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Fraktal

Getting a probe to the sun

Question

I've recently begun considering launching a flyby probe to the sun, making a quick pass just below 16 km to grab science, then transmitting it back to Kerbin once at a safe distance. However, I'm struggling with actually finding a way to get there.

The intended payload:

  • 1 x HECS core
  • 1 x thermometer
  • 1 x barometer
  • 3 x Z-200 battery
  • 2 x DTS-1 antenna (Antenna Helper says this gives 18% coverage at Moho for DSN level 2, so I'm hoping it's just enough to reach the sun)
  • 2 x Solar panel 1x6 (retractable)
  • 4 x Thermal Control System (small)
  • 1 x Heat shield (1.25m) at the very front, to be pointed radial-in during final approach

Strapping this payload to an FL-T800 tank with a Spark gives about 4100 m/s; however, the maneuver node says (and the community dV map concurs) I need nearly double that amount to reach the target periapse. I am not trying to achieve orbit, just a close flyby. How do I get it, without nukes or ions?

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27 minutes ago, Fraktal said:

I've recently begun considering launching a flyby probe to the sun, making a quick pass just below 16 km to grab science, then transmitting it back to Kerbin once at a safe distance. However, I'm struggling with actually finding a way to get there.

The intended payload:

  • 1 x HECS core
  • 1 x thermometer
  • 1 x barometer
  • 3 x Z-200 battery
  • 2 x DTS-1 antenna (Antenna Helper says this gives 18% coverage at Moho for DSN level 2, so I'm hoping it's just enough to reach the sun)
  • 2 x Solar panel 1x6 (retractable)
  • 4 x Thermal Control System (small)
  • 1 x Heat shield (1.25m) at the very front, to be pointed radial-in during final approach

Strapping this payload to an FL-T800 tank with a Spark gives about 4100 m/s; however, the maneuver node says (and the community dV map concurs) I need nearly double that amount to reach the target periapse. I am not trying to achieve orbit, just a close flyby. How do I get it, without nukes or ions?

Note, you can't "flyby the sun". Well you can but it takes a bit more dV than orbiting it. You START in orbit of it, as soon as you leave Kerbin.

The easiest way to get a close flyby, orbital-mechanic speaking, is to go up to Jool and use it to slingshot yourself as backward as you can do it. you may even be able to use Tylo to go even further. then rocket yourself the rest of the way if necessary. Shouldn't take 2x the dV of a Jool transfer burn I think, though that's 100% a gut feeling.

Good luck keeping your probe from overheating :D

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1 hour ago, Fraktal said:

making a quick pass just below 16 km to grab science

A quick pass it will indeed be, at 16 km....

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3 hours ago, Gargamel said:

A quick pass it will indeed be, at 16 km....

And warm! I did it with a xenon powered probe, and one of the solar panels blew up from overheating.

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I did a close flyby of Laythe once and ended up going Sun-retrograde, so it should be possible ... it's probably just going to be a matter of setting up the right flyby.

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You can also do it with ion engines and a really, really long burn.  For best results, put a nuke below your ion stages.

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A gravity assist from Jool is probably the most efficient way to lower your Perikerbol (Perikelion?) if you're not worried about time, but I generally just go direct with excessive delta-v and a clean Kerbin-retrograde ejection. You don't need to get it that low just to skim the low orbit region for a science pass.

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Low orbit is one thing, it's KER that says 16 km is high atmosphere altitude.

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I think that near space is just below 1000km, no need to go so close. For that I retrograding you need a bit more than 5000 dV.

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Got my facts wrong. Wiki says 18 km is low atmosphere altitude. High is 600 km.

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Posted (edited)
3 hours ago, Fraktal said:

Low orbit is one thing, it's KER that says 16 km is high atmosphere altitude.

Spoiler

Then KER is lying.

High atmosphere on the Sun is 600km, the ambient temperature is 4000K, and there is an actual atmosphere. Slamming into it at 86000m/s is guaranteed to destroy your craft, without cheats.

LOW atmosphere ("flying") is around 16km. Pressure is .25 kPa.

Ninja'd by the OP. :D

 

Edited by bewing

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I've never made it that close to the sun, but a tip I'll give you is to use repeated mun flybys to exit kerbin for about 770m/s (perfect run) and then burn so your aphelion is about Duna height and then retro from there. For me it cut down dv cost to 4.8k due to sloppiness in retroburning. 

If you are seriously considering going that close to the sun, the ablative shield isn't going to help that much, a fairing and an (uninflated, yes this sounds strange but it works) inflatable shield might do better, not to mention solar array are done for pretty far up from 16km, so your best power option might just be RTG.

As for comms, if you have any strong relays like the RA-50, one of those at Eve or Moho should help dramatically.

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On 3/19/2019 at 6:50 PM, Fraktal said:

Strapping this payload to an FL-T800 tank with a Spark gives about 4100 m/s; however, the maneuver node says (and the community dV map concurs) I need nearly double that amount to reach the target periapse. I am not trying to achieve orbit, just a close flyby. How do I get it, without nukes or ions?

Forget Hohmann transfers.  You'll want a bi-elliptic transfer, instead.  Bi-elliptic transfers are more efficient than Hohmann transfers whenever the origin and destination orbit radii are different by a factor greater than 12.  The drawback is that the manoeuvre suffers from vastly increased flight time.  Given that you want a perihelion of sixteen kilometres, you certainly qualify for the proper radius difference.  The transfer works by first raising your apoapsis (out past Eeloo for the greatest gains) and then taking advantage of the increased orbital distance to get a cheap transfer to a low solar pass.  It gets more efficient the farther you raise the apoapsis, though of course you will eventually reach escape velocity, so there is an upper limit.

Keep in mind two things:  first, the sun has an atmosphere, as others have mentioned, and as you come in from higher and higher apoapses, your periapsis speed will also be higher.  You may not be able to survive unless you first circularise outside the sun's atmosphere and then drop in, which may require a redesign--it's something to remember because you need not only to survive to get down there, but also to get back out again.

Second, atmospheric science from the sun isn't that valuable.  Go ahead and get it if you're a completionist--I am, so I'm sympathetic to your mission--but I would be remiss if I didn't mention the realities of what you will get for your efforts.  The science multiplier for low space science at the sun is 11--the second-highest in the stock game for space science--and for overall science, Eeloo and the Jool system are the only places with better multipliers.  The multiplier for atmospheric science at the sun (high and low) is 1--the second-lowest in the stock game for atmospheric science, and for overall science, only Kerbin itself offers less of a return.

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8 hours ago, Zhetaan said:

Second, atmospheric science from the sun isn't that valuable.  Go ahead and get it if you're a completionist--I am, so I'm sympathetic to your mission--but I would be remiss if I didn't mention the realities of what you will get for your efforts.  The science multiplier for low space science at the sun is 11--the second-highest in the stock game for space science--and for overall science, Eeloo and the Jool system are the only places with better multipliers.  The multiplier for atmospheric science at the sun (high and low) is 1--the second-lowest in the stock game for atmospheric science, and for overall science, only Kerbin itself offers less of a return.

I see. I think I'll have to make due with low space science for now.

It's less about being a completionist and more about the fact that in this save, I'm playing with 10% science gains for the purposes of developing functional designs with parts from as few tech tree nodes as possible. I've already wrung Kerbin, the Mun and Minmus completely dry, have a rover on its way to Ike and am waiting for a transfer window to Gilly, as I would like the Mainsail for launching a manned mission to Duna but need about 81 science to unlock it. Hence the solar mission.

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