How do you interplanetary travels?

[KingPhantom]

i have a question. i play ksp since over 200h... but i don't know how to do correct interplanetary travels...

probably i have to high targets:

i want

- all the science

- with return

- probably heavy payload

- burntimes under 6mins

- single launch from kerbin

whats the corrct TWR?

yesterday i sent a probe lander with rover to dres. i used a orange tank and four of the second smallest small-tanks on top of 4 LV-N engines. i had a TWR of 0.44 and 7000m/s deltaV... it will be a rough trip back. i have 2800m/s and what the lander will bring back from the surfe left for the return... but i forgot the science jr. so i havent the big payload...

second case:

for a manned jool mission i created a craft that pushes a interplanetary stage and a manned lander with rover and 10'000m/s (main stage) and 1000m/s lander into LKO... but the TWR is 0.11m/s and thats rally bad with the burntimes...

how do you fix that? i want a craft with 10'000m/s and a good twr that i dont have to wait for transfer windows... that should be able... i dont have any troubles to reach duna or eve with that. but the other planets are the problem...

important note: i dont land this crafts back on kerbin. they remain in LKO for refuel.

[psyper]

I'm sure people will post more specific details but as a general idea you'll want to take a lot of dv with you - I would have one ship which has multiple orange tanks (or even the new larger ones) and radial mounted nukes - 4 would probably cut down your twr so you're not taking a day and a half to do a burn maneuver. The main ship will need a transmitter maybe if you're planning to transmit the 100% sciences and a research lab (if i remember their name correctly this early in the morning) to reset your experiments. You'll need an all purpose craft which is capable of landing and launching from the different bodies you'll be visiting. I would have a mix of engines - nukes for low gravity to save on the fuel ratios and powerful engines for helping getting off high gravity wells - the powerful engines can then be toggled in an action group - I find they are also handy if you mess up an approach and heading too fast in to a moon! The lander will have at least two sets of instruments so you can at least get two good readings in different biomes before having to redock with the lab. Also perhaps look in to getting a resource mod such as Kerbonite or Kethane so you can top up your fuel supplies along the way.

[Pecan]

Make it huge, Huge, HUGE.

Single launch interplanetary is always fairly huge.

If you want a heavy payload it's just going to get worse. Huge.

Trying to push all that stuff with a high TWR will take Huge engines.

It's going to need masses of fuel if you don't go during a transfer window, so you're making it harder for yourself. Make it HUGE for all the extra fuel you'll need.

TL;DR - you're trying to do everything the hardest way so the ship will have to be HUGE.

TWR is best 'as low as you can go' but, yeah, I wouldn't have the patience for something that's 0.11 either (not m/s, it's just a fraction). I can cope with 0.2 but usually aim for 0.5. Let that determine how much you can take in a single launch and for best results go during a transfer window - that's what they are for!

[KingPhantom]

the launch is not the problem. i have a really big lifter with upgrade possitilities. he pushes the one monster i already have in a 75km LKO

i think i have to go bigger... i use 6 LV-N and will try to use 8... i have some ideas for the beast... here is an early picture of the craft...

but that means to me that my way i go is the right... huge and powerfull

i use big tanks and small tanks radial to it...

so i can put the payload on top and on the bottom.

[Wanderfound]

Much easier way: first, launch a refuelling station. Just lift the biggest fuel tank you can, with a probe core, battery, solar panels and docking ports on it.

Then, lift your interplanetary vehicle.

Rendezvous and dock with the fuel station, refuel, go.

LV-N's make it easier, but not if you want short burns. An LV-909 or Poodle will do it much quicker, and has sufficient fuel efficiency if they don't have to deal with the Kerbin gravity well (i.e. orbital refuelling).

It can even be done with super-high TWR sporty stuff, but that's for after you've got the basics sorted. Do it easy before you try it hard.

[Laie]

Generally, if you're using highly efficient LV-Ns, you're looking at a TWR of no more than 0.5 even for a relatively short-ranged ship. If you want high delta-V, you'll end up with TWRs of about 0.25-0.15. That's the starting TWR -- by the time you're underway to your destination, you've probably shed 20-30% of your weight and TWR has improved accordingly.

If you want a 2000m/s burn to be done in less than six minutes, you need a starting TWR of at least 0.65 -- that's a ship of little fuel and many engines, it may get there but not back.

You'll have to tackle this just like a launch from Kerbin: use staging, dropping empty tanks (and possibly engines as well) as you go along. This will be big and expensive; assembling it won't be easy and getting it into orbit will take many tries. I don't say that it can't be done, but it will probably take you several evenings, and you may find that the burn still lasts twenty minutes (real-world time) because of lag. Going with a (relatively) smaller and simpler ship and just sitting through a fifteen-minute burn will be much easier and you can be on your way tonight.

But for what it's worth, here's my solution. I admit that it was not worth the hassle.

