Best engine for interplanetary burns?

[Treldon]

Hi,

I am planning my first trip beyond Kerbin SoI, most likely to Duna.

My question is the following: which engine/setup should I use for the interplanetary journey?

1: Normal rocket engines. Pros: Fast, some are already on the craft (for getting to orbit, circularization) Cons: lots of mass for engine and fuel.

2: LV-N. Pros: low fuel consumption, moderate(?) speed. Cons: even more mass, slower

3: Ions: Pros: light. Cons: slow, high part count

Thanks in advance

Edited November 21, 2014 by Treldon

[Wanderfound]

It depends on what you're moving.

On a lightweight probe (Fine Print satellites etc) the LV-N's efficiency is outweighed by its bulk and weight. On anything bigger, the LV-N is the clear winner, but on really big stuff it may be painfully slow. In that case, you may take the efficiency hit in favour of speed and go for a KR-2L or similar. On landers, where bulk is a factor, LV-909's do very well, as do the little Rockomax engines.

One engine is always more fuel efficient than two or more of the same.

If you're sending something big, it'll be a lot easier to launch if you refuel it in orbit. If you're just sending a probe, consider using a spaceplane to give it low-cost headstart; launch the spaceplane into a massively elliptical orbit, drop the probe when you're nearly out of fuel, keeping just enough to drop your periapsis down to 30km for aerobraking. Set it up so that the probe does its interplanetary burn at periapsis for maximum Oberth. If you land the spaceplane at KSC, it'll only cost about √3,000 to do this.

Ions are efficient, but they're unpleasant to use. Much too slow, even at 4x time.

Edited November 21, 2014 by Wanderfound

[zekes]

I love LV-Ns, but if you're really careful you can get Ions to work well. you just need a LOT of fuel tanks.

I managed to make a 230 tonne battleship, powered by both LV-N and Ions. It's got 12 km/s D/V On the Ions, and about 2.4 km/s on the LV-Ns, and both are useful, depending on the kind of burn needed.

[Treldon]

Yeah, I guess I should have specified the mission. I am planning on sending a lander/spaceplane (not necessarily SSTO) with 1-2 kerbals and perhaps a small probe on board (rover optional, unlikely)

[Laie]

Short answer:

In almost all use cases, LV-Ns will suit you best. The saved fuel makes for a much smaller (=cheaper, simpler) rocket on the launch pad.

If you care about cost, Xenon fuel is pretty expensive. If your vessel weighs more than a few tons, launching many tons of conventional fuel will be a lot cheaper than Xenon.

If you care about part count, I can't help you. Only you know the limits of your hardware and your patience.

--

Longer answer: What do you mean with "fast" or "moderate speed"? If this is about the duration of your transfer burn, my best advice is to aim for a Kerbin-TWR of 0.15 or better; my own preferred figure is 0.2, which means an eventual acceleration of 2m/s², which means the ~2000m/s transfer to Jool can be done in about 1000 seconds or 17 minutes. That is still doable in a single burn, though some apoapsis-pumping won't hurt. With a TWR below 0.15, apoapsis-pumping becomes mandatory; with a TWR below 0.1, you will have considerably efficiency losses from the long burns (like a 1200m/s maneuver taking 1400m/s to execute).

Hint: you can use the LV-Ns for circularization, and probably as the last stage of your lifter as well. Refuel in orbit or drop empty tanks before you proceed with your mission.

Edited November 21, 2014 by Laie

[Zylark]

For Space-Truckin' between the planets, only the LV-N is suitable of the stock engines. Reasonable thrust with a reasonable fuel consumption.

Chemical rockets are so thirsty that you'll have to chug along massive amounts of fuel, and that is no good.

Ion engines have very low thrust, so if your vessel have a mass of more than a couple of tonnes, you better have the patience of a very patient saint.

[eddiew]

As is said above; LV-N is your friend for anything heavy. I have a habit of putting 2 or 4 on my interplanetary stages, but that does of course add weight, so it's more efficient to just stick with one. More is nice if your delta-v is plentiful.

Ion drives can be great for ultra-light probes. A core, antenna, a few stack mounted small batteries, 6-8 extendible solar panels, and some DMagic orbital science instruments can be put together for under a ton, and deliver you a TWR of ~0.2 with ~5k delta-v per tank of Xenon. Sure, they can take 10-15 minutes to make their burns, but you can get a time acceleration mod to let you speed that up no problem - such tiny things easily survive reduced physics accuracy during burn. At 4-6x warp I just let Mechjeb finish the manoeuvre for me while I get a cuppa and it's done by the time I'm back

In your case, I'd try to keep the probe's size down enough for it to use an ion drive. This way it'll be much easier to take as a payload - or indeed it'll be quite capable of making its own way to Duna, once you get it to orbit, so you could be ultra-efficient with your manned vessel and kick the probe out early to save your conventional rocket fuel.

[Pecan]

Yeah, I guess I should have specified the mission. I am planning on sending a lander/spaceplane (not necessarily SSTO) with 1-2 kerbals and perhaps a small probe on board (rover optional, unlikely)

LV-N or 48-7S for the transfer, 48-7S or LV-1/LV-1R for the lander, LV-1/LV-1R for the probe, if you take it. Spaceplanes are very inefficient (except for Kerbin and Laythe landings) and rovers almost completely useless so leave them off.

