Sun as final storage for radwaste?

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Tyndmyr
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Re: Sun as final storage for radwaste?

Postby Tyndmyr » Tue Oct 27, 2015 3:36 pm UTC

gmalivuk wrote:A single order of magnitude may justify "much", but not "much, MUCH".

And in any case, the claim was about delta-v, not mass ratio.


This is...pretty pedantic.

An order of magnitude, in this particular example, is quite significant. And yes, the delta-v difference is still significant, for the same reason. The original statement was quite reasonable and correct.

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Re: Sun as final storage for radwaste?

Postby Yakk » Tue Oct 27, 2015 5:52 pm UTC

Voyager(s) generated ~20 km/s delta-v in raw rockets relative to Earth orbit (from raw old fashioned thrust).

It ended with 17 km/s velocity (close to it).

Almost all of the 20 km/s was just enough to knock it over the hill to Jupiter from Earth orbit.

That means it earned about another 18.5 km/s from slingshots

Total is 38.5 km/s delta-v, including slingshots. And that is after you got it into orbit -- getting to orbit was another ~8 km/s delta-v.

So yes, counting slingshot maneuvers, we have blown past 30 km/s a while ago.
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Re: Sun as final storage for radwaste?

Postby Wolfkeeper » Tue Oct 27, 2015 10:12 pm UTC

Oberth effect and slingshots are not delta-v.

Delta-v is always and only directly generated by thrust, by definition.

And there's a huge difference between a delta-v of ~20km/s and ~30km/s. That's a difference of payload of a factor of at least 5 and more realistically 10-15. If Voyager had needed 30km/s it would have had to have been about a tenth the mass.

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Re: Sun as final storage for radwaste?

Postby gmalivuk » Wed Oct 28, 2015 12:43 pm UTC

Sure, but as I alluded earlier, slingshotting around Jupiter could get you to the Sun a lot cheaper than conventional chemical rockets for the entire 30km/s.
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Re: Sun as final storage for radwaste?

Postby Izawwlgood » Wed Oct 28, 2015 9:39 pm UTC

Can someone post some info on the Jupiter slingshot to the sun maneuver?
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Re: Sun as final storage for radwaste?

Postby Sandor » Thu Oct 29, 2015 12:59 pm UTC

Izawwlgood wrote:Can someone post some info on the Jupiter slingshot to the sun maneuver?

Starting from an orbit similar to the Earth's, you need 30 km/s delta-v to kill your orbital velocity and drop into the Sun.

However, that's not the most efficient method. Instead, you can put yourself into an elliptical orbit with the closest approach to the Sun (perihelion) the same as Earth (1 AU), and the furthest distance from the Sun (aphelion) some way further out. Once you reach aphelion, you then kill your orbital velocity and drop into the Sun.

For example, you could add 5 km/s (from 30 km/s to 35 km/s) to put yourself into an elliptical orbit with an aphelion of 2 AU. At aphelion your orbital velocity would be 17 km/s, so that's what you need to lose to drop into the Sun. The two orbit changes require a combined delta-v of 5 + 17 = 22 km/s, so you've saved 8 km/s.

In fact, the bigger the initial burn, and hence more eccentric the elliptical orbit, the more efficient it is. The most efficient way is a 12 km/s burn to very almost escape from the Sun, followed by a few m/s deep in interstellar space to drop back into the Sun. It would of course take a very very long time.

Here's some different aphelions you could chose, the delta-v needed to reach them, and the velocity at aphelion you need to kill to drop into the Sun:

Code: Select all

Target Aphelion     Delta-V Required      Velocity to kill     Total
of Elliptic Orbit   to reach the orbit      at Aphelion       Delta-V
-----------------   ------------------    ----------------    -------
1 AU                    0 km/s               30 km/s          30 km/s
2 AU                    5 km/s               17 km/s          22 km/s
3 AU                    7 km/s               12 km/s          19 km/s
5 AU (Jupiter)          9 km/s                8 km/s          17 km/s
10 AU                  10 km/s                4 km/s          14 km/s
19 AU (Uranus)         11 km/s                2 km/s          13 km/s
Light year             12 km/s                0 km/s          12 km/s

There is another short cut. If you pick an aphelion of around 7 AU to 10 AU, you need about 10 km/s of delta-v to reach your elliptical orbit. As you pass Jupiter on your way out, you'll be doing something like 11 km/s. Jupiter orbits at 13 km/s. If you imagine your 11 km/s split into radial (10 km/s) and tangential (5 km/s) components, then you approach Jupiter like this. The Sun is 5 AU below the bottom of this diagram, and Jupiter is orbiting anti-clockwise.

Code: Select all

        < 13 J -------......_ _ _ _
   ^
   10
< 5 \
     \
      \

If you then subtract 13 km/s from the tangential components to convert from Solar to Jupiter co-ordinates, and if you've been careful with your aiming, you get a Jupiter flyby like this. Relative to Jupiter you come in 13 km/s and leave at 13 km/s in a different direction.

Code: Select all

        _.------ -13 >
      .'
     /   J
    /
 ^ /
10/
  -8 >

If you add the 13 km/s to your tangential velocity after the flyby to convert back to Solar co-ordinates, you find the flyby has stopped you dead in space (relative to the Sun). From now on it's just a long, long fall to a fiery end.

That's a Jupiter slingshot to the Sun maneuver.
Last edited by Sandor on Thu Oct 29, 2015 9:02 pm UTC, edited 2 times in total.

