how can you understand best trade off between Dv and TWR?

[Smallish]

I've been playing with Kerbal engineer and started trying to optimize a main stage rocket, but have noticed that I get better Dv on some builds (atmospheric) and worse TWR, now obviously if the TWR is less than 1 it's not better, so how do I know what is the best setup. I can sometimes get some results like 1.4kDv and 1.8 TWR and another setup will give 1.7kDv and 1.6 TWR for example.

[capi3101]

Delta-V is always more important than TWR. Always.

That said, if your TWR is too low at launch, you will not be going into space.

There really is no "best" setup - it's far too dependent on what you're trying to do to give you a single cut and dry answer. The best any of us can say is that its whatever works for you. Basics are simple - 4500 m/s of delta-V to make orbit, TWR best kept somewhere between 1.8-2.2 in flight but it can be lower if necessary at launch (1.6-1.7 at launch for asparagus setups, 1.2 for single-stage rockets, in-between for everything else).

[Smallish]

ok, fair enough, Ive been doing some tests and they have surprised me. Of course, added to the TWR and Dv ratio is also the burn time... My head hurts trying to get to grips with it all : )

So it seems the only way to get the best return is to test all the different setups?..No wonder NASA spends so much money : )

Edit: Also, if you have a good Dv with a horrible TWR of say 1.1 and an average Dv with great TWR, say 1.8..what should I choose? the Excellent Dv by default?

Edited January 10, 2014 by Smallish

[Streetwind]

Less thrust means you spend more time fighting gravity, means you need more dV to get out of the gravity well.

Super simplified fantasy example:

- You can spend 200 seconds gaining 10 m/s per second, or 100 seconds gaining 20 m/s per second, for the same fuel cost.

- In both cases, you could reach a theoretical speed of 2,000 m/s. Thus your rocket has a dV of 2,000 m/s, regardless of which configuration you choose.

- However, every second spent ascending, you lose 5 m/s of your gain to gravity pulling downwards on your rocket.

- In case 1, you spend 200 seconds ascending, thus you lose 200 * 5 = 1,000 m/s to gravity, leaving you with 1,000 m/s final velocity.

- In case 2, you spend 100 seconds ascending, thus you lose 100 * 5 = 500 m/s to gravity, leaving you with 1,500 m/s final velocity.

Clearly, the high thrust option is better. It produces 50% more speed out of the same dV. In fact, even if the specific impulse of the high thrust engines was worse, and thus there was less dV available to the high thrust option, it would probably still end up with more final velocity. That is why the Mainsail engine is great despite its poor Isp rating. It simply catapults you out of the gravity well so much faster than other engines that you save more fuel than you lose.

Of course this comes with two caveats:

- If you launch out of an atmosphere, then you will incur atmospheric drag, which grows exponentially with speed. Ostensibly the fastest launch is the best launch, but go too fast in an atmosphere and you spend more fuel fighting air resistance than you would spend fighting gravity while going slower.

- If there is less gravity, then there is less to be gained by leaving it behind fast. And in orbit, there is no gravity at all (because the orbit itself counteracts it). So for orbital maneuvers, TWR is almost entirely irrelevant, and Isp is everything that matters. Of course, you might still want some thrust in order to avoid having half-hour burns for your maneuver nodes, and to be able to slingshot effectively.

The takeaway is that you want to optimize your launch stages for TWR (and potential landers as well, especially if they need to relaunch), and your orbital/transit stages for dV. Just don't optimize them too much, or you end up with a rocket that burns itself up in the atmosphere and later can't maneuver accurately.

[Smallish]

Thank you for the answers. So I ask, what do people here do to choose their ideal TWR/Dv setups? I'm looking at the numbers and am really boggled trying to make sense of the trade offs with TWR, time and Dv...I just made a judgement about a skipper driven basic rocket and a mainsail and my guess was way off...I'm thinking that my builds are horribly inefficient at the moment.

[tavert]

Delta-V is always more important than TWR. Always.

I absolutely disagree with this. Once you're in orbit, this is the case. However for takeoff and landing, the delta-V cost is a function of TWR. For airless bodies I've done the exact calculations here: http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

Although dV cost of landing or takeoff on airless bodies strictly decreases as TWR increases, engines are pretty heavy in KSP so there's actually an optimal TWR for best payload fraction of a single-stage lander (for a given size lander, this translates to an optimal number of engines - more is not always better). I put together a tool here http://forum.kerbalspaceprogram.com/threads/61659-Wolfram-Web-App-Optimal-Single-stage-Lander-Design-Tool to do this analysis.

