Flight Question Slingshot around the sun

[wil]

Is it possible to slingshot around the sun to gain speed like in the star trek movie http://en.memory-alpha.org/wiki/Slingshot_effect

 

[TMac3000]

I don't see why not, but I couldn't tell you how. In any case, it sounds pretty dangerous:shifty:

 

[Dude]

I'm not a hard core Trekkie, and had to look at the page to get the details.

"an encounter with a previously-uncharted black star required the crew to utilize all warp power in reverse to break away from the star's powerful gravitational attraction, creating a whiplash effect."

OK, so they did some sort of propulsive maneuver when near the star. You can certainly do something similar in Orbiter. The energy of an orbit (neglecting perturbation, etc) remains constant -- conservation of energy, ya know -- and is the sum of the kinetic energy (related to velocity) and potential energy (related to position). Kinetic energy = (1/2) mass * velocity^2.

The velocity squared part is the important thing here. Any delta-vee will get you a lot more bang for the buck if you're already moving fast. That's why burns are often done at or near periapse, where the velocity is fastest. If you do a delta-vee at periapse, you don't change your potential energy any but add kinetic energy.

And, when you're orbiting the Sun and very near it, you'll be moving VERY fast ... which means you'll get a lot of payoff for your delta-vee. If you want to get out of the solar system altogether, one great way to do it is to get into an orbit that passes close to the Sun then do a prograde burn once you're there.

Now, without doing that burn ... probably not. When you sling around Jupiter, for example, you're taking advantage of Jupiter's motion. But the Sun (at least in Orbiter) has no motion, if I understand correctly how it works -- it's just defined as The Center Of Everything, around which everything else revolves.

 

[C3PO]

To gain energy from that slingshot you need to be on a hyperbolic trajectory heading out of the solar system. If you remain in solar orbit, you will not gain anything except maybe the [ame="http://en.wikipedia.org/wiki/Oberth_effect"]Oberth effect[/ame].

 

[Urwumpe]

Is it possible to slingshot around the sun to gain speed like in the star trek movie http://en.memory-alpha.org/wiki/Slingshot_effect

Yes, you can gain speed, but only relative to the sun since you are fastest at perihelion, the point closest to the sun. It is no slingshot or gravity-assist, just a plain orbit then. The fastest satellites of Earth have been the Helios probes getting really close to the sun, reaching 90 km/s at their closest approach to the sun.

The sun is a poor choice for a gravity assist within the solar system, since are not approaching it from outside the sphere of influence.

The Memory Alpha stuff is nonsense, especially because of warp factors and stuff, but at least it makes a good movie.

 

[mhutiubih]

Simple answer is no, I do not see how you can enter into an object's pull and start to move away from it without losing all the energy it initiaslly gave to you.

It's like throwing a ball up into the air and saying that it will have a greater velocity when it comes back down. It will have the same velocity as when you threw it.

And as far as escaping the orbit by making a certain approach just means the sun will not keep you in perpetual orbit. It doesn't mean you get free energy.

if you want to learn about what type of systems propel spacecraft through space please check out my blog

spacepropulsion.blogspot.com

----- Post added at 19:15 UTC. ----- The previous post was at 19:12 UTC. -----

I forgot to mention that when people say "slingshot," they don't mean use the planet's gravitational pull to gain momentum, they mean use the planet's gravitational pull to change direction.

 

[jedidia]

Simple answer is no, I do not see how you can enter into an object's pull and start to move away from it without losing all the energy it initiaslly gave to you.

You can gain some of its orbital momentum, but since with a star that would only be relevant for interstellar travel. Plus, it's not a slingshot if you don't com from the outside of the sphere of influence.

I forgot to mention that when people say "slingshot," they don't mean use the planet's gravitational pull to gain momentum, they mean use the planet's gravitational pull to change direction.

Not correct. You can gain momentum from a slingshot, as described above. The planet will pull you along with it in its orbit, that's the momentum you get out of it.

 

[mhutiubih]

Ok as far as slingshotting around a single body with no other bodies influencing, you cannot gain any net momentum. that is, your entering momentum will equal your exiting momentum. If there is a body within orbit of a larger body lets say the sun, then you can steal momentum from the body orbitting the sun as long as you can make an approach capable of exiting a potential orbit with that body.