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Re: Sun as final storage for radwaste?

Postby Yakk » Thu Oct 29, 2015 1:35 pm UTC

Note that this operation speeds Jupiter up a very (very) (very very) small amount relative to the Sun; you use gravity to transfer kinetic energy from yourself to Jupiter (in the Sun's frame of reference).

In effect, you use Jupiter as a really big rocket exhaust (or maybe Jet? What do you call Propeller propulsion in the more general sense, swimming? as the matter you "exhaust" was already there, you didn't bring it with you), where you push/pull the exhaust using gravity instead of traditional E-M/Exclusion principle based forces.
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Re: Sun as final storage for radwaste?

Postby Izawwlgood » Thu Oct 29, 2015 1:57 pm UTC

Oh, I see. Out of curiosity, why can't you just use any planet out from Earth to do the maneuver? Would you need to skim too close to Mars to make it work?
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Re: Sun as final storage for radwaste?

Postby schapel » Thu Oct 29, 2015 2:49 pm UTC

Sandor wrote:That's a Jupiter slingshot to the Sun maneuver.

We should just shoot radioactive waste into Jupiter directly. There's no solid surface, so the waste would fall into the center where the heat would melt everything and allow it to dilute through Jupiter's core. Jupiter must already have quite a few radioactive elements in its core, so we'd be adding just a bit more.

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Re: Sun as final storage for radwaste?

Postby Wolfkeeper » Thu Oct 29, 2015 3:36 pm UTC

Izawwlgood wrote:Oh, I see. Out of curiosity, why can't you just use any planet out from Earth to do the maneuver? Would you need to skim too close to Mars to make it work?

Mars is too small, you'd never get the ~30km/s change, you'd be underground and it wouldn't have worked. There's a few factors, but it generally works best on a planet with a large escape velocity, Mars is only 5km/s.

Either Jupiter or Saturn would work, or another planet further out. Uranus or Neptune which aren't as big, but off-hand I think they should work because your orbital speed goes down with distance, they're big enough. Pluto wouldn't work- not enough escape velocity.

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Re: Sun as final storage for radwaste?

Postby Sandor » Thu Oct 29, 2015 4:24 pm UTC

Wolfkeeper wrote:Mars is too small, you'd never get the ~30km/s change, you'd be underground and it wouldn't have worked. There's a few factors, but it generally works best on a planet with a large escape velocity, Mars is only 5km/s.

What you can do is use Mars and Earth to get you out to Jupiter in the first place. Rosetta took 2.2 km/s of delta-v and 4 flybys to almost reach Jupiter's orbit. That's instead of the 9 km/s delta-v it would take without flybys.

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Re: Sun as final storage for radwaste?

Postby cyanyoshi » Thu Oct 29, 2015 4:31 pm UTC

Mars can probably be used to slow the spacecraft down enough with enough flybys, but why go to Mars when you could just use Earth itself? That's what MESSENGER did.

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Re: Sun as final storage for radwaste?

Postby gmalivuk » Thu Oct 29, 2015 5:37 pm UTC

Sandor wrote:However, that's not the most efficient method. Instead, you can put yourself into an elliptical orbit with the closest approach to the Sun (perihelion) the same as Earth (1 AU), and the furthest distance from the Sun (aphelion) some way further out. Once you reach aphelion, you then kill your orbital velocity and drop into the Sun.

For example, you could add 5 km/s (from 30 km/s to 35 km/s) to put yourself into an elliptical orbit with an aphelion of 2 AU. At aphelion your orbital velocity would be 17 km/s, so that's what you need to lose to drop into the Sun. The two orbit changes require a combined delta-v of 5 + 17 = 22 km/s, so you've saved 8 km/s.

In fact, the bigger the initial burn, and hence more eccentric the elliptical orbit, the more efficient it is. The most efficient way is a 12 km/s burn to very almost escape from the Sun, followed by a few m/s deep in interstellar space to drop back into the Sun. It would of course take a very very long time.
For some reason I had completely forgotten about this option. For what turns out to be the second time in this thread.
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Re: Sun as final storage for radwaste?

Postby Wolfkeeper » Fri Oct 30, 2015 2:05 am UTC

Sandor wrote:
Wolfkeeper wrote:Mars is too small, you'd never get the ~30km/s change, you'd be underground and it wouldn't have worked. There's a few factors, but it generally works best on a planet with a large escape velocity, Mars is only 5km/s.

What you can do is use Mars and Earth to get you out to Jupiter in the first place. Rosetta took 2.2 km/s of delta-v and 4 flybys to almost reach Jupiter's orbit. That's instead of the 9 km/s delta-v it would take without flybys.

Oh sure, you can do multiple passes, I didn't mean to imply otherwise. I was just talking about a single pass.

The minimum is probably enough delta-v to reach the Moon's Lagrange points, you can get anywhere in the solar system from there using virtually no delta-v; it's just often very, very slow; you could dump the stuff into the sun or spin it off into interstellar space.

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Re: The sun as a nuclear waste incinerator

Postby Minerva » Sat Nov 07, 2015 1:36 am UTC

CutmanCometh wrote:Why can't we round up all the spent fuel rods from nuclear power plants, load them on a rocket, and fly them into the sun?


Because that's a huge resource of valuable nuclear fuel, it's good stuff and we shouldn't waste it!
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