How do i know when my rockets are too powerful and the drag is holding them back?

[Varasalvi]

Hello.

Strapping too much power on your rocket isn't smart, your launch will be so fast that your rockets are fighting too much against the drag and wasting fuel.

This is what i've heard in many tutorials and guides, but none of them have specified where the limit is or how i find that out. If anyone could explain this to me, it would be greatly appreciated. But please, try to avoid unnecessarily complicated words, i'm not a complete beginner in the game, but my space vocabulary is limited

Edited July 5, 2014 by Varasalvi

[technicalfool]

Kerbin's atmosphere and terminal velocity at different altitudes

Atmospheric pressures of different celestial bodies

Atmospheric calculations

Generally if you are going up faster than you would be free-falling at a given height, then you are going too fast.

Hope that helps!

[Varasalvi]

Sure does! You'd think i would be able to find this myself, i guess i just had the wrong keywords.

Thanks!

[GoSlash27]

What "technicalfool" said. 2Gs should be your goal during the vertical boost phase. Less than that and you're wasting DV fighting gravity. More than that and you're wasting DV fighting drag. I taper that off as I tip over into the gravity turn transstage by the function 2sin Pitch until I leave the atmosphere.

Best,

-Slashy

[Varasalvi]

What "technicalfool" said. 2Gs should be your goal during the vertical boost phase. Less than that and you're wasting DV fighting gravity. More than that and you're wasting DV fighting drag. I taper that off as I tip over into the gravity turn transstage by the function 2sin Pitch until I leave the atmosphere.

Best,

-Slashy

DV, luckily, i just learned two days ago what Delta-V is

I taper that off as I tip over into the gravity turn transstage by the function 2sin Pitch until I leave the atmosphere.

This is where you lost me though, hehe. It doesn't matter, i think i get the idea. Thanks!

[Pecan]

The simple way: if you see 'atmospheric effects' during launch - white contrails then red plasma - you're going way too fast.

[stringyquark]

Hope this helps.

[rkman]

3 options:

- lots and lots of trial and error

- learn rocket science

- use a mod (Mechjeb or Kerbal Engineer)

[EtherDragon]

What "technicalfool" said. 2Gs should be your goal during the vertical boost phase.

That's very eloquently put! It's so simple, I knew that there was a complicated chart and I only remembered some key points of it. But now that there is a simple guide (2G's acceleration) that makes a lot of sense.

[Dkmdlb]

That's very eloquently put! It's so simple

Unfortunately it's also wrong.

While it is true, in an infinitely tall atmosphere, a TWR fixed at 2 would lead to a rocket attaining terminal velocity and never leaving it until it ran out of fuel; in a REAL atmosphere such as Kerbin's, atmospheric thickness falls off as you ascend, therefore causing terminal velocity to increase as the rocket ascends, and never be reached.

Therefore, as you ascend, terminal velocity is constantly increasing, and you constantly have to "catch up" to it.

A TWR = 2 rocket will NEVER keep up with the rate of increase of terminal velocity, however, even if it attains it briefly (before it drops its SRB's, for instance), as velocity will approach terminal velocity asymptotically with a rocket with TWR = 2.

What that means is, in a real atmosphere, since it takes exponentially longer for each incremental increase in velocity towards terminal velocity, a rocket's maximum velocity at TWR = 2 when thrusting straight upwards is terminal velocity, and terminal velocity is constantly increasing as the atmosphere thins, a rocket with TWR = 2 will theoretically never reach terminal velocity, so long as it is ascending straight upwards...

The obvious answer, then, is that a rocket needs a TWR that is GREATER THAN 2 in order to attain terminal velocity during the vertical portion of its climb...

http://forum.kerbalspaceprogram.com/threads/80821-TWR-and-Terminal-Velocity

[NecroBones]

Unfortunately it's also wrong.

http://forum.kerbalspaceprogram.com/threads/80821-TWR-and-Terminal-Velocity

It's not accurate, but reworded as a general "rule of thumb", it might be safe to say: "TWR 2 is a safe target, because you will not need to worry about terminal velocty".

[-Velocity-]

Install Mechjeb or some equivalent mod that gives you TWR (thrust to weight ratio) and delta V in the VAB and also during flight.

When in the lower atmosphere, you generally want your TWR to be about 2. A starting TWR a bit less than 2 is probably actually better, considering that the more TWR you have, the more engine mass you're having to move. Also consider that a rocket with a starting TWR of like, 1.7 will probably have a TWR of like 2.3 before first stage burnout. So you want to have your first stage to have something more like an average TWR of 2. So don't fixate on getting a TWR of exactly 2. It's OK (probably preferable) to have a starting TWR less than 2!!!! And you certainly DON'T want a TWR well in excess of 2- if you have that, you're wasting a lot of fuel lifting an overly-large engine.

If you REALLY want a first stage starting TWR of 2, you can always strap on some small boosters.

Edited July 7, 2014 by |Velocity|

[wasmic]

Of course, if you use FAR (which I greatly recommend, it's not harder than stock, just different and more realistic), you don't have to worry about terminal velocity at all, because it's very high at all phases of flight. In FAR, you should aim for a starting TWR of 1.2 to 1.6 (more isn't necessarily better).

[Wallace]

So if I tick that little doodad that says 'limit to terminal velocity' on the mechjeb ascent guidance it will keep me going at the right speed?

[Red Iron Crown]

So if I tick that little doodad that says 'limit to terminal velocity' on the mechjeb ascent guidance it will keep me going at the right speed?

Yes, it will modulate the throttle to keep you at terminal velocity if your rocket is capable of attaining it. Though if you spend a lot of time at less than 100% throttle you probably have too much thrust and are carrying too much engine mass.

[NecroBones]

So if I tick that little doodad that says 'limit to terminal velocity' on the mechjeb ascent guidance it will keep me going at the right speed?

Usually MJ does the right thing, but it can also cause an oscillation. I had this happen once:

[Pecan]

Usually MJ does the right thing, but it can also cause an oscillation.

i) Moar struts!

ii) Utilities: 'smooth throttle'

iii) If all else fails: disengage autopilot, shout and it, hit it with a branch, turn it back on.*

*Oddly enough this does work, 90% of the time. During the time its off you lose sufficient speed that when it comes back on at 100% it's not exceeding terminal velocity any more.

[-Velocity-]

Usually MJ does the right thing, but it can also cause an oscillation. I had this happen once:

<Censored>

Shame on you, there could be KIDS reading this thread!

[magnemoe]

Yes, it will modulate the throttle to keep you at terminal velocity if your rocket is capable of attaining it. Though if you spend a lot of time at less than 100% throttle you probably have too much thrust and are carrying too much engine mass.

Again with the exception that if you use SRB at least other than the trashcans mechjeb will limit trust then the SRB start to run out.

In this settings the SRB works as something between a booster and a first stage.

[Red Iron Crown]

Again with the exception that if you use SRB at least other than the trashcans mechjeb will limit trust then the SRB start to run out.

In this settings the SRB works as something between a booster and a first stage.

You're right, of course. I use SRBs so infrequently that the scenario you describe didn't occur to me. Thanks for the clarification/correction.