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what is the best engine for a 15.4 ton spacecraft?


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7 hours ago, GeneralVeers said:

Then we're at a bit of an impasse, because we seem to disagree on who's the one saying things that are patently false.

I could happily argue my way through any impasse, and the brick wall at the end of it. :D

However I only stepped back into this thread because of the confusion about fuel and oxidizer, mass, and what changes with a nuclear engine. As long as there is no more confusion about the meaning of Isp, then all is well.

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The thing bothering me about all this is the gravity losses for Duna takeoff.  Duna's near-vacuum, why calculate these things for a vertical initial ascent?  That 44% gravity loss when going vertical becomes more like 18% when you're putting all your acceleration to horizontal.  An ascent angle of 45 degrees (net acceleration not thrust direction) is more like 36%, and you might even go lower than that.

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1 hour ago, Kryxal said:

The thing bothering me about all this is the gravity losses for Duna takeoff.  Duna's near-vacuum, why calculate these things for a vertical initial ascent?  That 44% gravity loss when going vertical becomes more like 18% when you're putting all your acceleration to horizontal.  An ascent angle of 45 degrees (net acceleration not thrust direction) is more like 36%, and you might even go lower than that.

Same as ever: to get a non-vertical ascent, you need a high TWR. If you have a high TWR, you're carrying more engine than you need for your payload.

Of course it is a trade-off: ease of use vs cost. Go too low on TWR and you lose more to gravity than you gained in cost/payload/fuel efficiency or whatever, and that is precisely where the LV-N starts being a poor choice to put on the ground on Duna.

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On 2/16/2016 at 10:03 PM, Snark said:

Yup.  Unrealistic.  Because people don't land ships like that, generally.  You didn't.  Your craft was 15 tons, not 11.  If that's the example you mean, then use that example to prove your point, not a stripped-down version.

Done. Prepare to be disappointed again.

Two 15-ton landers. One with a Poodle, the other with two cancer-causing engines (lol). Shipped 'em both out to Duna, landed them both in the same spot (used cheats to get them there so they were both full payload and therefore the same mass sitting together on Duna). Launched both to a 60k circular orbit.

Here's the final numbers: Poodle used 964 total fuel units (fuel + oxidizer) to reach the finish line. Cancer only needed 452. Poodle needed more than twice as much fuel to reach the same orbit. On Duna. With a five-ton payload. Nukes win.

 

On 2/16/2016 at 10:03 PM, Snark said:

But the fact is that massive, low-TWR engines have a constrained "usability envelope" when it comes to powering landers.

Are you sure? One of those "constraints" just got busted.......

Next up: Minmus lander propelled by ion engines! (oh, God, Veers is getting worse rather than better..........)

 

On 2/16/2016 at 10:03 PM, Snark said:

So giving advice to a newbie, I'm not going to recommend "use nukes".

And my recommendation: definitely try them out. One LV-N will give a 15-ton ship plenty of zip--and great mileage. Two Spark engines or one Terrier will probably also do the job, the catch there is we don't know what the job is. ^_^

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4 hours ago, GeneralVeers said:

Done. Prepare to be disappointed again.

Two 15-ton landers. One with a Poodle, the other with two cancer-causing engines (lol). Shipped 'em both out to Duna, landed them both in the same spot (used cheats to get them there so they were both full payload and therefore the same mass sitting together on Duna). Launched both to a 60k circular orbit.

Here's the final numbers: Poodle used 964 total fuel units (fuel + oxidizer) to reach the finish line. Cancer only needed 452. Poodle needed more than twice as much fuel to reach the same orbit. On Duna. With a five-ton payload. Nukes win.

Not sure what you mean by "disappointed"-- if you can find a good use case that helps folks, great!

