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Laythe Ascent Vehicles (post yours here and help me save Jebediah)


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Jebediah is currently stuck on Laythe with his team of 2 other Kerbals. I've sent a rescue mission but I underestimated the Delta V required to get into orbit. I F5'd and did a test ascent before getting the team on board. My vehicle was underpowered and ran about halfway short on fuel getting back into orbit.

I'm just wondering if you guys can post screenshots of what you've used to ascend from Laythe? I really don't want to send another mission out there only to run into the same problem again. I know I could do the math but I don't have the dedication or inclination to put myself through the barbaric practice of relearning high school algebra again.

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My first landings on Laythe used the one-kerbal ship below. Landing was by four parachutes, so used no fuel (other than the deorbit burn). The two side tanks were dropped during the boost back to orbit, which the central engine/tank finished. I also had it set up so that the capsule could separate and continue burning to orbit with the RCS alone, but that proved unnecessary.

t5Xsw.jpg

My current Laythe crew came down to the surface (and will return to orbit) using the reusable SSTO rocket below. It required using a little fuel to deorbit and to do a last second braking burn during landing (and the current landing legs will not be strong enough). It will return to the station in orbit when it's time for the crew to leave (where it can be refueled and reused to bring down more kerbals.

pZ18cqE.jpg

D2ruOBT.jpg

I also have a 2-kerbal SSTO Spaceplane en route to Laythe that may be used to transfer future crews instead.

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Nasty business, Laythe rescues. Nasty, nasty, nasty. Fortunately for you, I have just the ship.

laythe_rescue.jpg

This state-of-the-art rescue mission has everything you need to get down to Laythe's surface, pick them up, and get them safely back to Kerbin. The mission profile is as follows:

1. Get into Kerbin orbit; this should leave you with a full orange tank's worth of fuel in the base of the rocket.

2. Get out to Jool. This should leave you with close to half a tank left in the orange tank.

3. Aerobrake around Laythe and get into a stable orbit just above the atmosphere; 70KM or so is fine. This should be a mostly free action.

4. Disengage the lander from the mothership and send it down to rescue your Kerbals. Remember, parachutes are your friend!

5. Get back into space and re-dock with the mothership; don't forget to re-pack those parachutes!

6. Fly the lot of it back to Kerbin.

7. Leave the transfer stage in low Kerbin orbit, and parachute-land that sucker back on Kerbin.

EDIT: If you want to bypass the need to perform hard calculations, Kerbal Engineer Redux is definitely the mod for you. That, combined with a delta-V map, can save you a lot of trouble.

Edited by SkyRender
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Nice idea using it as a refuelable space elevator.

I had considered jets for it, but was unsure about the air intake requirements. What is the rule of thumb? About double what is necessary on Kerbin?

Actually, assuming you're using turbojets to reach orbital speeds (plus an ion engine or a weak rocket to push periapsis out of the atmosphere), you only need *half* as many intakes! The basic reason: air pressure doesn't really matter, but gravity matters a lot, because gravity defines how fast you need to go.

The full reasoning starts here: Let's say you build a spacecraft with one turbojet that reaches orbital velocity plus a tiny bit at 30 km on Kerbin while going full throttle, but would flame out a meter higher. So you're going 2368 m/s orbit speed, which is 2193 m/s surface speed assuming you're flying East (which you should be!). Once you get to that speed, you'll climb a tiny bit since you're going faster than orbital velocity; throttle back when you climb, to avoid flaming out. Throttling back means you lost thrust, but you also lost drag; if you work out the equations, the amount you need to throttle back is (almost) precisely equal to the amount that drag falls, so you can maintain orbital velocity plus a tiny bit, and keep rising all the way to the top of the atmosphere. Then a tiny burst from a rocket (or even an ion engine) will push you into a stable 70km circular orbit.

Now take that same spacecraft to Laythe. You are facing the same atmospheric pressure at 23 km on Laythe as at 30km on Kerbin: you'll be able to go Kerbin's orbital velocity there, and you'll flame out 80 cm higher (rather than 1m higher). But you don't need to go that fast, you only need to reach 1878 m/s surface speed to reach Laythe's orbital velocity, 86% as fast. Since drag is quadratic, that seems to mean you can throttle back by a quarter (26% to be precise). But now you have more air than you need. So remove a quarter of the intakes, right?

