deltaV and gravity

[Kerbocracy]

General KSP science question: does deltaV potential of a given rocket change depending on where you are in a gravity well? My intuition tells me that for a given rocket, it should have less dV in a bigger gravity well (e.g. Jool vs. Kerbin) and also it should have less dV when it is closer to the body than when it is farther away. For a fixed amount of energy in a rocket, the rocket should change it's velocity (e.g. delta V) more or less depending on the gravity acting on it. But I'm not sure if this is the case in KSP: I'm using Engineer Redux to estimate dV on a rocket I'm building, and it allows you to select different planets. The TWR changes from planet to planet, but the dV for the rocket system remains the same across planets. I find this counter-intuitive; shouldn't a rocket be able to change it's velocity less in a larger gravity well?

I'm trying to build an Eve return craft so I'm scrounging for dV...

[SargeRho]

No. The ammount of deltaV you need to escape changes, not the one you have.

[Superfluous J]

You have the same amount of delta V in your rocket (with full tanks) no matter where it is be it on Kerbin, on Eve, in orbit, or whatnot.

The reason you can actually get going faster by burning all of your fuel if you're in orbit than you could if you were on the surface of Kerbin is because when you're in a gravity well, it's as if you are accelerating downward at a constant rate*. So, unless you're on the ground, you need to be accelerating about 10m/s just to lift off of Kerbin. If you sat there for 10 seconds, you'd use up 100 delta V, but not actually add much to your velocity.

That's why Thrust to Weight Ratio (TWR) is so important. When lifting off the surface of a planet, you want to get away from it AS FAST AS POSSIBLE to lower that delta V wasting surface gravity down to 0 as quickly as possible. However, on planets with atmospheres you don't want to go above terminal velocity either, which is why a TWR of about 1.6 - 1.8 or so is the target. You want to go fast, but not so fast that you fight air resistance.

*Clarification: You actually ARE accelerating downward at a constant rate. That's why if you jump off a ledge, you fall

[annallia]

SargeRho is right. Your amount of delta-v wont change based on gravity, only how much of it you need to escape said gravity.

[DChurchill]

Think of it in terms of potential energy. The amount of energy you have is based on how much fuel you have, how heavy your craft is, and how efficient the engines are in converting that energy to motion. How much energy you need to accomplish what you want is mostly based on the gravity of whatever system you happen to be in (and atmospheric drag if you're in atmosphere).

In a ship with no other forces acting on it (gravity or drag), the amount of dV that you have is how much your velocity will change if you use it all. Gravity and drag subtract from that total.

[Kerbocracy]

I a ship with no other forces acting on it (gravity or drag), the amount of dV that you have is how much your velocity will change if you use it all. Gravity and drag subtract from that total.

Thanks for the replies guys; it makes sense. I see that my dV on Eve is the same, but that the gravity well is deeper and steeper, so I spend the dV of my rocket and only get part way to escape compared to on Kerbin. In addition to the gravity well being deeper and steeper on Eve vs. Kerbin, the negative acceleration due to gravity following lift off is eating into my total dV, and worse so on Eve than Kerbin, correct? That is, immediately following lift off, on Kerbin I'm losing 9.8 m/s just hovering, and on Eve I'm losing 16.7 m/s doing same.

So I'm getting the sense that deltaV as it is expressed in Engineer Redux is like 'total delta V in a vaccuum', and I'm more concerned with something like 'effective delta-V' which is the amount a rocket system can actually change it's velocity on a given body and with a given ascent profile and subtracting gravity effects, drag, etc.... Perhaps harder to estimate.

On the Wiki when it say that you need 12.5km/s to orbit Eve from sea level, is this in total deltaV in a vaccuum or is it in effective deltaV? I worry that I might need much more than 12.5km/s... Right now my system has about 10.5km/s and 100,000kg. Don't want to go much heavier...

