Are LV-N's worth the trouble?

[KerikBalm]

In reality, the liquid fuel that a NTR uses is much less dense than liquid O2, and the tank needs to be able to withstand higher pressures, and needs more insulation.

I think this is the exact opposite case for the "standard" rocket using something like Kerosene/RP-1 - the O2 tank is the insulated and presurized one, and the liquid fuel can pretty much sit there at 1 atmosphere at room temperature.

[Chris_2]

In reality, the liquid fuel that a NTR uses is much less dense than liquid O2, and the tank needs to be able to withstand higher pressures, and needs more insulation.

I think this is the exact opposite case for the "standard" rocket using something like Kerosene/RP-1 - the O2 tank is the insulated and presurized one, and the liquid fuel can pretty much sit there at 1 atmosphere at room temperature.

I don't understand the relation between fuel density and tank pressure... The tank pressure depends very much on engine cycle used (expander, staged combustion, or even pressure fed etc.) and also of the specifics of the rocket engine (turbine inlet pressure, boost pumps used?)...

Also, the tank pressure depends on the tank. The stainless steel tanks of the centaur upper stage are extremely lightweight, but that is only because they are at high pressure. Because the tank walls are so thin, without the internal pressure they would collapse under their own weight ! (which obviously adds a lot of problems and costs during the transport of the stage, integration of the rocket etc because the tanks need to be kept pressurized at all times, but this is offset by the extremely high performance and light weight of the stage...)

Edited May 14, 2015 by Chris_2

[Meithan]

I've been experimenting with engine charts using the new 1.0 specs, and my general conclusion (for vacuum operations) is that:

The LV-N is king if your payload weighs more than 2-3 tonnes, you need 2500 m/s or more, and you're OK with a TWR of 0.5 or less.

Here's a comparison of the LV-N, the LV-909 and the LV-T30. The color of each pixel indicates which engine is more mass-eficient (i.e. will yield the smallest total ship mass) for the payload and delta-v of that pixel.

In the calculation the TWR has been restricted to be at least 0.4, which in my opinion is more than practical for interplanetary travel. The LV-N's high mass really becomes a hindrance above TWR of 0.5.

Here are the total mass curves for a payload of 8 tonnes (in the ballpark of the OP's Duna lab ship) for these three engines:

The discontinuous jumps is where the number of engines increases.

Note that for the LV-N I've been assuming it's paired to the Mk2 liquid fuel fuselages, which have a fuel-to-empty mass ratio of 6 (i.e., they carry 6 tonnes of fuel for each tonne of tank). For the rest of the engines I'm assuming the fuel-to-empty ratio is 8, which is that of the FL-Txx and Rockomax tanks. If all engines are instead paired with Mk3 tanks/fuselages (mass ratio of 8.3), the LV-N's case should be a bit better.

Edited May 14, 2015 by Meithan

[Jovus]

Um, the wet/dry ratios of all LFO tanks is 9, not 8.

[Meithan]

Um, the wet/dry ratios of all LFO tanks is 9, not 8.

Indeed (at least for the non-Mk3 tanks; for them it's around 9.3), but I'm defining alpha as the ratio of the fuel mass (not total fuel+tank mass) to the dry mass. Why? Just for convenience in the calculations. An alpha of 8 means the tank carries 8 tonnes of fuel for each tonne of empty tank.

[Jovus]

Gotcha. Your post is worded confusingly, at least to my mind. That explains why you reported lower number on the Mk2 and Mk3 as well.

[Temeter]

Those numbers fit my experience with nukes. You usually should go for a T/W between 0.5 to 0.3 (can go lower if you have the patience), although I recommend to go closer towards 0.3 for heavy freight and long range ships. Nukes generally become efficient upwards of 3k dV in a single stage. And of course, if the freight justifies the weight.

Edited May 14, 2015 by Temeter

[KerikBalm]

Indeed (at least for the non-Mk3 tanks; for them it's around 9.3), but I'm defining alpha as the ratio of the fuel mass (not total fuel+tank mass) to the dry mass. Why? Just for convenience in the calculations. An alpha of 8 means the tank carries 8 tonnes of fuel for each tonne of empty tank.

Methinks you are looking at the wiki with out of date numbers.

Also, when talking about the ratios, most people use the ratio of "wet mass" to "dry mass" ie (Fuel Mass+ Tank Mass)/(Tank Mass)

- because that is the relevant ratio for the rocket equation.

