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Anonymmm

Why is my spacecraft so inefficient - can it at least get tu Duna (and back)?

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Hello again,

I recently build a spacecraft which I called "Mun Giant" (-backstory to why I build it at the bottom of the page) and, surprisingly, it worked! And lifted off on my first attempt too, yay! However, while it gets the job done, I find that it has very very little delta v. Why is this so - can I improve the design? The "Mun Giant":

m0oJwhP.png

I highlighted all the fuel tanks and engines, and yes, if you have good eyes, you can probably spot the two Vectors at the back, but I use those only for suicide burns if I have enough fuel left and feel lazy to do a proper landing.

Ok, and now for my second question. Can I get this to Duna (and back - it can mine and refine fuel)? I do not know how to calculate delta v, but I heard that It requires dry a full mass, so here you go:

Dry mass: 74.265 t

Full mass: 172.265 t

Engines: On picture

Backstory: Well basically, I got a crazy contract for a Mun base on wheels with 6 000 units of fuel and capacity for sixteen passengers, plus all the usual stuff, antenna, docking port, etc. which in total paid 1 000 000  (money) and this was the result (cost about 600 000 with boosters). I now use it to fly between Mun and Minmus and do a bunch of rescue, mining and flag planting contracts which gets my a ton of money since I do all the contracts using a single spaceship and refill using drills and ISRUs.

Thanks for any comment,

Anonymmm

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Posted (edited)

If you have enough fuel to get to Minmus and refuel there, then you have enough to get to Duna and back -- because it takes about as much to get to Duna from Minmus as it does to get from LKO to Minmus.

And it's inefficient because it's big. :) If you want it to be efficient, you have to make it be just barely big enough to do everything you need to do -- but no bigger than that.

 

Edited by bewing
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To calculate your delta V in meters per second, take the ratio of full mass to dry mass (i.e. 172.265 divided by 74.265 in this case), and take the natural log. (This is the ln button on a calculator). Multiply the result by 9.81, and multiply that by the isp of the engines you're using (In this case 350). This gives a result of 2889 m/s (rounded to the nearest m/s), of delta V.

Starting from the surface of Minmus with a full tank, that's definitely enough to get you to Duna orbit or to the surface of Ike. Depending on your ship's re-entry characteristics, you might be able to get to Duna's surface as well. Your thrust/weight ratio is pretty anemic, though, so it would be a tricky maneuver.

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34 minutes ago, Allocthonous said:

To calculate your delta V in meters per second, take the ratio of full mass to dry mass (i.e. 172.265 divided by 74.265 in this case), and take the natural log. (This is the ln button on a calculator). Multiply the result by 9.81, and multiply that by the isp of the engines you're using (In this case 350). This gives a result of 2889 m/s (rounded to the nearest m/s), of delta V.

Starting from the surface of Minmus with a full tank, that's definitely enough to get you to Duna orbit or to the surface of Ike. Depending on your ship's re-entry characteristics, you might be able to get to Duna's surface as well. Your thrust/weight ratio is pretty anemic, though, so it would be a tricky maneuver.

Hey, thanks a bunch for the calculations, you helped me a lot! :)

Actually, there are two more Vectors at the end, so my TWR shouldn't be that bad... I mean, 4 Vector engines should be enough, right? :D

Thanks again everyone! :)

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6 hours ago, Anonymmm said:

Hey, thanks a bunch for the calculations, you helped me a lot! :)

Actually, there are two more Vectors at the end, so my TWR shouldn't be that bad... I mean, 4 Vector engines should be enough, right? :D

Thanks again everyone! :)

Ah, I was just looking at the ones you had the tooltips open for. Yeah, that should make the speed changes for re-entry pretty easy.

Can we see the back end of that thing? I have no clue how you can fit nine engines on it when there aren't really any visible in the picture.

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11 hours ago, Allocthonous said:

Ah, I was just looking at the ones you had the tooltips open for. Yeah, that should make the speed changes for re-entry pretty easy.

Can we see the back end of that thing? I have no clue how you can fit nine engines on it when there aren't really any visible in the picture.

Sure, here you go:

4PG213Y.png

:D

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You can aerobrake into Duna easily.    For landing it, chutes + thrust supported landing (for the final few metres)  or wings plus vernor thrusters (for the final few metres).

