What TWR do you aim for...

[Zutha]

So I've being using the engineer's plugin and parts (thanks for the recommendations) to view the thrust to weight ratio and delta v to build rockets. Very interesting I must say.

It's made a massive help on staging e.g. I aim for 3:1 for first 10k, then 2:1 for up to 70k.

After that though it's more fuel to TWR. So generally I use nuclear rockets for fuel efficiency and aim for 0.5 or above as I always have a an attached lander.

So my question(s) are:

What TWR or Delta v do you aim for in space?

What numbers affect your decision in rocket design?

[Peenvogel]

I'm no math guy, so I made a lot of tables with DeltaV/TWR/Mass etc. I'm not sure if I'm playing by the rules, but I've noticed that DeltaV is the most important one. I tested hundreds of rockets and the average DeltaV needed to get into circular Kerbin orbit is somewhere around 4500 Ms2. The TWR seems to be OK as long as it is higher than 1.01. It just goes very slowly, when you have less TWR, but the DeltaV seems to be a more or less stable number. But maybe I've been doing it "wrong" (I can get anywhere with this method) and some of the math people here have a better explanation.

[Pursuedtank]

Altitude (m) Velocity (m/s)

0_______________97.3

1000____________110

3000____________130

5000____________160

8000____________215

10000___________260

15000___________425

32000___________2250

Going up through the atmosphere you should have sufficient thrust to weight to get as close to these values as possible. I usually aim for a thrust to weight of 2. In space It doesn't matter how much you have and it is more of a trade off between efficiency and your time/patience.

The original table can be found here.

[ComradeGoat]

I want about 1.6-2.0 TWR for blast off. In space, as little as I can get away with, although I will often have jettisonable high thrust engines (thank you, devs, for the Skipper!) to start my burn away from Kerbin. The lower ISP probably more than cancels out benefits of the Oberth Effect, but I get impatient.

[Tommygun]

I try and keep it under 2to1 for the first stage to help keep it under 200 m/s, it helps with fuel efficiency.

I also keep the other stages under 3to1 or lower, I just need it to make orbit.

In space, I use whatever has the highest isp with enough thrust to get me somewhere in a reasonable amount of time.

Edited May 29, 2013 by Tommygun

[LeonG17]

I try to have at least 4000 m/s in the launch stages so I can get into orbit and still keep all the fuel for the interplanetary/moon travel.

[Endangerment]

With a twr of 3 for the first 10km I reckon you'd be pushing against terminal velocity or else throttling down which means you are carrying excess weight I'd suspect.

For my biggest launches (1000t +) I normally try and have a twr of about 1.5 up to 10km, then about 1.8 up to around 45-45km then it is normally around 1.1 or less for final ascent and circularisation. I then use tugs to get whatever it is I've launched out of LKO. They have nuclear engines so very low TWR

[Klajan]

Usualy I aim for 1.3 min and 2.0 max for my first stage and then I just try to never fall unter 1.2.

[erkha343]

How do you measure weight ingame?

[Zutha]

How do you measure weight ingame?

Add it up by hand or using a mod/plugin.

Search engineer on spaceport. It has a part you can attach to a craft in VAB to get the details. May be someone can provide a link as I can't remember the full name right now

[Pursuedtank]

Search engineer on spaceport. It has a part you can attach to a craft in VAB to get the details. May be someone can provide a link as I can't remember the full name right now

It's called the Kerbal Engineer Redux.

[rkman]

The TWR seems to be OK as long as it is higher than 1.01. It just goes very slowly, when you have less TWR, but the DeltaV seems to be a more or less stable number.

DeltaV certainly is important because that determines how far the craft can go (far as in: what orbit can it establish), that's why DeltaV is a "stable" number.

But another important issue is: how much fuel does it take?

I'd venture to guess that with a very low TWR it takes more fuel to get the same DeltaV as you'd get with a higher TWR.

Otoh there's not much of a point in having TWR>2 at launch because that makes the rocket go faster than terminal velocity in the lower atmosphere, wasting fuel on atmospheric drag.

I aim for TWR 1.3 to 1.8 at launch, and >2 above 10km.

[jebbe]

In this thread we had an extensive discussion about optimal ascent profiles. Bottom line: If you are aiming for fuel efficiency, you should stay close to terminal velocity (as stated before), and that is achieved by a TWR of roughly 2:1 in the beginning. However, while fuel drains, the TWR increases, and fortunately that is exactly what you need to keep up with the terminal velocity. For the orbital insertion you want to use a TWR as low as possible (light, high efficiency engines) - just high enough that you get into orbit without burning away from the prograde vector too much.

Edited May 29, 2013 by jebbe

[Brofessional]

You've pretty much got it right.

For doing stuff near bodies you want higher TWR, because you need to complete your burns before you orbit too far or because you need to slow down rapidly. Since Kerbals don't run out of food or anything, for interplanetary stuff the only thing that really matters is ISP. Though you'll typically want at least a 0.2 TWR out of impatience.

[GJames]

How do you measure weight ingame?

You can now (as of 0.20) see how much something weighs by clicking on the little "i" button on the far right of the map screen.