[Treldon]

Thanks for all the replies. For now I will stick 2 LV-Ns on a Jumbo-64, that should be enough for the transfer, then correct later if needed

[Jouni]

The biggest problem with ion engines is the small size of xenon containers. If you have a large ship, adding 20 ion engines to it is not a real problem. Adding 100-200 xenon containers might be.

The second biggest problem is providing enough electricity. If you're going to the inner system, one set of 1x6 solar panels per engine is usually enough, if you use batteries as a buffer. 2000-4000 units of electrical charge per engine should be enough for most purposes. In the outher system, you'll either need ridiculous solar panel contraptions or a huge pile of batteries.

[Wanderfound]

Yeah, I guess I should have specified the mission. I am planning on sending a lander/spaceplane (not necessarily SSTO) with 1-2 kerbals and perhaps a small probe on board (rover optional, unlikely)

In that case, you may want something like this with the VTOL jets replaced with Aerospikes for Duna use:

http://forum.kerbalspaceprogram.com/threads/90747-Kerbodyne-SSTO-Division-Omnibus-Thread?p=1529447&viewfull=1#post1529447

Although you may prefer to swap the VTOL for enhanced fuel capacity, as in this one: http://s1378.photobucket.com/user/craigmotbey/slideshow/Kerbal/Tutorials/Hangar%20to%20Landing

Edited November 21, 2014 by Wanderfound

[Red Iron Crown]

This thread needs more Mass Optimal Engine Type by Delta-V, TWR and Payload charts.

[Treldon]

Thanks for all the advices. The result:

Greetings from Duna

On the way there stopped on Ike as well

All that's left is to get them back from Solar orbit. But that's okay, it's not like they need to eat or anything

[numerobis]

The ion engine is a neat idea, but not really "fun".

The LV-N or the 48-7S are the two most fuel-efficient rockets, depending on the size of the payload. A small probe will do best with the 48-7S, anything else does best with the LV-N.

For a large spacecraft, the LV-1 is a great secondary engine. Toss one somewhere near the centerline (doesn't have to be right on it) and use it to make very accurate burns of less than 1m/s. That will let you carefully line up from very far out - e.g. for setting your periapsis on Laythe from "near Kerbin" space.

[Coga19000]

Well, as aforementioned, it depnds on your payload and mission type. If you have lightweight probes with enough electricity generation, have big delta- requirements, and don't mind the exorbitant cost and waiting, Ion engines are your friend (and look pretty cool) They are generally very specific on where they can be used effectively in a career file.

If you have anything bigger to haul around, the LV-N will more than do the par for you, with the second highest Isp and moderate thrust; they can also take you almost anywhere the Ion an take you, but the fuel requirements willl be a tad bigger. Generally, your standard-issue for most types of payloads.

Now, for a lander, the Ion engine gets almost automatically aborted fo the simple fact that it cannot provide decent deceleration for most celestial bodies' gravity; and even on the few where it does, God help you if you end up with a suborbital trajectory on the dark side of the planet.

Actually, if the lander is separate from the transfer stage (the thing that will haul you from planet to planet), you don't even need to bother with all that fuel-efficiency nonsense. What you need there is a low profile (small size), good thrust, and control on your thrust levels, as you will need to burn for small intervals, on enough power to actually see a difference. That means, go with conventional engines of small size (maybe radial-atached), with still aceptable Isp and enough thrust to be confortable.

[Kryxal]

Don't forget to make use of any excess fuel from your circularization stage, if you have an excess ... I routinely have maybe an extra 500 delta-v, so use that for my first apoapsis kick, and it's likely to have a better TWR. I make a point of getting that stage to then go into atmosphere for clean-up purposes...

[tsotha]

I'm not a fan of either LV-N or ion engines for a Duna shot. If you wait for the optimal window (and you're not using DR) the dV you need isn't that much more than what you need to get to the Mun. Fancy engines make a lot more sense if you're somewhere you need the extra efficiency.

[GoSlash27]

Honestly, the "best" engine for interplanetary burns is the Kraken drive unit. No waiting for transfer windows and journeys are reduced to days instead of years.

But in a realistic mission, the LV-N mass mover is the best IMO. Ions can out-perform them in terms of pure DV, but the LV-N gets you where you're going with a lot less hassle, complexity, and expense.

Best,

-Slashy

[Dispatcher]

Nuthin' sez lovin' like LV-N.

[davidpsummers]

Hi,

I am planning my first trip beyond Kerbin SoI, most likely to Duna.

My question is the following: which engine/setup should I use for the interplanetary journey?

1: Normal rocket engines. Pros: Fast, some are already on the craft (for getting to orbit, circularization) Cons: lots of mass for engine and fuel.

2: LV-N. Pros: low fuel consumption, moderate(?) speed. Cons: even more mass, slower

3: Ions: Pros: light. Cons: slow, high part count

Thanks in advance

I started using standard, reusuable (refuel in orbit), boosters to go from Kerbin Orbit to other planets. I have two basic models. One for small probes using ion engines and another for bigger ships using LV-N

Note, they are harder, but chemical work perfectly fine for interplanetary. Using chemical rockets, I sent unmanned missions to Eeloo and Jool on chemical rockets (I got LV-N late in my Career/Science mode) and manned missions to Duna and Eve.

All my heaving booster to lift anything of any real size from KSC to orbit are chemical rockets (nothing else has the thrust). I'm just getting my first space plane working to lift small stuff up to orbit.