When there's an atmosphere, drag makes the calculation more complicated, and the optimal gravity turn trajectory to get out of the atmosphere is also TWR-dependent. In a purely vertical ascent it's well-established that you don't want to exceed terminal velocity, and anecdotally I've seen with some designs you can actually reduce the delta-V cost of Kerbin ascent by imposing an acceleration limit of 22 m/s^2 through the entire gravity turn.

TWR influences the design tradeoffs of which engine is most efficient for any combination of delta-V and payload. See my charts here http://forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR for more detail on that. You can see how clusters of 48-7S engines outperform almost everything else right now, a sign of poor part balance in 0.22 and 0.23. Check the interactive spreadsheet version I linked to in that thread if you want a few more numbers.

Personally what I do when optimizing a design for ascent from a planet with an atmosphere is run experiments using MechJeb's ascent autopilot. Running the same ascent settings with slight changes to the design tells me whether the delta-V left once in orbit improves or not. It's the only repeatable way to make scientific conclusions for small design tweaks - with manual piloting, there's always uncertainty in how much of your performance was due to any changes you make to a design, versus flight-to-flight differences in your trajectory.

Edited January 10, 2014 by tavert

[capi3101]

http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29?p=346702&viewfull=1#post346702

and on a related note, http://forum.kerbalspaceprogram.com/showthread.php/33381-0-20-2-Zenith-rocket-family-%28modernised-for-0-20-x-with-perfect-subassembly%29

I still like to design my own boosters despite having this particular resource available. Temstar's method for guesstimating how much thrust you're going to want (first link) works reasonably well for other types of staging, incidentally...though you can't count on having as high of a payload fraction. Numbers that work for me are 4% for single-stage rockets (easiest to build with payloads less than 45 tonnes), 8% for serial-staged rockets and 12% for onion-staged. It also helps to learn how to run the Tsiolkovsky equation backwards to figure out how much fuel you're going to want for a given target delta-V.

[blizzy78]

So I ask, what do people here do to choose their ideal TWR/Dv setups?

I always aim for 1.6-1.7 TWR at launch, with the dV necessary to get into orbit. To go places I usually use a transfer stage, but as far as the lifter is concerned, that transfer stage is considered payload.

[Kasuha]

Delta-V is always more important than TWR. Always.

Wrong. Or do all your interplanetary ships use one ion drive as the main means of propulsion because it means most dv?

You always need the right proportion of TWR/TWM and dv to get the job done. In space, you need enough TWM to get your burn done in reasonable time but there's no reason to have it higher. For liftoff, you need TWR > 1 - or in other words, you need enough TWM to be able to get to orbital speed before you fall back on the surface. And enough dv to get to orbital speed with all losses caused by burning out of optimal points and directions.

Personally I don't use mods so I rarely know my TWR or dv. If it works, it's ok. If it's too sluggish, I add engines. If it falls from the sky, I add fuel.

Edited January 10, 2014 by Kasuha

[MrZayas1]

I have noticed that it is much better to worry about delta V unless you are making a landing with a stage and cannot afford a slow acceleration rate. I only really worry if the burn is over 8 mins long, even if the delta V is 3000, hope it helps a bit but thats my suggestion.

[Smallish]

I thank you all for the input, I'm now trying to get to grips with the linked items...

One thing is for sure, my original beliefs are not at all correct, my rockets of the future will be more efficient.

I didn't think about mechjeb for testing different builds, I have not installed it due to it's ability to take away some personal "fun" in the testing stakes, but more and more I'm questioning each build I make to try and eek out the best builds. I can't help but think that in the end, this way of working will lead to mechjeb.

Can anyone tell me, does this 8 array use stock items http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29?p=346702&viewfull=1#post346702 ? I havent seen this available in my game, maybe I don't know how to build it or don't know what it uses...

[tavert]

Can anyone tell me, does this 8 array use stock items http://forum.kerbalspaceprogram.com/showthread.php/28248-Is-asparagus-the-best-staging-system-%28might-contain-science%29?p=346702&viewfull=1#post346702 ? I havent seen this available in my game, maybe I don't know how to build it or don't know what it uses...

Looks like it's using the structural tail connector part with 8x symmetry. He may have needed to turn on part clipping in the debug menu to build that.

[Seret]

TWR is a simple go/nogo factor. In other words, once your TWR is good enough, it's good enough. Adding more even becomes harmful. Delta v is a different story, you can't ever have too much.

So optimise for delta v, but make sure you've cleared the minimum TWR requirement. SRBs are great for lifting a sluggish TWR into the acceptable range.

[capi3101]

I absolutely disagree with this.

Wrong.