[sturmstiger]

There have been several good discussions on this before the Great Forum Purge, but in general it's a trade-off. If your TWR is too small (but greater than 1 at lift-off), you'll end up needing more deltaV. For the flight to Kerbin orbit, the waste of deltaV is due to increased amount of time spent in low atmosphere, which meant low ISP for your engines and increased gravity loss. For orbit transfers, the waste of deltaV is due to long burn that occur in non-optimal points (for example, the theoretical minimal delta V using a Hohnman transfer is calculated with a instantaneous burn). But if TWR is big, it means you could have carried more fuel (thus more deltaV), and you're "wasting" your engine's maximum thrust. Also if TWR is too high, the structural load may also be too high for your rocket / spacecrafts, which is why many real world rockets throttles downs during parts of their flight. Also during atmospheric flight the TWR should not be too big to so your velocity exceeds terminal velocity as mentioned by several other people.

In KSP, my launch vehicles (with their maximum intended payload) tend to have a lift-off TWR between 1.2 to 1.5, while upper stage TWR are usually between 0.8 to 1.2.

[Stelith61]

For an upper stage at 40+km you only need a 0.9TWR, But for lower stages I usually use about 1.3 or 1.5.

[RoboRay]

Departing Kerbin...

Ascent stages: starting TWR of 1.7 +/-0.4

Insertion stages: starting TWR of 1.0 +/-0.3

Injection stages: starting TWR of 0.3 +/-0.2

Remember that TWR increases during the burn, so it's OK to start a little lower than what you think to be the best choice.

[jebbe]

I have browsed through the optimal ascent thread again, and dug out this: The optimal TWR is 2+2*a/g, where a is your current acceleration, and g is the accelaration due to gravity. This will actually increase from two to something like 5 for most designs during the first tens of kilometers of the ascent. Afterwards you are basically out of the atmosphere, and these considerations do not matter any more - as said before, for the orbital insertion you should try to stick with light-weight, high-efficiency engines.

[Comrade Jenkens]

On the ground anything above 1 is fine.

I space it doesn't matter, all that does is dV. Saying that I usually try to get at least 0.1 with motherships and 0.4 with 'military' ships.

[UmbralRaptor]

~17-18 N/kg on launch.

Upper stages tend to be 12-16 N/kg, depending on how much I'm expecting to have pitched over.

After hitting orbit, other considerations are more important.

[Peenvogel]

DeltaV certainly is important because that determines how far the craft can go (far as in: what orbit can it establish), that's why DeltaV is a "stable" number.

But another important issue is: how much fuel does it take?

I'd venture to guess that with a very low TWR it takes more fuel to get the same DeltaV as you'd get with a higher TWR.

Otoh there's not much of a point in having TWR>2 at launch because that makes the rocket go faster than terminal velocity in the lower atmosphere, wasting fuel on atmospheric drag.

I aim for TWR 1.3 to 1.8 at launch, and >2 above 10km.

To be honest, I never pay any attention to the fuel values. When I build a rocket, I just keep an eye on the DeltaV and that always seems to work for me regardless the TWR (given it is higher than 1.01). I've launched several rockets that had little over 1 TWR in the first stage (the number goes up slowly as fuel gets out during ascent) and all that seems to differ is the time it takes before I get into orbit... low TWR values result in a very slow ascent. I don't know the exact time, but maybe in the range of 5 to 7 minutes. My fastest rockets however can reach Kerbal orbit in under a minute.

I've build a lot of rockets that had a high TWR value (3.5 or higher), but I found out that too much thrust pushes the lowers stages through the upper ones which lead to obvious malfunctions.

Again, I'm no math guy. I just test a lot of different rockets and meticulously process the data (using mechjeb) in Excel sheets. With the data I'm trying to make a DeltaV map for every location in the Kerbol system... I know there's a DeltaV map on Wiki, but I like to make one myself and compare it with the "official numbers"... see how close I can get.

Edited May 29, 2013 by Peenvogel

[tomf]

The optimum TWR is just above 2 during the more or less vertical ascent phase of the launch.

When launching vertically there are two sources of drag on your rocket, gravity and air resistance.

The gravity slows your craft by 9.8m/s (on kerbin's surface) every second you spend going up so to minimize gravity losses you want to go up as fast as possible.

The air resistance slows your craft by an amount prportional to the square of your speed, so it is zero when you are barely moving and climbs rapidly.

At terminal velocity (as given on the wiki etc) the two forces are equal and the combined drag is minimized - you aren't hovering burning fuel more than necessary and you aren't losing to much to the rapidly increasing drag.

Since a TWR of 1 provides exactly enough force to overcome g and air drag = gravity you want TWR = 2 to keep you balanced at terminal velocity upwards.

You want to be slightly over 2 because you still need to accelerate to to keep up with the increasing terminal velocity.

Once you start your turn the required TWR decreases. I expect it is proportional to the cos of the turn angle but I can't prove that.

[rkman]

To be honest, I never pay any attention to the fuel values.

That's understandable because as of yet in KSP there is no cost to fuel. Though you might end up with rockets that are way over-build.

In real life rocketry however the amount of fuel is a major consideration, and we will have to take it into account when career mode is implemented in the game.

[Wait- Was That Important?]

I aim for 2 meters per second per second in space at minimum with nuclear engiNes, as otherwise those thousand meter per second burns drag on forever.

When I use the upper stage on my rockets instead of a Nerva though, I can get up to 50 m/s squared. It has a good kick to it!