Jupiter Gravity Assist-Sun Oberth Effect Sedna Flyby

[fredinno]

I have a plan to create a probe to Sedna (I just came up with this off the top of my head).

The probe would carry Solar Panels and stirling RTGs, along with 5 DS4G Ion thrusters. The wet mass of the probe would be about 12.5T, and would carry a sunshield.

1. Send the probe to Jupiter with an SLS Block II directly. A Block I may also be used, albeit with an Earth Gravity Assist.

2. Use a Jupiter gravity assist to send the probe hurling towards the sun.

3. At Sun Perpasis of 8 Solar Radii, use the ion drives to send the spacecraft to solar escape velocity.

4. After initial burn, dispose of sunshield. Turn on 2 of the 5 ion drives to increase velcocity.

5. When the spacecraft goes past Jupiter Orbit for the 2nd time, dispose of the solar panels. Switch to RTG power.

6. When Xenon Fuel is used up, eject the Xenon Tank and Engines. This leaves about 5 T of probe mass.

7. Sedna Flyby. ( Yes, I am aware the flyby would be incredibly fast.)

How long would such a mission take to get to Sedna? I know that getting to Jupiter directly would take 2.48 years (and 4.79 years for the Earth gravity assist option.)

Would such a mission be able to be realistically meet Sedna by its perpasis in 2060?

[HoloYolo]

Sedna will probably be one of the last places humankind visits here. It will be like Pluto when it was first found. We know absolutely nothing about it except that it's there, it's red, and has a distorted orbit. Also, ejecting engines is a bad idea. What if it's flying towards an asteroid and we can't change direction? It would probably take 50-500 years to get there, and the probe will become obsolete. If were going to even dream about going there, we gonna need rockets that are fast, and comm networks all around the solar system to compensate for the days of delay from Earth. A sun-dive is a very high risk, especially at 8 radiuses from the Sun. That's gonna be a heavy shield, due to the fact that it probably has to be made outta lead. So maybe in 500 years, but not today or even 100 years from today.

[fredinno]

Sedna will probably be one of the last places humankind visits here. It will be like Pluto when it was first found. We know absolutely nothing about it except that it's there, it's red, and has a distorted orbit. Also, ejecting engines is a bad idea. What if it's flying towards an asteroid and we can't change direction? It would probably take 50-500 years to get there, and the probe will become obsolete. If were going to even dream about going there, we gonna need rockets that are fast, and comm networks all around the solar system to compensate for the days of delay from Earth. A sun-dive is a very high risk, especially at 8 radiuses from the Sun. That's gonna be a heavy shield, due to the fact that it probably has to be made outta lead. So maybe in 500 years, but not today or even 100 years from today.

Solar Probe +, being launched by NASA in 2018, is 0.5 T, and passes at 8.5 Solar radii. No heavy shield here.

Ejecting engines in which the fuel is depleted anyways is a bad idea?

[HoloYolo]

Solar Probe +, being launched by NASA in 2018, is 0.5 T, and passes at 8.5 Solar radii. No heavy shield here.

Ejecting engines in which the fuel is depleted anyways is a bad idea?

Well, I thought maybe right after the sun-burn is complete, that the collect ions and refuel. Also, it doesn't need to jettison them in the first place because it can't use them.

[Dkmdlb]

When Xenon Fuel is used up, eject the Xenon Tank and Engines. This leaves about 5 T of probe mass.

So after all the burns are done you're going to jettison the engines? What is this, Interstellar? Other than that, this seems like a good time.

[fredinno]

Ok, so just jettison the tanks. Good to know.

[Guest]

Sedna is almost twice the distance from the Sun as Pluto right now and using the trajectory you postulate a probe would easily take longer to get there than New Horizons took to get to Pluto, plus it takes Jupiter being in position which means a few potential years of waiting. Furthermore, the spacecraft doesn't exist. You're talking 5~10 years planning and 20+ years of mission time; all the while Sedna moves further away from us (although its 11,400 year orbit means it's moving pretty slow). Now, I'm all for making long-term plans (because it's something we humans kind of suck at) but that's pretty crazy, especially since the probe will likely be in hibernation for much of the trip and will have to be extremely robust to survive the environment for that long.

I'm sure it could be done but there are much more important, closer targets to consider before we decide to check out Sedna. In the timeframe above (and probably within the budget for such a mission) we could launch several large telescopes and put them into an array, which would be much better for science overall.