Poor choice of words on my part. And may I say it's nice to see the two of you actually agree on something for once.

[Dispatcher]

... To go places I usually use a transfer stage, but as far as the lifter is concerned, that transfer stage is considered payload.

This is a very useful practice which I always use for translunar and interplanetary missions (since I have yet to assemble large craft in orbit). I find that for Kerbin lift off, if I design for dV, the TWR tends to take care of itself. Any problems at that point can be reduced or eliminated by further tweaking. The tools mentioned in previous posts will help.

[draeath]

Oh quick note - if you have FAR and/or Deadly Reentry Continued installed, going too fast is certainly a big deal when you're launching from atmosphere. At high speed the aerodynamics will fight your orientation changes, and you might have heat related explodiness problems if your speed is suitably extreme.

(early in my career mode when I really only had SRBs, I put a heat shield above my parachute on the pod so that I didn't burn up on the way up )

[Soda Popinski]

Oh quick note - if you have FAR and/or Deadly Reentry Continued installed, going too fast is certainly a big deal when you're launching from atmosphere. At high speed the aerodynamics will fight your orientation changes, and you might have heat related explodiness problems if your speed is suitably extreme.

(early in my career mode when I really only had SRBs, I put a heat shield above my parachute on the pod so that I didn't burn up on the way up )

What's interesting about this is, Vanilla KSP, you should shoot for a TWR of about 2 to keep you at about terminal velocity. With FAR installed, terminal velocity for any sanely built rocket (looking at you Whackjob), is about 400 m/s at the surface. So you would think going with a higher TWR would be good, but as dreath mentioned, the aerodynamics tend to make you less stable. Because of this Ferram often suggests a lower TWR of about 1.3 to keep you stable at the thicker part of the atmosphere.

To this I say, add MOAR FINS! I just have a bunch of vertical stabilizers on my heavy lifter boosters (with a few on my core stage). By the time they're gone, I'm passed the thicker part of the atmosphere and can really speed up. With this strategy, I keep my TWR at about 1.8-2, so I don't lose too much to gravity drag.

[Keldor]

As far as stability in FAR is concerned, you just want to keep your center of drag below/behind your center of mass. Think of the CoM as a pivot, which the rocket swings around. Then the end with more drag wants to go around to the back. Adding tail fins, as you said, adds more drag to the end of the rocket, thus pulling it around to the back as it moves through the atmosphere. Conversely, having a big fat payload at the top of the rocket adds drag up there, which will make it want to flip over so the center of drag goes behind the center of mass.

Edited January 10, 2014 by Keldor

[draeath]

At high speeds in atmosphere, it will actually resist turning against the airflow. This tends not to be an issue below mach 1, but once you break the sound barrier it will fight you until things thin up. On the other side of the coin, this means it helps keep you straight once you go supersonic.

[Superfluous J]

While designing my mission, I know the dV and TWR needed for every step, and I design each step to have the dV and TWR needed with the lowest mass possible. It's one of the joys of this game that I keep learning new ways to shed mass while still being able to achieve my goal.

I only care about TWR during launches off of bodies or if it's absurdly low for transfer stages. While I've done a 20 minute burn (5 minutes at 4x physics warp is still 5 minutes) I don't care to again and will happily lose some dV to speed up that part of the game.

[SSSPutnik]

Sufficient TWR during non atmospheric descent also important. (To avoid lithosphere braking).

[regex]

Once in orbit it really depends on how you play. I like to keep my transfer stages, nuclear or not, to at or better than 0.3 TMR Kerbin gravity. This means that burns to pretty much everywhere take less than fifteen minutes (usually eight to ten). Other people use different transfer methods than direct single burns and what method they use helps determine the TMR they're comfortable using.

[Thrombo]

(To avoid lithosphere braking).

This made me giggle.

[Taki117]

I dont have any facy maths to back me up, but here lately I've been using a set of boosters witha TWR of about 1.2, then my center stack has a TWR of about .6-.7 (can be as high as 1.12 depending on

payload) with just a Little less dV than I need to get into orbit, then have a orbital insertion/transfer stage with a TWR of .8-1.1 to get me where I need to go. I have yet to not make it to orbit, and I rarely leave debris behind. I play with FAR among other mods so while it may require less dV to get to orbit, I can't use asparagus staging because it throws off the aerodynamics. Typically I use Onion staging instead.

[SuperBigD60]

I imagine if you're having trouble making it to orbit, I would suggest simply stacking more fuel tanks wherever it is you're using a mainsail.

Those guys can lift three full orange tanks and change, so adding more fuel usually helps.

At least if you're still at a loss for what to do. That's just my go-to "I dunno" solution.