Taking off again is the more demanding bit, since you need thrust > weight and you'll be twice as heavy with fuel. But this ship has plenty of thrust.

I'm just wondering if you can convert it into something NERV powered, which will cost most of the TWR but give you loads more Delta V.    If it is made into a spaceplane,  the TWR requirement for Duna goes down due to wing-supported flight.

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7 hours ago, AeroGav said:

You can aerobrake into Duna easily.    For landing it, chutes + thrust supported landing (for the final few metres)  or wings plus vernor thrusters (for the final few metres).

Taking off again is the more demanding bit, since you need thrust > weight and you'll be twice as heavy with fuel. But this ship has plenty of thrust.

I'm just wondering if you can convert it into something NERV powered, which will cost most of the TWR but give you loads more Delta V.    If it is made into a spaceplane,  the TWR requirement for Duna goes down due to wing-supported flight.

Well, I am very lucky. Just spend the whole day trying to design an upgraded nerv powered version, which still has quiet a few problems and guess what I find on the forum! :) Behold the mighty "Duna Giant A" everyone:

NyKuwn9.png

This is still a very WIP projects. And first of all, let me just state that this is not (should it be?) an SSTO - I get it to orbit using a pair of 200 000 mk 3 fuselage rocket "boosters". And it has quiet a few problems... so, since you were wondering and I never build an interplanetary ship nor anything like an SSTO before, let me throw out some questions:

1) How do I land this at Kerbin? I have been trying to fit it with some aeroplane engines and intakes, plus parachutes, but it's so heavy that I would need just a motherload of them to fly this thing. And the worst part, it can't even glide, needs tons of thrust to stay in the air and not pitch down and fall . So, should I fit it with tons of engines and intakes or tons of parachutes or maybee just leave it more or less without anything and hope the crew cabin survives the crash landing?

2) How do I take off from duna? This is my current propulsion concept:

N8kIarP.png

...however, the Vectors only have the fuel for like a ten second full thrust burn, since I want to take as much liquid fuel only tanks as possible. Is ten seconds enought(to get into low duna orbit - I will obviously also use Nervs all the way)?

3) Considering that this weight about 250 tons (if I remember correctly) should't I add some more Nervs so my TWR isn't that bad?

4)  So, could this be an SSTO or should I just stick with the more sort of space shuttle idea?

 

I know that I am kinda demanding huge things, so if you don't have the time, just don't answer :)

All comments are very much appreciated,

Anonymmm

 

 

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Posted (edited)

29 minutes ago, Anonymmm said:

1) How do I land this at Kerbin? I have been trying to fit it with some aeroplane engines and intakes, plus parachutes, but it's so heavy that I would need just a motherload of them to fly this thing. And the worst part, it can't even glide, needs tons of thrust to stay in the air and not pitch down and fall . So, should I fit it with tons of engines and intakes or tons of parachutes or maybee just leave it more or less without anything and hope the crew cabin survives the crash landing?

How heavy is it again?  It looks like it has 2 pairs of big S wings .  That's a little low for landing offworld, but should be flyable on Kerbin.

Is the lack of lift due to not being able to pitch the nose up enough?    

You should be able to pitch the nose at least 10 degrees above prograde.    The trick with airplane design is getting enough pitch authority to do this in all conditions whilst still being stable in all conditions.   Note that flying with 10 degrees of pitch is acceptable when you're landing, but if flying to orbit like a spaceplane (you are not, at the moment) this is very draggy.   You want enough wing to be able to cruise with the nose only 5 degrees above prograde, at reasonable altitudes.  

Re: Jet engines.  -  if you are using them to take off like an SSTO,  use some Rapiers.   Put some Whiplash pods on decouplers to use as boosters to get past mach 1 up to the sweet spot of the rapier engines when launching out of kerbin.    If you are just rocket launching out of kerbin, forget about jet engines for landing.     That's just extra weight which raises the stall speed and cuts delta v. 

Concentrate on improving the handling and lift so it can glide properly and land in a field without damaging itself.       My massive Jool 5 spaceplane touched down at under 25 m/s on the island airstrip, though it was empty at this point, no fuel, rover gone, tylo lander gone, and all the jet engines jettisioned ! 

 

 

If you want to land near KSC,  that's a skill you can learn later, but it's all about controlling drag in re-entry with nose angle.  If you come down far from KSC you'll never have enough fuel to fly all the way back, nor enough patience. 