Actually, it gets better. There are two other effects to keep track of: most important, thrust depends on your speed, quite significantly. Let's say our spacecraft has a single turbojet. Full throttle at 2193 m/s, it delivers about 58 kN, which gets us to Kerbin orbital speed at 30 km. We can reduce that thrust by 26% to achieve Laythe orbital speed at 23 km, so we need 49 kN. Full throttle at 1878 m/s, the turbojet delivers 126 kN, so we can reduce throttle to 39%, and remove 60% of the airflow. The thrust numbers here are approximate; I'm not using exactly the same curve that the game uses, so throw in an error margin just to be sure (and you might be way overdoing it).

Lastly, airflow depends on your speed; it's linear in (600m/s + v) where v is your surface speed. At 1878 m/s, you get a bit over 88% of the airflow you'd get at 2193 m/s. So to remove 60% of the airflow, you slow down and reduce 54% of the intakes. Round that down to removing half the intakes just to be sure.

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Actually, assuming you're using turbojets to reach orbital speeds (plus an ion engine or a weak rocket to push periapsis out of the atmosphere), you only need *half* as many intakes! The basic reason: air pressure doesn't really matter, but gravity matters a lot, because gravity defines how fast you need to go.

The full reasoning starts here: Let's say you build a spacecraft with one turbojet that reaches orbital velocity plus a tiny bit at 30 km on Kerbin while going full throttle, but would flame out a meter higher. So you're going 2368 m/s orbit speed, which is 2193 m/s surface speed assuming you're flying East (which you should be!). Once you get to that speed, you'll climb a tiny bit since you're going faster than orbital velocity; throttle back when you climb, to avoid flaming out. Throttling back means you lost thrust, but you also lost drag; if you work out the equations, the amount you need to throttle back is (almost) precisely equal to the amount that drag falls, so you can maintain orbital velocity plus a tiny bit, and keep rising all the way to the top of the atmosphere. Then a tiny burst from a rocket (or even an ion engine) will push you into a stable 70km circular orbit.

Now take that same spacecraft to Laythe. You are facing the same atmospheric pressure at 23 km on Laythe as at 30km on Kerbin: you'll be able to go Kerbin's orbital velocity there, and you'll flame out 80 cm higher (rather than 1m higher). But you don't need to go that fast, you only need to reach 1878 m/s surface speed to reach Laythe's orbital velocity, 86% as fast. Since drag is quadratic, that seems to mean you can throttle back by a quarter (26% to be precise). But now you have more air than you need. So remove a quarter of the intakes, right?

Actually, it gets better. There are two other effects to keep track of: most important, thrust depends on your speed, quite significantly. Let's say our spacecraft has a single turbojet. Full throttle at 2193 m/s, it delivers about 58 kN, which gets us to Kerbin orbital speed at 30 km. We can reduce that thrust by 26% to achieve Laythe orbital speed at 23 km, so we need 49 kN. Full throttle at 1878 m/s, the turbojet delivers 126 kN, so we can reduce throttle to 39%, and remove 60% of the airflow. The thrust numbers here are approximate; I'm not using exactly the same curve that the game uses, so throw in an error margin just to be sure (and you might be way overdoing it).

Lastly, airflow depends on your speed; it's linear in (600m/s + v) where v is your surface speed. At 1878 m/s, you get a bit over 88% of the airflow you'd get at 2193 m/s. So to remove 60% of the airflow, you slow down and reduce 54% of the intakes. Round that down to removing half the intakes just to be sure.

Wow, this is one of the best things I've read on this board.

I do love it when someone is able to a take complex problems and digest them into an understandable format, that sir is a real skill.

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Thanks; I'm fascinated by the KSP model for jets for some reason. I had actually wondered about this very question, so your post finally made me get out the math while eating dinner. My Laythe single-stage plane I built before I understood how jets worked, and it had way, way too many intakes -- nearly 50 per jet, precisely because I was worried it wouldn't be enough at the destination. I could have saved a tonne on intake mass alone, it turns out, not to mention the resulting rocket fuel and wing mass savings.

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Hows about a 3 seater VTOL SSTO Spaceplane with a probe core!

Ok, maybe not the easiest to build, but easily the most suitable and fun way to collect yo kerbals. After tracking down mine in orbit around Laythe, I took it for a spin for some happy snaps.

11204232694_68e6d3f7a5_c.jpg

11204239214_78e431c6da_c.jpg

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Thanks for everyone's help.

I landed my guys back at Kerbin this morning. I ended up using a mk1-2 verticle lander with 4 jets and 2 rockets, engaged the rockets and ejected the jets when aero dropped to .05 and managed a 50k orbit with a thimble of fuel left. I had a big fuel deposit in orbit from my previous failed rescue, so rendezvoued that up and made my way home.

3r40.png

Edited by FlamedSteak
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