AND because we are on a planet inside the sun's solar system, we move with the sun as a satalite. So we cannot use the sun to propel us outside the solar system.

But yes, thank you for the correction jedidia. I was not considering slingshoting around a body orbiting another body.

 

[wil]

Gaining speed

IF not the sun what is the best planet to orbit to gain speed

 

[Dude]

Is it possible to slingshot around the sun to gain speed like in the star trek movie

Star Trek called that a slingshot. Well, Star Trek also had a planet populated by Nazis. On another planet, they ran into the Greek god Apollo. :lol: And I suspect that if they hadn't called this particular maneuver a "slingshot" -- which it wasn't -- a lot of confusion could have been avoided. In any case, I'm not going to get too hung up on that word, at least not when it's coming from Star Trek.

A slingshot is when you use a body's mass and motion to give a spacecraft a boost. Picture Marty McFly on his skateboard, approaching a pickup truck from the side, then grabbing the tailgate and whipping around so that he shoots forward alongside and past the truck really fast ... using the motion of the truck to increase his own speed. That's the general idea.

That's one way to use a big celestial body to gain speed. It's not the only way. But if that, specifically, is what you want to do, you'll need a planet. Jupiter is your best bet, because (within the Solar system's frame of reference) out of all the things in the Solar system that are moving, it's got the most mass. And, out of all the big things that are moving, it's the fastest. And conveniently, out of all the really big planets, it's the one that's closest to Earth.

IF not the sun what is the best planet to orbit to gain speed

If you're just looking to gain speed, the Sun is better -- if you can get close in, which isn't so easy. But take a look at the attached scenario. You start off at periapse, one million km from the Sun. Your orbit's apoapse is at Earth's orbital radius. Now, hit prograde autopilot to keep your nose in place and do a 2000 m/s burn. If you don't have BTC MFD or some other delta-vee calculator, just burn for two minutes. Watch what happens to your orbit -- you go from ellipse through parabola to hyperbola. You've reached and exceeded solar escape velocity. 2000 m/s, from low Earth orbit, isn't enough even to get you to the Moon, but used here it gets you all the way out of the Solar system. And if you want some serious speed, don't stop at 2000 m/s. Burn until the tanks are dry, and you'll be at Neptune's orbital radius in less than a year and still moving like a bat out of you-know-where when you get there.

Again, it's not a slingshot, even if Trek called it that, but it is closer to what they did -- got close to a star, turned the engines on full, and got a big impact.

BEGIN_DESC
High Ecc heliocentric orbit, DV at periapse -- demo.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 60000.0
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 50.00
END_CAMERA

BEGIN_HUD
  TYPE Orbit
  REF Sun
END_HUD

BEGIN_MFD Left
  TYPE Orbit
  PROJ Frame
  FRAME Ecliptic
  REF Sun
  TARGET Earth
END_MFD

BEGIN_MFD Right
  TYPE Orbit
  PROJ Frame
  FRAME Ecliptic
  REF Sun
  TARGET Earth
END_MFD

BEGIN_SHIPS
GL-01:DeltaGlider
  STATUS Orbiting Sun
  RPOS 0.0 0.0 1000000000.0
  RVEL 513484.964 0.0 0.0
  AROT -179.99 -89.50 0.05
  AFCMODE 7
  PRPLEVEL 0:0.900000 1:0.898687
  IDS 0:540 100
  NAVFREQ 0 0 0 0
  XPDR 482
  AAP 0:0 0:0 0:0
END
END_SHIPS

 

[Urwumpe]

IF not the sun what is the best planet to orbit to gain speed

The ideal choice for gravity assists is Jupiter, high mass, high density and still pretty high orbital velocity.

But you don't really orbit it. You fly past it (hyperbolic orbit) and while you fly past it, its gravity accelerates you. From the point of view of Jupiter, you will be as fast when you leave its gravity well, as when you entered it, just flying in a different direction. if you selected the right directions, you gained speed as seen from the sun.

If you want to get fast relative to the sun, pass the sun in a hyperbolic orbit with really low perihelion. not fast enough?

