Interplanetary Thrust?

[Glaran K'erman]

Hey guys, trying to figure out the best way to desing my first interplanetary station. Specifically, how worried should I be about my TWR? I can't decide between the trade-off of delta-v and twr, any help? Never done an interplanetary mission before, so I don't know what to expect. What kind of burn times am I looking at? What kind of time-frames should I expect between nodes? But first I don't even know if it'll move in any sort of useful way so let's take care of that first.

Stats as it would be in LKO now:

.72/2.72 TWR with 4779 DV

2144t

Used TweakScale by the way to make the dream of building a station capable of carrying out all science! Obviously this is maybe a bit on the "cheap" side but just trying to have some fun with this mission while I get my planet-traveling feet wet.

If you guys want screens or something else let me know, playing right now actually so Ill be popping between KSP and the boards.

Edited July 2, 2015 by Glaran K'erman

[YNM]

TWR isn't quite important once you're free from the surface. Quite important if you want to get things done fast though (this is one example how low TWR crafts works best, RL example. Notice the long time to compete those...).

[Grumman]

TWR isn't a big deal once you're in orbit. A TWR around 0.03 makes things tedious, but 0.72 would be fine as long as you're not planning on landing it anywhere. Just make sure that any landers that are piggy-backing a ride have sufficient TWR to get off whatever planet they'll be landing on.

[Randazzo]

Those TWR's are actually pretty high, and that's plenty enough for a trip to Duna and back. Once off the surface you can get away with as low of a TWR as you're willing to put up with. Aside from burn times your main consideration for low TWR's is whether or not you'll have enough time to complete your insertion burn to whatever your target is.

This thing had a TWR of .23 (max of .6something) and required two burns of ~7 minutes to escape Kerbin and an 8+ minute insertion burn.

Also, 2144 tons or 21.44 tons? If 2144, that's a pretty impressive TWR for it.

[Glaran K'erman]

Great! Thanks all and yes that is 2144, as in two thousand but was actually off by 200t! TweakScale is literally a dream machine and that TWR is totally owed to that. Again a bit "cheap", but hey, given enough time Kerbals would make everything huge right?

But even better I can now switch a lower TWR config I had before that put it at about .4 but gave me 6800 DV. Sure now I can play with that a bit to get it over 7k for sure.

[Gaarst]

As said above, TWR does not change much when you're in orbit, or in interplanetary space. The only notable consequence will be the burn time for manoeuvres. So do not worry too much about it (still avoid powering a 200t ship with a single ion engine)

What you want to look at is the delta-v. Try to get it as high as you can, but do not add too much mass to your ship or it will take you 10 flights to assemble it in orbit. Kerbin escape takes roughly 950 m/s delta-v from LKO and insertion burns from interplanetary space can go up to 3500 m/s (for Moho). You can get detailed delta-v information for interplanetary transfers on this map.

For burn times, it depends on your TWR. For example, with a ship having around 0.20 TWR, I did a 25 mins insertion burn at Moho (3600 m/s). If you're using ion propulsion, which is the most efficient, expect to have burn times approaching an hour unless you use dozens of ion engines on your ship.

Time frames between node (interplanetary travel times, if I understood well) can range from less than 100 days for Duna or Eve to a few years for Eeloo.

[Glaran K'erman]

Figured this being a post about thrust here is a look at the engines, big Poodles rock.

[Edax]

TWR isn't quite important once you're free from the surface. Quite important if you want to get things done fast though (this is one example how low TWR crafts works best, RL example. Notice the long time to compete those...).

Wont having low thrust on a heavy object in space make it far more difficult to accurately use maneuver nodes? Thrusting too long after the node could cause dramatic drift and cause orbital insertion burns to cause eccentric orbits thus wasting fuel, right?

[Wemb]

Wont having low thrust on a heavy object in space make it far more difficult to accurately use maneuver nodes? Thrusting too long after the node could cause dramatic drift and cause orbital insertion burns to cause eccentric orbits thus wasting fuel, right?

Yes, but that may not be a problem - a lower twr may well mean a lot more dV, which means you can afford to make small adjustments - and, of course, small engines are great for making small adjustments.

Also, if you're going to another planet, being very accurate when you're starting out from Kerbin isn't massively important - what is important is not missing the transfer window - so burns that take lots of time to complete, when you're heading toward something that's orbiting quickly (the inner-most planets) might be a problem. Doing a course-correction to a long burn when you're 6 months out isn't.

Wemb

[Ace_Warbringer]

Wont having low thrust on a heavy object in space make it far more difficult to accurately use maneuver nodes? Thrusting too long after the node could cause dramatic drift and cause orbital insertion burns to cause eccentric orbits thus wasting fuel, right?

make sure you are splitting the time for your burn around the maneuver node. If your burn is one full minute, begin your burn at 30 seconds before the maneuver.

this sorta balance's the error induced by the nodes assumption of a instant burn.

[ajburges]

You're right, long burns are not accurate to manuver nodes. Manuver nodes are point optimizations of burns. An accurate burn node would require calculus to integrate mass and velocity WRT time.

There are two camps to interplanetary nodes. Getting enough thrust for a single burn in an acceptable timeframe, or multiple burns for additive work over multiple orbits. Like so many things in orbital manuvers, it's a tradeoff of speed and efficiency.

If your stage has a Kerbin relative TWR .33 most interplanetary burns can be done within a quarter orbit. This offers a reasonable accuracy compromise. Even with a TWR that high, you can also make use of Pe kicks for more work efficiency. With enough planning, you can use a sublunar elliptical orbit to give a perfect departure time and angle.

Some people use Minmus as a way station for interplanetary flight. What it losses in efficiency, it makes up for in easy refuel, much more relaxed departure windows, and a down payment on most the dV required to leave Kerbin SoI.

In-fact, if you can't work out aerocapture logistics for a minimus mining operation to fuel an interstellar tug operation, it is more fuel efficient overall to perform interplanetary transfer from Minmus orbit.

[Rosco P. Coltrane]

Also, be careful with your Pe. If your orbit is too low, you risk entering Kerbin atmosphere because the way your orbit changes when you burn for a long time. The solution is burn less time in each pass of your apoapsis kicks... hence more passes... and it gets tedious pretty fast.

How low can you be depends of course, but anything below 100 km is a gamble, so I'd say expend some dV getting to 100.