How many Nukes?

[HeliosPh0enix]

I’m working on a Jool 5 mission, but am having a hard time designing the mothership that will ferry all of my landers to, around, and back from the Jool system. I am trying to use nuclear engines, but I’m having a hard time balancing the dV and TWR. I’m using the big Mk3 liquid fuel sections, and I don’t know how many nukes I need. Too many, and I have a usable TWR but not enough dV. Too little, and I have a decent dV but not nearly a large enough TWR to actually execute burns on time. Does anyone know of a good ratio of nukes to mass/amount of liquid fuel to maximize efficiency with a decent TWR? I have about a 50t payload for reference. 

[AeroGav]

1 hour ago, HeliosPh0enix said:

I’m working on a Jool 5 mission, but am having a hard time designing the mothership that will ferry all of my landers to, around, and back from the Jool system. I am trying to use nuclear engines, but I’m having a hard time balancing the dV and TWR. I’m using the big Mk3 liquid fuel sections, and I don’t know how many nukes I need. Too many, and I have a usable TWR but not enough dV. Too little, and I have a decent dV but not nearly a large enough TWR to actually execute burns on time. Does anyone know of a good ratio of nukes to mass/amount of liquid fuel to maximize efficiency with a decent TWR? I have about a 50t payload for reference. 

I build a lot of liquid fuel only SSTO space planes and they generally have 0.4 to 0.5 to 1 TWR in orbit, after the fuel used getting to orbit has burned off.

 

A 2k DV burn to Duna or similar can take 8 minutes according to the maneuver node planner,  so i start my burn 4 minutes early.        As a result you do half of your burn before the maneuver node and half after it , so overall the burn happens "on time" more or  less, and the craft takes the expected trajectory.

Of course,   if your burn times are long this can get in the way of making your burn close to PE to maximise Oberth effect.

So it's generally better to do split burns -  make a 900 m/s burn a few orbits before the ejection maneuver at PE,   and do the rest of the burn at PE when the transfer window comes up - a much smaller burn is needed to put you on the interplanetary trajectory, since your AP is already on the verge of going out the system.

For a single mk3 liquid fuel fuselage (long)   I would use between four and six nukes,

 

Edited May 6, 2018 by AeroGav

[StrandedonEarth]

For my current career, I've mostly eschewed nukes except using a single LV-N for interplanetary probes or 1-seater ships. For everything else, I'm using Rhino-powered transfer stages, often refueling them in orbit after using some of their propellants for the orbit injection stage. Then my departure from LKO (about 240km orbit) is in the 2-3 minute range.

One option would be to use a Rhino for the LKO ejection stage, and then nukes for the Jool system maneuvers

[GoSlash27]

You should shoot for 0.5G minimum acceleration. In order to answer your question, I need to know the DV required for the trip.

A single nuke can accelerate:

1t of payload 7100 m/sec 
2t of payload 5800 m/sec
5t of payload 2800 m/sec.
 

Best,
-Slashy

Edited May 7, 2018 by GoSlash27

[blakemw]

I usually use 1 or 2, with 4x physics warp to speed things up. If I need more thrust than that I use an earlier stage with a LF/Ox engine to provide that deltaV. In particular nukes aren't fun for the ejection burn from Kerbin, the rocket equation still makes it easy to deliver LF/Ox to LKO, so just use LF/Ox. The further you've had to deliver fuel (in terms of deltaV), the more valuable a high ISP becomes.

As a general rule, you want at least 15 minutes burn time out of a nuke - equivalent to 4 Mk1 Fuel Fuselages per nuke. I normally go with about 6 Mk1 Fuel Fuselages per nuke. If the burn time is shorter than 10 minutes you're probably better off just using an efficient LF/Ox engine.  When playing Career, a LV-N costs 10000 which will buy nearly 4 Kickbacks, so the cost of a LV-N will fund a significant amount of lifting power, if you look into it, then a multiple-LV-N solution is often beat by removing all the LV-Ns except one, and using those funds to upgrade the lower stages.

Edited May 7, 2018 by blakemw

[Lego_Prodigy]

I can show you my latest design of vertically launched spaceplane, which uses nukes for engines

Its intended for inter planetary travel, namely to duna. but can also land on airless moons vertically.

[Laie]

20 hours ago, HeliosPh0enix said:

Too many, and I have a usable TWR but not enough dV. Too little, and I have a decent dV but not nearly a large enough TWR to actually execute burns on time.

That's the kind of question where there is no single right answer. One can make do with a pretty low TWR if one is willing to plot many more maneuvers, quite apart from the patience of sitting through long burns.