Edited by AeroGav

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35 minutes ago, Anonymmm said:

...however, the Vectors only have the fuel for like a ten second full thrust burn, since I want to take as much liquid fuel only tanks as possible. Is ten seconds enought(to get into low duna orbit - I will obviously also use Nervs all the way)?

Duna's gravity is tiny, less than a third of Kerbin's,  and the atmosphere is very thin, so the NERVs will give full thrust at ground level.     If you can get it flying like a proper airplane, those 7 nervs will make it climb fast.   Orbital velocity's only about 700 m/s or something.   Honestly I might forget about those Vectors.     They are very heavy and have terrible vacuum ISP, even for a chemical engine.    

Here's a landing and takeoff from Duna, with an orange tank.  Probably still lighter than your ship but you get the idea. 6 nervs.

Lots of vernier lift engines.   On kerbin landing speed is 30m/s, without vernier lift motors.  I think you can still download this craft from my Kerbal X,  the "Wyvern"

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@AeroGav

Thanks a bunch for all the detailed answers! You are really helping me a lot - I remember that the last thing I was doing before I read you replies was attaching even more jet engines and intakes so it could fly better and then stuffing it with almost a hundred of those big parachutes so I could fly back to KSC an then cut the engines and slowly fall down on the chutes - now I realise how stupid that plan was :D ! So, as you told me, I added more lift (actually, if you look closer at my last picture, you can see that it had two pairs of three big delta wings joined using the move tool, not only two pairs of "normal" delta wings, but that's just a detail), removed the jet engines (thank god you told me to do that), removed the Vectors and, since the last craft was very unpractical anyway, implemented all these tips of your in a new version. By the way, you know what was ultimately the BIGGEST mistake in the design? Try to guess, I will post the picture of the new version without the mistake and the answer will be down below - hint, it has something to do with control surfaces. So, my new version, prepared to launch to space (I put it on the runway, but that's just for it to look better):

bREssXP.png

Yeah, if think it's pretty obvious. My first concept had no vertical control surfaces :D:D:D. All the control problems and mysterious falls are now explained! So, anyway, here is the propulsion:

VfCYpEB.png

...and the launching stage:

wkqKjZZ.png

Yeah, haven't test the whole launching concept much yet... maybe you cold give me some tips on that?

So, overall, the "DG - B" handles wonderfully in the air at both low and higher speeds, I am more then capable of pulling off an excellent glide and landing with it and using the few chutes at the back it brakes to a stop almost instantly. It also has a very good delta v, I think about 7500, if my calculations are correct. The only thing I am worried about really is TWR (it has 11 Nervs, yes, but also weights 290 tons )... so, am I ready to go to Duna (and maybe even further) with it? If you or anyone else has any more tips (especially about the whole booster launching contraption), please tell me, I will be more than happy to try to implement them.

 

Thanks again @AeroGav,

anybody else, please feel free to contribute too if you want to, all ideas welcome,

Anonymmm

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Posted (edited)

1 hour ago, Anonymmm said:

he only thing I am worried about really is TWR (it has 11 Nervs, yes, but also weights 290 tons ).

That's a thrust to weight ratio of 0.85 to 1 on Duna gravity.  So it cannot takeoff vertically with full tanks.

Horizontally it should be fine,  unless it has the aerodynamics of a brick, you should be seeing lift to drag ratio of 4:1 below mach 1, 2:1 above it, but by the time you get supersonic on Duna,  orbital freefall effect will already be starting to support your weight.

With a 3:1 lift:drag ratio,  in horizontal flight,  the wings of a 300 ton vessel will create 100 tons of drag.  If you got 100 tons of horizontal thrust, you can stay up.

Recommend you install Kerbal Engineer mod so you can simulate all this from the VAB.

The hardest thing on Duna is not breaking up on landing.   Despite the lower gravity, the thin air means landing speeds are two or three times higher, and the whole place is hilly, like the highland biome on Kerbin. 

What you need are a good amount of Vernor engines in the belly to reduce landing speed.

Coming in to land, you're going to be nearly empty, so mass = 170 Tons

Duna gravity is about 3 m/s,  so 170  x  3   =  510 Kn Downward force

Each Vernor is 12kn Thrust,  510 divided by 12 is about 42 - in other words, about 42 of them would be enough for you to stop the plane completely in the air and land on thrusters alone.     That is silly.

