What is the most efficient way to send an object to the sun?

[ar81]

Imagine you have a low thrust tow vessel orbiting Earth. You attach a payload that needs to be delivered to the sun, but the tow vessel should not leave Earth SOI.

1.What is the most fuel efficient way to send the payload directly into the sun, given that scenario?

2.Is there any other more fuel efficient way to do it by sending the tow ship through a series of slingshots in the inner solar system, where payload goes to the sun and tow vessel goes back to Earth orbit?

 

[Artlav]

If the tow vessel have like 60 km/s of DV - decelerate, detach, accelerate back.

For realistic vessels the only path is thru slingshots in the outer system - once around Jupiter, payload to the Sun, tow vessel back home (or never there).

It'll be a while before we will dump garbage into the Sun, i guess.

 

[cjp]

If your payload has no engines, then thrust can only come from the tow vessel. If the tow vessel is not allowed to leave earth SOI, then all thrust has to be applied within earth SOI. The rest of the trajectory of the payload will be determined by the initial orbit (when leavin earth SOI) and gravity.

Note that, after the payload and the tow vessel separate, they are still more or less in the same orbit. So, if the tow vessel is not allowed to leave earth SOI (and ultimately hit the sun too), then it must apply thrust to change its orbit back to an earth-bound orbit, after it separates from the payload.

You mention two scenarios, where the second one involves slingshots. So I assume that in the first scenario 'directly' means 'without slingshots'.

Without slingshots, the most efficient way is to let the payload follow a Hohmann orbit, with aphelium at the place where earth is when you leave it, and perihelion on the sun's surface. To enter such an orbit, you need a lot of delta-v: you need to almost completely cancel out the velocity of earth w.r.t. the sun.

With slingshots, I suggest going for Jupiter. It's by far the most effective planet for sling-shotting, and unless the inner planets are helping you to reach Jupiter, I'm afraid they won't be of much help.

My experience (by playing with Orbiter) is that a single slingshot around Jupiter is not enough to reach the sun. What I did was to apply some extra thrust at perijove, but if your payload doesn't have engines, the alternative could be to do multiple slingshots around jupiter. This requires some careful planning however.

 

[ar81]

I am thinking as if I owned a tow ship to get rid of space junk in orbit around Earth with 4 VASIMR engines that provide very low thrust (my guesstimate is something as low as 20 to 40 Newtons).

If it spends too much fuel or too much time, it may not be profitable.
So I guess a "profit efficient" mission profile would be needed.

Unfortunately Orbiter does not provide tools for low thrust navigation.

http://www.adastrarocket.com/aarc/Technology

I am thinking that this could be a very interesting concept for some orbinomics...
But it would require trajectory planning to make it cost-effective and profitable.

 

[francisdrake]

Interesting problem! I try to compare 2 options:

1. Brute force: Earths orbital speed is 30 km/s. I you cancel that out and are away from Earths SOI, the spacecraft will drop straight into the sun. As your scenario starts in an Earth orbit, you have already 8 km/s orbital speed, provided the maneuver starts at a suitable point, so the resulting path points 'backwards' to Earths vector around the sun.

2. Jupiter swing-by after Venus-Venus-Earth gravity assist:
This is probably the cheapest one, regarding the dV.
Basically you need only the dV to reach Venus, which is around 7 km/s (starting in Earths orbit). Source see http://www.projectrho.com/rocket/rocket3o.html.

After the Venus and Earth swing-bys and a flight time of 6 years (like Gallileo) the spacecraft reaches Jupiter, where it performs a retrograde swing-by, which is designed to cancel out Jupiters orbital velocity around the sun.

After leaving Jupiters SOI the spacecraft hangs 'dead in space' and starts dropping into the sun. I would assume it takes 2 or 3 more years until it plunges into the sun.

This option takes quite a long flight time, but I guess is hard to beat concerning the dV budget. I am not sure if any other retrograde swing-by (around Venus or Mars) could provide enough dV to effectively cancel out the orbital speed of the planet, but maybe multiple fly-bys could do the same job.

 

[ar81]

Where did I come up with this idea?
If you played orbiter, you may think space is mostly empty, but it is far from that.
Do you want to see all objects around Earth with Google Earth?

