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Help a noob with rocket optimization please :)


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Greetings friends! :)

I'm in dire need of some advice/pointers regarding rocket building...

I'm playing in career mode, and just unlocked that heavenly atomic engine that seems to consume almost no fuel at all <3 ...

So my main issue, is the first stages of the launch, (I understand and use some concepts quite well like staging, coupling, aspargus'ing, etc...) but... before i clarify it better, pretty much my rocket doesn't seem to gain any decent speed (barely goes over the 120-150 m/s range) before passing the 30-36km barrier

To sum it up:

I was planning an aspargus system to go to Duna + Ike(duna moon, or am I mistaken?) + return, with 7x Rockomax jumbo-64 fuel tanks where the 6 outer ones had "Mainsails" on them... and a Skipper in the center for vacuum operations, then I had a landing -> return stage with just 1x "Poodle" strapped under 1 wide Rockomax X200-16... which i tweaked to the same rockomax x200-16 + 4x FL-T800 radially mounted.

Then i faced a slight issue... it would burn my aspargus faster, and make a lot harder to push it up past beyond the mun.

Now... my main problem arose when i tried to achieve a solution...

Tried with 14x Kerbodyne S3-14400 tanks (7 + 7 on top) with the same exact aspargus system (6 outer + 1 center (x2 in length)) and tested with both... Kerbodyne KR-2L engines, or S3 KS-25x4 engine clusters... and guess what... with the KR-2L at least, i burn through the 3 outer aspargus stages just to get into orbit... (i actually reach orbit with about 75% fuel left) but i end up feeling like it's too few remaining fuel (?) i get there with about 2.5k - 3k Delta-V left, and it just feels too few fuel, what do you guys think?

(my previous "weaker" aspargus used to reach orbit still with 3 fuel tanks, being that 1 was the central one at 100%, and the others around 25%)

Also, just on a side-note... think 2x Atomic engines are enough to push up a 30~60k Ton Payload out of a regular (non-eve'ish) planet?

You can check some of my projects here> Mkorny, and you can check all the info on the mechjeb screens that are up

Since I'm currently at work am unable to add any pics, so i linked my profile with the details of one of my missions.

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Wow, that seems like a lot of overkill to get to Duna & Ike. Some bits of advice that may or may not help (I'm not quite sure what the problem is):

  1. Use efficient engines (High ISP) for transfer stages, like Poodles, 909's, and nukes.
  2. Refuel in orbit
  3. Send your mission on several smaller rockets
  4. Keep your parking orbit low

And to your other question, yes, two nukes can get 30 - 60 tons almost anywhere, with enough fuel. For that matter after you're in orbit, any engine can get you anywhere with enough fuel.

But if you're having trouble getting that monster you're describing to Duna, I'd recommend starting smaller and see if you can get that to work.

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Well, my main issue was really that i find it odd that a ship this massive, lifts at a VERY VERY slow rate and wanted to know if that's normal considering i'm only lifting around 40 tons payload, aswell as the amount of fuel it burns out just to get into kerb. orbit :)

The question about nukes being able to lift that much, was because at start i failed horribly, and repeatedly, at getting back from mun lol... didn't even have enough remaining fuel to get back to kerb. orbit :(

Also, on another side note, do you think a cluster of say... 5/7/9 engines to be more efficient than a massive one? maths seem to point that way, but would love to hear more on it from someone more experienced :)

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Oh lord. I just took alook at your other projects, and I must say they are a model of inefficiency.

First off let me give you some tools in your toolbox.

This is a dV map. It will help with figuring out how much dV you need for a transfer. I use it alot.

This is a transfer time calculator. It will give you a launch time based on various parameters and a dV estimation base on the transfer times.

This rocket design and flight path will get you to the Mun and back with fuel to spare. By extension it should also get you to Minmus.

This Mod will help with figuring out how much dV you actually have. Also, This one will eliminate the need to add the part.

Now that you have the tools required for the job, I leave the rest up to you.

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The way I look at things, everything needs to be in proportion. So if you add fuel to a launcher, you must also add more engine somewhere. For example, at launch, the Thrust to Weight Ratio (TWR) should be about 2:1.

