Is there any reason to not use nukes for interplanetary anything?

[tsotha]

Try as I might, I can only split the first 900m/s or so. If you can divvy the entire transfer into small chunks, I'd like to know how you do it.

Somebody on this forum (don't remember who, sorry) created a table which allows you to do bigger splits. The key is knowing when you have to start, since your orbits will start to take nontrivial amounts of time toward the end.

[maltesh]

The problem is that once you've put on about 900 m/s in LKO, you're on an escape trajectory from Kerbin, and thus, won't be coming back for another periapsis kick.

[Laie]

The problem is that once you've put on about 900 m/s in LKO, you're on an escape trajectory from Kerbin, and thus, won't be coming back for another periapsis kick.

Finally someone who knows his stuff. Tsotha, sit down, you got an F.

the people who never put more than like two NTRs on their interplanetary ships are causing themselves a lot of hassle for very minimal gain.

Yep. Case in point, my current vessel has six nervas and 1975m/s; reducing the number to three would increase that to 2044m/s. However, the six-nuke vessel takes me to Jool with a little to spare, while the three-nuke version ends up short despite it's higher delta-V. If I was just playing for myself I'd pack more fuel *and* nukes and be done with it. But I'm currently trying to make a new best score in Laythe Capitalism; three percent here or there really matter.

[-Velocity-]

Try as I might, I can only split the first 900m/s or so. If you can divvy the entire transfer into small chunks, I'd like to know how you do it.

Say I need 750 m/s to get to the Mun, and I want to split it into roughly two. I create a manuever node where I think I will make my final burn, and make it a 300 m/s-400 m/s maneuver. This creates a dotted line that I can make a SECOND manuever node on, that I will execute on the orbit AFTER I make the first maneuver, and this second node will also be placed at the same point (which will be periapsis of the orbit after the first maneuver has been done) as the first node. Now, on this second node, I create whatever delta-V maneuver is necessary to reach the Mun.

It's really simple, but it can be harder to time, because you're thinking two orbits ahead instead of just one. I'll make an example and show you a screenshot.

You can also think THREE or more orbits ahead and split your maneuver into three or more parts, which I have done on occasion for some ion-powered spacecraft with extremely low TWRs. Basically, it becomes simply a matter of thrusting at your periapsis and only at your periapsis over and over again, slowlying increasing your orbital eccentricity till your apoapsis is just outside the torroidal zone defined by the path of the Mun's SOI around Kerbin, then waiting for a random alignment with the Mun so that an additional small thrust at Kerbin periapsis will butt you into the Mun's SOI on an encounter with the characteristics you desire.

There is no loss of efficiency when doing this either.

Edited February 19, 2015 by |Velocity|

[Laie]

I think you mean something like this:

For my current Jool maneuver, that approach costs about 1120m/s compared to the nominal 1050m/s of a single maneuver -- but of course, trying to do that as a single node would be even lossier.

Edit: and before you complain about the lack of periapsis-kicking, that was an attempt to reach Eve early in the career. In that case, getting underway -NOW!- was cheaper than raising apo and starting later.

Edited February 19, 2015 by Laie

[-Velocity-]

Here- here's a three-part Hohman transfer to the Mun, broken down into 3 equal parts that are like 280 m/s each-

So, like I said, even with a very low TWR, you can still make many of your burns at the optimal point in your orbit.

[Laie]

Here- here's a three-part Hohman transfer to the Mun, broken down into 3 equal parts that are like 280 m/s each

That's ye goode olde periapsis kick. The question was what you do after the first 900ms.

[-Velocity-]

I think you mean something like this:

For my current Jool maneuver, that approach costs about 1120m/s compared to the nominal 1050m/s of a single maneuver -- but of course, trying to do that as a single node would be even lossier.

Edit: and before you complain about the lack of periapsis-kicking, that was an attempt to reach Eve early in the career. In that case, getting underway -NOW!- was cheaper than raising apo and starting later.

