Best engines to get large payloads to Laythe.

[astroadrian99]

Hey guys. I've been playing KSP for a while now but I have never been able to get from to kerbin to Laythe with a stock rocket. I am trying to take all I need with me and I follow the moto "go big or go home". So I started out with a small lander for Laythe and one thing led to the other and now I have a huge rover that can cross laythes oceans, a ton of science instruments plus a six manned lander (in total, everything weighs 220tons). So I have been wondering, what is the best kind of engine that could give this ride to Laythe. I would also have to be able to bring back 90 tons to kerbin. I have a design that might work but it is so slow that the burn takes 2 hours and I still can't manage to get it in an orbit that's higher than 75 km. Can anyone help me? Is it even possible to do this? How much Delta-V would it require? How much fuel? Thanks for the help.

Edited May 11, 2014 by astroadrian99

[cantab]

Well nukes will need the least fuel, but you'll want a big cluster to get a reasonable TWR. That can send the part count up. An alternative is the KR-2L. You'll need a lot more fuel, but with one KR-2L giving as much thrust as 42 nukes you might be able to really bring the part count and burn times down.

[Superfluous J]

Yeah Nukes is almost always the way to go. If you've got KER or MechJeb installed, get your TWR around 1 for Kerbin and the burns shouldn't be that bad.

If you've not got those installed, just throw a bunch of engines at it and if it takes too long, revert the flight and throw more engines at it

[astroadrian99]

Well nukes will need the least fuel, but you'll want a big cluster to get a reasonable TWR. That can send the part count up. An alternative is the KR-2L. You'll need a lot more fuel, but with one KR-2L giving as much thrust as 42 nukes you might be able to really bring the part count and burn times down.

About how many nukes do you think I'll need, and how much fuel?

[Jouni]

You need roughly 2000 m/s to reach Laythe, 1500 m/s to return back to Kerbin, and 500 m/s for margin of error, assuming that you aerobrake at Laythe and Kerbin. Let's add another 500 m/s, if you're not comfortable with doing the return burn directly from a low Laythe orbit. Overall, you'll need at least 4500 m/s of delta-v, which is split quite evenly between the journey there and return.

The payload is 220 tonnes to Laythe and 90 tonnes back. Let's assume that you need another 20 tonnes for command pods, structural stuff, SAS modules, and RCS fuel. With nuclear engines, the 2250 m/s delta-v requirement for one-way trip means that you'll be burning about a quarter of the ship's starting mass.

Let's guess that the total mass of the ship is 480 tonnes. If you aim for initial TWR 0.2, you need 16 nuclear engines, or 36 tonnes of engine mass. This means that the ship will have 204 tonnes of fuel tanks, or 5.67 orange tanks. Those tanks will hold a bit over 180 tonnes of fuel. On the way to Laythe, you'll burn 120 tonnes of fuel, leaving you with 60 tonnes for the return trip. The initial mass of the ship for the return trip is 230 tonnes (it has lost 120 tonnes of fuel and 130 tonnes of payload), leaving you with about 2370 m/s of delta-v.

The margins are quite low for that design, so you may want to add more fuel and more engines just in case. As a rough guideline, each additional orange tank should come with two nuclear engines.

With initial TWR 0.2, the escape burn from Kerbin will take about 15 minutes. It's a bit too long for a low orbit, so I'd do from a higher orbit (let's say 600 km). If you can't launch your ship directly there, you'll have to refuel it before the start of the mission.

[SSSPutnik]

Have a look at the Jool 5 challenge thread here.

[astroadrian99]

You need roughly 2000 m/s to reach Laythe, 1500 m/s to return back to Kerbin, and 500 m/s for margin of error, assuming that you aerobrake at Laythe and Kerbin. Let's add another 500 m/s, if you're not comfortable with doing the return burn directly from a low Laythe orbit. Overall, you'll need at least 4500 m/s of delta-v, which is split quite evenly between the journey there and return.

The payload is 220 tonnes to Laythe and 90 tonnes back. Let's assume that you need another 20 tonnes for command pods, structural stuff, SAS modules, and RCS fuel. With nuclear engines, the 2250 m/s delta-v requirement for one-way trip means that you'll be burning about a quarter of the ship's starting mass.

Let's guess that the total mass of the ship is 480 tonnes. If you aim for initial TWR 0.2, you need 16 nuclear engines, or 36 tonnes of engine mass. This means that the ship will have 204 tonnes of fuel tanks, or 5.67 orange tanks. Those tanks will hold a bit over 180 tonnes of fuel. On the way to Laythe, you'll burn 120 tonnes of fuel, leaving you with 60 tonnes for the return trip. The initial mass of the ship for the return trip is 230 tonnes (it has lost 120 tonnes of fuel and 130 tonnes of payload), leaving you with about 2370 m/s of delta-v.