1.Download this file: http://adn.agi.com/SatelliteDatabase/SatelliteDatabase.kmz
2.Double click on the file and it will install a Google Earth addon.

It displays:
1.Active satelites
2.Inactive satellites
3.Debris
4.Rocket bodies.

You will notice that active satellites are a minority.
We already have seen and impact of satellites in 2009.

U.S. Satellite Destroyed in Space Collision
http://www.space.com/news/090211-satellite-collision.html

This animation predicts the trajecory of the debris.

Animation shows spread of debris from satellite collision

It is clear to me that to get rid of space junk is necessary, and the only engine I can think that could do the job is the VASIMR due to its efficiency that would allow to stay very long time in space without refueling.

Is there any other existing engine that could do it?

 

[tori]

Don't forget to include the time needed to rendezvous with all of your junk items in orbit in your profit calculations. With minimum dv you're looking at weeks and weeks of collecting scrap before dipping it into the Sun.

By the way, why the Sun? What's wrong with recycling? Deorbit your debris (most of the time with less than 1 km/s dv). It burns up, metal oxides rain down, the biosphere reprocesses them, couple eons later we mine 'em up again. All that without streams of ejected debris cruising through deep space at dozens of km/s, obscuring our radars, endangering our deep space assets and wasting resources (both the scrap metal and the energy needed to eject it) away.

 

[ar81]

You may have some military satellites that governments may not want to deorbit so other can study them. I considered that sending them outside of solar system may not allow the tow ship to return, so the logical thing is to use the sun. If a single spot may throw the mass of Everest, dumping a small scrap sat with toxic or dangerous substances or devices into the sun should not be a problem.

 

[tblaxland]

You may have some military satellites that governments may not want to deorbit so other can study them.

A more controlled deorbit into the Pacific should cover that eventuality.

I considered that sending them outside of solar system may not allow the tow ship to return, so the logical thing is to use the sun. If a single spot may throw the mass of Everest, dumping a small scrap sat with toxic or dangerous substances or devices into the sun should not be a problem.

Why does it need to make it to the sun? If not the Earth, why not the Moon, or why hit anything at all? A slingshot around the Moon could send it into an inclined solar orbit, never (or at least unlikely) to be seen again.

 

[ar81]

Why does it need to make it to the sun? If not the Earth, why not the Moon, or why hit anything at all? A slingshot around the Moon could send it into an inclined solar orbit, never (or at least unlikely) to be seen again.

And how do you make the tow vessel to return to Earth? Remember it has very low thrust, so it would rely mostly on a continuous burn, so its trajectory would be mostly like a spiral, not the normal transfer you could have.

To enter such an orbit, you need a lot of delta-v: you need to almost completely cancel out the velocity of earth w.r.t. the sun.

Why should it cancel so much velocity? You only may need a prograde lower orbit ejection at a certain point or a retrograde ejection aiming at upper orbit.

To me it is clear that perihelion must be inside sun's atmosphere, so drag could cause some sort of aerobraking. So it seems to me that the key should be to know what should be that ejection direction when it gets out of Earth SOI.

I came to think that if I send the payload aiming towards higher orbit in retrograde direction, 45 degrees from Earth's orbit line, it could be more effective than a simple retrograde trajectory to make the orbit more elliptic and to make it fall towards the sun. Of course this is just my guess.

Another possibility would be to get out of Earth SOI and use Earth itself for subsequent slingshots, using non ecliptic plane as stated here:
http://www.smad.com/analysis/SEP_Mission_Paper.pdf
SEP (Solar Electric Propulsion) also known as ion engine does not seem to me as viable to me as the VASIMR engine.

SEP
http://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/1-how-does-ds1s-engine-work.html

VASIMR
http://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html

I may like to know what do you think about it.

 

[tblaxland]

And how do you make the tow vessel to return to Earth? Remember it has very low thrust, so it would rely mostly on a continuous burn, so its trajectory would be mostly like a spiral, not the normal transfer you could have.