But if you went for about 60% of that amount -  say 26 thrusters - you'll reduce the stalling speed a lot.

You'll need a bit of oxidizer for those things.   

Each Vernor uses half a unit per second, so with 26 = 13 oxidizer per second.   30 seconds continuous firing would need 390 oxidizer.   The idea is you're using then at the last moment to flare for touchdown.     After stopping, the oxidizer storage is good to have, it can power the fuel cells to keep your drills going overnight.

 

Landing technique -

Keep rate of descent to about 5m/s on the last few hundred metres.   Don't be fooled by the low gravity - your ship has a lot of inertia and there isn't much lift (thin air), so get into a high rate of descent near the ground you're going to smack in hard.

As you slow down, keep pitching up to maintain lift, until pitch angle reaches about 10 degrees.    After that, leave pitch angle where it is.  Pitching up more won't allow you to get much more lift, but raises the risk of tail strike/tumbling after touchdown.  Instead, activate your Vernors (RCS) and control rate of descent by tapping the K key (RCS translate UP).

It is a good idea to organise the landing gear so that the ones that touch the ground first are just behind your centre of gravity, as close as possible.  This minimises the rotation torque from the landing impact, means less chance of tumbling on landing.

 

 

 

Edited by AeroGav
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1 hour ago, Anonymmm said:

you or anyone else has any more tips (especially about the whole booster launching contraption), please tell me, I will be more than happy to try to implement them.

I'm not great at boosters, your design looks good to me (symmetrical).     The only further advice i can give is on minimising drag.  Unless you're going to convert to a horizontal launch, you don't need to worry about that right now.

All i would say is that the most important rule is that every fuselage section, be it a 1.25m engine nacelle or the mk3 , must have something pointy on its front and back attachment nodes.   Every time you join one part to another, the part you are adding must be of the same diameter as the attachment node you're putting it on or you need to use a size adapter to ensure a smooth transition.

I can't see how you're doing the back end, I have a subassembly here that's the lowest possible drag way to attach 6 nukes to a mk3 fuselage.   It uses a mk3 engine mount , with nukes directly attached to the 1.25m nodes and a triple adapter on the central 2.5m node.  All the engines have cones on the back for low drag.  This does put 19 tons on the back end of your craft so i'd personally hang the remaining nukes radially further up the fuselage, maybe on engine pods above and below the wing?

https://kerbalx.com/AeroGav/Streamlined-6-Nuke-Mk3-engine-mount

20170313095643_1_zpsuoxx31iu.jpg

copy the file to (substitute Shuttle challenge for whatever your current game is called)

C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\saves\SHUTTLE CHALLENGE\Subassemblies

 

One final thing, you've got a lot of engines on the back end,  with most of the fuel closer to the front, is the centre of gravity still ok with the fuel tanks empty?  Get RCS Build Aid mod, it shows a red indicator of where your empty CoM will be.

 

 

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@AeroGav,

thanks again! Just wanted to let you know that I read everything, just didn't have much during the past few days, so no KSP :( However, I will try to install the engine mount and all the other stuff as soon as I possible. 

 

Happy KSPing,

Anonymmm

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Posted (edited)

11 hours ago, AeroGav said:

I'm not great at boosters, your design looks good to me (symmetrical).     The only further advice i can give is on minimising drag.  Unless you're going to convert to a horizontal launch, you don't need to worry about that right now.

All i would say is that the most important rule is that every fuselage section, be it a 1.25m engine nacelle or the mk3 , must have something pointy on its front and back attachment nodes.   Every time you join one part to another, the part you are adding must be of the same diameter as the attachment node you're putting it on or you need to use a size adapter to ensure a smooth transition.

I can't see how you're doing the back end, I have a subassembly here that's the lowest possible drag way to attach 6 nukes to a mk3 fuselage.   It uses a mk3 engine mount , with nukes directly attached to the 1.25m nodes and a triple adapter on the central 2.5m node.  All the engines have cones on the back for low drag.  This does put 19 tons on the back end of your craft so i'd personally hang the remaining nukes radially further up the fuselage, maybe on engine pods above and below the wing?

https://kerbalx.com/AeroGav/Streamlined-6-Nuke-Mk3-engine-mount

 

One final thing, you've got a lot of engines on the back end,  with most of the fuel closer to the front, is the centre of gravity still ok with the fuel tanks empty?  Get RCS Build Aid mod, it shows a red indicator of where your empty CoM will be.