Once in orbit, for transfers, it's the opposite situation; you need high ISP engines but don't worry about thrust.

But the important thing is to have fun getting a feel for what works, and watching the beautiful spirals in the meantime. ;)

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You might like to look at the tutorial linked in my signature - chapter 5 deals with manned lunar landings and interplanetary satellites, although you may find some of the stuff in the earlier chapters helpful too. Chapter 6 has a 'Kerpollo' vehicle capable of Minmus, Mun, Ike, Gilly, Duna or Pol landing, while chapter 7 has a low-part 40t launch vehicle based on it.

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Once in orbit, for transfers, it's the opposite situation; you need high ISP engines but don't worry about thrust.

If you have to do an interplanetary burn from LKO, it's really difficult to get it done with thrust below about 0.3. For instance, a TWR of 0.2 in LKO gets you about 2 m/s2 of acceleration. A trip to Jool typically takes an initial burn of about 2,000 m/s, so that's about 1000 seconds (16.7 minutes) of acceleration, or about half of an orbit. If you split the burn: half before the node and half after, you'll be starting the burn pointing almost directly radially inward (down!) You'll almost certainly end up lowering your periapse below the atmosphere before you get to the second half of the burn and end up lithobraking on Kerbin rather than heading towards the outer planets.

tl;dr: For interplanetary burns, thrust is important.

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If you have to do an interplanetary burn from LKO, it's really difficult to get it done with thrust below about 0.3. For instance, a TWR of 0.2 in LKO gets you about 2 m/s2 of acceleration. A trip to Jool typically takes an initial burn of about 2,000 m/s, so that's about 1000 seconds (16.7 minutes) of acceleration, or about half of an orbit. If you split the burn: half before the node and half after, you'll be starting the burn pointing almost directly radially inward (down!) You'll almost certainly end up lowering your periapse below the atmosphere before you get to the second half of the burn and end up lithobraking on Kerbin rather than heading towards the outer planets.

tl;dr: For interplanetary burns, thrust is important.

In this situation though you could split the burn into 3 orbits, each burn time only being about 5.5 minutes long, which when executed would mean a 2.5 minute burn before and after th node (And periapsis on subsequent orbits) which isn't that bad at all.

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In this situation though you could split the burn into 3 orbits, each burn time only being about 5.5 minutes long, which when executed would mean a 2.5 minute burn before and after th node (And periapsis on subsequent orbits) which isn't that bad at all.

Sure, but interplanetary burns are somewhat finicky with regard to departure time, and with 3 orbits, each of which is longer than the previous, you're talking many hours or even days between when you start the burns and when you finally finish ejection. You'll have to pre-adjust your phase angle to compensate (and burn a couple hundred extra m/s.) Also, for Jool transfer, you'll still have a final burn that's over 1000 m/s because your speed for the pumping orbits can't be higher than escape velocity.

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Sure, but interplanetary burns are somewhat finicky with regard to departure time, and with 3 orbits, each of which is longer than the previous, you're talking many hours or even days between when you start the burns and when you finally finish ejection.

Actually, I find ejection angle far more finicky than phase angle, but I really can't disagree with anything you said.

When i do this, I start off making a single maneuver node to do the transfer. Then, I bump the maneuver node backwards in time, with the time I bump it back depending on whether I'll need one or two passes to get about 800 m/s delta-v. That keeps me below the Mun's orbit, and I've had too many times that the Mun has flung my craft out of Kerbin's SoI at the wrong velocity and angle to be tempted by the improved efficiency that a higher apoapsis would get me. Also note that the higher the apoapsis, the more you have to adjust your initial burn, since you're increasing the orbital period. Once I've got the first maneuver node, I just set up one or two subsequent maneuver nodes at periapsis that all sum up to the proper transfer velocity, maybe with a small amount extra due to inefficiency on the last burn (though I've never had to do a couple hundred).

