Best place for ISRU in the Jool system? Is it Laythe?

[juanml82]

So, I'm running a Jool-5 and I have set up a karbonite refinery in Vall, but I wonder if that's really the best choice. Here are my toughts:

Scoop Karbonite from Jool's atmosphere: simpler, as the tanker doesn't need to land by the refinery, but refueling ships in the system must descend all the way to Jool or the tanker must go to them. My karbonite scooper is still on the way, so I haven't tested this.

Vall: I thought it was the best choice, as the lack of atmosphere should cut delta-v from take off. But it takes over 1 km/s to land there, so tankers landing and going up again would need to keep a lot of fuel for landing.

Bop & Tylo: nothing to say

Pol: the light gravity makes it great for landing and taking off, the problem is getting there and back

Now Laythe: I haven't tried (yet) to get karbonite from the atmosphere, but it may seem a better idea than Jool, as it's not that down the gravity well and incoming ships to refuel can aerobreak to save fuel.

And now that I think about it, a surface refinery could work - but the problem is the limited land on that moon. In theory, it takes more dV to reach orbit from the surface than from Vall. But the tanker could be an MK3 spaceplane, which uses the far more efficient air breathing engines to take off. And it would also need to keep far less fuel to deorbit. Did someone try that?

What do you people think?

[Kryxal]

The deorbit burn is going to be done with the tanker mostly empty, so delta-v is not such a large issue then (though you'd better know how much you need). Still, I'd rather use an orbital tanker than a lander, all things considered. This might be different for you if you're comfortable with precision landings or using mechjeb.

[juanml82]

The deorbit burn is going to be done with the tanker mostly empty, so delta-v is not such a large issue then (though you'd better know how much you need). Still, I'd rather use an orbital tanker than a lander, all things considered. This might be different for you if you're comfortable with precision landings or using mechjeb.

Yeah, orbital scoopers are better and save the tedium of landing, refueling, taking off, rv, landing again. But I personally feel them kind of cheating. It's sort of free fuel, so for gameplay value I usually try to limit to either surface miners or sending fuel from Kerbin

[Landge]

I haven't been able to build a viable large Mk 3 spaceplane. My Mk 2's capacity is too limited and would need to make several trips.

My personal favorite is Vall, if you are going for bulk surface drilling. Bop would probably be the easier of the two to land and takeoff. That is if you can fine a relatively flat drilling site. Utilizing both moons, ferrying fuel up to orbital fuel stations around each, you have travel to both the inner and outer moons covered.

Edited March 24, 2015 by Landge

[Norcalplanner]

I like Vall. It has lots of flat landing areas, is conveniently located, and refueling there definitely doesn't feel cheaty at all. Laythe would be my second choice, but I wouldn't do atmospheric scoops - I'd use a combination of parachutes, jets, and rockets to minimize the dV requirements, and would probably site things on that great bay that Brotoro first popularized. Tail-sitter SSTOs are a bit easier to land if you're nervous about your ability to land on unimproved ground.

[tsotha]

Now Laythe: I haven't tried (yet) to get karbonite from the atmosphere, but it may seem a better idea than Jool, as it's not that down the gravity well and incoming ships to refuel can aerobreak to save fuel.

You can use Laythe or Jool to aerobrake on your way to one of the other moons. And Tylo is always good for a gravity assist.

An airless, low gravity moon seems like a better choice to me, since otherwise you lose too much fuel getting back into orbit. Unless you were planning to use jet engines, in which case Laythe is your only choice.

[juanml82]

You can use Laythe or Jool to aerobrake on your way to one of the other moons. And Tylo is always good for a gravity assist.

An airless, low gravity moon seems like a better choice to me, since otherwise you lose too much fuel getting back into orbit. Unless you were planning to use jet engines, in which case Laythe is your only choice.

But, OTOH, you also save a lot of fuel by landing in a body with an atmosphere if your tanker is supposed to be reusable. It takes some 2400 m/s to take off (without jets) from Laythe, but it may take some 100 m/s to land there. Vall, OTOH, takes some 1,000-1,200 m/s to both take off and land.

Then again, that's the theory. Laythe is mostly ocean and the atmosphere makes pinpointing the landing harder. And while using jets is more efficient, getting a heavy MK3 tanker to Laythe seems like challenge on its own. Plus, I don't think there are a lot of smooth places to make a long landing run, so such a spaceplane should also need retrorockets to brake.

[Kryxal]

Retrorockets may not be needed ... so long as there's an engineer on board, parachutes could be used so long as you have the balance right.

[Maukse]

For me, Vall of course.

Remember, 1000 m/s dV on an empty lander, it's not that much fuel ! I do have an orbital station on Laythe with a lander, and a refueling tanker making roud trips from Vall surface to laythe orbit.

The most important is to manage efficiently your fully loaded tanker. My design is not satisfiying yet (lv-n based, high isp as vall has no atmospere, but yet inefficient low take off twr) but once you have the right manoeuvre for transfert (hello aerobraking !) you can ship some fuel !

A question : with a twr of 1.3 at take-off, is it worthy to add 48-s engines for a small twr boost ? Or it won't make up for the low isp ?