The numbers were kinda interesting-- plugging in the fuel consumption you give, and assuming vacuum Isp, I come up with 1330 m/s of dV for the Poodle and 1280 m/s for the nukes, which seems off (I'd expect the Poodle to need somewhat less dV than the nuke.  Not hugely, but some, and certainly not more.)  What altitude were you launching from?  Just now tried a test of my own with a 15-ton Poodle ship from 1800m, did several launches, and pretty consistently came in at about 895 units of LF+O (within plus/minus about 4 units, it was remarkably consistent).  That comes out to 1215 m/s, which feels more reasonable to me, though it's not really apples-to-apples if it wasn't from the same elevation you launched from.  (Didn't have the patience to build a nuke ship and test that from the same place.)  The shape of the craft also matters, somewhat-- the Poodle ship will be going faster when it's lower, so cares about air resistance more than the nuke ship would.  An un-aerodynamic shape would hurt the Poodle more than the LV-Ns.

So it comes out to about twice as much fuel by mass, not all that surprising given the relative Isp.  The nuke has 2.29 times the Isp, the Poodle uses about 2x the fuel.  The disparity (2.29x vs 2x) seems reasonable, given the increased gravity losses from the nuke.  So the numbers are roughly what one would expect, no big surprises there.

But... the Poodle's carrying 4.25 tons less engine than the nuke-powered ship.  It uses about 4.48 tons of fuel (my number) to get to orbit.  So it uses 2.22 tons more fuel than the nukes do, but it's got an extra 4.25 tons to spare.  Meaning it can loft a couple more tons of payload to orbit than the nukes can.  Poodle wins.  :)

Of course, saying "nukes win" or "poodle wins" is kind of beside the point (not to mention starting to sound a little Charlie Sheenesque), because it completely depends on what is meant by "winning".  Lower fuel mass consumption?  Higher payload to orbit?  Lower lander mass for same payload?  Buildable at lower tech level?  Style?  Context is everything.  Depending on what the player is trying to do, any of the above might be the "win" definition.

Clearly the nukes win for "lower fuel mass consumption," which comes as no surprise to anyone-- that's the whole point of nukes, after all, it's the one thing they're good at.  So if you've got a setup where that matters more than anything else (e.g. you plan to shuttle back and forth from surface to orbit repeatedly, and you don't have an ISRU setup to replenish fuel), then yeah, they're going to be attractive.  On the other hand, there are other scenarios where they'll lose out.  Player in early career, where tech is an issue, cost is an issue, ship size is often really an issue?  Smaller, lighter, low-tech landers win over nukes.  Got an ISRU setup on the surface and are hauling fuel to orbit?  Higher payload fraction means Poodle wins over nukes.  Got an ISRU setup on Ike and you have fuel coming out of your ears and it doesn't really matter one way or the other?  Everybody wins, just build whatever looks coolest.  :)

(Which I suppose would be the nukes, hands down... they're pretty to look at, and it's hard to look a lot less cool than a Poodle.  It's the minivan of rocket engines.)

Edited by Snark
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6 minutes ago, Snark said:

The numbers were kinda interesting-- plugging in the fuel consumption you give, and assuming vacuum Isp, I come up with 1330 m/s of dV for the Poodle and 1280 m/s for the nukes, which seems off (I'd expect the Poodle to need somewhat less dV than the nuke.  Not hugely, but some, and certainly not more.)

Call it human error. I'm pretty sure my two test ships didn't follow exactly the same path.

 

6 minutes ago, Snark said:

What altitude were you launching from?

Don't remember. Some ordinary plain on Duna. Doesn't matter, because the two ships started right next to each other.

 

6 minutes ago, Snark said:

Just now tried a test of my own with a 15-ton Poodle ship from 1800m, did several launches, and pretty consistently came in at about 895 units of LF+O (within plus/minus about 4 units, it was remarkably consistent).

Sounds about right. That's only about 50 fuel less than my run used.