I don't know which tricks you know, if you plan for gravity assists or whatever. I don't have the time to spell out all the possibilities, either. BUT: If you do the ordinary periapsis-kicking, where you raise AP over several turns, and finish it off with a single departure burn, I recommend a TWR of no less than 0.2 at the beginning of said burn. Better 0.3.

I'd still recommend to "split" that last burn -- put the last 200m/s or so in a separate maneuver, and leave a gap of a minute or two between the main burn and the final one. You will probably need to adjust it a bit in the time between the burns, though the magnitude should stay about the same. That last maneuver will probably sit 1/3rd of the way to Munar orbit and can be done at much lower TWR, say, 0.15 or even less. In your case, that might resolve to a single nuke. That's fine -- it will suffice for the remainder of the mission.

You may still want to check how much bigger your mission needs to be if you want to get it under way on (say) a Skipper or even a Rhino, which solves a lot of complications in a single stroke. With a 50t payload and a nuke for Joolian maneuvering, I suspect that a conventional booster to get it underway won't need to be all that large.

[GoSlash27]

Laie,

Actually, there *is* a single right answer, but you have to know what your minimum acceptable acceleration, payload, and DV are. 

 Given those, you can calculate precisely how many engines and fuel tanks are required.

Best,

-Slashy

[Not Sure]

“Unusable TWR” 

No such thing! Just gotta learn how to do split burns. 

But a twr of 0.3 will get a Kerbin escape in a single burn without losing too much efficiency.

[lugge]

Well, I'm having the same problem at the moment.

I'm playing with USI-LS, thus, a manned Duna mission with Lander and Ike-probe (with seat) is about 33t payload.
I cannot decide if I shall use 2 or 4 nukes but I think I will go with 2.
TWR will then be about 0.15. I never went with this little TWR, however, the same mothership shall later be used for a Moho/Eve/Kerbin roundtrip (given that I can design a sufficient  lightweight Moho-ISRU-lander for refueling ;-) ).

I already testet the design in a "simulation".
0.15 seems to be working, however, for Jool I would split the burn in two burns.

[CrashyMcCrashFace]

For the "cruise" stage my Duna station/lander is 34 tons with 9 nukes and 5,200 units of fuel. TWR is 0.64.

[RizzoTheRat]

There's some great stuff from @Streetwind on this thread about dV, Mass Fraction and ISP

 

 

 

A work around to this is to use a tug within Kerbin SOI.  On my last career (not got there this career yet) I nicked an idea from @Red Iron Crown (I think), and had a tanker/tug that refuelled from a my Minmus base, docked with interplanetary vessels in LKO, towed them to an Apoapsis near the Kerbin SoI, and refuelled the ship before undocking and heading back to Minmus.  The lower powered interplanetary ship then only needed a shorter burn at the Periapsis, with plenty of help from Oberth, to it's destination planet.

 

[blakemw]

On 5/8/2018 at 12:20 PM, RizzoTheRat said:

A work around to this is to use a tug within Kerbin SOI.  On my last career (not got there this career yet) I nicked an idea from @Red Iron Crown (I think), and had a tanker/tug that refuelled from a my Minmus base, docked with interplanetary vessels in LKO, towed them to an Apoapsis near the Kerbin SoI, and refuelled the ship before undocking and heading back to Minmus.  The lower powered interplanetary ship then only needed a shorter burn at the Periapsis, with plenty of help from Oberth, to it's destination planet.

3

There's a generally more scalable approach than tugs: Once you're in the end game and using Mammoth engines, a Mammoth based rocket can easily SSTO to Minmus, and even before then, cheating a little with some SRBs makes it pretty easy for smaller rockets (Rhino, Twin-Boar or Skipper scale) to basically SSTO to Minmus. The way to do this with a staged rocket, is you either empty out all the upper stage tanks or crossfeed them down into the Mammoth (for Xenon, just leave them full). Then you land on Minmus and get fueled up by an ISRU rover (which should dock using a Klaw). At this point configure crossfeed for staged usage as required. This approach can easily give an extra 3000-5000m/s of effective dV by re-using the launch stages.

So once you have refueling at Minmus, the single-stage-to-minus /multi-stage-from-minmus is a very powerful technique, with the biggest benefit being that by refueling on the surface of Minmus huge amounts of fuel can be shamelessly expended. (I developed this technique when I got interested in using Triple-Mammoth from Minmus type vessels and the tugs and tankers were becoming problematically large).

Edited May 9, 2018 by blakemw

[HeliosPh0enix]

Unfortunately, I cannot do any of these Minmus refueling operations, as the Jool 5 challenge requires the ship to leave straight from LKO.  However, I will be sure to use that in the future in my career save.  @RizzoTheRat, thank you for the Mass Delta-V link, it is extremely helpful!