Are super long burn times managable?

[cephalo]

So I have been working the last few weeks on a Jool 5 challenge mission, in an attempt to gather all science in the Jool system in one mission. I cobbled together all the separate parts on a transfer stage that has six nukes. It's a huge ship, and its TWR is infinitesimal. The burn time for my Jool transfer ended up being almost 6 hours, and I'm not sure how to approach that!

Is it possible to break up a burn time like that; is it just a matter of laboriously babysitting 50 or so partial burns? And if not, is this something that can be fixed by slapping on more nukes, or do I really need more powerful engines? Nukes seem to be the most efficient, dv wise, but maybe when a ship gets too big they become unworkable? What is the maximum burn time that I should consider tolerable?

Edited September 14, 2015 by cephalo

[Empiro]

I don't know how big your ship is, but if you're actually trying to visit every single biome in Jool, you should be using drills to get the fuel you need. You shouldn't need quite as large of a craft to do this.

There's no way to break up such a long burn. The total delta-V to Jool is about 2000 m/s. The escape velocity for Kerbin is about 950. No matter how you break up your burns, after you get to escape velocity, you'll never come around again for another chance. You'd need to complete your burn in solar orbit, which means that you're spending more fuel to get Jool, but it should still be possible if you're patient.

[Foxster]

Alt> will help a little and there are some warp mods out there too.

I'd really think about some re-fuelling along the way though.

[Kyrt Malthorn]

If you aren't aware, you can access physics timewarp (the 2x-x4 range available in atmo where you can still use engines) by holding Alt+timewarp. Very handy I find for long burns necessary for something like a medium sized ion powered probe. A burn that normally takes 20 minutes, only have to sit through 5 min on 4x speed.

However... physics warp wrecks havoc with large fragile craft and the game's performance behaves similarly to a boulder dropped in the ocean. I was a little concerned watching one of my poor CPU cores spike for that long. Oh, I can't wait for 1.1 multi-threaded physics...

I kinda gave up on massive ships because I was sick of sitting through hours of burning. The game started to feel remarkably like work at that point. I decided these were my options:

1. Get MechJeb to do it for me.

This isn't my thing.

2. Get kOS and script the burns to be made automatically.

I'd feel better using this than MJ. I'm pretty comfortable with scripting usually, but I'm not ready to dive into that mod just yet.

3. Make smaller things. Fly with better TWR.

This was my choice.

[Snark]

Yes, nuke burns are long, but 6 hours seems pretty extreme-- what sort of dV are you talking about, and what ship mass, and how many nukes?

Assuming that the dV isn't something insane, then the only way you'd have a burn time that long would be if your TWR is really ridiculous, like a single nuke pushing a gargantuan ship. I'd suggest adding some more nukes; a low TWR is good, but that seems to be taking it to extremes, and you can afford a little more mass for some engines.

The main limitation on really long burns is your patience, and (to a lesser extent) trying to maximize Oberth effect-- you'd ideally like to do your whole burn when you're at very low Kerbin orbit, which is impractical when your burn is hours long. If you can get your burn down to an hour or less, then you can get at least some Oberth advantage by making multiple passes-- i.e. do a burn of a few minutes to raise your apoapsis, then coast around, do another burn to raise it again, possibly a 3rd or 4th time, so that you're always doing your burns at low periapsis and getting maximum Oberth advantage. Then on your last pass you do a final big burn at periapsis to send your ship on its way.

Depending on how your numbers work out, the benefit you get from better utilization of Oberth may more than offset the price you pay (i.e. extra mass) for adding a few extra nukes to the ship-- could end up being a net dV win, not to mention a lot less tedium.

[Vegetal]

6 hours with 6 nukes? I can only imagine the gargantuan size of your ship bro.

The only solution I think I have is a lot of staging. Doing this in a single stage is probably insane. I'm thinking about staging DURING the burn actually...

[fireblade274]

I remember seeing a graph somewhere saying that above 1300t, ion engines become more efficient fuel wise then nuclear. That doesn't mean its practical though. my largest ship has 4 nukes and has a burn time to jool of about 45 min, so your craft must indeed be massive; probably too massive. You should definitely consider making a smaller ship and even then if your trying to visit all jool moons you WILL need a drill to refuel on each landing. You should post a picture as im quite interested in how large this is

[wasml]

I've been watching this thread with interest (http://forum.kerbalspaceprogram.com/threads/133735). Will require some MM patches to add to the engines I want to use (other than ion) but should be worth it - I get impatient even with 15 min burns.

