Thrust/weight ratio + fuel amount

[dino1984]

What thrust/weight ratio do you assume for your launches?

And additionally - how do you calculate how much fuel to take?

[Wemb]

Between 1.5 and 2 would be pretty normal at launch - Rockets with much below 1.5 will struggle to get off the pad, and nearer 2 or above are over powered and you'll either have to throttle back (meaning you're wasting fuel) or will burning up/become unstable of you fly them too fast in the atmosphere.  SRBs on your rocket can push your initial TWR fairly high, but normally they would be dumped fairly quickly - before the lack of steering or overspeeding would become an issue.  The aim of them is to get you off the pad and moving as fast as possible  - the less time your spend at the bottom of Kerbin's atmosphere the better.

For calculating how much fuel you need, your best starting point is the delta-V maps - see this thread. 

They tell you how much 'dV' is needed to do the various maneuvers needed to get somewhere from Kerbin.  delta-V is the amount your rocket can change it's velocity by - and can be calculated knowing the mass of your rocket, the types of engines it has and the amount of fuel you have brought with you.  To get useful data on that, installing somethng like KER is an essential tool for designing appropriate rockets for a particular mission. Thought the maths needed to calculate it all is here:

http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet#Delta-v_.28.CE.94v.29

Wemb

 

[dino1984]

Thanks, that's the science I need.

[Plusck]

To get rough figures on a purely stock install (no MechJeb, no KER), I made a basic spreadsheet for simple rockets (just the rocket equation repeated on every line, with space to add in Isp and payload, then either total fuel or required dV).

I never got around to developing my spreadsheet to cover multiple stages and asparagus fuel routing, so I generally used this tool: https://garycourt.github.io/korc/, again only to get a rough idea of an efficient arrangement and a basic ballpark figure for the cost. Don't leave the tool running while you play though because it completely hogs the processor (I'm testing its suggestions for a 40t payload right now and for the last few minutes it's been using 80-100% of all eight CPU cores...).

[Snark]

There are other considerations for TWR besides just fuel efficiency.  For example, piloting.  Unless you're letting a mod do all the piloting for you, it can be tricky to judge how much of an eastward nudge (and when to do it) for a well-executed gravity turn.  It takes a lot of practice to get good at it.

One way to make it easier on yourself to climb that learning curve is to choose the TWR you like and stick to it religiously.  If all your rockets have the same launchpad TWR, then they'll all follow fairly similar flight profiles in those crucial first couple of minutes of flight time, which makes it easier for you to learn how to judge the turn.  (In my case, I like to use 1.5 as my TWR, it works well for me.  Pick one that works well for you.  YMMV.)

[GoSlash27]

I use 1.4 t/w minimum. I calculate the fuel by reversing the rocket equation.

Best,
-Slashy

[dino1984]

One more question. Do you consider your orbit-insertion engine to have TWR >1 ? As you're already in vacuum, it should not be a 'must', but will I be able to reach 7,9 km/s (RSS+realism overhaul).

Edited March 30, 2016 by dino1984

[Plusck]

39 minutes ago, dino1984 said:

One more question. Do you consider your orbit-insertion engine to have TWR >1 ? As you're already in vacuum, it should not be a 'must', but will I be able to reach 7,9 km/s (RSS+realism overhaul).

No, not at all. It's much more comfortable to play with a high TWR, and a touch more efficient in pure dv terms, but decidedly inefficient in terms of cost and/or total engine mass and/or payload fraction.

I haven't tried RSS/RO but the principles should be exactly the same. If your ascent is efficient, you'll be nearly horizontal on a very shallow suborbital trajectory (i.e. a long time still to reach Ap) by the time you get near to orbital height. A relatively low TWR should be sufficient over that long time to Ap to push your velocity up to a full orbit.

 

However, I can't help but wonder: are you sure that RSS+RO is right for you at this stage? From what I have gathered it is difficult, with very little room for error. If you're wondering about attaining orbital velocity, you might be better setting yourself efficiency challenges in stock (such as getting to orbit with less than 3200 m/s) while doing other missions, rather than undertaking what might seem to be a Herculean task all on its own...

[dino1984]

Thanks for answer!

 

5 minutes ago, Plusck said:

are you sure that RSS+RO is right for you at this stage?

I'm ambitious   After all it gives a lot of satisfaction to even put things into LEO, not mentioning lunar missions and so on...

[Snark]

1 hour ago, dino1984 said:

One more question. Do you consider your orbit-insertion engine to have TWR >1 ? As you're already in vacuum, it should not be a 'must', but will I be able to reach 7,9 km/s (RSS+realism overhaul).

+1 to Plusck's answer above.

One way to think of it:  Everything is a tradeoff.  Getting more of one thing means getting less of something else.  So a general philosophy is "don't provide something unless it's actually needed", because otherwise it's costing you more than it's worth.

For example:  TWR is important for getting off the launchpad, because you have to go up and you're fighting directly against gravity.  Therefore, TWR is one of the most important considerations for getting off the launchpad, and (to a lesser degree) is often a relevant consideration for your 2nd stage as well.

But by the time you've climbed high enough that you're going mostly horizontally... well, TWR isn't so important then.  If you're going mostly horizontal, you're not fighting gravity, so TWR simply doesn't matter, as long as it's not so low that you catch up to your Ap.

And the important point there is that in such a situation, having a high TWR is not only not-helping, it is actively hurting you.  That's because the really high-TWR engines tend to be heavy and not very fuel-efficient.  Heavy = dead weight = bad.  Low fuel efficiency = wasting fuel = lower dV = bad.  So when you are in a place where you no longer need the TWR so much, what you really want is an engine that is as light and fuel-efficient (i.e. high vacuum Isp) as possible.  Those engines tend to be low-TWR.