 

6 minutes ago, Snark said:

But... the Poodle's carrying 4.25 tons less engine than the nuke-powered ship.  It uses about 4.48 tons of fuel (my number) to get to orbit.  So it uses 2.22 tons more fuel than the nukes do, but it's got an extra 4.25 tons to spare.  Meaning it can loft a couple more tons of payload to orbit than the nukes can.

But it didn't. You said "use the actual Duna lander". That means a five-ton payload. Stop moving the damn goalposts.

Wanna go bigger payload? Let's go bigger payload. My Munar mining ship. Four nuclear engines, radial mounts. Total mass with full load of fuel and ore? SEVENTY TONS. Boo. Yah. Oh, and she has an ISRU on board. Stylin'!

This monster burns about 1200 fuel to reach low Munar orbit. Took off the LV-N's and slapped a Poodle on. This turned out to be a bit of a problem, because the Poodle engine stuck out past the landing gear. Made for a comical landing. Had to balance on the tiny engine nozzle, like those alien ships in Bugs Bunny cartoons that land on a pinpoint. :lol: Anyway, Poodle no longer needed the LV-N's; took those off. Poodle did need oxidizer. Added that. Turned out no weight was saved.

So, two ships. Both 70 tons. Almost the same thrust (250 Kn vs. 240). Almost the same TWR. Poodle had a slight advantage here because of that. Yet Poodle burned 1900 fuel to reach the same orbit.

Same mass. Same mass fraction. Same thrust. Same TWR. Different performance. Nukes better.

For pretty much any mass of payload, anywhere except Kerbin, Eve, or Laythe (or Jool? lol!) I've been able to build nuclear ships that perform better than O2 cookers in terms of total ship mass, operational range, and fuel consumption. Especially for a lander that's planning to land on one planet several times and collect lots of science. Lower fuel consumption by the lander means less fuel needed on the mothership, and THAT factor outweighs almost all other design considerations.

Where do LV-N's fall short? Cost in credits. Higher in the tech tree. No good for tiny space probes. No good in dense atmosphere. And that's about it. None of these things have been a problem in either Science or Career mode.

 

6 minutes ago, Snark said:

Of course, saying "nukes win" or "poodle wins" is kind of beside the point (not to mention starting to sound a little Charlie Sheenesque), because it completely depends on what is meant by "winning".  Lower fuel mass consumption?  Higher payload to orbit?  Lower lander mass for same payload?  Buildable at lower tech level?  Style?  Context is everything.  Depending on what the player is trying to do, any of the above might be the "win" definition.

That's just it. We weren't given a context, cuz JWOC didn't say what he's doing with this 15-ton ship of his. Probably top secret spy missions. We have no context, so off the reservation we go! :lol:

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2 minutes ago, GeneralVeers said:

But it didn't. You said "use the actual Duna lander". That means a five-ton payload. Stop moving the damn goalposts.

Sure it did.  What moving goalposts are you talking about?  We're talking about exactly the test you proposed, yes?  As in:

5 hours ago, GeneralVeers said:

Two 15-ton landers. One with a Poodle, the other with two cancer-causing engines (lol).

Which means that the Poodle has a bigger payload because less of the mass budget is going to the dead weight of the engines.  The 15-ton nuke ship gets the nukes, plus 9 tons to play with, which includes fuel and payload.  The 15-ton Poodle ship gets the Poodle, plus 13.25 tons to play with, which includes fuel and payload.

A poodle weighs a lot less than an LV-N.  Which means you've got more payload available out of that 15-ton budget.

Which ship "wins" depends on what your definition of "winning" is.  In the above scenario, both ships get to orbit quite easily.  The LV-N gets to orbit while using less fuel.  The Poodle gets to orbit with more payload mass.  "Use less fuel" and "have more payload" are both useful things.  Which one is more important depends on the particular situation.

8 minutes ago, GeneralVeers said:

Wanna go bigger payload? Let's go bigger payload. My Munar mining ship. Four nuclear engines, radial mounts. Total mass with full load of fuel and ore? SEVENTY TONS. Boo. Yah. Oh, and she has an ISRU on board. Stylin'!