 

 

wouldn't it be easier to use radial attachment points?

Like this

Spoiler

20170313220937_1.jpg

20170313220933_1.jpg

 

Edited by Numerlor

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23 minutes ago, Numerlor said:

wouldn't it be easier to use radial attachment points?

Like this

  Hide contents

20170313220937_1.jpg

20170313220933_1.jpg

 

I can think of an experiment to prove why this is not optimal.   Stand by for the world's slowest rocket race.

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I think you should you swap out your nose cones (if you have them unlocked that is) for the special pointed nose cones. (cant remember name though). If the description is to be believed (its sometimes wrong :P), they decrease drag, which means less fuel needed while in atmosphere, which means more fuel for actually getting to duna.

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Sound didn't record, but you just want to see the result, right?

On the left,  nukes mounted with radial attachment points.  On the right is my subassembly, with mk3 mount and tri adapter.   The radial attachment points do not reduce the flat plate drag from the back of the mk3 fuselage.    Both ships have cones attached to the back of the nuke engines for low drag.  

This test was run with hack gravity and infinite fuel cheats, obviously.    The radial attachment point solution is lighter, but at high speed (well, not actually all that fast) the more aero one wins out.

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Posted (edited)

10 minutes ago, AeroGav said:

Sound didn't record, but you just want to see the result, right?

On the left,  nukes mounted with radial attachment points.  On the right is my subassembly, with mk3 mount and tri adapter.   The radial attachment points do not reduce the flat plate drag from the back of the mk3 fuselage.    Both ships have cones attached to the back of the nuke engines for low drag.  

This test was run with hack gravity and infinite fuel cheats, obviously.    The radial attachment point solution is lighter, but at high speed (well, not actually all that fast) the more aero one wins out.

 But with attachment points you can add one more engines which could help at taking off from duna etc. althrought I have to say that I didn't expect that (and reply fast as that)

So attachment points are for power and your design is for more ΔV

Edited by Numerlor

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On 3/12/2017 at 9:28 AM, AeroGav said:

What you need are a good amount of Vernor engines in the belly to reduce landing speed.

^ This.  I've found that landing on Duna airplane-style isn't too bad... if I've got some belly Vernors.  Not only do they add lift (thus reducing landing speed), but also they help in slowing down (because I generally pitch up pretty hard when flaring for landing, which causes the Vernors to point somewhat forwards, which means they're braking me in addition to adding lift.)

The Vernors also help quite a bit in taking off again-- specifically, in getting the nose up.  Just make sure that the Vernors are fore-and-aft instead of being all clustered in the middle of the ship.  Gives a lot of pitch control that way.

19 hours ago, Anonymmm said:

I will try to install the engine mount and all the other stuff as soon as I possible. 

Some other suggestions:  Unless you've got some RP reason for wanting not to do this, consider tweaking the design so that you ditch any mass you don't need as soon as you don't need it.  For example, mount your engines on stack decouplers that have fuel crossfeed turned on.  This lets you ditch the (very heavy) engines that are otherwise just dead weight.  For example, after you've finished taking off from Duna, you don't need the Vectors any more until you land at Kerbin, right?  So ditch 'em as soon as you've spent the oxidizer; they're just dead weight, otherwise.

Next, your staging pattern from the launch pad.  If I'm reading your screenshot correctly, you're firing all your engines (including the LV-N's) right off the launchpad, yes?  Suggestion:  Don't.  Set up your staging so that you lift only on those radial boosters, and save the LV-N's for later.  Reason #1:  LV-N's get abysmally horrible Isp at full atmospheric pressure, which means you're wasting precious fuel and not really adding any significant thrust to your ship.  Reason #2:  if you want to maximize a ship's dV, you should generally spend all the low-Isp fuel before you spend the high-Isp fuel.

Next, it looks to me as though you're burning all four of those big radial boosters simultaneously, and also ejecting them simultaneously when they run out of fuel.  Suggestion:  Asparagus them.  Set up the fuel pattern so that your four Mammoths are draining two of the big stacks completely, before getting to the second pair of stacks.  (You can do this without rejiggering your design; all you need to do is set up fuel feeds appropriately.)  Then, when the first pair of Mammoths runs dry, you eject just those two booster stacks, and then continue burning the remaining two Mammoths until they run dry of their fuel.  This way, you save quite a bit of dV by not lugging around all that extra dead weight.