Other comments: If you're going to be doing a long burn on your last orbit, you might want to avoid too low a parking orbit. I usually use an 80k parking orbit for interplanetary transfers, and every once in a while if I'm not paying attention, I'll wind up with the last pass of my orbital transfer burning long enough it actually pushes the trajectory into the atmosphere, which is bad. However, trying to do the transfer with a low TWR in a single burn just makes that issue even more pronounced.

Basically, except for timing the departure, any issues that the multi-pass burn has would be worse trying to do the burn in a single pass at the same TWR.

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Building an efficient rocket made easy:

Break up your launch into 3 phases for planning. Each phase can be covered by whatever staging scheme you prefer (or even just a single stage), but the engine requirements are different for each.

The 3 phases are boost, transstage, and insertion.

The boost phase gets you from the pad to your gravity turn kick downrange. It's straight up and lifting the whole rocket, so it needs good thrust. Weight isn't an issue for this phase. You want at least 2:1 thrust to weight. Your DV in this phase is 1,500.

The transstage takes you from the gravity turn to your apoapsis. It requires good Isp in atmosphere and should be as light as possible while maintaining at least 1:1 thrust to weight. Your DV is 2,000

The insertion stage circularizes your orbit and carries out any intercept, rendezvous, and docking. Thrust isn't important here, but light weight and high Isp in vacuum are critical. 1:2 thrust to weight or even less will work just fine. DV is 1,500.

The process starts with the payload. See if you can lighten it up to give your launch vehicle an easy time. Send it up empty and fuel it in orbit/ make it modular and bring up the pieces separately, etc.

Once you've got your payload set, you're ready to start. Warning, math ahead...

The 3 equations you will be working with:

A) e^(ÃŽâ€V/9.81Isp) = Rwd

Given a DV you need to achieve and the Isp of the engine you intend to use, this will spit out the required mass ratio (Rwd)for your stage. "e" is the natural log base, approx. 2.718

Now plug that mass ratio in here:

B) 9(Rwd-1)(Me+Mp)

________________ = Mft

(9-Rwd)

Me is the mass of your engine

Mp is the mass of your payload

This gives you the mass of your tanks when fully fueled. But is your engine powerful enough to lift it?

Note* This equation only works with small and large radius tanks. Other tank sizes require more

C) 9.81Rtw*Mt=T

Given a desired thrust to weight ratio (Rtw) and total mass (Mt), this spits out how many kilonewtons of thrust you need (applies only to launches from Kerbin).

Find the number of engines you need and repeat the process. You can experiment with different engines and staging schemes to find the lightest stage that will do the job.

You do this first for the injection stage carrying the payload. Then you apply that stage+ payload to the transstage. Finally you apply both of these stages + the payload to the boost stage. Once you've got a firm grasp on what each stage needs, go and build your rocket.

That's really all there is to it.

Best,

-Slashy

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An example of what this approach can yield:

TRACstack_zps1a134aff.jpg

A stack of "TRAC" utility rovers bound for orbit aboard a Shotput/ Courier launch vehicle.

That little booster is capable of taking that big chunk of hardware all the way to orbit at 80KM, execute an intercept/ rendezvous/docking and still have enough DV left to execute deorbit...

TRAC2_zpsd7d59764.jpg

A "courier" delivering a stack of TRAC rovers to a Startrader shuttle outbound for Eve.

I also launched that Startrader aboard a Shotput booster, although I did away with the insertion stage in that case since the Startrader already had all the required hardware aboard to complete it's own orbital insertion.

Edited by GoSlash27
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This Mod will help with figuring out how much dV you actually have. Also, This one will eliminate the need to add the part.

This.

Even if you don't use mods, install it temporarily as a teaching tool. Understanding the Rocket Equation is one thing. Getting an intuitive feel for it is something best done playing around in the VAB and getting realtime feedback on the delta-V.

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An example of testing in stages.

The payload, this complex lander, was built and flight tested by itself.

0GgzMzC.jpg

A core stage was then added and a suborbital test done

(No picture.)

Finally, the booster, in this case a six asparagus launcher, was built and flight tested.

439ilSm.jpg

This combination reached Minmus on the mid stage and achieved its goal of landing there.

s1NXKuX.jpg

MP3bFwS.jpg

When I do come up short or want some additional range, such as the ability to reach Duna, I will add a ring of SRBs on a design for that extra punch to reach 5,000 meters before going to the liquid engines.