Also, I'm probably getting a bit confused because I was trying to estimate a rocket's effective dV empirically by taking off from kerbin, straight up, and seeing what my velocity above the navball reads when I'm out of fuel. My 10.5km/s rocket was finished at about 8.8km/s navball reading. Is that a good measure of effective dV (e.g. total - drag/gravity from kerbin)?

[DChurchill]

So I'm getting the sense that deltaV as it is expressed in Engineer Redux is like 'total delta V in a vaccuum', and I'm more concerned with something like 'effective delta-V' which is the amount a rocket system can actually change it's velocity on a given body and with a given ascent profile and subtracting gravity effects, drag, etc.... Perhaps harder to estimate.

Depends also on how efficiently you fly the rocket. dV losses to steering come into play as well (lateral losses due to your engine gimbaling away from centerline or when your force vector isn't aligned with your velocity vector) and when you do your gravity turn. So Engineer can't know how good of a pilot you are. Mechjeb has a tool that measures your ascent stats and will give you what your losses are due to gravity, drag and steering.

That number for dV for Eve is the total dV you need, given a fairly optimal ascent profile. If you fly well, that's how much dV you need to get to orbit. 10.5 is not enough to get you there from sea level. Kerbin needs ~4,400 dV to get to a 100 km orbit. That orbit is ~2,240 m/s, so you waste ~2,160 in overcoming gravity and drag. If I build my rocket with ~5,000 dV, then I'm ok with some left over for orbital maneuvers. You don't want to overdo the reserve, though.

Edited November 4, 2013 by DChurchill

[DMagic]

Also, I'm probably getting a bit confused because I was trying to estimate a rocket's effective dV empirically by taking off from kerbin, straight up, and seeing what my velocity above the navball reads when I'm out of fuel. My 10.5km/s rocket was finished at about 8.8km/s navball reading. Is that a good measure of effective dV (e.g. total - drag/gravity from kerbin)?

Delta-V is just a sort-of weird way of measuring the total energy output of your rocket (though that's not quite right). But part of the confusion is that delta-v is a function of ISP, and ISP is a function of the atmospheric density. So you'll have a different delta-v depending how dense the atmosphere is and mods like engineer or mechjeb can't quite deal with this. You can calculate delta-v for a specific ascent profile, but it will be a little bit different if you don't fly that same profile.

Another problem, and this one prevents you from calculating it based on initial and final velocity like you describe above, is the effects of gravity and atmospheric drag. The amounts lost to drag and gravity are highly dependent on how you fly and your rocket design. Flying straight up means you are fighting gravity the whole way, so you have huge gravity losses (and yes, gravity loses are worse on planets with higher gravity). Flying almost straight sideways means you will have huge drag loses. That's why there is so much discussion about ideal ascent profiles. The ideal profile will minimize your gravity and drag loses, but it's a little hard to calculate exactly how much you lose, and it can be difficult to actually follow that ascent profile, too. TWR is another matter, and there are about a thousand threads about ideal launch TWR.

[Specialist290]

Thanks for the replies guys; it makes sense. I see that my dV on Eve is the same, but that the gravity well is deeper and steeper, so I spend the dV of my rocket and only get part way to escape compared to on Kerbin. In addition to the gravity well being deeper and steeper on Eve vs. Kerbin, the negative acceleration due to gravity following lift off is eating into my total dV, and worse so on Eve than Kerbin, correct? That is, immediately following lift off, on Kerbin I'm losing 9.8 m/s just hovering, and on Eve I'm losing 16.7 m/s doing same.

That's exactly correct, and the effect you've been noticing is called "gravitational drag." Actually calculating it is rather complicated (although you can have a look at the math over at this page on Atomic Rockets), but you're right in that a higher TWR will ultimately result in lower gravity losses because of the fact that you're spending less time accelerating against gravity.

At the same time, of course, you also have atmospheric drag, which will also reduce your effective delta-v as you try to ascend through the atmosphere, which gets especially significant if you try to surpass terminal velocity. Ultimately, for takeoff from Kerbin, you want a happy medium that accelerates quickly enough to negate most of your gravitational drag while not so quickly that you end up generating too much atmospheric drag.