Trying to figure out deltaV vs weight

[Amoun]

Hey guys

I am getting a little more serious about KSP and trying to calculate delta V (well....looking at it via mechjeb) for different stages rather than slapping rockets on it and "seeing if it makes it".

The question is: when I was using the deltaV of mechjeb, I noticed that dV would increase with the amount of fuel I add.....ok...makes sense...but then I thought why would it increase so much when I am also adding weight? So I slapped a giant tank on it (right now its just a pod with 4 small fueltanks and small engines) and dV also goes up. Now, obviously, the engines won't lift the huge tank, yet dV is still there. So I was wondering, how do you compensate for that mass? I am trying to make a Mun lander with different stages and escape pod in case the guys need to abandon the base and go home.

Edited August 31, 2013 by Amoun

[blizzy78]

You've already answered it yourself: Adding more mass reduces the thrust-to-weight ratio (TWR). Going below 1.0 TWR makes your rocket stay on the pad instead of lifting off. Of course adding more mass will decrease your total delta-v, but the impact on TWR is much harder.

As for mass compensation, this should be obvious: You add more engines to gain more thrust.

Perhaps this might still be a little too advanced for you, but in case you're feeling adventurous, you can try my online engine layout configuration calculator to work out engine configurations given a specific payload mass. (see signature)

Edited August 31, 2013 by blizzy78

[Amoun]

Hmmm......so as long as I stay above TWR of 1, it will be effective?

What I am wondering is if the TWR and dV go hand in hand during the calculation. So say you have TWR of 1 and a dV of saaay...(whatever) 2000. If your TWR drops to 0.8, does dV automatically become 0? Or can you still have a positive dV but have a TWR below 1?

(excuse the nub q's, I tend to overthink)

[blizzy78]

You need a TWR of greater than 1 to lift off. In reality, you want a bit more than that or you will be fighting gravity for too long, wasting fuel in the process.

Total delta-v is not dependent on TWR, though. TWR is just that, thrust divided by mass. Delta-v on the other hand is a function of your engine's specific impulse and dry/wet masses of the vessel. It does not matter if your engine is slow (low thrust) or fast (high thrust), as long as the specific impulse (fuel efficiency) stays the same. You will just be accelerating faster/slower, but the total potential change in velocity stays the same with equal specific impulse.

[Amoun]

@blizzy

Yup, just played around with it.

Slapped a giant tank, TWR went below 0 and dV dropped.

But I see what you mean and that makes sense. I am trying to get a feel for TWR vs dV for when I make return vehicles.

Thank you

p.s. I know about your engine layout calculator. I think it's awesome.

edit: I cant give you rep, it's not popping up for some reason =\ I'll try again later.

[blizzy78]

I guess that's because some forum functions are broken right now since the update. Check the Forum forum for more.

[Zeroignite]

You might find the Tsiolkovsky Rocket Equation helpful.

ÃŽâ€v = g₀ * I_SP * ln( wet mass / dry mass)

Basically, for a given engine, the more fuel mass you have relative to the mass of everything else, the more delta-v you'll have.

[RadHazard]

An important thing to note is that you only need a TWR > 1 for lifting off Kerbin. Once you're in space, you can get by with TWR as low as 0.1-0.2, if you don't mind splitting up your burns into multiple parts. When making landers, the minimum TWR you need to take off or land is equal to the ratio of the body's surface gravity to Kerbin's surface gravity. For example, Duna has a surface gravity of 2.94 m/s^2 compared to Kerbin's 9.81 m/s^2, so a Duna lander would only need a TWR > 0.3 to be able to take off.

If you're trying to get a lot of dV, a good rule of thumb is to use the nuclear engines for any ship larger than a few tons. Tavert has a great series of charts that can help you decide what engines to use.

[Amoun]

@RedHazard

huh......yeah I remember seeing that chart and other than thinking that it looks pretty I did not get into it. Once I started paying attention to the TWR it started to make more sense. Thank you for reminding me of them, they are extremely useful as I friggin' love efficiency.

I am not sure what you meant about parts of the burn? As in longer burns/multiple burns in the same "spot"?

So far I got the engineering redux and that thing is delicious. TWR for all the bodies and dV for atmo. love it.

@Zero

I tried to fiddle with it but, I won't lie, I feel lazy manually calculating everything...............for now. I say for now because the more I advance in the game, the more involved I become, like, for example, as I posted before: I used to slap things together and then saw if it worked

[Zeroignite]

@Zero

I tried to fiddle with it but, I won't lie, I feel lazy manually calculating everything...............for now. I say for now because the more I advance in the game, the more involved I become, like, for example, as I posted before: I used to slap things together and then saw if it worked

Yeah, in KSP I never do the math by hand... MechJeb gives me a nice instant readout. Knowing the underlying math is useful to getting an intuitive sense for optimization though