VLMWyAk.jpg

(See the Tutorial thread on how to get 5 sided symmetry.)

Edited by SRV Ron
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A simple three stage example of testing to orbit;

tIPGLRs.jpg

And the test where SRBs are added to the same design for additional range.

ZAadMVs.jpg

Build simple, test in stages, look for that ideal launch profile, and you can go a long ways without having to build huge complex designs that require refueling or assembly in orbit.

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2 Atomic engines will never lift up a 30 ton payload(on kerbin at least). the max it can lift up on kerbin is about 12.2tons

Hey m8, yeah I am aware of that :) Just wanted to confirm whether it would be able to lift off from mun or some other body with low gravity (like mun, minmus, ike, etc...) :)

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Hey m8, yeah I am aware of that :) Just wanted to confirm whether it would be able to lift off from mun or some other body with low gravity (like mun, minmus, ike, etc...) :)

Minmus definately, with the mun, you might need multiple nukes depending on the weight of your payload and how much you want to carry up into orbit

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Sure, but interplanetary burns are somewhat finicky with regard to departure time, and with 3 orbits, each of which is longer than the previous, you're talking many hours or even days between when you start the burns and when you finally finish ejection. You'll have to pre-adjust your phase angle to compensate (and burn a couple hundred extra m/s.) Also, for Jool transfer, you'll still have a final burn that's over 1000 m/s because your speed for the pumping orbits can't be higher than escape velocity.

But the transfer windows aren't that narrow. If it takes more than about 3 orbits anyway you should take a look at increasing TWR. Also, you will probably need to make a mid course correction anyway, so any error in your escape burn can be corrected during the Mid course correction burn.

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Hey m8, yeah I am aware of that :) Just wanted to confirm whether it would be able to lift off from mun or some other body with low gravity (like mun, minmus, ike, etc...) :)

Nukes, with their high efficiency, are good for long range flights such as to Jool or Moho. For landers, an LV-909 or smaller engine will be more efficient since they are far lighter then the heavy nuke.

Note, the little Rocomax is more then enough to land Bob on Minmus and had plenty of fuel for his return to Kerbal.

h669f6P.jpg

Yo1TNvB.jpg

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You're running into the same problem I run into with refueling stations. Your fuel is so heavy that you burn most of it getting to orbit. Try launching your interplanetary ship with just enough fuel to get to orbit. (large tanks tweaked to be only partly full)

Then refuel it in orbit. Your tankers are just lifting fuel and not your landers and other stuff.

As for nukes, I don't use them. The poodle is normally my benchmark for moving large ships. It takes about 4 nukes for a bit more thrust, but they're a lot heavier. If I make up that extra weight in fuel I usually end up with more dV.

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OK!

Finally tweaked and tweaked and tweaked... and got a VERY good ship design (imo) :D

Have 4x RT-10 Solid boosters, then Rockomax jumbo tank + 5x Vt-L30's (at this point i reach a 100km parked orbit) and then have my nuke + FL-T400 Tank and it gets me anywhere within kerbin CoI :D

Also, to note that I currently went down to a 4 ton lander :D so... down from like 10k+... now lander + tank + nuke i have under 12k

Thanks a huge lot for all the tips guys! : )))

PS: how do I set the topic as Answered?

Edited by Mkorny
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OK!

Finally tweaked and tweaked and tweaked... and got a VERY good ship design (imo) :D

Have 4x RT-10 Solid boosters, then Rockomax jumbo tank + 5x Vt-L30's (at this point i reach a 100km parked orbit) and then have my nuke + FL-T400 Tank and it gets me anywhere within kerbin CoI :D

Also, to note that I currently went down to a 4 ton lander :D so... down from like 10k+... now lander + tank + nuke i have under 12k

Thanks a huge lot for all the tips guys! : )))

PS: how do I set the topic as Answered?

Would love to see a pic (Or a link to a mission report) but to set the topic as answered edit the first post and then Go advanced.

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