LV-N vs LV-909

[RedKosmos]

Hello, and thank you for reading this post.

I have an question about which engine would work better in Mun's gravity, the LV-N or the LV-909. In TWR comparisons (on Kerbin), the atomic engine loses every time， but the vac isp for the LV-N is more than twofold of the LV-909. I would like to know which would work better on the Mun. Thanks!

[J0hnsmine]

I'm 90% sure its the 909, someone correct me if i'm wrong.

[Giggleplex777]

What are you trying to land on the Mun?

[tavert]

It depends on the total payload of your lander, and your mission profile (total delta-V requirement). Are you doing an Apollo-style mission, or a single craft that does the whole Munar injection, landing, ascent and return?

For a large enough payload, the higher specific impulse makes up for the LV-N's heavier mass. You only need a TWR of 0.166 g's to land on the Mun. Have a look at http://imgur.com/a/kfnng for some data on which engine to use at which payload mass and delta-V requirement. You could theoretically even use an LV-1 if your lander is small enough. And the new Rockomax 48-7S (which wasn't included in that chart) is also a really good choice for small craft.

[Crush]

When you are building a lander, thrust-to-weight ratio is much more important than fuel-efficiency. A better TWR of your whole craft allows you to decelerate much more quickly, which makes landings much safer and easier. And the later you brake, the less fuel you need because you spend less time accelerated by the gravity of the planet. The same applies to the start: a high TWR means less loss of Delta-V due to gravity-drag.

Besides that, due to it length, the LV-N is hard to integrate into most lander designs, while the compact LV-909 is practically made for being placed between landing legs.

Keep in mind that while the LV-N has the best fuel-efficiency, it has by far the highest mass compared to its thrust. This doesn't matter much when you accelerate a mass of tens of tons during an interplanetary transfer, but when you have a small craft which weights just a few tons, it really starts to matter. Twice the fuel efficiency is pointless when it means more than twice the total mass. Also, every additional kg of payload means more power required for every single stage of your rocket.

Edited July 26, 2013 by Crush

[Kanddak]

I use the LV-N for basically all of my Munar landers, lowering the legs by putting them fins.

But I've only compared the delta-V directly. I haven't tried computing how much I lose to gravity drag by having a worse TWR and then comparing whether the nuclear lander is truly more fuel-efficient or not.

[tavert]

I use the LV-N for basically all of my Munar landers, lowering the legs by putting them fins.

But I've only compared the delta-V directly. I haven't tried computing how much I lose to gravity drag by having a worse TWR and then comparing whether the nuclear lander is truly more fuel-efficient or not.

I have

http://forum.kerbalspaceprogram.com/showthread.php/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

Any higher than TWR of 2 (relative to body in question, so 0.332 g's for Mun) doesn't give you much additional benefit. Come in low and control your vertical speed by pitching higher than retrograde.

Edited July 26, 2013 by tavert

[SRV Ron]

It's the difference between the 0.5 tons of the LV-909 and the 2.25 tons of the LV-N when it comes to landing. Easier to have 1.75 tons of less weight for such a landing. Also, less problems for installing lander legs for an LV-909 design.

I have landed probes using thrusters alone on Mun. No landing legs either. I just set it down on the 100 unit thruster fuel tank.

Where does the LV-N have the advantage? In long range travel where the double fuel efficiently will overcome the disadvantage of its extra weight.

Edited July 26, 2013 by SRV Ron

[Crush]

The only situation where I would consider landing on LV-N's would be when it would be an interplanetary mission and my lander were also my engine-section. It just doesn't make sense to have 1.75 additional tons of engine unless it also saves you at least 1.75 tons of fuel.

[numerobis]

If you use the LV-N as your last launch stage at Kerbin, for the transfer to Mun, the landing, and the return to Kerbin, it is worth it if you land a heavy payload like the 3-kerbal cockpit. Any shorter or lighter payload and the LV-909 wins in terms of total mass.

[lammatt]

for Mun and other low gravity places which doesnt require a lot of dV to land... i use the rockmax radial ; so that i can have a lower ground clearence (u know... the lander legs are short)

otherwise use the LV-909 if you dont need a huge dV to land

LV-N is good for orbit ejection/insertion...

[Alistone]

I have

http://forum.kerbalspaceprogram.com/showthread.php/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

Any higher than TWR of 2 (relative to body in question, so 0.332 g's for Mun) doesn't give you much additional benefit. Come in low and control your vertical speed by pitching higher than retrograde.

I usually have a heavy lander and use the LV-N almost exclusively. If you maneuver your periapsis to 5km-12km above the mun then you can "suicide burn" retrograde at full throttle at periapsis (your maximum speed for excellent oberth effect) and simply adjust your nose to slightly above the horizon to keep your vertical speed in a controlled descent.

If you want to orbit and choose your landing spot you can circularize instead of completely eliminating your horizontal velocity.

Then it is a fairly short controlled descent that is just as efficient as a mechjeb suicide burn.

[RedKosmos]

I'm building a ship piece by piece on the mun. I would like to use the atomics for the boosters, but their size is a big hassle in the construction process. Also I'm going to need a device to push my rocket from a laying state to an upright state. (a large rocket) the main rocket motor for this rocket is the poodle. (because of its size)

[RedKosmos]

From what I see, I need to go with the lv-n. now the problem is fitting it on the ship. Solve one problem and create another, haha. The 909 is a staple for my manned landers. Thanks for all the input! I really did learn a lot for my future missions from yall.

[AlexanderB]

The crossover point is FL-T800 tank with the LV-909 engine, versus a FL-T400 tank with the LV-N engine.

If you need more Delta V than that, then the LV-N wins, if you don't need as much, then the LV-909 wins.

[numerobis]

By the way, you didn't mention the 24-77 or the (new) stacked equivalent. Those are even lighter than the LV-909, and thus a good choice if you are landing a light payload on the Mun.

[WhiteWeasel]

Depends really, If its a one or two way trip, 909's. If its going to be reused a lot like a shuttle or skycrane the LVN's will save you a bunches of refueling trips.

[RedKosmos]

By the way, you didn't mention the 24-77 or the (new) stacked equivalent. Those are even lighter than the LV-909, and thus a good choice if you are landing a light payload on the Mun.

I have not had a chance to play the new patch yet, so I am not familiar with any of the new parts.