More on the Starship ‘Slingshot’ Maneuver

by Paul Gilster on November 20, 2012

Although we ordinarily think of Stanislaw Ulam in connection with pulse propulsion — and in particular with the Orion concept, in which nuclear devices are exploded behind the spacecraft — the scientist was also investigating other propulsion ideas. It puts yesterday’s discussion of Freeman Dyson’s ‘gravitational machines’ into context to realize that Ulam was writing about obtaining ‘a velocity arbitrarily large — that is, close to the velocity of light…’ using gravitational slingshots around astronomical objects in papers written as early as the late 1950s. There is, as Greg Matloff and Eugene Mallove remind us in The Starflight Handbook fantastic energy available in the orbital motion of celestial bodies. How best to tap it?

Michael Minovitch was already working at UCLA in the early 1960s on how to use a gravity ‘slingshot’ to affect spacecraft trajectories, thus extending the capacity for exploration. Dyson’s futuristic paper, as we saw yesterday, described gravitational machines involving white dwarfs and, quite presciently for the time, neutron stars back in 1963, just a few years after Ulam’s studies had concluded that gravitational maneuvers were possible but impractical for high velocities. One can easily object to Dyson’s thinking by asking how even an advanced civilization could manipulate neutron stars and, for that matter, how it might use such a transportation system. But Dyson’s thinking was aspirational. He wondered what might be possible for extraterrestrial cultures because this was a way of looking into possible human futures.

Matloff and Mallove quote Dyson on the matter: “There is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do, provided it is physically possible.” It’s a thought that quickens speculation. And if we put our minds to it and run it through a science-fictional process of extrapolation, maybe a culture that already has interstellar travel would find it useful to have a kind of automated propulsion system that could routinely fling a payload out at almost 30 percent of the speed of light. Exactly how they might manage it and what their intentions would be will remain a mystery to a culture at our technological level.

A gravitational machine using neutron stars would subject the spacecraft using it to stupendous tidal forces, but a culture that can master neutron star engineering would perhaps find a way around these as well. In any case, in a time when we are thinking about ‘interstellar archaeology’ as a new mode for SETI, double star gravitational accelerators make one more possible artifact, or perhaps I should say, one more astronomical object near which we might look for signs of extraterrestrial engineering. A Kardashev Type II civilization might thus reveal its presence not in the form of a beamed radio or optical signal but rather through the works of its engineers.

But let’s get back closer to home. Yesterday I mentioned Krafft Ehricke’s own ideas on gravitational maneuvers in the form of a deep-space probe he thought could be accelerated to speeds that would make a mission to the inner Oort Cloud possible in a 50-year time-frame. In a 1983 paper in JBIS, Matloff and Mallove had initially confirmed Ehricke’s figures, assuming a close solar approach (to within 0.01 AU of the Sun’s center, or two solar radii) and factoring in no powered perihelion maneuver. But in a subsequent paper, Matloff went on to revise the numbers, finding what he calls a ‘subtle error’ in the original calculations.

Image: Interstellar theorist Gregory Matloff. Credit: CUNY.

Working with Kelly Parks, Matloff computed the velocities obtainable by a worldship falling toward the Sun and grazing the solar photosphere at 0.005 AU from the Sun’s center, with a 10 km/sec propulsive maneuver applied at perihelion. The result: “The hellish penetration of the solar atmosphere at 600 km/sec would occur in a relatively short but sizzling half-hour period. With the 10 km/sec maneuver, the starship would exit the Solar System with a hyperbolic excess velocity of 110 km/sec…” Doubling the perihelion maneuver to 20 km/sec gets you up to 156 km/sec as you exit. This is the sound of anti-climax, a nice bump up over Voyager-style speeds, but one that still leaves you with an Alpha Centauri crossing that takes close to 8000 years.

We can do better than this with a ‘Sun-diver’ maneuver involving solar sails, but that is a topic for another day. The paper is Matloff and Parks, “Interstellar Gravity Assist Propulsion,” JBIS 41 (November, 1988), pp. 519-526.

tzf_img_post

{ 19 comments }

djlactin November 20, 2012 at 11:25

I can understand how the slingshot works when a body slings around an object moves is moving (e.g. Voyager around Jupiter), but how does the maneuver work when the barycenter is (at least in relative terms) stationary? Intuitively, a probe skimming the sun would achieve a large velocity, but would lose it while exiting the sun’s gravity well. Please explain.