The wet over dry mass for all Mk2 and Mk3 LFO or LF tanks is 8 now

The wet over dry mass for all non-spaceplane part LFO tanks is 9 now (that is 8 tons of fuel for every ton of tank, 9 tons full, 1 ton empty)

I don't understand the relation between fuel density and tank pressure... The tank pressure depends very much on engine cycle used (expander, staged combustion, or even pressure fed etc.) and also of the specifics of the rocket engine (turbine inlet pressure, boost pumps used?)...

Apple's and Oranges....

I'm talking about storing cryogenic liquids... not fuel pressure to the combustion chamber.

Water boils at 100 C right? Not if you increase the pressure to 10 atmospheres....

Likewise if you want to store O2/N2/CH4/Xe/H2, you don't have to keep it as cold, if you can keep it under higher pressure.

The more pressure the tank can tolerate before a valve has to release pressure from the tank (due to the liquid heating up and starting to boil, raising the tank's pressure)

Edited May 14, 2015 by KerikBalm

[EventHorizon429]

Honestly I think the Nerv itself is fine as it is, what needs to change however is that there needs to be more types of dedicated LF-only tanks and dedicated heat sync parts for the Nerv engine as the only real good LF parts we currently have are the Mk2 and Mk3 fuselages, which unfortunately don't fit in with our conventional sized 1.25m, 2.5m and 3.75m parts, and the only decent heat sync worthy parts we have are solar panels and wing parts. Squad, if you're reading this, please implement these in the next major update.

[Brotoro]

The pressures in rocket propellant tanks are not particularly high. High pressure tanks would need to be much stronger, and therefore much heavier. The Space Shuttle external tank was pressurized at about 1.5 atmospheres for the LOX tank and 2.3 atmospheres for the LH₂ tank. Even rockets like the Atlas that depended on internal tank pressure to maintain the structure of its balloon tanks was only pressurized at around 4 atmospheres.

[KerikBalm]

None of those tanks are designed to hold their propellents for long periods of time, as you would need for a return trip from mars, for example.

[Meithan]

Gotcha. Your post is worded confusingly, at least to my mind. That explains why you reported lower number on the Mk2 and Mk3 as well.

Also, when talking about the ratios, most people use the ratio of "wet mass" to "dry mass" ie (Fuel Mass+ Tank Mass)/(Tank Mass)- because that is the relevant ratio for the rocket equation.

Yes, I feared that my definition might be confusing in light of the generally accepted meaning of "fuel mass ratio". I might consider switching to it before I post the calculations. I went this way because it's "more natural" in the calculations: the fuel mass in a tank can be computed as alpha*dry_mass instead of (alpha-1)*dry_mass. I just find the first form cleaner. It's an OCD thing of mine.

Methinks you are looking at the wiki with out of date numbers.

The wet over dry mass for all Mk2 and Mk3 LFO or LF tanks is 8 now

The wet over dry mass for all non-spaceplane part LFO tanks is 9 now (that is 8 tons of fuel for every ton of tank, 9 tons full, 1 ton empty)

I am indeed pulling the values from the wiki (the Parts page), and I haven't checked in-game to see if they're correct. So you're telling me they're not?

It will hardly make a difference, though. I already assumed a wet-to-dry ratio of 9 (alpha of 8) for all engines except the LV-N, for which I used a wet-to-dry ratio of 7 (alpha of 6). As long as the actual values don't deviate much from this, the results will be similar.

[GoSlash27]

I've been experimenting with engine charts using the new 1.0 specs, and my general conclusion (for vacuum operations) is that:

The LV-N is king if your payload weighs more than 2-3 tonnes, you need 2500 m/s or more, and you're OK with a TWR of 0.5 or less.

Here's a comparison of the LV-N, the LV-909 and the LV-T30. The color of each pixel indicates which engine is more mass-eficient (i.e. will yield the smallest total ship mass) for the payload and delta-v of that pixel.

http://i.imgur.com/rnaKmfgm.png

In the calculation the TWR has been restricted to be at least 0.4, which in my opinion is more than practical for interplanetary travel. The LV-N's high mass really becomes a hindrance above TWR of 0.5.

Here are the total mass curves for a payload of 8 tonnes (in the ballpark of the OP's Duna lab ship) for these three engines:

http://i.imgur.com/qnDyxWym.png

The discontinuous jumps is where the number of engines increases.

Note that for the LV-N I've been assuming it's paired to the Mk2 liquid fuel fuselages, which have a fuel-to-empty mass ratio of 6 (i.e., they carry 6 tonnes of fuel for each tonne of tank). For the rest of the engines I'm assuming the fuel-to-empty ratio is 8, which is that of the FL-Txx and Rockomax tanks. If all engines are instead paired with Mk3 tanks/fuselages (mass ratio of 8.3), the LV-N's case should be a bit better.

^Reposted for truthiness.

