Optimal TRW from Kerbin?

[Jestersage]

The only thing I am sure is that, given the designed maximum payload, it should not have less than a TRW of 1. That being said, TRW of 1 is slooow... So what is the best TRW would you consider?

I am guessing it would actually be a bell curve in terms of Delta-V, but can someone give me a ball part? The ship I am building currently have a TRW of 1.42 with the payload.

[The Aziz]

With TWR=1 you would hover, not move at all. (I tried it, accidentally)  I tend to go above 1.5, but not much more than 2, however I am still learning how this works, even though I spent helia lot of hours in this game without even looking at TWR.

Just now, The Aziz said:

I fail at editing.

 

Edited April 7, 2017 by The Aziz

[bigcalm]

It doesn't matter hugely, as long as it's above 1.  Basically, with a TWR of < 1 you won't actually lift off - gravity wins.  With a TWR of just over 1, you'll just about move off the ground - most of your fuel will be wasted in gravity losses in this situation - you're not really using that fuel to do much more than hover.  With a massively high TWR, you'll accelerate very quickly, but potentially then run into atmospheric problems - (unless you're just using solid rocket boosters, you can always throttle back) - and becomes a question of how aerodynamic your craft is. 

[Reactordrone]

A lot of people aim for about a 1.5 TWR on the pad so 1.42 should be ok. In terms of delta-v a higher thrust to weight ratio will usually give lower delta-v to orbit but at the cost of higher price/heavier rockets.

[Streetwind]

@Jestersage There's no single absolute answer to this question.

For example: a rocket with a high TWR consumes less fuel to reach orbit.
However, the same rocket could accept a lower TWR by loading more fuel, and potentially end up with more dV left in orbit. The details depend on the exact rocket and can only be generalized in the broadest of strokes - namely, don't drop below 1.25 atmospheric liftoff TWR, or you will always lose out.

TWR also influences the way the rocket flies. A high TWR rocket can go sideways sooner and harder, cutting down on gravity losses; but at the same time, a high TWR rocket is more difficult to steer. Both because turning over sooner means that even small deviations from the perfect angle get strongly magnified, and because going fast in the lower atmosphere tends to make many rockets so stable that they struggle to change their course at all.

An atmospheric (not vacuum!) liftoff TWR of 1.42 is actually in a pretty nice spot, IMHO. Some people prefer slow and others prefer fast, but that number definitely works without any immediate drawbacks, so I wouldn't change a thing about it. However, if that's vacuum TWR, your atmospheric value is probably a little too low and your rocket could stand to lose a little weight.

[bewing]

Streetwind's answer is great. But I like to use plenty of cheap SRBs as a first stage -- and then my remaining liquid booster stage tends to be very low TWR, because it's always extra heavy. Which is why the first stage needs to be overpowered -- to get that second stage up close to orbital speed. So I am perfectly happy with a 2 to 2.5 TWR off the pad.

 

 

[Urses]

As i understand all other above +my belly feeling

<1 = nope you don't go to Space Today

=1 and somewhere low above maybe maybe not but reeealy inefficient

1.25-2,5 You have a Good Spot

>2,5 you go up but you get problems like stearing, melting, drag

I Hope i get ot right.

Urses

[Superfluous J]

I personally think anything above 1.3 TWR is excessive. Higher is funner, but not as efficient.

[VaPaL]

I prefer my TRW a little lower, normaly I keep it between 1.15 and 1.3, I prefer a slow ascent. Mainly because I don't like to see any aero effects during ascent, not to mention reentry effects!

Streetwind answer is really good, but if you want to complement it, theres also this thread:

 

[Foxster]

I haven't a clue about the maths regarding this but from practical experience I find the most efficient (and less tedious) way to orbit from Kerbin is as high TWR as you can get away with before the craft either melts or becomes unsteerable. 

[Reusables]

Gravity drag tends to play major role on ascent, as it consists at least 1km/s from 3.4km/s of dv to LKO. If you don't hit the sonic barrier too fast, aerodynamic drag won't be too big (typically under 100m/s for streamlined ones)

In my experience, the major drawback for high TWR is often the price of the engine - which is one of the most expensive parts. Also they comprise quite a bit of the dry mass of the first stage. Thus reduction of engine gives more dv in less cost.

Though I don't think TWR under 1.3 will be better. Compared to TWR 1.5, it will need about 1.7 times more time to fight the gravity drag.

[Kryxal]

If SRBs are useful on a rocket, I usually attach pairs and stage them independently.  That way, I can run a pair at full and a pair at 70% or some such, and not have such excessive thrust at the end.

[Freshmeat]

2 hours ago, Kryxal said:

If SRBs are useful on a rocket, I usually attach pairs and stage them independently.  That way, I can run a pair at full and a pair at 70% or some such, and not have such excessive thrust at the end.

SRBs are (almost) always useful. If you don't light a couple of Kickbacks on launch, you should ask yourself seriously why not.

[Spricigo]

10 hours ago, bewing said:

 I like to use plenty of cheap SRBs as a first stage -- and then my remaining liquid booster stage tends to be very low TWR, because it's always extra heavy. Which is why the first stage needs to be overpowered -- to get that second stage up close to orbital speed. So I am perfectly happy with a 2 to 2.5 TWR off the pad.

Similar procedure in my space program and similar TWR off the pad.