The margins are quite low for that design, so you may want to add more fuel and more engines just in case. As a rough guideline, each additional orange tank should come with two nuclear engines.

With initial TWR 0.2, the escape burn from Kerbin will take about 15 minutes. It's a bit too long for a low orbit, so I'd do from a higher orbit (let's say 600 km). If you can't launch your ship directly there, you'll have to refuel it before the start of the mission.

Really!? That's all the delta-v required. I honestly thought it was about 6500 m/s to get to Laythe one way. I never would have thought it was a total of 4500 m/s. I'll later try to build that tug, after school.

[SSSPutnik]

4500dV is pushing it and is ideal amount. You will need more for course adjustment too within Jool system I think.

Edited May 6, 2014 by SSSPutnik

[Jouni]

The ideal amount is less than 3500 m/s. Jool intercept is around 2000 m/s from LKO, and if you aim directly at Laythe, you'll need only a little bit more to circularize after aerobraking. Some people have managed to do the return burn from low Laythe orbit to Kerbin for less than 1100 m/s.

[astroadrian99]

4500dV is pushing it and is ideal amount.

Ok. I'll have to try it then. Hate I have school and can't play it now but oh well.

[astroadrian99]

The ideal amount is less than 3500 m/s. Jool intercept is around 2000 m/s from LKO, and if you aim directly at Laythe, you'll need only a little bit more to circularize after aerobraking. Some people have managed to do the return burn from low Laythe orbit to Kerbin for less than 1100 m/s.

Shoot if I would have known. I have been calculating by using a delta v map thing and it kept telling me I needed about 6500 m/s one way.

[SSSPutnik]

He is going to Laythe, so once you have Jool intercept you will need to adjust course for Laythe after entering Jool SoI. Then aerobrake at Laythe and circularize.

Burning direct depart from Kerbin low orbit is tricky but doable.

[Kasuha]

Shoot if I would have known. I have been calculating by using a delta v map thing and it kept telling me I needed about 6500 m/s one way.

Delta-v maps are misleading, don't trust them too much. All you need to travel to Laythe in terms of dv is to get Jool intercept - the rest is freefall and aerobraking, without spending any fuel except for corrections. Of course, you need to adjust your entry so you really fall on Laythe, or get some other means of slowing down (Jool aerobraking, Laythe or Tylo gravity slingshot that slows you down etc).

6500 m/s it is including Kerbin liftoff, I guess.

Similarly when returning, all you need is Kerbin intercept. Except if you are using deadly reentry and need powered braking instead of aerobraking.

[astroadrian99]

Delta-v maps are misleading, don't trust them too much. All you need to travel to Laythe in terms of dv is to get Jool intercept - the rest is freefall and aerobraking, without spending any fuel except for corrections. Of course, you need to adjust your entry so you really fall on Laythe, or get some other means of slowing down (Jool aerobraking, Laythe or Tylo gravity slingshot that slows you down etc).

6500 m/s it is including Kerbin liftoff, I guess.

Similarly when returning, all you need is Kerbin intercept. Except if you are using deadly reentry and need powered braking instead of aerobraking.

Oh ok, now I get it

[cantab]

This is my usual basic delta-V map, but it's not very good for the Joolian system since it assumes you're going via low Jool orbit, which is silly unless you want to go to low Jool orbit for the science.

http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png

These ones on reddit are more comprehensive, if a bit trickier to use. For interplanetary travel you'll typically need to use multiple maps, "linking" them on the transfer orbits. So say you want to go from Ike to Gilly, follow the Duna map from Ike to a Duna/Ike-Eve transfer, then the Eve map from the Eve-Dunar transfer to Gilly. I honestly didn't believe it when they told me you'd need just 1100 m/s to get from Laythe orbit to Kerbin, but as per above comments apparently that's right.

http://www.reddit.com/r/KerbalAcademy/comments/1qu5jv/deltav_charts/

And remember for any delta-V map you may be able to provide delta-V by aerobraking which means you don't need that figure from your engines. This is typically indicated by an arrow pointing in the direction aerobraking is possible.

[Soda Popinski]

Depending on your piloting skills, you may want to have a bit more than 4500 m/s. In my own ongoing Jool mission (link in my sig), I used about 3600 m/s just to get to Vall. In my own defense, I suck. Plus my build took about 25 minutes of thrust broken into 3 legs to leave Kerbin orbit, so there was some inefficiency there (used 2215 m/s out of expected 1963 m/s). I play with Deadly Re-entry, so I couldn't aerobrake very hard (9 aerobrake orbits through Jool).

[iDan122]

I just use the big and humongous new engines to get there, and from 2-4 nukes to get back

Like I did here:

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