If you have enough dV to get the tow vessel back to earth after any boost method proposed in this thread (including your idea of lowering the periapsis into the sun's atmosphere), then your will have enough to get it back after injecting the payload on a lunar slingshot. I envisage it like this:

1. Tow vessel injects stack onto lunar flyby (approx 4 km/s dV)
2. Mid course corrections (approx 0.030 m/s dV)
3. At pericynthion, the stack separates. The payload continues on its way to solar orbit. The tow vessel burns retrograde to lower its perigee enough to return to Earth (approx 2 km/s dV ???)
4. Tow vessel returns to Earth and aerobrakes to lower apogee, perhaps using several passes to reduce thermo/aerodynamic stress.
5. Tow vessel raising perigee to achieve stable orbit (approx 0.2 km/s dV ???)

Roughly a total dV of 6 km/s and steps 1 & 3 could be done in smaller chunks over consecutive apsides. The next best option in this thread is the VVEJ flyby option which would require about 3 km/s insertion dV and at least the same lower the tow vessel perigee back down again.

 

[Ark]

Putting space debris into the sun sounds expensive, can't we just make Half Section do it for us?

 

[ar81]

It would not be so expensive if tow vessel does not leave Earth SOI.

 

[Urwumpe]

It would be far cheaper if we return the mess to earth, even from geostationary orbits.

maybe somebody else can check my "math", but even with just attempting to use multiple Venus/Earth flybys for lowering the perihelion, it would be cheaper to de-orbit it to earth, even in a controlled way.

 

[Sky Captain]

I think rendezvousing with each piece of scrap to collect it would be extremely time consuming, maybe the largest pieces like failed satellites and spent rocket stages could be collected that way but there are thousands of tiny bits of scrap few cm in size and smaller. These tiny pieces are actually more dangerous because they are numerous and more difficult to track than large bits of scrap and they are so many that rendezvousing with each of them would take forever.

A more efficient method would be to use some other means like powerful laser beams to clear orbital space from tiny pieces of junk.

 

[Ghostrider]

A more efficient method would be to use some other means like powerful laser beams to clear orbital space from tiny pieces of junk.

Lasers are lame. Wave motion guns, however...

 

[Artlav]

Maybe launch a cloud of water ice into retrograde orbit at evening side?
It will sweep all the tiny bits and evaporate once in sunlight.

Better yet, launch it suborbital - put a pump-cannon on the coast to shoot water into retrograde suborbital trajectory, effectively a big particle net for tiny stuff. Make sure to move all the needed satellites out of the way first though.

 

[tori]

There was a similarly themed discussion on space.com forums.

 

[cinder1992]

I think that some sort of "rotational launcher" would work, as an example, remove the cap on the arrow's angular velocity and spin round real fast, what happens when you release a cargo?.. WHOOSH! it goes flying off on a tangent.

 

[ar81]

But remember that profit and cleaning space is the key.

In the discussion at space . com they talked about a tank with about 15 tons of oxygen. To bring it up you may need about 150 tons of fuel and a discardable rocket to bring the tank to orbit, and the last stage of the rocket and fairings would be another object added to the pile of orbital junk.

If the tank clears the scene it still would be discardable, for you'd need another tank to refill, so it becomes useless.

This is why I think a reusable long-duration-between-fuel-refilling orbital collector that remains in space would be better.

Due to the fuel efficiency that a variable Isp provides, I think using VASIMR could be a suitable propulsion system. Changing orbit inclination is what usually consumes more fuel, so fuel efficiency is very important to reduce the need of refilling costs using discardable rockets.

This is what wikipedia says:

http://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket

The ISS orbits at a relatively low altitude, so as to make it easily accessible from Earth. The downside of this, however, is that the ISS experiences fairly high levels of atmospheric drag, making periodic boosts of altitude necessary. Currently, altitude reboosting by chemical rockets fulfills this requirement. If the tests of VASIMR reboosting of the ISS goes according to plan, the increase in specific impulse could mean that the cost of fuel for altitude reboosting will be one-twentieth of the current $210 million annual cost.[4] Hydrogen is generated by the ISS as a by-product, which is currently vented into space.

So VASIMR collector could refill with ISS byproducts.

Even if an ion engine could be fuel efficient, you may need a certain charge/mass ratio and therefore you may need mercury, and for electrostatic ion thruster you need xenon. It is easier to get hydrogen than mercury or xenon as propellant in space.

So I am inclined to prefer VASIMR for the garbage collector in terms of profitability. I am trying to think in terms of today's realistic technologies. What do you think? Notice I am thinking about "profitable".