LV-N atomic engine question

[creator1629]

hello, im fairly new at this game and was wondering regarding interplanetary travel. it tends to require a lot of delta v so i decided to use the atomic engine which has a high isp and is therefore very fuel efficient. what im wondering though is what is the right ratio of atomic engines to fuel?

i tried a set up of 1 jumbo 64 fuel tank, a tricoupler and 3 atomic engines, but when doing maneuvers that require more than a thousand delta v, it seems to take forever and by the time im finished, i seem to be a bit far from where the maneuver node started so the accuracy seems to suffer.

i also tried a set up of 7 FL-T800 tanks each with its own atomic engine, but when going all the way to moho i found i didn't have enough fuel to return to kerbin.

i guess my question is what is the ideal ratio of atomic engines to amount of fuel carried to maximize efficacy. or rather, what do you guys use on your interplanetary ships?

[Sovek]

Any time you are dealing with a long burn time (in excess of 10s), its best to do a 50/50 burn, that is take the estimated burn time, half it and burn at that time BEFORE the node. So say you have a 30s burn time coming up. Start burning 15s before the node. This should help.

[Francesco]

I like to have a TWR of 0.5-0.6 when using LV-N engines: that way, I can usually fractionate long escape burns (1.6-2 km/s) in two parts.

but that's just my preference.

in general, you don't want to burn for more than 3 or 4 minutes - that is, for more than 2 minutes before and after your ejection angle.

also, I find the 50/50 rule to be quite imprecise: if you create a maneuver node and look at the expected burn time, you'll see that it slowly decreases from its original value; that's because as you burn fuel, the TWR of your ship rises.

so, I like the "3/7" rule better: given an estimated burn time per the maneuver nodes, I'll start burning at 3/7 of that time before T+, and put the remaining 4/7 after.

note that this applies more specifically to long burns (e.g., Kerbin escape) as opposed to short, correction burns (e.g., inclination changes)

Edited April 27, 2013 by Francesco

[Sovek]

Any time you are dealing with a long burn time (in excess of 10s), its best to do a 50/50 burn, that is take the estimated burn time, half it and burn at that time BEFORE the node. So say you have a 30s burn time coming up. Start burning 15s before the node. This should help.

[Eric S]

i guess my question is what is the ideal ratio of atomic engines to amount of fuel carried to maximize efficacy. or rather, what do you guys use on your interplanetary ships?

Once you're in orbit, it becomes a matter of efficiency vs patience, as fewer engines will almost always be more efficient than more engines because you don't need to accelerate/decelerate the mass of the extra engines. The only exception is if you don't have enough thrust to perform a maneuver in the time allowed, and if you plan ahead well, that doesn't happen.

I tend to use two atomic engines in all my designs so that I can radially mount them (inline, they're a pain in the butt). Sure, it means half hour burn times on some of my bigger ships, but I don't actually mind that.

[creator1629]

oh ok thanks for that. i had never thought about it that way, yeah you do lose the fuel weight when doing a burn so a 3/7 rule would make a lot of sense. but how do you break your burns into 2 parts? like when trying to get an intercept with moho, how do i go about splitting the burn, do i do one burn to bring my orbit close to mohos orbit then a second one to actually make contact? or is there another technique?

[rosenkranz]

I think he meant by fractionate, that the burn is split into two periapsis kicks. The first half raises your apoapsis and the second time around reaches escape.

[shand]

so long as you have (i think i worked it out at) 5 tonnes of fuel per engine its more efficent then other liquid fuel rockets. that answers your initial question, but to expand on what others have been saying. i tend to split my manuvures into 100-200 deltav for the first orbit (100km-roughly 300-500km AP) then from there i'll either circularise and carry out the escape burn on the next opportunity (if i've done it right, instantly) or do a few more Pe kicks.

the lower your Pe, the shorter time you have to carry out the burn... so raising your Pe makes things EASIER, but not more efficent.

[baarnold]

What I would do is make a sky-lift sort of configuration where you have your Jumbo 64 Rockomax fuel tank and attach 4 FL-T200 tanks and create 4 fuel lines from the main tank to those tanks and then attach your nuclear motors to the bottom and add your launcher to the bottom. You can also do this with smaller fuel tanks or change it depending on where you're going and how much you're carrying.

[karolus10]

You can make this easier by transfer burn to elliptical orbit few orbits before you intended to made one burn transfer. When you came back to next maneuver, You will need less ÃŽâ€V in final burn .