[NovaSilisko]

It would be much better to just design a lighter payload and launch it directly to Sedna using a Jupiter gravity assist. The longer you're in transit the longer you have for something to go wrong on the spacecraft. You also avoid the need for a heavy sun shield and all the thermal requirements of the spacecraft having to go so insanely close to the sun.

When it comes to spacecraft, simpler is generally better. And cheaper, too.

Edited June 15, 2015 by NovaSilisko

[cryogen]

Is it actually feasible to have SEP that close to the sun at all? I'm wondering how much mass is needed for thermal design, and what that does to the power / mass ratio.

[Streetwind]

This is a topic close to my heart

Also, huh. Looking at this, I was convinced this would never work. The DS4G has so much power draw and transforms almost all of it into Isp, not thrust, making it inconceivable that it could accelerate a 12.5 ton spaceship sufficiently during one short periapsis kick... right?

Except that the rocket equation says that burning just 100 kg of fuel provides over 1.5 km/s of dV to a 12.5 ton vessel, and the burn takes less than 43 hours with this 5-engine setup... With an apoapsis at Jupiter and a periapsis at 8 solar radii, how much dV do you even need to reach solar escape velocity? My amateur math says: about 804 m/s. So just a one-day burn will do it? With ion drives? Wait what, did I get my math wrong? Someone please check.

DS4G thrust = 2.5 N

DS4G Isp = 19,300 s

Isp defined as: takes this long to burn 1 kg propellant at 1 N thrust

---> five of them burn 5 * 2.5 kg every 19300 s

100 kg / 12.5 kg = 8

19300 s * 8 = 42.889 h

dV = 19300 s * 9.81 m/s² * ln(12500 kg/12400 kg) = 1520,75 m/s

solar radius = 700,000 km

aphelion: Ap = 800,000,000 km

perihelion: Pe = 5,600,000 km, but another solar radius must be added to convert height above surface to distance between orbit and primary, so:

mathematical perihelion: Pe = 6,300,000 km

eccentricity: E = (Ap - Pe) / (Ap + Pe) = 0.984373

semi-major axis: SMA = (Ap + Pe) / 2 = 403,150,000 km

Sun standard gravitational parameter: SGP = 132,712,440,018 km³/s²

speed at perihelion: SQRT[ ((1 + E) * SGP) / ((1 - E) * SMA) ] = 204.454 km/s

Sun escape at perihelion: SQRT[ (2 * SGP) / Pe ] = 205.258 km/s

Of course, for such a plan, the devil is in the details. Can you even drop your periapsis to 8 solar radii with a single Jupiter assist? That's a really crazy eccentricity that's required here, even for a craft that already has its periapsis at Earth. Remember, you can't run the ion drives out at Jupiter. They require so much power that they will be utterly useless outside Mercury's orbit, if solar panels are all there is to power them. All the RTGs ever launched to space by all nations combined won't run a single one at quarter thrust, much less all five at full. Carrying chemical propulsion assistance for the Jupiter maneuver will require more launch mass.

Next, you're asking for a spacecraft to continually produce and process one and a quarter megawatts of power while in direct proximity to the sun, for a minimum of one full day, with at least the solar panels in full exposure at all times. That strikes me as pretty difficult to keep things cool. In fact, if I had to guess, such a heat load might simply melt the spacecraft.

Then, how much time does it take? Flying out to Jupiter is a 5-year trip, dropping back down to sun periapsis probably takes another 6 at minimum. So that's 11+ years just to begin the burn towards Sedna. Which is more than twice as far away as Pluto is. So even if the probe could get the same speed as New Horizons, the current record holder for fastest spacecraft ever, it would still take another ~10 years to pass Pluto's orbit... and the second half of the trip would take longer. 35 to 40 years, all in all, probably. That's an insane amount of time to fly! The scientists who built the instruments will be retired by the time it arrives. To reduce the travel time, the question is: how much speed beyond New Horizons can the DS4G's impart on the craft while it recedes away from its source fo power, before their huge power draw makes them impossible to operate? And even then, getting below 25 years is probably unrealistic, considering the huge lead-up time to even reaching perihelion.

Finally, how to get the data back? New Horizons is already struggling with transmission speeds of less than 2 kB/s out at Pluto. And Sedna is over twice as far away, with an ultra-fast flyby only making it farther and farther by the minute.

So yeah, not entirely convinced by the technical feasibility, but I must admit that I was genuinely surprised how practical some of the less practical-looking parts turned out to be. Quite interesting!

EDIT: hilariously relevant here! https://xkcd.com/1244/

Edited June 16, 2015 by Streetwind