A. A. Jackson November 20, 2012 at 13:02

Dang it I should have remembered Ulam.
Bet Dyson heard the idea by way of Feynman , who at Los Alamos, used to talk to Ulam all the time.

Thinking back on the gravity assist physics… makes me think the process should have , probably was , thought of in the 19th century.
I seem to remember that celestial mechanics people had figured out that Jupiter could remove long and intermediate comets from the solar system, for good! That is put them on interstellar escape orbits.
It is a classic restricted three body problem.
I am going to guess Félix Tisserand.

Ariff November 20, 2012 at 14:29

Another thought on this topic. A society that thinks in the long term could potentially use these sling-shot maneuvers to move planetary or stellar bodies too. One such maneuver wouldn’t have much impact, but given a sufficient number of them you could modify a planet’s orbit, which may be very useful as your star evolves. I have no idea what the numbers are though and it may be that the “sufficient number” is way too large.

Greg November 20, 2012 at 15:10

So what temperatures are to be expected on a close slingshot like this? And if it mostly light and plasma’s, I’m thinking magnetic fields and reflective materials would allow the probe to last through such a close encounter.

ljk November 20, 2012 at 18:23

A. A. Jackson said on November 20, 2012 at 13:02:

“I seem to remember that celestial mechanics people had figured out that Jupiter could remove long and intermediate comets from the solar system, for good! That is put them on interstellar escape orbits.”

Makes one wonder if any comets we have in our Sol system are from other star systems? And if someone might have used any for interstellar Worldships?

How long would it take a comet a few miles across from our system to get to Alpha Centauri using the Jupiter slingshot method?

william November 20, 2012 at 20:20

“I can understand how the slingshot works when a body slings around an object moves is moving (e.g. Voyager around Jupiter), but how does the maneuver work when the barycenter is (at least in relative terms) stationary? Intuitively, a probe skimming the sun would achieve a large velocity, but would lose it while exiting the sun’s gravity well. Please explain.”

it doesn’t just involve gravity wells only; path chosen makes a diff.

Astronist November 20, 2012 at 20:46

@djlactin: The close approach to the Sun is not in fact a slingshot manoeuvre. When doing a flyby of the barycentre of the Solar System, the point is that you fire the rocket engine at perihelion. Engine burns lower down in a gravitational well have greater leverage on the resulting hyperbolic excess velocity at infinity. However, as Paul says in the last para but one, the advantage is not great when aiming for interstellar speeds.

Rob Henry November 21, 2012 at 2:36

Djlactin asks “how does the maneuver work when the barycenter is (at least in relative terms) stationary?”, and in this context my answer is twofold.

1) the quote from above “With the 10 km/sec maneuver, the starship would exit the Solar System with a hyperbolic excess velocity of 110 km/sec” is a pretty clear statement that they were also considering the Oberth effect.

2) It is my guess that the solution to the maximum possible slingshot effect from our solar system is to end the series of manoeuvres when the highest velocity is attained at which a retrograde approach to Jupiter can be redirected almost straight at the sun. Thence it could go in any direction, though without an additional Oberth manoeuvre there would be no reason for such an extreme solar approach.

Rob Henry November 21, 2012 at 3:22

Oh djlactin, I have just realised that the figures they gave for the Oberth boost implied that they envision entered the solar atmosphere with a hyperbolic excess that is very close to zero. This is implies that they see very little potential gain from slingshot manoeuvres themselves!

Volucris November 21, 2012 at 4:18

djlactin, the name of the game is http://en.wikipedia.org/wiki/Oberth_effect

Basically rockets apply constant force, force times velocity equals power, and since a craft will have large velocity at the closest approach, it can do work towards it’s kinetic energy at high power.
The “extra” energy is sapped from from the final total energy of reaction mass, for example, it can be left behind trapped deep in the (negative potential energy) gravity well, while the craft continues it’s merry way out of the system.

Rafal November 21, 2012 at 10:13

Forgive me if this is a naive question. Is is possible to use gravitational maneuvering not only for speeding up but also for decreasing your speed? If it’s possible, then it could reduce the need for fuel for any interstellar probe. As it could use the gravitation of a destination star(s)/planets to reduce the speed.

