Sun contract

[blag]

Hi,

I'm seeking some help about a contract I accepted at the beginning of my career save.

I need to make a rescue mission of a spacecraft around Kerbol, but I'm having some hard time planning it.

Album gallery/vhCS2oj will appear when post is submitted

For what I've done now, I think I'll need at least 14k dV from LKO (2k to put my Pe around Kerbol, and twice 6k to circularize and get back to Kerbin). Will 14k dV be enough ?

I think I'll go with the 3x nuke one, but I find my spacecraft really big for what it''s intended to do. Is there a better way ?

 

Edited April 30, 2018 by blag

[Snark]

Moving to Gameplay Questions.

40 minutes ago, blag said:

For what I've done now, I think I'll need at least 14k dV from LKO (2k to put my Pe around Kerbol, and twice 6k to circularize and get back to Kerbin). Will 14k dV be enough ?

It's a little hard to tell for sure from the screenshot-- is the target's orbit circular, and pretty much zero inclination?

Assuming that it's in circular orbit at 2.4 million kilometers, then a bit of orbital math tells us this:

• Circular orbit speed at target orbit:  22060 m/s
• Pe speed for transfer ellipse:  28754 m/s

So in terms of matching speed with the target at Pe, looks like it's closer to 7K m/s than 6K.  So it would probably be best to allow 16K rather than 14K.

One bit of advice:  make sure you have plenty of deployable radiators on the ship.  You're getting within 2.4 million km of the sun-- it gets hot there.  The deployable radiators auto-rotate to keep themselves edge-on to the sun, and pull heat from your whole ship.  You'll want them.

Regarding your ship designs:

First, the ion ship is massive, massive overkill.  You don't even vaguely need that many solar panels.  Nor do you even vaguely need that much xenon.  Remember, the key to getting high dV is to keep the mass down, and you're lugging along far more than you need.

Also, bear in mind that a pretty good design principle to get high dV ships is to use multiple stages-- and put the higher-dV stages last.

For example, consider this little ship:

That's 11,600 m/s of dV right there, in a tidy little package that only costs 43K funds.  If you give it some droptanks like this:

...now you've got a ship with a total of 19, 964 m/s, for under 74K funds.  And it weighs just 3.3 tons, meaning it's easy to launch.  Just loft that to orbit, give it a decent LFO stage to give it a good hard shove to send it on its way, and Bob's your uncle. 

(Actually, now that I come to think of it, I realize I neglected to put a heatshield on it, which you'll definitely want for the return to Kerbin.  So that would add a bit of mass... but not much.  Also, I didn't put any parachutes on it; if you want to actually land the rescue ship itself, you'd want that, too.  There are a lot of ways to slice the design-- my point is, if you keep it super lightweight, you can get insane amounts of dV out of an ion engine and a modest amount of xenon.)

If you're wondering "why so few solar panels", remember that you'll be expending most of your dV down close to the sun, where the sunlight is seriously intense.  You don't need gobs of solar panels.  Even at Kerbin's distance from the sun, you only need about six of the regular folding solar panels to keep an ion engine running, if you have them oriented properly.

As for your LV-N design:  you could do that, too, if you want.  Main issues I see with that design:  you're lugging along too much dead weight.  You're using LFO tanks, even though nukes only need LF to run, meaning you're carrying double the mass of fuel tank as you need.  Plus you've got three LV-N engines instead of just one, which is a whole bunch of dead weight eating into your dV.  To build a high-dV nuclear-powered ship, my advice to you would be to use a single LV-N; use LF-only fuel tanks; and use droptanks (e.g. have radially attached fuel tanks that you jettison as you drain them).

As shown above, you can do this mission with a pretty small ion-powered ship.  An LV-N powered ship might look something more like this:

What you're looking at, here:  an LV-N with 4 tons of LF for the final stage.  It has six radial drop tanks, each containing 4 tons of LF.  It drains them in pairs, jettisoning each pair when that pair of tanks becomes empty.

That's 30K funds, 36 tons, has a total of 13800 m/s of dV.  A little shy of what you'd want for the full mission, but if you can stick it on top of a hefty LFO-powered stage (say, a Rhino with a 36-ton tank), that'd do the job, too.

[blag]

Thanks a lot for your full length  answer.

 

16k dV ? damn, that's worst that what I calculated... u_u

Totally forgot about the radiator, never get this close of Kerbol before (and if I can avoid cooking Lodlan as a crisp, that's better).

 

For the Ion design (and the nuke one too), I was trying to put it on track straight from LKO : this explain the 0.3 T/W ratio, the 9 engine and the omnidirectional x4 XL solar panel ( with a 100e/s output)

But you're right, a LF/LOx engine is a better choice for this 2k dV ejection shoot, and will allow me a smaller T/W (but then that mean a lot more tweaking to match the target speed, and kerbin one on the way back)

With staging, I get the following spacecraft, that's able to make the 2.5k dV burn for the 2.10^9 Pe and circularize (take a 3h burn but well, it's efficient XD)

 

 

 

[Snark]

20 hours ago, blag said:

With staging, I get the following spacecraft, that's able to make the 2.5k dV burn for the 2.10^9 Pe and circularize

Great, glad it's sorted out! 

A few miscellaneous suggestions:

• Is there any chance you could use the 1.25m probe core instead of a HECS?  Reason I ask:  the HECS has a low temperature tolerance, only 1200 K.  The 1.25m core can go up to 2000 K, quite a bit tougher.  Something to consider.
• You'll probably want to put a fairing around the upper ion-powered stage-- otherwise aero will clobber you during launch.
• Skipper may be overkill for a vessel of that size-- perhaps consider a Poodle instead?  Will boost your dV from that stage quite a bit (not only saves weight, but has a significantly higher Isp), while still getting comfortably >1g acceleration.
• Don't forget to reduce the ablator on the heat shield-- you don't need anywhere near a full load.  About 30-40% should be plenty.  Saves a fair bit of weight.
• You may want to consider moving those radiators-- putting them right next to the solar panels like that risks having the panels get shaded at an inconvenient time.  My guess is that you can probably make do with just a couple of radiators-- perhaps move them up to the sides of the probe core?

Also, when considering potential overheating issues:  by far the most explode-prone component in the system will be... the EVA kerbal.  Kerbals have a pretty low temperature tolerance, considerably lower even than science instruments.  I'm not sure how long an unprotected kerbal will be able to stand direct sunlight, that close to the sun.  Maybe they'll be fine.  If you do run into problems, though, two suggestions.  First, when preparing to transfer the kerbal across, try to park your ship as close and conveniently as you can, to minimize the length of time the kerbal needs to be exposed.  Second, you can help matters by positioning the rescue ship such that the kerbal is shaded as much as possible while transferring.  (Perhaps it won't be a problem anyway, just something to be aware of as a backup plan.)

Also, bear in mind that the target kerbal's derelict is going to start heating up the moment you get within physics range (2.3 km).  Not sure how big a problem it'll be-- perhaps okay, since crew pods tend to have a fairly high temperature tolerance.  Probably it'll be fine.  However, just in case... you might want to do a quicksave while you're still safely out of physics range.  That way, if overheating there does turn out to be a problem, you can revert, then try for a faster approach so that the target derelict doesn't have as long to heat up before you arrive.

20 hours ago, blag said:

take a 3h burn

Well, that's what physics warp is for.    Sure, 45 minutes is a long time to twiddle your thumbs even at 4x, but it's a heckuva lot better than 3 hours...