Well, sure.  This is on the Mun.  Where gravity is barely more than half of Duna's.  Nukes work great on the Mun, I use 'em all the time.  No argument there.  The lower the gravity, the better nukes are gonna be.

26 minutes ago, GeneralVeers said:

Especially for a lander that's planning to land on one planet several times and collect lots of science. Lower fuel consumption by the lander means less fuel needed on the mothership, and THAT factor outweighs almost all other design considerations.

Well, sure, no argument with you there.  But there are different ways to keep fuel usage down.  One way is to go for higher Isp, which is what nukes do.  The other approach is to keep the ship very lightweight, so it simply doesn't need much even with less-efficient engines.  I routinely build very lightweight science landers where the mass of the entire lander, including fuel, is less than the mass of a single LV-N.  They need very little fuel.  And because the lander itself is so lightweight, it helps keep the mothership weight down, too.  Smaller scale mission = cheaper.

22 minutes ago, GeneralVeers said:

Where do LV-N's fall short? Cost in credits. Higher in the tech tree. No good for tiny space probes. No good in dense atmosphere. And that's about it.

Well, that, and you lose the option of "going small" with manned landers, as mentioned above.  But yes, those pretty much line up with what I said in my last post, so as far as I can tell we're violently agreeing with each other, here.

31 minutes ago, GeneralVeers said:

We weren't given a context, cuz JWOC didn't say what he's doing with this 15-ton ship of his.

Actually, we kinda were, because he kinda did.  He specifically said that this ship was not going to land and would be orbital only (to which I replied, "yeah, use a nuke, then").  Which means that this entire back-and-forth discussion is totally moot, as far as the OP's question is concerned.  :)

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I understand what you folks are arguing about, but I haven't the foggiest idea of *why* you're arguing about it.

 The NERV is "better" in some cases and the Poodle is "better" in others. There is no single engine that is "best" in all situations the OP may be describing and we don't have enough info to narrow it down to one engine. So... why all the fuss?

 

 

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Well, here's why all my fuss, GoSlash: I'm a big fan of Mythbusters and GLaDOS, all of whom have this thing about testing.

In several of my posts in this thread, I've done actual tests that disprove pretty much everything Snark has been saying (guess he's been getting SNARKY about it........ :lol:) At every turn, I've been able to design landers that perform better with nuclear engines. Frequently I've actually been able to save weight with them too. Actual test results trump every theory in the book.

Why all of his fuss? Who knows.

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9 hours ago, GeneralVeers said:

Well, here's why all my fuss, GoSlash: I'm a big fan of Mythbusters and GLaDOS, all of whom have this thing about testing.

In several of my posts in this thread, I've done actual tests that disprove pretty much everything Snark has been saying (guess he's been getting SNARKY about it........ :lol:) At every turn, I've been able to design landers that perform better with nuclear engines. Frequently I've actually been able to save weight with them too. Actual test results trump every theory in the book.

Why all of his fuss? Who knows.

The trouble is that you aren't busting any myths here.

The facts are quite simple: with an Isp of 800s, the nuke gives a total amount of thrust over time which is more than twice that of a Poodle with its Isp of 350s, per unit of mass of propellant.

However, the LV-N weighs 3t and produces 60kN thrust, while the Poodle weighs 1.75t and produces 250kN thrust.

Therefore, to get exactly the same amount of thrust, 6 Poodles (10.5t of engine) = 25 LV-N (75t of engine) = 1500 kN.

Under Duna's gravity, 6 Poodles or 25 LV-Ns give a TWR of 1 for a total of about 490 tons. To get 1400 m/s dv, the LV-N would need 88.5t of fuel + tank, while the Poodle would require 177.4t of fuel+tank. After adding the weight of the engines, that gives the LV-N 326.5t of maximum effective payload, and the Poodle 302.15t. Just over 24t advantage for the LV-N.