*This* is what's most important. It's not just which engine has a higher specific impulse or which engine weighs more, it's about which engine yields a lighter overall stage for the preceding stage to push.

Mass efficiency is the key to successful missions.

Thanks for this, Meithan! My calculations have put the crossover point at 1500m/sec, 7 tonnes payload per engine, and .5G acceleration. Longer trips, heavier payloads, and lower accelerations favor the LV-N, while shorter trips, lighter payloads, and higher acceleration favors the LV-909.

*edit* the most generalized, simplified version: If you're going to Eve, Duna, or anywhere closer, use the LV-909 (or Poodle, depending). If going farther with any serious payload, consider the LV-N.

Best,

-Slashy

Edited May 14, 2015 by GoSlash27

[Meithan]

My calculations have put the crossover point at 1500m/sec, 7 tonnes payload per engine, and .5G acceleration. Longer trips, heavier payloads, and lower accelerations favor the LV-N, while shorter trips, lighter payloads, and higher acceleration favors the LV-909.

*edit* the most generalized, simplified version: If you're going to Eve, Duna, or anywhere closer, use the LV-909 (or Poodle, depending). If going farther with any serious payload, consider the LV-N.

Beware that I only included three engines in the previous chart. Here's what happens with more engines (for minimum TWR of 0.5):

Note: thanks to Slashy I found two engines had incorrect specs for 1.0.2; I've since corrected this and recomputed the chart.

The LV-909 is the best engine only in a narrow-ish band (and the vertical scaling is logarithmic). For lighter payloads and/or smaller ÃŽâ€v's, small engines like the 48-7S are actually more efficient (because they're very light).

Also note that for the intermediate-to-high-mass (> 5 t) low-ÃŽâ€v (< 2000 m/s) region the Aerospike becomes the best option (and it completely dominates the LV-N everywhere at a TWR of 0.8). I don't remember the aerospike being a good vacuum engine, but I admit I never really tried.

Note that I purposefully left the LV-1 (and its radial variant) out of this chart because it outperforms both the LV-909 and the 48-7S in a large portion of the low-mass low-ÃŽâ€v quadrant, but the solutions involving it require literally dozens of LV-1's (for example, pushing 5 t of payload with 2000 m/s of ÃŽâ€v and a minimum TWR of 0.5 needs 30 LV1's).

Edited May 15, 2015 by Meithan

[GoSlash27]

meithan,

You probably want to verify your thrust and Isp numbers with the 1.02 config files. I ran an analysis a couple weeks ago at a couple of these points and came up with some different results.

My figures had the Rhino and Poodle as the best engines to use at 1,500 m/sec and .5G acceleration..

Best,

-Slashy

[Meithan]

meithan,

You probably want to verify your thrust and Isp numbers with the 1.02 config files. I ran an analysis a couple weeks ago at a couple of these points and came up with some different results.

I followed your advice and just checked my values against the .cfg file and I indeed had incorrect values for the RAPIER and Aerospike engines (e.g., Aerospike had 390 s Isp, but it's actually 340 s). Thanks for pointing out this discrepancy! I've now corrected my tables.

With this correction, at 1500 m/s delta-v and min accel. 0.5g, these are the best engines as a function of payload mass according to my calculations:

payloads up to 0.4 t: 24-77

0.4 - 3.4 t: 48-7S

3.4 - 20.5 t: LV-909

20.5 - 29.0 t: Poodle

29.0 - 40.9 t: LV-909

40.9 - 57.9 t: Poodle

57.9 - 61.3 t: LV-909

61.3 - 100 t: Poodle

Here's the corrected chart (and I've updated my previous post):

My figures had the Rhino and Poodle as the best engines to use at 1,500 m/sec and .5G acceleration.

The Poodle and LV-909 now seem to alternate as best options above 3.4 t. But I'm not seeing the Rhino yet, not even as second best engine.

For what payload are you finding the Rhino is a close contender at 1500 m/s and 0.5 g?

[GoSlash27]

I followed your advice and just checked my values against the .cfg file and I indeed had incorrect values for the RAPIER and Aerospike engines (e.g., Aerospike had 390 s Isp, but it's actually 340 s). Thanks for pointing out this discrepancy! I've now corrected my tables.

With this correction, at 1500 m/s delta-v and min accel. 0.5g, these are the best engines as a function of payload mass according to my calculations:

payloads up to 0.4 t: 24-77

0.4 - 3.4 t: 48-7S

3.4 - 20.5 t: LV-909

20.5 - 29.0 t: Poodle

29.0 - 40.9 t: LV-909

40.9 - 57.9 t: Poodle

57.9 - 61.3 t: LV-909

61.3 - 100 t: Poodle

Here's the corrected chart (and I've updated my previous post):

http://i.imgur.com/LATm48am.png

The Poodle and LV-909 now seem to alternate as best options above 3.4 t. But I'm not seeing the Rhino yet, not even as second best engine.