Incorrect, you did not interpret my post correctly. See my above screenshot.

One place I can think of where you CAN'T break down your maneuvers into separate parts for maximal efficiency is of course capture burns. I remember one time I was approaching Eeloo with a big nuclear-ion manned ship, and I had to start my orbital insertion maneuver before I even entered Eeloo's SOI, or I would have just flown right on past it. I think that the Dawn spacecraft may be doing something similar right now as it approaches Ceres, just based on how it's taking it over a month just to fly a distance less than that from the Earth to the Moon.

- - - Updated - - -

That's ye goode olde periapsis kick. The question was what you do after the first 900ms.

I was only considering the question of going from Kerbin to the Mun. So you mean doing your ENTIRE burn to some distant planet from low Kerbin orbit?! You need some kind of transfer window planner for that, if that would even work. Your orbital path around the Sun doesn't even appear to you for the first time until you're on a path to leave Kerbin's SOI (though I think MAYBE I've gotten it to appear for me before, but I don't know how), so I wouldn't have a clue how to do everything in just a single burn; it's impossible to target something beyond Kerbin if you don't even know where a maneuver is going to take you.

Edited February 19, 2015 by |Velocity|

[Laie]

I was only considering the question of going from Kerbin to the Mun.

Well, this thread deals with nukes and interplanetary. I'd say you're a few hundred m/s short.

So you mean doing your ENTIRE burn to some distant planet from low Kerbin orbit?! You need some kind of transfer window planner for that, if that would even work.

Not the entire burn. But as Maltesh said, you can only do periapsis-kicks for the first 900m/s of your transfer; in the case of Jool/Moho/Eeloo, you're left with well over 1000m/s that still need to be done. If there is a way of splitting that burn, I'd very much like to learn about it. My example of splitting the maneuver doesn't count: that's still a single burn, and lining up several maneuvers end-to-end is a lot of trouble. Whether that burn lasts five minutes, or fifteen, has a tremendous effect on your dV expenditure.

The latter is the point I've been going on about for the last two pages: trying to reduce weight by saving engines is a good idea, but only up to a point. If you take it too far, that long burn when leaving Kerbin will waste so much fuel that all weight savings are for nought, or even wasteful. TWR can be way too low, and this is not only a question of patience.

Edited February 19, 2015 by Laie

[Starhawk]

that approach costs about 1120m/s compared to the nominal 1050m/s of a single maneuver

I'm just doing another split-in-half (well, 900 and 1080) transfer burn to Jool myself.

It seems to me that the efficiency loss is because the orbit precesses a bit as you spend the seven Kerbin days to orbit, and then your periapsis is no longer quite at the optimal point for the next maneuver node for the next burn. I noticed I had to adjust the position a fair bit after coming back around. But the efficiency loss is fairly small if you plan for the length of time in orbit. Unless you're doing the Laythe Capitalism Challenge. Then it's a lot.

Happy landings!

[EDIT] I'm pretty darn sure there's no way to split that second maneuver up!

And yeah, if your burns take too long, you're doing too much of them away from the maneuver node/periapsis and you can lose lots that way.

Edited February 19, 2015 by Starhawk
clarity

[AlexinTokyo]

You can get the right final departure burn time of a Mangalyaan maneuver with a bit of calculation. Red Iron Crown (I think - too lazy to check) posted a spreadsheet in the tutorials section that does the calculations for you.

Of course, as has been pointed out, you can't split any further when the second-to-last burn takes you right to the edge of the SoI.

[Kryxal]

There's not too much calculation involved ... plot the first burn to determine the orbital period, re-plot for a full-up transfer burn that length of time into the future for actual node placement, make your initial burn at the place of the second node, at the time determined by the orbital period desired.

[Starhawk]

You can get the right final departure burn time of a Mangalyaan maneuver with a bit of calculation. Red Iron Crown (I think - too lazy to check) posted a spreadsheet in the tutorials section

Red Iron Crown's thread on it can be found here.