Also Adding more LV-N engines improve overall trust but not stage efficiency.

[capi3101]

what im wondering though is what is the right ratio of atomic engines to fuel?

Depends on the target and how fast you want to be able to accelerate. An increasing number of engines will have a negative effect on your delta-V, but only in as much as they add dead-mass to your rocket; the trick is to find an acceptable balance. My own designs incorporate 5 LV-Ns; you get 350 kN of thrust that way, which is a decent amount for interplanetary missions. A round trip to Moho can easily be accomplished with three X200-32 tanks (12 FL-T800s equivalent). If you set it up so all five engines are drawing off of one set of tanks, you should be able to get there and back no problems.

That 30/70 ratio they're talking about is the amount of time before and after the maneuver node; I haven't tried that my own self but it does make sense given that you are losing mass in your burn (thus your rate of acceleration is increasing with time) and it jives with my observations that I never seem to wind up where the node predicted either with a 50/50 burn. Might have to do the math on that one.

[tomf]

I think the best time to burn is going to be affected by whether you are burning pro- or retrograde, If you are burning prograde then you will get to your maneuver node earlier than expected so front loading your bun more before periapsis will ensure as much delta-v is given while you are at minimum planetary distance.

Doing calculations to prove this is a little beyond me though.

[Francesco]

I think he meant by fractionate, that the burn is split into two periapsis kicks. The first half raises your apoapsis and the second time around reaches escape.

exactly, periapsis kicks.

That 30/70 ratio they're talking about is the amount of time before and after the maneuver node; I haven't tried that my own self but it does make sense given that you are losing mass in your burn (thus your rate of acceleration is increasing with time) and it jives with my observations that I never seem to wind up where the node predicted either with a 50/50 burn. Might have to do the math on that one.

just to be totally clear, it's not 30% and 70%: it's 3/7 (roughly 43%), which is just a fancy way of saying, "less than half" (of the estimated burn time.)

[rosenkranz]

If you have a mod that will tell you your TWR or max acceleration, I'd say you want a TWR (relative to Kerbin) of at least .4 or better. That translates to about 4m/s2 acceleration. My most recent experience with a ship with a TWR of .1 or about 1m/s2 and that was not happy

Ended up scrapping that mission . Jool will have to wait for next time.

For the next attempt I got the TWR upto .5 and refueling at minmus and they burned out just like they knew what they were doing Got a NAV SAT Carrier, a rover, and a base segment headed out to Duna

[ThatBum]

You'd want to do your burn at the fastest point in your orbit (periapsis), to maximize the Oberth effect.

[creator1629]

wow thanks a lot for all the info guys, this is really helping me develop my interplanetary ship.

i have another question though, a lot of people seem to mention that a ship with a TWR of 0.4-0.5 or higher is a good way to go about it, but what if the ship has a lower TWR like 0.1 or 0.2 but the majority of the weight is due to fuel rather than other equipment, does that fact help balance it out in the end?

also which destination in the solar system is the most fuel/delta v intensive, because if i design my ship to be able to go there and return, i will know it can go anywhere

[Francesco]

if you're patient, once you are in LKO, TWR doesn't really matter: with a 0.1 or 0.2 TWR, you can do a series of periapsis kicks.

most delta-v intesive destinations: depends on whether you plan to simply get in orbit around them, land on them, or land and get back to Kerbin from them.

in general, I would say they are Moho, Tylo, and Eeloo, with Dres as a runner-up.

[creator1629]

thanks francesco. yeah my purpose is for the interplanetary ship to carry a small lander to the target, drop it off to land, have it take off and re-dock with the interplanetary ship, and then return to the orbit of kerbin.

when you say a series of periapsis kicks, do you mean i do a short burn at the periapsis, stop, let the ship complete the orbit until it reaches the periapsis again, do another burn, stop complete another orbit... in a sequence like that?

[Scotius]

Exactly like that It's the best way to get huge ship with low thrust out of Kerbin's SoI. Sometimes you need to do more than 5 minute burns to get escape speed - trying to make it in one waste fuel, because you start and end burning outside of optimal window.

[Francesco]

although a thing I should add, which I haven't seen mentioned yet, is that by doing periapsis kicks, for every kick after the first one, you're going to be burning quite higher than the typical 70-80km LKO: coming in towards periapsis, you'll be as close as you can to Kerbin (which is what you want to take advantage of good ol' Oberth effect) for quite a short time.

so this kinda mitigates the efficiency of the maneuver, but it's still the best course of action anyway.

