# Theoretical top speed using multiple gravity assist

1. Jul 8, 2012

### Rocket_man

Basicaly wanting to know the maximum velocity achievable if we sent out a craft purely to go as fast as poss using gravity assist + initial escape burn. Obviously a time limit would be needed so lets say 10years of gravity assist manouveres.

How fast could we go? Would the gforces involved destroy the space craft?

Thanks

2. Jul 8, 2012

### Staff: Mentor

The finite size of the planets will limit your velocity: you cannot come too close, or the spacecraft will crash. Given enough time, I would expect a velocity similar to the orbital velocities of the inner planets plus their escape velocity, which would be a sum of ~50km/s for mercury. However, mercury is quite deep within the potential well of the sun. If you look for the total energy of the space craft, Jupiter should be better. It could accelerate the spacecraft to ~70km/s, if you somehow manage to approach it quick enough.

Acceleration within a uniform gravitational field is impossible to observe within the space craft (unless you look what is going on outside) - and the gravitational field of planets is extremely close to uniform for reasonable space craft sized. The space craft would not feel any stress.

3. Jul 9, 2012

### Rocket_man

70km/s is a lot faster than any of our exploratory spacecraft. I guess only a deep space mission requires massive velocity. (edit: just read that Juno will be setting new record achieving 150,000mph on its mission)

Im struggling to get my head around the space craft feeling no forces during a slingshot manouvre?. Surely the planet is changing its velocity & direction & you cannot do either of those things without a force being applied. How are these forces not felt?

Last edited: Jul 9, 2012
4. Jul 9, 2012

### xAxis

Because they act on each part of your body equally. If you are driving in the car for instance, when you hit the brake, the force acts on the car which decelerates, but your body continue to move. Now imagine that instead of breaking, you want to reduce the speed by introducing the gravitational field from behind. Your car would decelerate, but so would your body by exactly the same rate.

5. Jul 9, 2012

### DaveC426913

The craft is in freefall the entire time. Just like in Earth orbit, the craft and anything in it all experience the same forces, therefore no net force is felt.

The forces that will stress a craft are tidal forces, where unequal forces might be pulling on different parts of the craft. That would require either a very large craft, or a steep gravitational well.

6. Jul 9, 2012

### Danger

I'm thinking that an object could get close to c if it dips into the ergosphere of a black hole for its gravity whip. I wouldn't want to be in it, though; the aforementioned tidal forces would be outrageous.

7. Jul 9, 2012

### DaveC426913

http://en.wikipedia.org/wiki/Neutron_Star_(short_story)
Won him the Hugo Award.
You can read how he survived.

8. Jul 9, 2012

### Staff: Mentor

If it is possible at all, it would require much more than 10 years. Therefore, a shorter path would be quicker for any target in the solar system.

I would not want those black holes in the solar system.

9. Jul 9, 2012

### Danger

Actually, Larry is one of my all-time favourite authors. His "hard" SF is just about impossible to beat. I was actually security chief for an SF convention at which he was the guest of honour. He and Fuzzy Pink were great to be around. I remember him whapping a monk in the chest with a flippy-flier hard enough to knock him into a toilet in the con suite. Good times.
Two of the funniest things that he ever wrote were both in "Protector". Stuff to keep in mind if you're ever being chased by someone in a Bussard ramjet.

10. Jul 9, 2012

### DaveC426913

Protector is my favourite of all. Well ... after Ringworld.

11. Jul 9, 2012

### jbriggs444

12. Jul 9, 2012

### Danger

But General Products ships included inertial damping.

13. Jul 9, 2012

### DaveC426913

Not sure why rotation would have been a problem. It was on a hyperbolic course; it would have rotated less than 360 degrees.

No. The whole point of the story was the mystery that something reached through the hull and squashed the first explorers and they didn't know what it was.

Frankly, I think the big problem with the story is the speed at which the flypast occurred. I am skeptical that the range in which tidal effects were dominant would have lasted more than a fraction of a second. Far too short to do anything about it.

14. Jul 9, 2012

### Danger

Apparently, I'm not remembering it properly. Keep in mind, however, that I haven't read it since it came out in '68. (The anthology that includes the short story, which was written in '66.) I just went and dug it out of my bookcase, so I'll read it tonight to refresh my memory.
Anyhow, I suspect that furtherance of this should probably take place in GD; we're a bit off-topic for physics.

15. Jul 9, 2012

### jbriggs444

Found a reference.

http://en.wikipedia.org/wiki/Neutron_Star_(short_story [Broken])

"
In the "Afterthoughts" section of the Tales of Known Space collection, Niven writes: "I keep meeting people who have done mathematical treatments of the problem raised in the short story 'Neutron Star', .... Alas and dammit, Shaeffer can't survive. It turns out that his ship leaves the star spinning, and keeps the spin." If this is true, it does more than kill Shaeffer: it kills the entire story premise, for the Laskins' ship also would have acquired and kept a similar spin, which the puppeteers could hardly have failed to notice. It is also unclear how the Laskins' ship returned to its starting point; unless the puppeteers sent another ship to retrieve it, it would have had to do so through normal space, a journey of years.
"

Last edited by a moderator: May 6, 2017
16. Jul 9, 2012

### Danger

That's reminiscent of the embarrassment suffered by Ben Bova. (There was no reason for him to be embarrassed, but he was.) He's one of the other supremely hard SF writers, and a physicist by schooling. An entire short story of his was premised upon the coldest spot in the solar system being the back side of Mercury. 30 or 40 years later, it was discovered that Mercury isn't tide-locked to the sun and rotates slowly relative thereto.

17. Jul 9, 2012

### DaveC426913

I would not have thought that were the case. His passage past the star should be symmetrical - i.e. you could film it and play it forwards and backwards and not be able to tell which was reality.

But of course that's probably what Niven thought too, until his mathematician fans got hold of the story...

18. Jul 10, 2012

### Staff: Mentor

A spin would not violate time-symmetry - the reversed process would be a spaceship spinning in the opposite direction, losing its spin during fly-by. And the symmetric process would be a spaceship spinning in one direction, reaching the closest point without rotation and spinning in the other direction afterwards. I think this should be possible.

19. Jul 10, 2012

### DaveC426913

I know.

But likewise, a million shards of glass could spontaneously form into a glass shape and leap off the floor onto the counter.

20. Jul 10, 2012

### Staff: Mentor

Yes, but a spinning object would not have a lower entropy than the non-spinning one.

Consider a fly-by where the deflection angle is small, and the spaceship consists of two connected masses:

In general, I would expect a rotation of the outgoing spaceship, as you have a variable torque all the time, which depends on the position and the rotation angle at the same time. I am too lazy to evaluate this analytically.

21. Jul 10, 2012

### DaveC426913

Indeed. A character in one of Larry Niven's books (Protector I believe) made good use of this to evade capture.

Fusion ships were lifepod, supplies and powerplant strung together like mile long beads on a chain. The pursuee headed for a high density star and disconnected his ship before entering the fly-by resulting in three independent bodies following the same path. The pursuers didn't have time, and their ship was flung all over the place.