For what payload are you finding the Rhino is a close contender at 1500 m/s and 0.5 g?

Meithan,

Thanks again for your work on bringing this awesome resource back to us in 1.02. You rock out loud!

My calculations show that the Rhino achieves over 57% payload fraction in this scenario at 232 tonnes payload, making it the single most mass-efficient engine in this instance.

Your analysis is much more comprehensive than mine, since it provides stage mass for all possible configurations at a given t/w.

I am limited by iterative analysis at discrete points, so I'm looking for highest payload fraction in common scenarios. Your results show which engine is most mass-efficient at any given point, but doesn't show exactly how mass- efficient it actually is.

If we think of your plot as a topographical map, my data would represent the third dimension of "height" for each engine. Unfortunately, I can only see results for the highest point whereas you can see the lay of the land, which is actually more useful IMO.

Best,

-Slashy

Edited May 15, 2015 by GoSlash27

[Meithan]

Meithan,

Thanks again for your work on bringing this awesome resource back to us in 1.02. You rock out loud!

No need to thank, really. I'm planning on releasing the full analysis soon.

My calculations show that the Rhino achieves over 57% payload fraction in this scenario at 232 tonnes payload, making it the single most mass-efficient engine in this instance.

Ah, my chart was only calculated to 100 t, that's why I wasn't seeing the Rhino. Indeed, I find that for payloads from 174 t to 232 t, the Rhino is the best engine for 1500 m/s and min accel 0.5g. Here are the details output by my program:

==Mass analysis for KR-2L engine ==
Payload = 232.000 tonnes
Delta-v = 1500 m/s
Minimum TWR = 0.50

Total ship mass: 405.660 tonnes
 Payload: 232.000 t
 Propellant: 146.809 t
 Tankage: 18.351 t
 Engines: 8.500 t
Number of engines: 1
Minimum accel: 4.93 m/s^2
Maximum accel: 7.73 m/s^2
Payload fraction: 57.2%

At least we're getting the same payload fraction, which is a good sign .

Your analysis is much more comprehensive than mine, since it provides stage mass for all possible configurations at a given t/w.

I am limited by iterative analysis at discrete points, so I'm looking for highest payload fraction in common scenarios. Your results show which engine is most mass-efficient at any given point, but doesn't show exactly how mass- efficient it actually is.

If we think of your plot as a topographical map, my data would represent the third dimension of "height" for each engine. Unfortunately, I can only see results for the highest point whereas you can see the lay of the land, which is actually more useful IMO.

You're quite right, that engine chart does not tell the whole story. It's meant to summarize some of the results for a wide range of conditions.

I've also been calculating more detailed curves that do show how different engines compare for a given dv. Here's an example for a 10 t payload and a minimum TWR of 0.5:

I'm still working on the presentation; these have a lot of info.

[Rune]

I will peruse those charts when/if they come out! I also have a feeling the aerospike increased in utility a lot. It'll give the 909 a run for its money in TWR now, and the isp is right there with the high-efficiency engines... while it retains an awesome sea level isp. Used to be too heavy to really be worth it, but now it must be a decent choice even if your ship will only see atmosphere rarely. What can I say? I'm a sucker for second-best.

Also, I'm very glad to see the Poodle is now the go-to engine for your everyday interplanetary transfer, if you are impatient (me, I'm a fan of 1m/s^2 ejections on nukes).

And you know what this (awesome) analysis also doesn't show? The fact that all isp's got a nerf except the LVN means that for the really big dV budgets (>4-5km/s), the Nerv is now literally the only choice: the tankage ratio in ksp imposes a theoretical ceiling of 7.something km/s for chemical engines (don't worry, that's tankage only, you'll sweat to break 4km/s if you add the engines and stuff), while the LV-N's keeps almost unchanged at >16km/s (of course, you'll again be lucky to get half of that in a real design).

Rune. Thanks for showing us your efforts!

[GoSlash27]

No need to thank, really. I'm planning on releasing the full analysis soon.