[Comrade Jenkens]

The more engines you have the lower the efficiency because of the mass the additional engines add. However I usually use at least 3 NERVA engines due to having not much patience or time for long burns. My warship designs are even less efficient and often use upwards of 10 NERVAs to increase the TWR to above 1 at the cost of range.

[RoboRay]

when you say a series of periapsis kicks, do you mean i do a short burn at the periapsis, stop, let the ship complete the orbit until it reaches the periapsis again, do another burn, stop complete another orbit... in a sequence like that?

Yes, exactly. This is what your resulting orbits might look like if you do a lot of short kicks...

although a thing I should add, which I haven't seen mentioned yet, is that by doing periapsis kicks, for every kick after the first one, you're going to be burning quite higher than the typical 70-80km LKO: coming in towards periapsis, you'll be as close as you can to Kerbin (which is what you want to take advantage of good ol' Oberth effect) for quite a short time.

so this kinda mitigates the efficiency of the maneuver, but it's still the best course of action anyway.

Actually, it doesn't. The Oberth effect is derived from the kinetic energy of the fuel you're burning. While you do have a higher kinetic energy in low circular orbits than in high circular orbits, an elliptical orbit with a low Pe is actually better than a circular orbit at the same altitude, because you're moving relative faster at Pe than you would in a circular orbit. That kinetic energy has to go somewhere when you burn the fuel, so it's added to your craft's velocity. It's pretty much always more efficient to do Pe kicks than one long burn, not only because you're applying more of your thrust close to the ejection angle but also because your velocity is higher when you're making the burn near Pe on each successive orbit.

Edited May 1, 2013 by RoboRay

[capi3101]

Periapsis kicks - interesting concept; wish I'd known about it before I left for Duna this last trip. About how long of a burn are we talking about?

[Francesco]

Actually, it doesn't. The Oberth effect is derived from the kinetic energy of the fuel you're burning. While you do have a higher kinetic energy in low circular orbits than in high circular orbits, an elliptical orbit with a low Pe is actually better than a circular orbit at the same altitude, because you're moving relative faster at Pe than you would in a circular orbit. That kinetic energy has to go somewhere when you burn the fuel, so it's added to your craft's velocity. It's pretty much always more efficient to do Pe kicks than one long burn, not only because you're applying more of your thrust close to the ejection angle but also because your velocity is higher when you're making the burn near Pe on each successive orbit.

dat picture

I guess I should have phrased it better.

there's no doubt that periapsis kicks is the best "technique" here, I was just noticing that for all the burns following the first one, your altitude is going to change quite a bit during the burn itself, compared to the first kick (which you perform starting from a circular orbit.)

but then, it's not really the altitude per se that matters, but rather your velocity, right?

so during the subsequent burns, coming towards periapsis, you're gonna be more efficient as you're travelling faster.

so yeah, let's just pretend I didn't say that to begin with

what about this, RoboRay: what's your rule of thumb for PE kicks - how long does each one last, at the most?

[RoboRay]

Heh, yeah, those pictures just seem to pop up all the time here.

Right, it's the velocity that matters, not your altitude. The general rule of "burn as low as possible" is just a side effect of lower orbits having more kinetic energy (higher relative velocities), and most people are referring to the parking orbit from which they are starting the transfer burn. But an elliptical orbit gives you an even higher relative velocity at the same Pe, so each kick actually becomes a little more efficient than the last due to the fuel possessing more kinetic energy on each pass.

Departing Kerbin, I try to do my burns for 20 degrees or two-minutes on either side of the angle, depending on if I'm using Protractor or the Maneuver Nodes. Assuming your parking orbit is 100km or less (or, about 30 minutes long), that works out to about the same distance around the planet. Beyond that, you're expending a lot of thrust off-axis, making it less effective.

It's really bad if you don't do kicks for long burns. Think about it... if your orbital period is 30 minutes and you're making a 30 minute burn, half of your burn will have part or all of your thrust pushing you away from the direction you're trying to go. You're still increasing your Ap (and your Pe, too, which is just wasted fuel) from the planet, but the effect on your transfer-orbit velocity (in respect to the sun) is partially counterproductive. This makes your transfer burn even longer than what you should need, so you must burn even more fuel.

Edited May 1, 2013 by RoboRay