[tsotha]

Finally someone who knows his stuff. Tsotha, sit down, you got an F.

Yep. Case in point, my current vessel has six nervas and 1975m/s; reducing the number to three would increase that to 2044m/s. However, the six-nuke vessel takes me to Jool with a little to spare, while the three-nuke version ends up short despite it's higher delta-V. If I was just playing for myself I'd pack more fuel *and* nukes and be done with it. But I'm currently trying to make a new best score in Laythe Capitalism; three percent here or there really matter.

Ah, well. That's a good point, though I don't understand why you're asking questions if you already know everything.

[-Velocity-]

Well, this thread deals with nukes and interplanetary. I'd say you're a few hundred m/s short.

Not the entire burn. But as Maltesh said, you can only do periapsis-kicks for the first 900m/s of your transfer; in the case of Jool/Moho/Eeloo, you're left with well over 1000m/s that still need to be done. If there is a way of splitting that burn, I'd very much like to learn about it. My example of splitting the maneuver doesn't count: that's still a single burn, and lining up several maneuvers end-to-end is a lot of trouble. Whether that burn lasts five minutes, or fifteen, has a tremendous effect on your dV expenditure.

The latter is the point I've been going on about for the last two pages: trying to reduce weight by saving engines is a good idea, but only up to a point. If you take it too far, that long burn when leaving Kerbin will waste so much fuel that all weight savings are for nought, or even wasteful. TWR can be way too low, and this is not only a question of patience.

I'm confused, and not sure what you are referring to. Are you referring to the fact that if you complete your burn to a distant planet within Kerbin's SOI- the closer the better- you save delta-V from the Olberth effect? If THAT is what you are referring to, yea, you can't split that up, and the higher the TWR, the better. But I personally have trouble planning maneuvers to other planets when in another planet's SOI, because in my experience I can't see my path around the Sun until I've made a burn that puts me past escape velocity, so I'll always make a transfer burn to another planet after I leave Kerbin's SOI. Hmm... maybe if I plan a second burn it will allow me to see my solar orbit?

[Norcalplanner]

I don't know if anyone has mentioned it, but I'd like to plug NecroBones' MRS mod, simply because it contains the quad-nuke. It's a single 2.5m part with the weight and thrust of four LV-Ns, and works well as part of a main rocket stack. It really helps reduce part count, increase structural stability, and maintain proper aerodynamics.

[Laie]

because in my experience I can't see my path around the Sun until I've made a burn that puts me past escape velocity,

It should be sufficient to have a maneuver on the flightplan that would take you out of Kerbin SOI -- or at any rate, that's enough for me to see a projected orbit around the sun. If your projected orbit ends at "Kerbin Escape", you've misconfigured something.

[Superfluous J]

I'm confused, and not sure what you are referring to. Are you referring to the fact that if you complete your burn to a distant planet within Kerbin's SOI- the closer the better- you save delta-V from the Olberth effect? If THAT is what you are referring to, yea, you can't split that up, and the higher the TWR, the better. But I personally have trouble planning maneuvers to other planets when in another planet's SOI, because in my experience I can't see my path around the Sun until I've made a burn that puts me past escape velocity, so I'll always make a transfer burn to another planet after I leave Kerbin's SOI. Hmm... maybe if I plan a second burn it will allow me to see my solar orbit?

Nailing an encounter (or at least a very near encounter that will require minimal tweaking via a mid course correction) with another world via a maneuver node in LKO is an essential skill in the game. It's as hard - and as rewarding - as all the other essential skills like rendezvous, docking, landing, etc.

[Kibble]

I don't use nukes cause of suspension of disbelief

[Laie]

I don't use nukes cause of suspension of disbelief

They are/were a real thing, and even better than the ones we have in the game. The only reason for not using them is that people have qualms about launching them into space.

[tater]

I don't use nukes cause of suspension of disbelief

Because it's not yet the late 1950s?

[juanml82]

I'm just doing another split-in-half (well, 900 and 1080) transfer burn to Jool myself.

