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To Spaceplane, Or To Not Spaceplane


Scarecrow71
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I am in the plannng phase for my Jool-5 run as part of the Kerpollo Challenge, and I know what I want to build as the mothership.  What I intend on doing is having 6 completely expendable landers, one for each moon and then one to put in Jool's upper atmosphere to collect science with.  I am sure I could save some weight by using a single lander, but I gotta have all that fuel anyhow...and I want the Level Three nod here (a different Kerbal landing on every moon).

So, the question here is what type of lander should I use?  Should I go traditional, with a can or one of the Munar modules?  Or do I use spaceplanes to land and return?  Are there advantages or disadvantages to using either?

I've seen a lot of people use a Spaceplane as the mothership with landers in the cargo bay, but I really want to do something different than that.

Any thoughts or ideas?

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3 hours ago, Zacspace said:

You don't need high TWR in orbit. Am I wrong to assume you're using mechjeb to plot and pilot your interplanetary transfers? I know it just slams you into the atmosphere if your burn takes too long because it tries to do it all in one go. Consider either doing the transfer yourself, splitting it up into multiple burns, or starting from a higher orbit so that you'll have enough time with smaller/fewer engines. The mass of extra engines (and their fuel) adds up quick.

I do use MechJeb to plot burns...but I always have to do correction burns because MJ tends to plot and not burn correctly.  And while I can do correction burns, I'm just not good enough yet to do the primary plot/burn outside Kerbin's SOI.

As far as starting orbit, I was told 100km was the right height.  Is that nor correct?

3 hours ago, Zacspace said:

- I saw your Eeloo encounter was very energetic it probably took a lot more dV to capture into Eeloo orbit than you expected. Try and keep your encounter with another planet close to your solar apoapsis and you won't need to overbuild so much (saving you mass). It's hard to get a direct Eeloo encounter in the first place, so I understand, but you pay for it in the end. For Jool specifically, if you get a nice, clean encounter you can gravity capture into orbit of one of the main 3 moons using a Tylo encounter for just a few hundred m/s (or none at all if you're very patient). It saves thousands of m/s over doing a capture burn into Jool orbit and then transferring to a moon.

Can you explain how to do this properly?  All the videos I see show correction burns to get a Pe close to what you want your orbit to be.  As far as capture from solar Ap...?  I'm not sure what you mean here?

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9 minutes ago, Scarecrow71 said:

As far as starting orbit, I was told 100km was the right height.  Is that nor correct?

Theoretically, a lower starting orbit is always more efficient due to the Oberth effect. In practice, long burn times introduce inefficiencies due to steering losses (the maneuver requiring you to point away from prograde, basically) that can pretty easily outweigh the gains from starting lower, and that's before you even consider what you're leaving on the table by bringing more rockets than you need. There's not really a "best" altitude. There's probably a most efficient altitude for doing a one-shot transfer in any given craft, but I truly have no idea how you'd find out what it is besides trial and error. If you need more time, you need more altitude.

 

32 minutes ago, Scarecrow71 said:

Can you explain how to do this properly?  All the videos I see show correction burns to get a Pe close to what you want your orbit to be.  As far as capture from solar Ap...?  I'm not sure what you mean here?

Yeah, so your trajectory inside your target's SOI should be where you want your orbit to be, just like you say. But between leaving Kerbin orbit and arriving at your destination, you're in solar orbit. What your orbit around the sun looks like, and where along your solar orbit your encounter takes place, determines how fast you're going to be going when you enter your target's SOI. You want to be going slow, both so that there's less velocity to burn away, but also so that you're more influenced by the planet's gravity, which makes it easier to slow down. To do that you want to basically make sure your orbit around the sun and the planet's orbit around the sun don't cross at a large angle. Ideally you want to encounter your target near the tip of your orbit, like if you were docking with it. Like I said, it's hard to do it with Eeloo, but you should have an easier time tuning your Jool encounter because Jool has a huge SOI and long transfer windows. Once mechjeb makes your maneuver node, try wiggling it around and adding/removing a little velocity to get it just right.

As far as doing the capture into Jool with gravity assists goes, I do have a picture of that

Spoiler

nn9xMnd.png

if you fiddle around enough with prograde/retrograde and radial in/out while plotting your course correction, you can fine tune exactly when you to get to Jool to make sure the moons are in a position you want them to be when you get there. since the moons orbit in resonance, you should be able to copy this exact trajectory if you wanted, but the important part is meeting Tylo in pretty much that part of its orbit around Jool, where it's going the same direction you are. That's going to give you the biggest slow down. because it'll give you the most time inside Tylo's SOI, being influenced by its gravity. Here, all I did was encounter Tylo and it threw me directly into a Jool orbit for free, and one that encounters Vall even, which is where I was going. I made sure the Vall encounter was "low energy" in the way I described above (the Vall encounter is really close to my Jool Pe), and it only cost me 400m/s to get from a Jool flyby into low Vall orbit.

Obviously we're getting a little advanced. You don't need to be super precise like I was here to capture with Tylo, but it helps. It's also possible to use Laythe's atmosphere to aerobrake if you bring a big enough heat shield. It's easier than aerobraking in Jool's atmosphere. Either way should save you a lot of dV and let you make a much smaller rocket.

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And I'm still running into the problem of not being able to put enough engines on this thing to get it off the ground.  No matter how many tanks and engines I put on it, I cannot get enough TWR to get off the launchpad.  What am I doing wrong here?

f4Pt5gK.png

I try using the Kerbodyne S4-512 tanks with any combination of Mastodons and Mammoths, and with asparagus staging the best I can get is a TWR of 0.78.  I have tried every configuration I can possibly think of with the liftoff stage, and I cannot get off Kerbin.  What am I doing wrong?

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Well, I got off the ground.  This game really goes backwards in that bigger is not better, which goes against everything we ever learn in games.  Levelling up and getting better equipment should mean that the bigger stuff does more and works better...but this is not the case.  I scaled down the tanks and was able to get off the ground.  Unfortunately, I still can't do a Jool-5 in one launch; I run out of fuel.  I think I've reached the limit of what I am capable of in this game.

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On 11/28/2021 at 8:03 AM, Scarecrow71 said:

Well, I got off the ground.  This game really goes backwards in that bigger is not better, which goes against everything we ever learn in games.  Levelling up and getting better equipment should mean that the bigger stuff does more and works better...but this is not the case.  I scaled down the tanks and was able to get off the ground.  Unfortunately, I still can't do a Jool-5 in one launch; I run out of fuel.  I think I've reached the limit of what I am capable of in this game.

Try a center core of 3x the longest 3.75m tanks with a Mammoth. Then 6x the same boosters around the outside, with fuel lines into the center. Add SRBs to the outer ring to improve the initial TWR. 6-12 Kickbacks will do it.

I've done a lot of heavy lifting with that combo. I just did the math, and it should easily get 442 tons to orbit. That might actually be too much, so you can maybe go with 2.5 of the longest tanks, and/or swap the center engine for a Rhino.

I recommend keeping the trajectory vertical until the SRBs burn out so that they stage cleanly, and limiting the acceleration to 30 m/s, because the TWR gets insane.

If you run out of vertical space in the VAB, you can grab the whole craft and shift it up and down to clip through the floor or ceiling.

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