[panzer1b]

Personally i like to have a minimum of 2 KN of thrust for every ton of vessel. This means a nuke for every 30 tons. That said, i like to have ~3kN/ton, its more pleasant and most vessels that im not trying to single stage to half the planets in the jool system tends to have (when i either refuel in LKO, or just do not NEED to have 7.3K dV with a SSTO fighter).

Finally, if you have excessively bad TWR, there is no option but to do a very inefficient burn. Ive learned from experience that once you rop below a certain value, the paper dV becomes meaningless since you get nothing out of the oberth effect anymore. You can actually attain more effective range when you have a higher thrust even if that higher thrust ship has less total dV in LKO then math would tell. Its really a balance you need to go for, too much TWR and you loose dV cause you are dragging useless weight, too low TWR and you end up dragging orbits out so much that you loose any benefit from the oberth effect.

[Kryxal]

I made a ship with an ion engine and a capsule, and it didn't need that long a burn to go meet something below Moho's orbit ... I think you need a better TWR. 6 hours= 21600 seconds, maybe 3000 m/s for transfer, your TWR must be under .02!

[cephalo]

My miscalculation comes from the fact that I've built the ship in pieces. I didn't realize my TWR would be so low because I didn't do the math. I just made a bunch of pie in the sky components for each mission. I can't do drilling because that's against the rules for the Jool 5 challenge (There is a subcategory for that though).

My components are all in orbit and all transferable, so I can probably make a new transfer stage and move everything over. I wonder if I can just use Rhinos instead of Nukes. I'll try everything.

[I_Killed_Jeb]

You'll be surprised how reasonably small the drop in DV is when using the Rhino in a big enough craft

[cantab]

To make a single ejection burn you need the length of that burn to be small compared to the period of your orbit, so that you're burning close to prograde. So if your burn will be long you'll need to make your orbital period long, by raising your orbit. If your burn will be six hours then you'll need to first get to somewhere well above geostationary orbit, but probably below the orbit of the Mun. You can raise your orbit with low TWR by constantly burning prograde and therefore spiralling outwards.

Departing from such a high altitude is of course inefficient, but as a general rule low-TWR ships have the delta-V to spare.

Now that we have hold manoeuvre SAS, long burns on a well designed ship are hassle free. Start them going then do something else while the burn happens. I flew a proof-of-concept mission from LKO to Munar orbit with a 32-ton ship pushed by a single ion engine delivering a Kerbin TWR of 0.01. More practically my Moho mission wasn't quite so bad but was still using long burns.

[Geher]

I had the same problem as you. That's when I stopped using Nukes and started using the "Rhino". It has 2 000.00 kN thrust and a relatively high ISP of 340s. (Nukes have 60.00 kN thrust and 800 s ISP). It makes the ship much bigger (and the launcher even more). But as long as your launcher doesn't get so big that your PC starts to melt and has a high "second to frame"-rate, you're really saving time.

Also, as some said before: try to stage some tanks during the burn. I generally use an extra large tank in the middle and lot's of small tanks around that which get asparagus staged.

Edited September 14, 2015 by Geher

[technion]

In a similar vein to the Rhino discussions, I started strapping an orange tank with a skipper on each side of my Jool rockets. Everyone at the time made a point of why you should be using Nukes, but damn, the thing is that the d-v you lose trying to make massive corrective burns due to the inaccuracy of your ejection always ended up cancelling out the benefit.

One LV-N on one tank - that's what I used for the second half of the mission, once the Tylo and Laythe landers are gone.

[Kryxal]

If you like cleaning up your mess via impact, you can always attach probe cores and solar panels to the LFO boosters. A bit left over translates into FAR more delta-v once you drop the boosters, and you can plow them into Jool if you managed the whole way, or orbit back to Kerbin in solar orbit, or if you have enough thrust stop short of SoI change...

[gavman420SSJ]

I remember seeing a graph somewhere saying that above 1300t, ion engines become more efficient fuel wise then nuclear. That doesn't mean its practical though. my largest ship has 4 nukes and has a burn time to jool of about 45 min, so your craft must indeed be massive; probably too massive. You should definitely consider making a smaller ship and even then if your trying to visit all jool moons you WILL need a drill to refuel on each landing. You should post a picture as im quite interested in how large this is

not necessarily, i did it in an earlier version with no refuel. iirc i used all nukes and detached some as i went to keep twr around .33