However, to lift off from the surface you'd really need a TWR of (say) 1.5. That gives a total ship mass of 317t, and to get 1400m/s dv the LV-N would require 57t of fuel + tank to the Poodle's 115t. After adding engine weight of 75t and 10.5t, respectively, that gives the LV-N 185t of maximum effective payload, and the Poodle 191.9t. Just under 7t advantage for the Poodle.

So yes, taking off from the surface of Duna is quite precisely at the point where the advantage switches from chemical to nuclear rockets. If you need more dv, then multiple LV-Ns are going to be better. If you need less dv than that, they will be worse.

There are no myths to be busted - the numbers are what they are.

 

However, where the numbers get more interesting, and where the practical application can be interesting, is when you start considering gravity losses.

Gravity losses are going to be bad for both the Poodle and the LV-N with a TWR of 1.5, to start with.

But gravity losses are going to differ between the two ships as soon as they start burning fuel.

With a quarter of the fuel gone, the LV-N ship has a TWR of 1.7. The Poodle ship has a TWR of nearly 1.9.

With half the fuel gone, the LV-Ns have a TWR of 1.8. The Poodles 2.25. At three quarters gone, the LV-Ns have a TWR of 1.92, the Poodles 2.8.

So total burn times are obviously going to be very different. The LV-N will burn for longer and rack up greater gravity losses. Inevitably.

 

And of course, this isn't considering the cost. For 4% less payload, the LV-N ship will cost 250,000 in engines alone, the Poodle ship only 7,800.

Edited by Plusck
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Going back to the OP's question, I would strongly recommend this tool by Meithan: http://meithan.net/KSP/engines/

Plugging in a payload of 15.4 to 20 tonnes at Duna for 1000 to 3000 m/s dv gives this result for the "winner":

Spoiler

ZU9OCLT.png

The blue spike on the left corresponds to 3 LV-Ns, and the blue area on the right to 4 LV-Ns. So yes, the OP's ship is precisely at the edge here.

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And after using the Optimizer, test the ship to make sure it actually does what you think it will. Cuz this is Kerbal Space Program, and anything that can go wrong, will, and anything that can't go wrong will sometimes find a way to go wrong anyway.......

 

7 hours ago, Plusck said:

The trouble is that you aren't busting any myths here.

The facts are quite simple: with an Isp of 800s, the nuke gives a total amount of thrust over time which is more than twice that of a Poodle with its Isp of 350s, per unit of mass of propellant.

However, the LV-N weighs 3t and produces 60kN thrust, while the Poodle weighs 1.75t and produces 250kN thrust.

This is not actually the case. You need to take the fuel load into consideration......and that adds a lot of weight to all O2 cookers.

 

7 hours ago, Plusck said:

But gravity losses are going to differ between the two ships as soon as they start burning fuel.

With a quarter of the fuel gone, the LV-N ship has a TWR of 1.7. The Poodle ship has a TWR of nearly 1.9.

With half the fuel gone, the LV-Ns have a TWR of 1.8. The Poodles 2.25. At three quarters gone, the LV-Ns have a TWR of 1.92, the Poodles 2.8.

So total burn times are obviously going to be very different. The LV-N will burn for longer and rack up greater gravity losses. Inevitably.

Inevitably. Except that the actual liftoff tests I did, and documented in this thread (documented?? when did this turn into a Senate hearing?? :lol:) didn't produce a measurable difference even though the Poodle had twice as much thrust as the nuclear setup.

Oh, wait, there is one measurable difference. Which I already mentioned but which is worth repeating. Whatever the above ships in your description just did, if they want to dock with their motherships and then do the same thing again? Poodle will need to refuel with roughly twice the mass of fuel as the nuke, forcing its mothership to carry a much larger fuel load. Where does she get that fuel?

Usually from Kerbin.

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