The escape velocity relative to the sun in the context of the galaxy is [math](\sqrt{2} - 1) \cdot 225 \frac{km}{s} = 93.19 \frac{km}{s}[/math].

if you would fall from the edge of the galaxys gravity well, you could pass the sun in 1 AU distance at [math]\sqrt{2 \cdot {29.6}^2 \frac{km^2}{s^2} + \cdot 93.19^2 \frac{km^2}{s^2}} = 102.16 \frac{km}{s}[/math]

Reduce this to still tolerable 0.25 AU and you will have:

[math]\sqrt{2 \cdot 4 \cdot {29.6}^2 \frac{km^2}{s^2} + \cdot 93.19^2 \frac{km^2}{s^2}} = 125.27 \frac{km}{s}[/math]

Of course, you don't really want to know the time that it takes from somewhere far out of space until you buzz past the sun at less than 0.5 permille of the speed of light. And no, Orbiter does not permit simulating this.

 

[Screamer7]

Yes, you can do it with IMFD.
Use the course program. Select Jupiter or Saturn as the target. Then decrease the mjd until you see a hyperbolic flight path around the sun towards your target.
Go to the config option in the IMFD screen and change GET to mjd.
Then use the orbit/eject program and select course.
Included is a scenario for a high speed intercept for Jupiter leaving Earth:

BEGIN_DESC
Orbiter saved state at T = 12037
END_DESC

BEGIN_ENVIRONMENT
System Sol
Date MJD 51981.7401538776
END_ENVIRONMENT

BEGIN_FOCUS
Ship GL-01
END_FOCUS

BEGIN_CAMERA
TARGET GL-01
MODE Cockpit
FOV 60.00
END_CAMERA

BEGIN_HUD
TYPE Surface
END_HUD

BEGIN_MFD Left
TYPE User
MODE Interplanetary
Scenario Old2
MapMFD V5
Reference Auto
Target none
Center GravityRef
Data 0 1 1e-006 1 0 0 0 0 1 0 0 0
MassLimit 1e+020
CMode 0
Config 1 1 1 1 0 0
ExtMode 0
Periapis none
END
CorMFD V4
Reference Sun
Target Jupiter
Source Earth
ActiveProg 1 1
DataA 0 3 0 0 0 0
DataB 1 100 51981.74015365357 0 0 1.764965859867198 0 52281.37484024108 0
DVProg 0 0 0 1
AdvConf 0 0 1 0 0
Guidance 0
END
EjectMFD V5
Reference Auto
Data 1 1.5 3 1 0 51981.74015365357 45812.1
Guidance 0
END
BaseAprMFD V2
Reference Auto
Target none
Source none
DataA 0 0 120000 0.10821 0.366519 1 1 51981.60089574447 51981.60089574447 0
DataB 0 3 0 1 0 1
END
SlingMFD V4
Reference Auto
Source none
Data 0 1 1 3 0 1 51981.60089574447 0
END
LaunchMFD V4
Target None
Data 0 1 1 3 1 1 51981.61111102188
END
CF1_DataA 0 0
CF1_DataB 0 10 120000 2 20 150000
CF1_SecTgt
mfdShare -1
mfdProgram 3
END_MFD

BEGIN_MFD Right
TYPE User
MODE Interplanetary
Scenario Old2
MapMFD V5
Reference Sun
Target jupiter
Center Sun
Data 0 1 2.115080248270106e-010 1 0 1 0 2 1 0 0 0
MassLimit 1e+020
CMode 0
Config 1 1 1 1 0 0
ExtMode 0
Periapis Earth
END
CorMFD V4
Reference Auto
Target none
Source none
ActiveProg 0 1
DataA 0 3 0 0 0 0
DataB 1 1 0 0 0 0 0 0 0
DVProg 0 0 0 1
AdvConf 0 0 0 0 0
Guidance 0
END
EjectMFD V5
Reference Auto
Data 0 1 3 0 1 51981.71343705741 10
Guidance 0
END
BaseAprMFD V2
Reference Auto
Target none
Source none
DataA 0 0 120000 0.10821 0.366519 1 1 51981.71343705741 51981.71343705741 0
DataB 0 3 0 1 0 1
END
SlingMFD V4
Reference Auto
Source none
Data 0 1 1 3 0 1 51981.71343705741 0
END
LaunchMFD V4
Target None
Data 0 1 1 3 0 1 0
END
CF1_DataA 0 0
CF1_DataB 0 10 120000 2 20 150000
CF1_SecTgt
mfdShare -1
mfdProgram 4
END_MFD

