LV-Ns at Duna, ISP

[Laie]

Just because the topic comes up every once in a while: the ISP is 700@450m and 740@2000m.

[Red Iron Crown]

The LV-N gets a bad rap for its atmospheric Isp, but it's not really much of a penalty anywhere but Eve. On Kerbin, for example, its Isp exceeds that of the best chemical engines by 1700m of altitude, and it keeps gaining efficiency the further it climbs.

[Laie]

The LV-N gets a bad rap for its atmospheric Isp, but it's not really much of a penalty anywhere but Eve. On Kerbin, for example, its Isp exceeds that of the best chemical engines by 1700m of altitude, and it keeps gaining efficiency the further it climbs.

Yeah, it's always the same song... I just happen to be at Duna right now, and thought I'd provide some numbers. Something to quote and point to the next time the topic comes up, which is sure to happen.

[SRV Ron]

The LV-N is great for long range interplanetary burns. That is where its efficiency comes into play as an advantage.

For landing and takeoff from moons and small planets, its mass becomes a disadvantage compared to less efficient but much lighter engines. Far easier to push a 1/2 ton engine to a destination then one that weighs 2.25 tons. And, overcoming that extra mass of your lander on takeoff to orbit will burn up as much fuel as the much lighter but less efficient engine of comparable thrust.

[LethalDose]

The LV-N gets a bad rap for its atmospheric Isp...

Well, it's also got a horrendous TWR. But you're right on the rest: the atmospheric Isp is balanced out by the time you get any altitude.

[Jouni]

For landing and takeoff from moons and small planets, its mass becomes a disadvantage compared to less efficient but much lighter engines. Far easier to push a 1/2 ton engine to a destination then one that weighs 2.25 tons. And, overcoming that extra mass of your lander on takeoff to orbit will burn up as much fuel as the much lighter but less efficient engine of comparable thrust.

The LV-N is a good lander engine, if the payload is large enough or the lander requires a lot of delta-v. It's a bit difficult to use properly, because landing with a low TWR is different from what we're accustomed to. You may also need non-standard design choices to use the LV-N efficiently. Instead of having a transfer stage or a mothership carry the lander to the destination, you land with the main ship, after leaving unnecessary modules in orbit.

[Pecan]

The LV-N is great for long range interplanetary burns. That is where its efficiency comes into play as an advantage.

For landing and takeoff from moons and small planets, its mass becomes a disadvantage compared to less efficient but much lighter engines. Far easier to push a 1/2 ton engine to a destination then one that weighs 2.25 tons. And, overcoming that extra mass of your lander on takeoff to orbit will burn up as much fuel as the much lighter but less efficient engine of comparable thrust.

The point being, that if you've already got LV-Ns for the transfer, there's no point in adding extra engines just for landing.

Travert's mass-optimal engine thread is my bible, as always.

[Norcalplanner]

The LV-N is a good lander engine, if the payload is large enough or the lander requires a lot of delta-v. It's a bit difficult to use properly, because landing with a low TWR is different from what we're accustomed to. You may also need non-standard design choices to use the LV-N efficiently. Instead of having a transfer stage or a mothership carry the lander to the destination, you land with the main ship, after leaving unnecessary modules in orbit.

That's exactly what I did with my Jool-5 Kethane challenge ship. The main drive section detached to become the lander, albeit on a somewhat large and inefficient scale.

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[Wanderfound]

The LV-N is a good lander engine, if the payload is large enough or the lander requires a lot of delta-v. It's a bit difficult to use properly, because landing with a low TWR is different from what we're accustomed to. You may also need non-standard design choices to use the LV-N efficiently. Instead of having a transfer stage or a mothership carry the lander to the destination, you land with the main ship

Yup.