Edit: just saw that Slashy summed-up interesting works about landing/take-off Twr. No point trying to pump my 1.3 twr so my design might be not too bad.))

Edited March 25, 2015 by Maukse

[FleshJeb]

A question : with a twr of 1.3 at take-off, is it worthy to add 48-s engines for a small twr boost ? Or it won't make up for the low isp ?

Someone made a graph of TWR efficiency on airless worlds (probably travert). I think 2-3 was pretty efficient. At 1.3, you'll be losing quite a bit to gravity drag. If you're only using the 48-7s for the first 5-10 seconds, it's probably worth it.

Assuming 2 * 48-7S per 1 * LV-N, the combined ISP is 487.

[Maukse]

Someone made a graph of TWR efficiency on airless worlds (probably travert). I think 2-3 was pretty efficient. At 1.3, you'll be losing quite a bit to gravity drag. If you're only using the 48-7s for the first 5-10 seconds, it's probably worth it.

Assuming 2 * 48-7S per 1 * LV-N, the combined ISP is 487.

Well, 1.3 twr allows a 55degree angle, so with cos(55), I get something like 57% of my acceleration on the horizontal plan (so it's like a 43% loss to gravity drag)

I I add a 48-7s for each lvn, my twr goes to 1.85 (+50%)

Then, I can use a 33degree angle, with cos(33), I can get almost 84% of my acceleration on the horizontal plan.

so it's 0,57 vs (1,5)x0,84 ... => 0,57 vs 1,26 => 2,2 time more usefull thrust with the additionnals 48-7s

the joke is fuel consumption : 100% vs 214%

no clear winner here, especially if we consider the dead weight added on the return trip (mostly empty)

[FleshJeb]

Well, 1.3 twr allows a 55degree angle, so with cos(55), I get something like 57% of my acceleration on the horizontal plan (so it's like a 43% loss to gravity drag)

I I add a 48-7s for each lvn, my twr goes to 1.85 (+50%)

Then, I can use a 33degree angle, with cos(33), I can get almost 84% of my acceleration on the horizontal plan.

so it's 0,57 vs (1,5)x0,84 ... => 0,57 vs 1,26 => 2,2 time more usefull thrust with the additionnals 48-7s

the joke is fuel consumption : 100% vs 214%

no clear winner here, especially if we consider the dead weight added on the return trip (mostly empty)

That doesn't take into account the curvature on a small, low-grav world. Get up fast, turn off the 48-7Ss, and aim flat. A constant-altitude takeoff, if you will.

Found the study I was looking for: http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

As you can see, 1.3 TWR is pretty bad. 2.0 is definitely a sweet spot.

[magnemoe]

You can use Laythe or Jool to aerobrake on your way to one of the other moons. And Tylo is always good for a gravity assist.

An airless, low gravity moon seems like a better choice to me, since otherwise you lose too much fuel getting back into orbit. Unless you were planning to use jet engines, in which case Laythe is your only choice.

Yes, I prefer Bop, Pol works just as well but the flight time is longer, dV cost is pretty much the same. Laythe should also work however it require an cargo spaceplane who can land on uneven terrain. Anything can land on Bop or Pol. Spaceplanes don't use much fuel so you are probably better off refueling in orbit. Use the Laythe base as spare fuel storage.

[Red Iron Crown]

Assuming 2 * 48-7S per 1 * LV-N, the combined ISP is 487.

I get 550s for 2 48-7Ss and an LV-N in vacuum. How did you calculate it?

[Jouni]

Vall TWR 1.3 means that we have 20 tonnes of mass per LV-N. According to tavert's charts, we need 1000 m/s to take off, which means 2.39 tonnes of fuel.

If we add one 48-7S engine per LV-N, TWR goes up to 1.94, while delta-v to orbit goes down to 860 m/s. Fuel usage increases to 2.91 tonnes. If we shut down the 48-7S after 68 seconds and use only the LV-N for the remaining 111 seconds, we get 860 m/s of delta-v for the same 2.39 tonnes of fuel. The actual delta-v requirements are probably a bit higher, if we shut the 48-7S down early, but using it for the first 50 seconds or so will probably save us some fuel.

With two 48-7S engines, TWR becomes 2.57 and delta-v to orbit 840 m/s. The break-even point for fuel usage is now at 39 seconds, so we will probably save fuel, if we use the 48-7S engines for the first 30 seconds or less.

The estimates are based on the assumption that we aim for a 0 km equatorial orbit. In practice, we want to gain some altitude as well, so the numbers will be a bit different. Still, I'd guess that having a single 48-7S engine per LV-N and using it to boost the TWR for the first 40-50 seconds of the ascent could save us some fuel (around 0.5% of launch mass). With two 48-7S engines per LV-N, we would probably end up wasting fuel.

[FleshJeb]

I get 550s for 2 48-7Ss and an LV-N in vacuum. How did you calculate it?

60kN  +  2(30kN)
-------------------- = 487s
60kN      2(30kN)
------  + ---------
 800s       350s

My mass flow (denominator) calculates to the same volumetric flow as shown on the wiki.

Edited March 27, 2015 by FleshJeb
formatting

[Red Iron Crown]

Ye gods, I have an error in the spreadsheet I've been using, well, forever. Thanks for pointing that out, FleshJeb.