and least time are literally the inverse of each other.

I feel like you are being purposely vague because you think it's helpful. It's not.

#### metastable

Please note that those are competing parameters that need to be specified in order for the answer to be meaningful. Most distance and least time are literally the inverse of each other.

I feel like you are being purposely vague because you think it's helpful. It's not.
Sorry for the sloppy language. I'm not aware of any other methods that could in theory get a craft to the same arbitrary 30,000km/s velocity relative to the earth's surface using less fuel, so I wondered if anyone here knew of such a method?

#### DaveC426913

Gold Member
If you plan to coast to the galactic centre, you're still going to have a heckuva time dodging all the stellar gravity wells you pass through.

Perhaps a more oblique approach would achieve the desired effect. Blast north, out of the galactic plane. Then all the mass of the galaxy will be pulling you in the same direction.

EDIT: Ah. If the goal is to facilitate fly-bys of other destinations, then leaving the galactic plane will be ... counter-productive.

#### russ_watters

Mentor
Sorry for the sloppy language. I'm not aware of any other methods that could in theory get a craft to the same arbitrary 30,000km/s velocity relative to the earth's surface using less fuel, so I wondered if anyone here knew of such a method?
I suppose not, but please note that the acceleration will be really slow after the rocket stops firing. If I did the calc right, it's 1/1000th of a g, so it would take about a thousand years to reach that speed.

#### PeterDonis

Mentor
As far from earth as possible in the least time with the least energy.
As @russ_watters pointed out, you can't have all three of these at once. You need to pick two. I assume "least energy" is one, so that leaves either "most distance" or "least time", but you have to pick one.

#### PeterDonis

Mentor
I'm not aware of any other methods that could in theory get a craft to the same arbitrary 30,000km/s velocity relative to the earth's surface
And this is yet a fourth criterion "highest speed", in addition to "most distance", "least time", and "least energy". And you can still only have two. Which two?

#### metastable

And this is yet a fourth criterion "highest speed", in addition to "most distance", "least time", and "least energy". And you can still only have two. Which two?
I'm not aware of any other methods (besides a ~215km/s engine boost from earth surface to cancel the vehicle's galactic orbital motion) that can get a 10kg vehicle mass (containing a trapped electron) launched to >30,000km/s with respect to a trapped electron orbiting earth, using less propellant energy.

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