Ah, my chart was only calculated to 100 t, that's why I wasn't seeing the Rhino. Indeed, I find that for payloads from 174 t to 232 t, the Rhino is the best engine for 1500 m/s and min accel 0.5g. Here are the details output by my program:

==Mass analysis for KR-2L engine ==
Payload = 232.000 tonnes
Delta-v = 1500 m/s
Minimum TWR = 0.50

Total ship mass: 405.660 tonnes
 Payload: 232.000 t
 Propellant: 146.809 t
 Tankage: 18.351 t
 Engines: 8.500 t
Number of engines: 1
Minimum accel: 4.93 m/s^2
Maximum accel: 7.73 m/s^2
Payload fraction: 57.2%

At least we're getting the same payload fraction, which is a good sign .

You're quite right, that engine chart does not tell the whole story. It's meant to summarize some of the results for a wide range of conditions.

I've also been calculating more detailed curves that do show how different engines compare for a given dv. Here's an example for a 10 t payload and a minimum TWR of 0.5:

http://i.imgur.com/wSjQloym.png

I'm still working on the presentation; these have a lot of info.

Meithan,

That serves as confirmation for both of us. Our models agree precisely

*proof of concept dance*

Best,

-Slashy

[NikkyD]

There are some things that you don't consider.

The 909 is so short that you need less fairings which means less weight, the LVN is really long and require larger fairings and thus more weight.

[KerikBalm]

Maybe your design has no fairing around the engines to start with?

But on another point:

I think you should also make an optimal "fuel mass" chart.

In the past, I've looked at Tavert's charts, and disregarded the "mass optimal" engine, and done the math instead to fine a "fuel optimal" engine.

If I'm going to make a re-usable design, like a single stage Mun lander that docks with a fuel depot, or a tug to eject payloads from the Kerbin system, or a fuel tanker to bring fuel from an ISRU base to a LKO fuel depot... or something to ferry fuel to a Duna fuel depot (pre-ISRU) - then I want something that will consume the least amount of fuel.

With those charts;

If engine X is 2.5 tons heavier than engine Y,

but engine Y uses 2.4 tons more fuel than engine X,

Then it will say engine Y is better.

If I use them each 3 times, then engine Y will need 3*2.4 tons more fuel, while engine X simply masses 2.5 tons more.

In total, I'd have to lift 3*2.4-2.5=4.7 more tons of stuff to orbit with engine Y, and engine X is superior for my purposes.

This mainly concerns making efficient landers, so the orbiting fuel depots don't need refills so often, but it should also apply to orbital tugs (which can quickly repay the mass penalty of spamming LV-Ns in the form of much less reaction mass needed before they are ready to go again,

[Meithan]

I also have a feeling the aerospike increased in utility a lot. It'll give the 909 a run for its money in TWR now, and the isp is right there with the high-efficiency engines... while it retains an awesome sea level isp. Used to be too heavy to really be worth it, but now it must be a decent choice even if your ship will only see atmosphere rarely. What can I say? I'm a sucker for second-best.

Yes, the aerospike is now somewhat frequently among the top runner-ups, so it can be selected even if it's not optimal (e.g., for aesthetic reasons) without incurring a big mass penalty on your ship.

Example: for 2000 m/s delta-v and a 20 t payload with min TWR of 0.7, the top 3 engine choices are six LV-909's, two Aerospikes or two Poodles, at total ship masses of 46.1, 46.7 and 46.7 tonnes, respectively. They're so close that any will do.

And you know what this (awesome) analysis also doesn't show? The fact that all isp's got a nerf except the LVN means that for the really big dV budgets (>4-5km/s), the Nerv is now literally the only choice: the tankage ratio in ksp imposes a theoretical ceiling of 7.something km/s for chemical engines (don't worry, that's tankage only, you'll sweat to break 4km/s if you add the engines and stuff), while the LV-N's keeps almost unchanged at >16km/s (of course, you'll again be lucky to get half of that in a real design).

Yeah, I also determine the maximum theoretical delta-v for all engines, which depends on the TWR restriction imposed (for no restriction, the best engine is simply the one with the best Isp):

The LV-N is off-scale both on the low and high TWR ends, achieving a maximum delta-v of 16336 m/s at a TWR of 0, 9541 m/s at 0.4 (quite reasonable), and quickly dropping to zero as TWR approaches 2.04 (the TWR of the engine itself, which is the limiting value).

In this particular plot I also included two curves for the PB-ION (the second one including enough solar panel mass to fully satisfy the electricity demand at Kerbin's distance), but I'm not sure I'll add it to the analysis since the problem is different enough that it deserves a separate analysis.

There are some things that you don't consider.

The 909 is so short that you need less fairings which means less weight, the LVN is really long and require larger fairings and thus more weight.

Yeah, I don't consider engine sizes in this analysis. Since "payload" is understood here as "anything that's not fuel, tanks or engines", you'd have to discount the weight of the fairings, if any, in determining the useful payload.

Edited May 15, 2015 by Meithan