How do i read KER?

[modybird]

Im new to ksp, downloaded it people said it would give me a good reading on if i would have enough "delta v" or if my ship is too big etc, where do i find all this info? Sorry if its a stupid question but i really dont know

 

Here is a picture of the KER stats

[Jas0n]

The delta V of a rocket is a measurement of how much acceleration it can produce (Someone correct me if I'm wrong). Basically the mileage on a car.

This is a delta V map. To find how much delta v you need, add up all the numbers from Kerbin to your destination. 

For example if you're going for a Minmus landing then return, you would need 3400 + 930 + 160 + 180 + 180 + 160 + 930  = 5940 m/s of delta V.

Note that you can save a lot of fuel by aerobraking.

In the KER window under delta V, the first number is how much delta V that stage has and the second number is the total delta V of the rocket up to that stage. 

[Starman4308]

Column second-most to the right is where KER is printing out its delta-V estimate.

It goes by stage, showing both the stage delta-V and cumulative delta-V. I'm not sure on where to set KER to show atmospheric delta-V*, but there's probably a toggle somewhere.

*Being in atmosphere means that your engines produce less thrust, and since you're still consuming the same amount of propellant, your Isp is dropping by the same factor. Very loosely, you can think of it as atmosphere pushing back on your exhaust gases. First stage type engines like the LV-T45 and Mainsail trade off some vacuum Isp to improve their sea-level thrust/Isp.

Overall, it looks like you might not know what delta-V is, which is a very important concept. Delta-V is essentially your ability to use engines to change your current velocity, and is determined by how much fuel you have and how efficient your engines are. You use delta-V to execute maneuvers; if you have no delta-V (no fuel) remaining, you're at the mercy of gravity, atmospheres, and lithospheres.

 

In the absence of gravity: if you are traveling 2000 m/sec to the right, and have 1000 m/sec of delta-V, you can use that 1000 m/sec to:

Burn prograde and hit 3000 m/sec right

Burn retrograde and hit 1000 m/sec right

Burn perpendicular to that vector and achieve a velocity of 2,236 m/sec (2000 m/sec component in the original vector, 1000 m/sec component perpendicular)

 

Delta-V can be calculated manually using the famous Tsiolkovsky Rocket Equation: dV = Ve * ln(mass of full stage / mass of empty stage).

dV is your delta-V, Ve is your exhaust velocity, ln is the natural logarithm, and it's important to remember that mass of the empty stage is not just payload, but also empty fuel tanks, engines, etc. Anything that isn't fuel.

 

Now, you may ask yourself, "where is that specific impulse everybody talks about?". That is in the Ve term. Specific impulse is equal to exhaust velocity divided by 9.8063 m/sec^2 (surface gravitational acceleration on Earth/Kerbin). As such, the Tsiolkovsky equation is often rewritten as:

dV = Gm * Isp * ln(full / empty)

 

That raises the question of why we use something so silly as Isp instead of just listing exhaust velocity. You can thank me and my fellow Yanks for still not switching over to MKS. You see, early in rocketry, people in the US were using ft/sec for exhaust velocity, while everybody whose nations had converted to sane units was using m/sec. To make things simpler, they just decided to divide by the surface gravity of Earth (9.8 m/sec^2, 32 ft/sec^2), getting a measurement that depended only on the unit of time, and at the end, they would just multiply back by that gravity to get exhaust velocity back.

 

The final major thing to note is what the equation looks like when you flip it around, set a desired delta-V target and ask "how much fuel do I need relative to the empty mass".

full/empty = e^(dV / Isp * Gm)

You'll notice that's exponential, the tyranny of the rocket equation. For large delta-V, Isp becomes hugely important, because you get to divide dV by Isp before taking that exponent.

It also imposes some fundamental constraints and requires staging: for stock (where a full fuel tank is 9x heavier than when empty), the absolute maximum you can get out of an engine without staging is Gm * Isp * ln(9).

[Alshain]

21 minutes ago, Jas0n said:

The delta V of a rocket is a measurement of how much acceleration change in velocity it can produce (Someone correct me if I'm wrong). Basically the mileage on a car.

Almost.  Acceleration is a measurement of the rate of change in velocity, while delta V is a measurement of the amount of impulse it takes to change velocity for a specific maneuver.

[modybird]

16 hours ago, Jas0n said:

The delta V of a rocket is a measurement of how much acceleration it can produce (Someone correct me if I'm wrong). Basically the mileage on a car.

This is a delta V map. To find how much delta v you need, add up all the numbers from Kerbin to your destination. 

For example if you're going for a Minmus landing then return, you would need 3400 + 930 + 160 + 180 + 180 + 160 + 930  = 5940 m/s of delta V.

Note that you can save a lot of fuel by aerobraking.

In the KER window under delta V, the first number is how much delta V that stage has and the second number is the total delta V of the rocket up to that stage. 

How did i reach minimus  and back with 4334 delta v when i needed 5940?

[Jas0n]

15 minutes ago, modybird said:

How did i reach minimus  and back with 4334 delta v when i needed 5940?

From your picture your rocket is not staged correctly. Stage 3 has both an engine and a stack decoupler. KER did the calculations based on that and completely ignored that stage.

[Starman4308]

Two things:

First, you're mis-reading the delta-V map a bit. On the way back from Minmus, you don't need to spend 930 m/sec again. On the way out, it's necessary to burn for 930 m/sec to turn your circular parking orbit into a highly elliptical transfer orbit; on the way back in, you can just let atmosphere brake you back down.

In addition to that, the delta-V guide is a rough estimate, and it's entirely possible to get to Minmus for less. The theoretical lower bound is going to be about 4000 m/sec to Minmus and back, and you could convince me it could be practically done for around 4700 m/sec with an excellent pilot and efficient launch vehicle.

Also, what Jas0n said; KER is dutifully reading the bad staging you have in the VAB and ignoring the first liquid stage (with the LV-T30 or LV-T45, not sure exactly which), because with the staging as-is, that stage is dropped the moment the engine is fired.

EDIT: Also, as a side note, your vehicle has a lot of really unnecessary fuel lines and struts. At most, you need one pair of struts to secure the SRBs to the liquid core stage, the fuel lines attached to the SRB are completely unnecessary (there's no fuel to transfer!), and I'm pretty sure the fuel lines from the side tanks on the lander are unnecessary*.

*At some point I should really get back to playing bone stock so I know what is and isn't still true of stock physics; I remember having to edit a line in physics.cfg to make radially attached tanks feed through.

Edited June 28, 2017 by Starman4308

[DStaal]

On 6/28/2017 at 7:12 PM, Starman4308 said:

First, you're mis-reading the delta-V map a bit. On the way back from Minmus, you don't need to spend 930 m/sec again. On the way out, it's necessary to burn for 930 m/sec to turn your circular parking orbit into a highly elliptical transfer orbit; on the way back in, you can just let atmosphere brake you back down.

That's a bit of a mis-representation.  You do need to have the 930 m/s somehow - but it doesn't have to be via fuel.  The point is that you need that much d/v to do that transfer, however you get it.  Areobraking to get all or most of it is just another way to get the d/v.  The total stays the same.

(General point is right, but it just sounded wrong to me.)