It seems to me that the efficiency loss is because the orbit precesses a bit as you spend the seven Kerbin days to orbit, and then your periapsis is no longer quite at the optimal point for the next maneuver node for the next burn. I noticed I had to adjust the position a fair bit after coming back around. But the efficiency loss is fairly small if you plan for the length of time in orbit. Unless you're doing the Laythe Capitalism Challenge. Then it's a lot.

Happy landings!

[EDIT] I'm pretty darn sure there's no way to split that second maneuver up!

And yeah, if your burns take too long, you're doing too much of them away from the maneuver node/periapsis and you can lose lots that way.

I don't split my burns more than some 800 dV - basically, my AP is roughly at the moon's orbit. That way, the orbital period is of about a day, so you don't miss the transfer window. I think the couple hundred dV saved in kicking the orbit until near the edge of Kerbin SOI could end up getting lost anyway because of a suboptimal departure burn.

That, and it becomes a bit tedious to keep kicking Pe, specially if I'm sending several ships to the same destination.

I'm confused, and not sure what you are referring to. Are you referring to the fact that if you complete your burn to a distant planet within Kerbin's SOI- the closer the better- you save delta-V from the Olberth effect? If THAT is what you are referring to, yea, you can't split that up, and the higher the TWR, the better. But I personally have trouble planning maneuvers to other planets when in another planet's SOI, because in my experience I can't see my path around the Sun until I've made a burn that puts me past escape velocity, so I'll always make a transfer burn to another planet after I leave Kerbin's SOI. Hmm... maybe if I plan a second burn it will allow me to see my solar orbit?

Go to your KSP install directory and look for a file called settings.cfg

Open it with notepad, look for a line called CONIC_PATCH_LIMIT = and make sure the value it's at least 3. Five or six is better.

[Laie]

I don't split my burns more than some 800 dV - basically, my AP is roughly at the moon's orbit. That way, the orbital period is of about a day, so you don't miss the transfer window.

Not missing the transfer window is merely a question of starting early enough -- up to 40-45 days in advance, if you're really really serious.

I think the couple hundred dV saved in kicking the orbit until near the edge of Kerbin SOI

More like a dozen, at best. Whether that's worth planning so far ahead is up to you.

CONIC_PATCH_LIMIT

Thanks, I forgot how it's called.

[Kibble]

The only reason for not using them is that people have qualms about launching them into space.

Exactly! Plus they have very few benefits over traditional chemical-thermal propulsion - low TWR, undense propellant if you use hydrogen, and if you don't use hydrogen the specific impulse drops rapidly to being not that much better than chemical-thermal. It doesn't make a good space tug either, cause hydrogen isn't storable. Plus you have to carry a massive shadow shield if the spacecraft is piloted, and keep everything within the cone it casts, including another spacecraft if you are trying to dock. Nuclear reactors are also massive, complex, messy plumbing nightmares.

[WuphonsReach]

I don't split my burns more than some 800 dV - basically, my AP is roughly at the moon's orbit. That way, the orbital period is of about a day, so you don't miss the transfer window. I think the couple hundred dV saved in kicking the orbit until near the edge of Kerbin SOI could end up getting lost anyway because of a suboptimal departure burn.

That's why I usually park things destined for other worlds at the 3000km altitude. Far enough out that your orbital period is measured in multiple hours, not tens of minutes. On most launches it is not hard to give a little extra dV in the circularization stage to kick the payload up to a 3000km orbit.

But I also use Remote Tech in place and have a flock of satellites at KEO (2869.75km). In case I forget to turn on my longer-range antennas before kicking the payload up to the 3Mm parking orbit, they will come within 500km of one of the KEO sats sooner or later.

The other factor is that I use KCT, which means I can only launch a payload every 3-24 hours (depending on mass of the previous launch). So by putting payloads for other worlds into a parking orbit ahead of time, they are properly configured, fueled and ready to go when the transfer window arrives.