BEGIN_SHIPS
GL-01eltaGlider
STATUS Orbiting Earth
RPOS 223393301.49 568463.52 43851143.72
RVEL 44977.648 -4.579 8907.137
AROT -0.03 -78.80 -64.51
AFCMODE 7
PRPLEVEL 0:0.724028 1:0.995378
NAVFREQ 402 94 0 0
XPDR 0
AAP 0:0 0:0 0:0
END
END_SHIPS

BEGIN_ExtMFD
END

 

[jedidia]

Ok as far as slingshotting around a single body with no other bodies influencing, you cannot gain any net momentum.

It has to be noted though that although you don't get any momentum, you get a higher average velocity, which can potentially shave some time off your trip. You could use the sun for that in orbiter, but the delta-v required to make such a close pass is ridiculous. Plus, it might get hot, of course...

 

[C3PO]

But take a look at the attached scenario. You start off at periapse, one million km from the Sun. Your orbit's apoapse is at Earth's orbital radius. Now, hit prograde autopilot to keep your nose in place and do a 2000 m/s burn. If you don't have BTC MFD or some other delta-vee calculator, just burn for two minutes. Watch what happens to your orbit -- you go from ellipse through parabola to hyperbola. You've reached and exceeded solar escape velocity. 2000 m/s, from low Earth orbit, isn't enough even to get you to the Moon, but used here it gets you all the way out of the Solar system.

The reason you get that massive effect in this scenario is that you start off with a Ecc of 0.9868 and when it's that high, changing the aopapsis takes very little energy.

 

[Urwumpe]

The reason you get that massive effect in this scenario is that you start off with a Ecc of 0.9868 and when it's that high, changing the aopapsis takes very little energy.

Exactly, the known Oberth effect.

Lowering the perihelion to 1000000 km takes a lot more than the 2000 m/s dV for ejection at perihelion.

 

[Dude]

The reason you get that massive effect in this scenario is that you start off with a Ecc of 0.9868 and when it's that high, changing the aopapsis takes very little energy.

True ... but unless you live on the Sun, any orbit that takes you that close to it from your starting point will have a very high eccentricity.

 

[C3PO]

Exactly, the known Oberth effect.

With a PeVel of ~514 km/s you get some extra DV from Oberth, but that wasn't really my point. When your Ecc is close to 1, any change in energy will have a massive effect on your ApD. When you do a TLI the ApD change is ~50% in the last 5% of the burn.

Lowering the perihelion to 1000000 km takes a lot more than the 2000 m/s dV for ejection at perihelion.

Starting from Earth it will take an impulse of ~27 km/s, plus the gravitational losses due to the long time of the burn.

 

[jedidia]

True ... but unless you live on the Sun, any orbit that takes you that close to it from your starting point will have a very high eccentricity.

It might take you a lot more DV to reach solar escape velocity from a circular orbit at 1 AU. But if you calculate how much DV you have to expend to get from that to the eccentric orbit, and then add how much DV you require for escape velocity, you'll ed up with pretty much the same number. Minus the Oberth effect Urwumpe mentioned.

What you are really doing when changing from the circular into the eccentric orbit is converting kinetic energy into potential energy (your velocity decreases, but you now start to fall towards the sun). Your overall energy stays the same. By the time you reach periapsis, all that that potential energy will have converted back into kinetic energy (your velocity went up again), so all you have to add is that bit of extra energy that you would have needed to reach escape velcity had you made a prograde burn instead of a retrograde burn at apoapsis. Again, minus the Oberth effect, of course.

You can't cheat conservation of energy.

 

[C3PO]

True ... but unless you live on the Sun, any orbit that takes you that close to it from your starting point will have a very high eccentricity.

Yes, but it's the eccentricity that makes the burn look so awesome, not the magnitude of the Sun's gravity.

With a perihelion of 1 AU you can go hyperbolic with 12.6 km/s instead of 27 + 2 km/s. There's not a lot to gain that way.

 

[Dude]

Starting from Earth it will take an impulse of ~27 km/s, plus the gravitational losses due to the long time of the burn.

Or you could do a retrograde sling around Jupiter, and kill your heliocentric angular momentum that way.