Tarmen November 21, 2012 at 15:33

ljk, Yes, good possibility of excavated comets being used as sling shot colonial world-ships in the past by others . And in the future by us too, maybe. Plenty of resources to last a long time for food and fuel. I think it may be the default method used by the more desperate sorts of colonists . The ones with no choice but to go, somehow, anyhow, now . Obviously many of them would fail to reach anywhere hospitable for their kind and all aboard would die, but some proportion would find a warm wet spot to ‘blossom’. Maybe there is a whole ecology of cometary life between the stars.
Hmm…….

william November 21, 2012 at 16:06

“Rafal November 21, 2012 at 10:13
Forgive me if this is a naive question. Is is possible to use gravitational maneuvering not only for speeding up but also for decreasing your speed? If it’s possible, then it could reduce the need for fuel for any interstellar probe. As it could use the gravitation of a destination star(s)/planets to reduce the speed.”

YES

A. A. Jackson November 21, 2012 at 16:12

@ljk
A very interesting question: Does the comet catalog have any definitive comets that have arrived from interstellar space.
The answer is no.
A bit of a mystery that, thus the catalog sets an upper limit of 10^12 comets per cubic parsec.
http://www.space.com/4759-enduring-mysteries-comets.html
Jupiter should capture one every 60 million years, 96P/Machholz may be one.

A. A. Jackson November 22, 2012 at 12:16

Addendum to my comment about interstellar comets.

There are comets in the catalog that have escape speed with respect to the Sun-solar system.
However it was calculated that these received a Jovian slingshot to remove them.

Once again I believe this was known-calculated in the 19th century…. which means that Gravity Assist has been known for a long long time.

Kelvin F. Long November 25, 2012 at 14:37

Hi all,
well done Paul for another outstanding discussion.
Although Arthur C.Clarke never published a technical paper in the red cover issues of JBIS, he did publish a letter titled “CORRESPONDENCE: An Optimum Strategy for Interstellar Robot Probes”.

Published in 1978,the letter discusses the concept of ‘gravity assistance’. The letter is too long for me to type out, but its worth reading if the history of the gravity assist is to be completed, and its connection to science fiction. He uses the gravity assist in his books ‘The Fountains of Paradise’ (end of the book) and ‘Rendezvous with Rama’.

Clarke says “….there may be an optimum velocity for robot probes, of around 0.02c, with corresponding transit times between stars of a few thousand years. Even if it was technically possible to build probes with cruise velocities of 0.5c, and thus transit times of a decade, such vehicles could investigate only a single star system, and for a very short (one day) period of time. They would be travelling far too fast to be deflected towards another system within reasonable range”. The letter contains some calculations Clarke did.
The letter was published in the following:

JBIS, Interstellar Studies, Volum 31, No.11, November 1978, p438.

Best wishes
Kelvin F.Long

Eniac November 25, 2012 at 20:41

Clarke says “….there may be an optimum velocity for robot probes, of around 0.02c, with corresponding transit times between stars of a few thousand years. Even if it was technically possible to build probes with cruise velocities of 0.5c, and thus transit times of a decade, such vehicles could investigate only a single star system, and for a very short (one day) period of time.

I do not know if Clarke discussed this or not, but if propulsion is on-board, the solution is simple: Only accelerate to 0.25 c and use the remaining 0.25c for stopping. Two decades is still better than millenia, and being able to observe the target for decades is MUCH better than one day and a chance at another day somewhere else, in another thousand years.

Eniac November 25, 2012 at 20:51

Gravitational fields are ineffective at high velocities, because they are strictly localized around stars.

If we do want to retarget probes, we should consider Lorentz turning in the galactic magnetic field. The field is weak, but present throughout the entire trip. A mesh of very fine wires charged to a high voltage could generate enough Lorentz force to permit a turning radius of tens of light years, if not less. That would allow plenty of possibilities for targeting multiple system, including an eventual return to the origin.

Rob Henry November 26, 2012 at 15:27

AA Jackson writes “Once again I believe this was known-calculated in the 19th century…. which means that Gravity Assist has been known for a long long time.” and I am not sure that one follows the other.

To me it is counterintuitive that that known slingshot effect is frequently worth the detour involved for a manmade probe. This slingshot effect is also close to nonexistent at the velocities at which we would wish to send probes to the stars. Thus, to my mind, the thought that the slingshot might be useful as a gravitational assist was still a brilliant insight.

Comments on this entry are closed.