Advice for Interplanetary Mothership bound for Duna and Nucelar Rockets

[-Velocity-]

As long as your ship is rather big, don't listen to the people telling you to put only one LV-N on your ship. When your ship weighs like 100 tons, adding a second LV-N onto it will only reduce the delta-V by about 4% or 5%. BIG FREAKING DEAL. In return, you will HALVE the time it takes to complete any maneuver, which makes your ship easier and more fun to fly, and may remove the need to split maneuvers up into multiple stages. Additionally, as the LV-N is long and narrow, it can sometimes be difficult to launch a ship ship with a single LV-N- you have to launch it with the entire weight of your interplanetary transfer ship resting on the LV-N. Of course you can strut it up and eventually find a way to launch it stably, but putting two LV-Ns- one on each side- just makes it so much easier to launch into orbit.

Personally, I just use as many LV-Ns as I need to get an acceleration of at least 0.7 m/s^2, down to a lower limit of 2 LV-Ns. Putting enough LV-Ns on a ship to give it more than like 2 m/s^2 acceleration is total overkill, unless its a lander for low gravity, airless worlds (where the LV-N makes an excellent lander engine).

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Why would you use LV-Ns for simple Duna trip? There's no reason. It can be safely reached with regular rockets.

If you use LV-Ns, it increases the amount of payload you can deliver to Duna, of course. You can make multiple landings on both Duna and Ike in the same trip.

[haltux]

What craft did you test? Did you made the same mistake of giving NERVAs less than a half of the propellant compared to LFO engines, because you just removed the oxidizer, but didn't add more fuel instead?

Yes I did. I admit it is rather stupid.

Still, volume does matter, in particular for the rocket stability at launch, and I find it a bit annoying to require twice the tank volume for the same weight. Ability to fill any tanks with fuel is really a must have for the next KSP version.

[heng]

yeeeeeessss.... in theory you could go to Duna in a 100ton ship with a single Ion engine and lots of patience...

but you would need to burn several times at PE. doing lots and lots of Kerbin orbits... and then hit the right ejection angle at precisely the right time. I have no clue how to plan this kind of maneuver in KSP. If someone could tell me, I'd be happy to try it... until then? TWR it is. :-)

Edited May 19, 2015 by heng
typo

[Hannu]

Space ships do not need high TWR values. 0.1 is more than enough.

Do you have some mod or separate software to calculate spiral trajectories? KSP's instantaneous maneuver node model does not work if burn lasts more than couple of tens of degrees around planet. I made once a little help software to integrate long burn trajectories but it was little annoying to daily use and I use again high parking orbits and about 3 m/s acceleration to avoid maneuver errors.

[KerikBalm]

ok, you're right but there is still no sense to use LVN with LFO if you have Spaceplane tanks.

Correct, the LV-N is best used with the LF-only tanks.

Even the FAT-455 (airliner) wings are better fuel tanks than LFO tanks with no O.

Still, the nuke Isp is good enough to more than compensate for the reduced mass fraction, and removing dead weight (in the form of Oxidizer) is always a very good idea... and LFO tanks with full oxidizer constitute A LOT of dead weight.

I don't know how you can reach this conclusion. Terrier is way better in terms of pure Delta-V for any ship except massive ones, unless Kerbal Engineering is completely wrong in its computation.

I trust my math over Kerbal Engineer any day... its pretty good, but its long had a history of not displaying dV correctly in certain cases (like airbreathers), and I've heard many cases of it not displaying dV correctly in the latest version.

Although it depends on your definition of massive.

If your total mass limit for engine+ fuel is 3.5 tons... a LV-909 is going to do a lot more with its 345 Isp and 3 tons of fuel, than the LV-N will do with its 800 Isp and 0.5 tons of fuel

It doesn't take a very big ship for the LV-N to *destroy* the LV-909 in a dV competition.

There's a reason the LV-N shows up so much in those mass optimal engine charts, and the LV-909 doesn't.

I suggest you go take a look at them

[lajoswinkler]

Do you have some mod or separate software to calculate spiral trajectories? KSP's instantaneous maneuver node model does not work if burn lasts more than couple of tens of degrees around planet. I made once a little help software to integrate long burn trajectories but it was little annoying to daily use and I use again high parking orbits and about 3 m/s acceleration to avoid maneuver errors.

No, I rely on Oberth effect and aerobraking. Never had any related problems. I'm not gonna use ion engines for anything larger than probes, I'm not that nutty.

[Anglave]

As others have already said, get and use Kerbal Engineer Redux It won't affect your game play at all. All of your craft can remain stock. But it gives you information during the design phase that's necessary for good design.

When you're doing interplanetary transfers, you really don't need much thrust. The nuclear engines don't provide much, but they're very efficient, which is why they're a good transfer engine. That being the case, don't lift more engines than you need (they're just useless mass). In this case, one. Two, if you want them to be radial and balanced.

If your Thrust to Weight ratio is very low, your burn time might be quite long (several minutes). If that's the case, you can make your transfer in several "kicks". Plot your maneuver (from LKO to Duna intercept), but only perform half of the burn (still centered around the maneuver, half before and half after). Then wait most of an orbit until you're just about to your maneuver node, and perform the other half of the burn.

I approve of the imagination you're bringing to the task. I'd probably design a different craft for each distant planet though, rather than trying to build one general purpose craft. The mission parameters for getting to Eve and Duna are fairly similar (though returning from their surfaces is VASTLY different!) But going farther (any of Jool's moons, for instance) is almost certainly going to require a different design.

Make your top stages as small as possible. Smaller. Every gram of your lander will increase the size of your interplanetary stage significantly. Every kilogram of your interplanetary stage will increase the lifter required, until it's all crazy huge. Do it with the smallest amount that can reasonably succeed at your mission goals, at every stage.

You've got a mammoth lifting only one Mk3 fuel tank and your payload. If I understand correctly, this will be your "LKO insertion" stage and maybe the beginning of your Duna transfer. If that's the case, it probably has WAY too high a Thrust to Weight Ratio. The Mammoth has a HUGE thrust, and reasonably good atmospheric ISP, making it a great lower-stage lifting engine, but it's overkill at the top of your trajectory. At the point you're circularizing your orbit and planning your transfer to Duna, you've lifted entirely too much engine for the job. You'd do better to use something with moderate thrust and a better vacuum ISP. (If you're using Mk3 parts, the Rhino might be a better choice?)

In general, you want the most thrust when you're on the launch pad (pushing against the full force of Kerbin's gravity, through the thickest atmosphere). Your thrust requirement will decrease as you ascend (because your craft will mass less, but also because of less gravity and less atmosphere) meaning you can use smaller, more efficient engines. Lifting the big heavy engines way up high isn't an advantage unless you really need all that thrust.

It's much smaller than your planned mission, but check out my first 1.0.2 Career Duna land-and-return craft, and if you're interested, I've collected a lot of general tips for designing and piloting interplanetary missions (including Duna Land-and-Return instructions) in to a KSP Knowledge Base. Check it out, let me know what you think!

Edited May 20, 2015 by Anglave