Usual TWR

[Dilir]

Hello!

I am designing a Matlab piece of code to compute optimal design rockets (mainly for fun, but i could be useful for a hardcore career). In the current state, with an imput of dv and payload mass, it will choose the most efficient design with either 1 or 2 stages (design=fuel + engine).

The main problem is that to be able to do that, you have to imput a TWR. The point being, the lower TWR, the cheaper / lighter rocket you will ends up with. But TWR to low will make manoeuvres difficult.

So my question is : what are the typical TWR you use (using Kerbin gravity), for theses manoeuvres:

- Circularization burn

- Kerbin orbit manoeuvre (or burn to get to mun / minmus SOI)

- Munar landing

- Minmus landing

- Escape burn

- Transfer to another planet

[Th3F3aR]

For Kerbin the common TWR used is (as far as i know) 1 < TWR < 2.5

[DrMonte]

1 means you stand in place.

>2 on Kerbin means you move too fast fighting atmosphere.

Best option is 2 but...

When you burn fuel in tank your rocket gets lighter and your TWR increases. So if your initial TWR was 1.8 the final might be 3.0. That is more obvious with bigger parts than with small ones. For example with KR-1x2 you can start with TWR 1.2 and end up with 6.

Bigger TWR is no big deal on airless bodies. You should keep >=2 there.

[Streetwind]

Are you aiming to hardcode the TWR value and need a sort-of-community-survey to choose a value that represents a good compromise for most people? Or is it an input field as well, and you're just not sure what to input?

For circularization, you can technically get away with fairly low TWRs, but it depends strongly on your player skill, and your rocket's ascent profile. If you fly up relatively steep and end up with just 1km/s velocity at your apoapsis outside the atmosphere, you're going to need a huge push quickly to avoid falling back down. On the other hand, if you get to apoapsis with 2km/s, you can circularize just fine with something like 0.2 or so. I'm fairly sure that almost no one designs their rockets with an upper stage TWR higher than 1.

For all other maneuvers once you're in orbit, TWR is no longer a necessity, but rather just a convenience. Technically, the longer the burn the more TWR you need to keep burn times down, but ironically you'll see people accept really low TWRs for the really high dV burns (largely because adding more engines costs you dV). Nuclear interplanetary transfer stages often fly with as little as 0.1, while most people don't do Mun transfers below 0.5. Basically, the lower dV transfers are easier to provide with a high TWR, so you'll see this counterintuitive phenomenon with TWRs actually going down with increasingly difficult burns, creating a doubly growing burn length.

Which of those provides a mathematically optimal design? Difficult to say, because these build decisions are only partially driven by math. I mean, you're building this tool in the first place because it's often really difficult to find a suitable combination of engines and tanks, right? A lot of build decisions are made because the vessel is assembled around a certain tank or engine, or needs to be a certain length or weight, or needs to have a certain shape for utility and/or aesthetics, and so on. All these things constrain the builder's flexibility and make it difficult to cater to both dV and TWR at the same time. And ultimately, because the former is a hard requirement and the latter is just convenience, the former pretty much always wins out.

TWR once again becomes a requirement instead of a convenience when you're landing, and for this, I have a rule of the thumb: for landing, I want at least twice the takeoff TWR, better three times. Trying to land with a local TWR of 2 or below is an exercise in pure frustration, since you basically need to burn all the way from orbit to touchdown (performing a reverse launch), and starting just a second too late can splat you into the ground. For a human being, the right moment is close to impossible to estimate without sheer dumb luck... or with a healthy safety buffer, which makes for a really inefficient (dV-wise) landing. Much rather you want a small deorbit burn followed by a coast phase down to the surface and short, hard deceleration started only when you're almost there. The higher your TWR, the shorter this final burn is and thus the easier it is for a human to eyeball correctly (and the less dV you waste fighting gravity).

Edited October 31, 2014 by Streetwind

[Dilir]

I don't intend to hardcode it. The purpose of the tool is to do something like this

- I want to go to X with payload Y, to do that, i need:

- Burn 1 with Dv=... and minimum TWR =...

- Burn 2 with Dv=....and minimum TWR=...

....

With theses data, the optimal rocket design will be a N stage rocket with engine A=..., engine B=.... fuel A=...., fuel B=...., ....

For now, it can compute the optimal single stage design (engine+fuel) for a tripplet (TWR, DV, payload), and choose between a 1 stage or 2 stage design to minimize total mass.

Concerning the math, I use procedural tanks designs, so fuel tanks are not a problem. Engine have discrete performance, so their are obviously efficiency jumps at some point, but the code take that into account. The main benefits is for multi-stage designs. It is not trivial to divide the total DV in the right way. My code will tell you how to do that, and which engine to put.

[Laie]

...

*slow clap*

As to me, most orbital maneuvers are made with whatever I have available at the moment. I strive to maintain a Kerbin-TWR of 0.2 or better at all times, and hardly ever go below 0.1, but that's a convenience thing -- see Streetwinds post above.

On landings, any TWR>1 will do (and unlike Streetwind, I don't consider it to be all that difficult -- someone is bound to post Komo-Not's how-to real soon now). The only place where I really cared about my lander's TWR so far was Tylo. Most other places have such a low gravity that my TWR of convenience_in_space would be way more than enough for the landing.

[mhoram]

Tavert created a few rockets with the help of optimizers. Have not seen the code he used, but here is one of them.

You might find these interesting: http://blizzy.de/asparagus/ and KSP Optimal Rocket Calculator

For orbital maneuvers I aim to have a minimal (Kerbin-Sealevel)-TWR of 0,2-0,5.

For landing on airless-bodies I aim to have a (Local-Sealevel)-TWR of at least 2 - if it is lower, it becomes a pain, but is doable.

About Escape burns: It depends on where I want go and if I want to do a two-burn-escape (first burn: to a highly elliptical orbit within Kerbins SOI, second burn: escape). In all cases I try to keep the duration of each burn below 5-6 minutes. The longer the duration gets, the more inaccurate the burn and the worse the usage of the Oberth-effect get.

An idea for the TWR-Input: have you thought about modelling it by two input parameters, namely TWR at launch and TWR at circularization with a linear interpolation?

Edited September 26, 2015 by mhoram
typo

[Starman4308]

Optimal TWR is a complicated bag of kittens.

During ascent from a body with thick atmosphere, you want a launch TWR of ~1.6-2.0. You want to be ascending at terminal velocity, which would suggest a TWR of slightly > 2 (1 for gravity, 1 for atmosphere, a sliver leftover for acceleration), but a number of factors reduce that. #1: as you burn fuel on your first stage, it's TWR goes up. If it's mostly a solid stage, this is particularly noticeable (look Ma, no throttle!), whereas for liquid stages, it's more subtle: while you can throttle to remain at terminal velocity, the moment you do, you have wasted engine mass: dead weight causing gravity and atmo drag.

For the upper stages, TWR depends on how nice a gravity turn you pull. An ideal gravity turn should have initial TWRs declining steadily from 2.0, as the dV cost of engine mass and the decreased need for thrust begin to counteract the need to minimize gravity/aero drag, but that depends on skill and the overall TWR profile.

[cantab]

For all those tasks except maybe the Mun landing 0.5g will be fine. (And it would be adequate for a Mun landing but I'd prefer more). Enough that burns won't take ages but not excessive engine mass. Ships using the LV-N or PB-ION might run somewhat lower.