[KingPhantom]

great. thanks for all the respones...

just some addings from my side: i have 200h KSP so i think i can do the advanced stuff .

and the only mod i use is MECHJEB. no other mod needed. i dont want to use kethan or how this fuel-engineering-mods are named... they dont fit the flow of KSP (in my eyes).

its good to know to what i have to aim. i will try to get this 0.5 TWR and will check if i can use 1 or 2 poodles as a kicker from LKO...

i saw on different mun and minmus missions that the poodle is not that bad in efficient. he doesn't need much fuel...

LV909 are my first choice in landers for every body (except gilly. there i use RCS)

[Wanderfound]

just some addings from my side: i have 200h KSP so i think i can do the advanced stuff .

Well, if you want to do it in style: SSTO spaceplane, Aerospike-driven TWR > 2, VTOL landing on Duna.

Be sure to post the screenshots once you're done.

Edited September 1, 2014 by Wanderfound

[psyper]

If you want to bend your rules for a single ship to the destination a little you could send a refueller ship first which can then leave at the destination and drain it when returning back to kerbin.

[KingPhantom]

Well, if you want to do it in style: SSTO spaceplane, Aerospike-driven TWR > 2, VTOL landing on Duna.

Be sure to post the screenshots once you're done.

SSTO Spaceplane is a thing i failed in every attemp... in EVERY... i dont know how you guys become that into orbit. everytime i have a spaceplan with a little bit of payload (science) in LKO i'm out of fuel :D

i will wait to 0.25 until i retry a spaceplane... until then i will try it with rockts. i'm more the rocket scientist (UT3 )

[cantab]

i have a question. i play ksp since over 200h... but i don't know how to do correct interplanetary travels...

probably i have to high targets:

i want

- all the science

- with return

- probably heavy payload

- burntimes under 6mins

- single launch from kerbin

whats the corrct TWR?

All reasonable goals except maybe the burn time.

2500 m/s should suffice for the ejection burn to anywhere provided you use sensible transfers. Don't need to be perfect just don't be silly. You'd need a TWR of around 0.7 for that in 6 minutes, which is rather high. A TWR of 0.5 is more sensible and will still give you an ejection burn of 9 minutes at worst.

for a manned jool mission i created a craft that pushes a interplanetary stage and a manned lander with rover and 10'000m/s (main stage) and 1000m/s lander into LKO... but the TWR is 0.11m/s and thats rally bad with the burntimes...

how do you fix that? i want a craft with 10'000m/s and a good twr that i dont have to wait for transfer windows... that should be able... i dont have any troubles to reach duna or eve with that. but the other planets are the problem...

10 km/s is overkill for Jool. You can aerocapture both ways, so 2000-2500 m/s out is enough, and you can get home with 1000-1500 m/s so long as you're sensible. Add a bit more for getting around the system of course.

If you must have that much delta-V, you'll need to consider multiple transfer stages. As you've found, in a single stage the TWR plummets.

And you don't necessarily have to wait for the exact transfer window, but try and go at a sensible time. There will be dates when it's really bad to attempt to go to a certain planet.

In terms of engines, if you're sending big things and want quick burns, use the KR-2L. The Isp's nearly as good as the best chemical engines and the TWR is massive.

[Laie]

i will try to get this 0.5 TWR and will check if i can use 1 or 2 poodles as a kicker from LKO...

For a kicker, the KR-2L is unbeatable. About 12 times a poodles thrust at twice the weight, and ISP is pretty good as well. Allows you to use big (and hence few) parts and make simple designs. That thing above is ~45 parts, and half of that is because I wanted it to be reusable (which also included a 15-minute burn on the four Nervas to bring it back to Kerbin -- so much for saving time).

Your interplanetary mission is probably already big in part count (mine sure are) and a 200-part kicker would only bog things down. What good is a three minute burn if it takes ten minutes real time to play out?

[KingPhantom]

ISP in two sentences?

it's the fuel need in vaacuum, correct?

the KR-2L is efficient?... is it possible to use only 1 KR-2L for the whole travel? so i don't need the LV-N?... hmmm... i just thinking about to build a interplanetary stage with the 1-2 biggest tanks and a KR-2L...

i only used LV-N for interplanetary because i read that they are the efficient and best way... but with the KR-2L is a lot of fun on the travel . but it needs too much fuel, isn't it? (as the LV-N, or the poodle)

edit: the highter the Isp(vac) the lesser the fuel usage?

Edited September 1, 2014 by KingPhantom

[Red Iron Crown]

edit: the highter the Isp(vac) the lesser the fuel usage?

Sort of. Isp is essentially thrust per unit of fuel, so for a given level of thrust a higher Isp yields lower fuel usage. It is possible, though, to have a high Isp engine consume more fuel than a lower Isp one, if its thrust is high enough. For example an LV-N will consume fuel faster than an LV-1 Ant engine even though it has much higher Isp (the LV-N delivers more acceleration per unit of fuel though).

LV-Ns are the most efficient orbital rocket engines, but their low thrust can make them a pain to use sometimes as the burn times get long pretty easily. The KR-2L has a great TWR and decent Isp, it's a good alternative if your ship is big enough to justify all that thrust.