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1 hour ago, Snark said:

The Vernors also help quite a bit in taking off again-- specifically, in getting the nose up.  Just make sure that the Vernors are fore-and-aft instead of being all clustered in the middle of the ship.  Gives a lot of pitch control that way.

I generally have enough reaction wheels to be able to swivel my vessels around when in space quite smartly, without using any RCS.   So on Duna, when you'll have (admittedly limited) aerodynamic control as well, the craft are if anything over-controlled.  I actually set actuation toggles on these thrusters to turn them off for anything except upward translation so i'm not wasting fuel in the course of normal pitch control.

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2 hours ago, Snark said:

Some other suggestions:  Unless you've got some RP reason for wanting not to do this, consider tweaking the design so that you ditch any mass you don't need as soon as you don't need it.  For example, mount your engines on stack decouplers that have fuel crossfeed turned on.  This lets you ditch the (very heavy) engines that are otherwise just dead weight.  For example, after you've finished taking off from Duna, you don't need the Vectors any more until you land at Kerbin, right?  So ditch 'em as soon as you've spent the oxidizer; they're just dead weight, otherwise.

No, I do not have any rp reasons for building my craft in the way I'm building it, however, I am playing in career mode, so since my delta v looks good, I feel like getting a little more refund is better than having a little more delta v. However, I might certainly use this tip when going to planets a bit further away, so thanks!

2 hours ago, Snark said:

Next, your staging pattern from the launch pad.  If I'm reading your screenshot correctly, you're firing all your engines (including the LV-N's) right off the launchpad, yes?  Suggestion:  Don't.  Set up your staging so that you lift only on those radial boosters, and save the LV-N's for later.  Reason #1:  LV-N's get abysmally horrible Isp at full atmospheric pressure, which means you're wasting precious fuel and not really adding any significant thrust to your ship.  Reason #2:  if you want to maximize a ship's dV, you should generally spend all the low-Isp fuel before you spend the high-Isp fuel.

Oh, sorry, don't worry about that, I just took a screenshot to show how I was progressing, but didn't actually prepare the launch, so the whole staging system on the screenshot is messed up...

2 hours ago, Snark said:

Next, it looks to me as though you're burning all four of those big radial boosters simultaneously, and also ejecting them simultaneously when they run out of fuel.  Suggestion:  Asparagus them.  Set up the fuel pattern so that your four Mammoths are draining two of the big stacks completely, before getting to the second pair of stacks.  (You can do this without rejiggering your design; all you need to do is set up fuel feeds appropriately.)  Then, when the first pair of Mammoths runs dry, you eject just those two booster stacks, and then continue burning the remaining two Mammoths until they run dry of their fuel.  This way, you save quite a bit of dV by not lugging around all that extra dead weight.

Ooooh... that's what Asparagus means! :D Erm... so... how exactly do you do that? :D Could you please maybe send some sort of link to a tutorial? Thanks!

 

And once again, thank you everyone for all the support I am getting, 

you are teaching me so much stuff guys! :)

 

Any other comments/ideas on my design from anyone are always welcome,

Anonymmm

 

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50 minutes ago, Anonymmm said:

 I feel like getting a little more refund is better than having a little more delta v.

Notice that you can just get ride from this extra deltaV you don’t need anymore because you craft become lighter and cheaper.

Also there is a domino effect, when you reduce the weight in a later stage it will also increase the delta V in the stage just before it, but you don’t need it, so you reduce the weight in this stage also. Soon you end up with a much lighter and cheaper craft.

 

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Ok, added the Vernors and oxidizer fuel tanks for them. Sadly, I realised that I can't use the engine mount, since I haven't unlocked the parts yet :( Yeah, sucks, but still:

OHAavTr.png

(The wheels are for a contract, again.)

Hopefully, tomorrow or during the weekend, I can finally get this to Duna.

Feeling hyped! :)

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Use a MK3 to 2,5 (remove the oxidizer if you don’t use it) behind that a stack adapter . If you still need more space for engines use tail cones to create more nodes.

300px-Cone_engine_mounts.png

 

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