 

24 minutes ago, Freshmeat said:

SRBs are (almost) always useful. If you don't light a couple of Kickbacks on launch, you should ask yourself seriously why not.

Launching the standard small satellite on top of the standard tweaked kickback is really convenient when you consider time and effort, while not necessarily cost effective.  Anyways the satellite will probably pay for the mission several times with the contracts done.

[Alshain]

17 hours ago, Streetwind said:

@Jestersage There's no single absolute answer to this question.

For example: a rocket with a high TWR consumes less fuel to reach orbit.
However, the same rocket could accept a lower TWR by loading more fuel, and potentially end up with more dV left in orbit.

Even this is not 100% accurate either.  A rocket that accelerates too quickly will reach suborbit long before a proper gravity turn can possibly be made.   That results in consuming more fuel.  The air resistance at high speeds prevent turning, and by the time you get out of the air it's too late.  Then you have a long orbital insertion burn, which is costly.

@JestersageMy personal recommendation is 1.5,  that's largely personal preference and it doesn't have to be exact, but no lower that 1.25, and absolutely no higher than 2 would be the best guidelines I could really offer.  Streetwind is correct when he says there is no good answer to this question.

Edited April 8, 2017 by Alshain

[Snark]

Relevant:

TL;DR:  these days, gravity losses are much bigger than aero losses unless you're going stupidly fast, so higher TWRs tend to be more efficient if you have an aerodynamic ship.

I always used to aim for around TWR 1.5, in the pre-1.0 days of the old souposphere.  The new aerodynamics favor higher TWR, though just how high depends on how aerodynamic your ship is.  These days, I tend to go for close to 2, at least for bigger ships.

Note that size matters.  Bigger ships experience less drag relative to their mass than small ships do, so they do better with higher TWR values.

[Streetwind]

7 hours ago, Alshain said:

Even this is not 100% accurate either.  A rocket that accelerates too quickly will reach suborbit long before a proper gravity turn can possibly be made.   That results in consuming more fuel.  The air resistance at high speeds prevent turning, and by the time you get out of the air it's too late.  Then you have a long orbital insertion burn, which is costly.

Well, with a high TWR rocket, you just turn over sooner and harder at launch, that circumvents this issue nicely. Look at some of the videos posted in relation to "minimum dV to orbit" challenges. The winners generally have huge amounts of thrust and pitch sharply right off of the pad. So there's absolutely no mechanic that invalidates the concept of "high TWR reduces dV cost to orbit".

Of course, I would still agree with you that moderate numbers are better. It's really hard to get the initial pitchover right with hyper-fast rockets - small errors get magnified and can make or break your ascent. Also, least dV to orbit doesn't generally coincide with least funds cost or maximum payload fraction. And when in doubt, those are probably more relevant to career mode play.

[Spricigo]

5 minutes ago, Streetwind said:

It's really hard to get the initial pitchover right with hyper-fast rockets - small errors get magnified and can make or break your ascent...

Actually doing it at the editor makes it much more easy and reliable.  

[CSE]

The cheap & cheerful thread is worth a read because it has some good thoughts about balancing the different design requirements, including TWR, to make the design an overall success.

To go into more detail for determining the ideal TWR for each stage, @GoSlash27 had a good post that laid out a mathematical design approach, which I can't now remember/find.

 

[Norcalplanner]

Wow, two of my threads linked in response to the same question. Thanks mucho. 

And just in case anyone is wondering how TWR interacts with efficient ascents where atmosphere isn't a factor, here's another thread:

 

[Mad Rocket Scientist]

It's worth noting that if you're landing somewhere, you want a much higher TWR, but if you're going to stay in space for the entire time, you want the lowest TWR you can stand.

[Gaarst]

10 minutes ago, Mad Rocket Scientist said:

It's worth noting that if you're landing somewhere, you want a much higher TWR, but if you're going to stay in space for the entire time, you want the lowest TWR you can stand.

You actually want the best compromise between a high TWR and a light engine.

Having too low TWR causes losses of dV when doing manoeuvres. It's not a lot when comparing to what you gain by using a lighter engine, but it's enough to be significant when considering electric propulsion (at least in real life, KSP's Dawn is extremely powerful so I don't know if it's still valid).

Edited April 8, 2017 by Gaarst

[Mad Rocket Scientist]

15 minutes ago, Gaarst said:

You actually want the best compromise between a high TWR and a light engine.

Having too low TWR causes losses of dV when doing manoeuvres. It's not a lot when comparing to what you gain by using a lighter engine, but it's enough to be significant when considering electric propulsion (at least in real life, KSP's Dawn is extremely powerful so I don't know if it's still valid).

True, although for many maneuvers it becomes unbearably slow before it becomes inefficient. For example, if you're making an ejection burn away from Kerbin, you can just timewarp around to pe again when you get to far away.

[Red Iron Crown]

From my experience:

To minimize delta-V to orbit: As high a TWR as the craft can stand without burning up in atmo.

To maximize payload fraction or minimize cost: TWR of around 1.3 or 1.4 at launch.

The latter is usually the more useful metric.

[StrandedonEarth]

On 4/8/2017 at 2:33 AM, Spricigo said:

Actually doing it at the editor makes it much more easy and reliable.  

Okay, now I'm going to have to try that. Just set the initial pitch attaching it to the launch clamps, and then fire-and-forget if you set it right?