[KingPhantom]

so a tons of fuel with deltaV (a real overkiller) will be the best to push it with a KR-2L through space .

thx for your help. i have to open my mind and use all the liquid engines...

the lv-n has a ISP of 800... its not reachable for everything else... the KR-2L is not way back with 380.... and it has a tons of thrust. so it dont use that more fuel as the LV-N... i will test it with a jool-5-probe.

@ aerobraking:

i'm extremly bad at aerobreaking. i have to check some videos about it. i don't know the ideal height i have to use for that. and its too risky. in the jool system i break at an orbit like Pol's to go through the moons from the outside... same @ eve. i break way above the planet and orbit in a really hight altitude.

[Red Iron Crown]

Switching to any of the other rocket engines from the LV-N will halve the available delta-V from a given amount of fuel, all other things being equal. Or put another way, any other rocket engine will use more than twice the fuel of the LV-N to complete the same maneuver. It's a trade-off for sure.

You can take some of the guesswork out of aerobraking with this online tool: KSP Aerobraking Calculator Better to err on the side of caution; you can always do another aerobraking pass if it doesn't lower your AP enough, but an accidental descent caused by aerobraking too deep can be fatal.

[Laie]

@ aerobraking:

i'm extremly bad at aerobreaking.

The only mod you use is mechjeb, right? Try it's landing helper: it has an option to show aerobrake nodes.

IMPORTANT: turn off / remove that aerobrake node before you allow mechjeb to perform the next maneuver. I don't know why, but it goes totally bonkers when such a node is anywhere on the flight plan.

[KingPhantom]

Switching to any of the other rocket engines from the LV-N will halve the available delta-V from a given amount of fuel, all other things being equal. Or put another way, any other rocket engine will use more than twice the fuel of the LV-N to complete the same maneuver. It's a trade-off for sure.

thats because the ISP, correct? LV-N has 800 and the second places have 390... it will use twice or more of fuel...

[Wanderfound]

thats because the ISP, correct? LV-N has 800 and the second places have 390... it will use twice or more of fuel...

Yup.

I'm sure that regex will drop by to scream about this shortly, but in current KSP:

Thrust: how much grunt does it have?

ISP: how thirsty is it?

ÃŽâ€V: given how thirsty it is, how fat your spacecraft is and how much fuel you're carrying: how much total push do you have?

[KingPhantom]

and the KR-2L isn't that thirsty as the other engines (except the LV-N)... hmmm thats really interesting. thank you all for that help! was important for me.

i'm happy to try it with other engines. i hope it will work ... the KR-2L looks really nice and more "realistic" like the one the apollo command module used (thats the real world model for it).

[Red Iron Crown]

Yup.

I'm sure that regex will drop by to scream about this shortly, but in current KSP:

Thrust: how much grunt does it have?

ISP: how thirsty is it?

ÃŽâ€V: given how thirsty it is, how fat your spacecraft is and how much fuel you're carrying: how much total push do you have?

Those things would all still be true if thrust scaled with changing Isp rather than fuel consumption scaling with changing Isp. That has to be one of the simplest realism-improving changes that could be made to the game.

[Wanderfound]

and the KR-2L isn't that thirsty as the other engines (except the LV-N)... hmmm thats really interesting. thank you all for that help! was important for me.

i'm happy to try it with other engines. i hope it will work ... the KR-2L looks really nice and more "realistic" like the one the apollo command module used (thats the real world model for it).

The downsides with the KR-2L are that it's big, it's heavy, it's expensive and (excuse the blasphemy) sometimes it has too much thrust.

That doesn't mean that you can't have some fun with them, though:

[Red Iron Crown]

it has too much thrust.

Words do not make sense together.

[cantab]

Switching to any of the other rocket engines from the LV-N will halve the available delta-V from a given amount of fuel, all other things being equal.

Of course the crucial factor is that not all else is equal. Chances are you've saved some dry mass. So even though the ship using less efficient engines will need more fuel as a percentage of its mass, it may not be that much heavier overall or could even be lighter.

By way of example a while back I did a couple of Duna ship sketches. Pretty simple jobbies with a three-man orbiter powered by either an LV-N or a pair of 48-7S's and a common two-man lander. The nuclear ship massed 16.7 tons while the 48-7S ship was 18.6, so an Isp less than half as good resulted in a mass increase of a mere 11%. And that's at a TWR of just .3 or so where the LV-N should be really "in its zone", if I wanted more TWR the 48-7S would have given the lighter ship.

PS: The stock KR-2L is lolhuge for a service module engine on an Apollo-style ship. The real SPS engine was a version of the AJ-10. Size-wise comparable to a Mainsail or Skipper, but much much lighter (most of the size was just a big bell for efficiency in vacuum) and with a thrust of about 90 kN, about the same as a pair of LV-909's.

Edited September 1, 2014 by cantab

[Laie]

it has too much thrust.

Words do not make sense together.

Seen that kicker stage two pages back? Some of its predecessors were torn apart by KR-2L power. The one in the picture could stand it... barely. Limited them to 80% anyway, as it makes no sense if 2/3rds of your thrust goes off in a 30 degree angle.