It means that those engines would be a lousy choice on the launchpad, but they're great for circularizing your orbit-- a much better choice, in fact, than a heavy/inefficient high-TWR engine.

[OhioBob]

I generally like my liftoff TWR to be between 1.3 and 1.5.  When I'm using an all liquid launcher, I prefer the low end of the range, about 1.3-1.4.  When using solids, I prefer being closer to 1.5.

Using a high TWR means that you can usually get to orbit using less Δv, but that's not always the best metric to go by.  If you are playing a career game where funds matter, using a low TWR generally means that your cost per ton of payload is lower.  This is because you can launch the same payload with a smaller and less expensive engine.  Granted, you'll be less efficient in terms of Δv, but who cares if you are saving funds.

As other have said, having a high TWR in orbit is not very important.  Of course if you are using the same upper stage that you used during launch, then you're going to have a relatively high TWR.  But if you are using a stage that is dedicated for use in space, then you generally want to use as small an engine as practical to save mass.

The factor that I take into consideration when selecting the TWR for an upper stage is the length of the burn.  If your burn is too long, inefficiencies are introduced because you are sweeping through a large portion of your orbit while burning.  The rule of thumb is that you don't want to pass through more than about 60 degrees of an orbit while burning.  For Kerbin orbit, this works out to about 5 minutes.  Therefore, I like to have a TWR large enough that I don't need more than a 5-minute  burn.  In general, if you plan to perform an ejection burn of 2000 m/s, then you need a TWR at ignition of about 0.5 to limit the burn to 5 minutes.  For smaller burns you can get by with a lower TWR, and for larger burns you may want a higher TWR.

 

Edited March 30, 2016 by OhioBob

[PnDB]

I have found -- at least in the earlier part of the game -- that it is challenging to get a good TWR at launch without starting to get really hot during ascent.  For example, starting with a TWR of 1.4 and using a decent gravity turn, I usually end up as a red glowing streak around 20-25km.  I don't get so hot that I explode, but sometimes it does get a bit toasty on the way to space.

Is it common practice to throttle back during the ascent to maintain the TWR one had at launch, or do folks usually just set the throttle to a single level and ride the fire all the way up?

[OhioBob]

3 minutes ago, PnDB said:

Is it common practice to throttle back during the ascent to maintain the TWR one had at launch, or do folks usually just set the throttle to a single level and ride the fire all the way up?

I keep the throttle at 100% all the way.  Pre-1.0, it was common to throttle back during ascent to keep the rocket from exceeding terminal velocity.  However, terminal velocity is now high enough that it is almost impossible to reach it unless you have insanely high TWR.  Therefore there is no reason to throttle back unless you are experiencing control problems, e.g. the rocket won't turn or it wants to flip around.  As long as you have a reasonable launch pad TWR, there shouldn't be a need to run at anything less than full throttle.

[Empiro]

I too also launch with a TWR of around 1.3 these days.

Since the air quickly gets thin with altitude, between your engines increasing in thrust and your rocket getting lighter, you don't need a very high TWR on the pad to get good acceleration toward the end of your stage.

[GoSlash27]

20 hours ago, dino1984 said:

One more question. Do you consider your orbit-insertion engine to have TWR >1 ? As you're already in vacuum, it should not be a 'must', but will I be able to reach 7,9 km/s (RSS+realism overhaul).

 dino1984,

 I use a t/w of 0.7 for my orbital insertion stage and 0.5 for transfer stages.

Best,
-Slashy

[dino1984]

16 minutes ago, GoSlash27 said:

 I use a t/w of 0.7 for my orbital insertion stage and 0.5 for transfer stages.

Do you pass Apogee during insertion burn or you manage to get orbit before Apogee?

[GoSlash27]

31 minutes ago, dino1984 said:

Do you pass Apogee during insertion burn or you manage to get orbit before Apogee?

dino1984,

 I maintain 45 seconds before apogee during the full- throttle portion of the insertion burn. I have trouble doing this with less than .7 t/w ratio.

Best,
-Slashy

[Wemb]

23 hours ago, dino1984 said:

One more question. Do you consider your orbit-insertion engine to have TWR >1 ? As you're already in vacuum, it should not be a 'must', but will I be able to reach 7,9 km/s (RSS+realism overhaul).

It doesn't have to - and you're right, once you're in space you're not fighting an atmosphere.  But!   If you're suborbital, you are running against the clock, and a too low thrust engine might not get you into orbit before you re-enter the atmosphere.  

 

As for the apogee.  Ideally, you would circularise your orbit (or, at least, get both Pe and Ap out of the atmosphere and above 70km) at the moment you hit the Ap.  This is, of course, not actually possible - but the best you can do is make the mid-point of your orbital insertion burn happen at the Ap.

It's entirely possible to get a stable orbit doing this burn before or after the Ap, but doing so will be slightly less efficient, since you'll be adding a 'vertical' (well, radial) component to the burn, instead of a purely horizontal (well,  pro-grade) burn.

The main problems with not doing the orbit burn exactly at the Ap is that (most likely) you'll not get the intended Ap and Pe you expected and, at worst, have to do another burn to circularise your orbit at another point.  Doing it at the Ap simplifies the whole process - your required vector will be exactly on the pro-grade marker, your final Ap/Pe will be what your current altitude is before you started the burn, it'll be easier to get the exactly Pe/Ap needed/to circularise and you minimise the amount of fuel needed to do it.

Wemb